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Topic: If A Plane Took Off A Conveyor Belt...
Username: Sovietjet
Posted 2005-11-29 07:30:34 and read 32767 times.

hey guys there is a huge debate over at another forum concerning this question...

Imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
There is no wind.
Can the plane take off?

Just wanted to know what you guys thought about it. I say it's not taking off.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: BA
Posted 2005-11-29 07:39:49 and read 32767 times.

As long as the aircraft is moving at lift speed regardless of how it is moving at that speed, it can take-off.

If the wings are generating enough lift, the plane will lift into the air.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Joness0154
Posted 2005-11-29 07:42:07 and read 32767 times.

No, the plane will not takeoff.

The reason? Relative wind.

Planes make lift by air moving over the wings. Whether the wind is blowing, or the engines pushing the plane forward, air is moving over the wings to provide lift.

If you are stationary on a conveyor belt, there is absolutely no wind moving over the wings, and hence no lift.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: BA
Posted 2005-11-29 07:49:59 and read 32767 times.

Oops, I think I misunderstood your question Sovietjet...

I thought you were stating if the conveyor built is run at a high enough speed to propel the aircraft up to lift speed, but I just noticed you meant running the conveyor belt in the opposite direction, while the aircraft moves forward at that same exact speed. Thus it stays stationary.

So what you're saying is if the plane is running on a treadmill.  

Then yes, it will not fly as it is not physically moving forward. It's position is remaining fixed the entire time.

[Edited 2005-11-29 07:51:02]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Diamond
Posted 2005-11-29 08:54:41 and read 32767 times.

It's not about how fast you can make the wheels spin. It's about how fast the air moves over the wings.

So the plane will roll along but never lift off.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Nighthawk
Posted 2005-11-29 09:15:22 and read 32767 times.

yes the aircraft would take off, the conveyor belt will cause the wheels to spin in the opposite direction of movement, but eventually the thrust from the engines will counter the push of the conveyor belt. The wheels will spin at an infinite speed, but this will not push the aircraft back, the force of the engines will move the aircraft forward and it will eventually take off normally. The aircraft will continue to move forward at takeoff speed, but the wheels may well be spinning at 1000+ mph.

Just because the wheels are turning doesnt mean the aircraft is moving with them!

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Joness0154
Posted 2005-11-29 09:26:22 and read 32767 times.

Hmmm interesting. The more that I think about it, the more I think it could happen. I don't really know how to put it into words though.

I guess you can say the thrust of the A/C is pushing against the air, and would therefore propel it forward, regardless if the conveyor belt was matching the speed of the wheels or not.

So, the speed of the wheels could be equivalent to, say, 100mph, but the airplane could actually be going 300mph, or have a 200mph relative wind.

So yes, I change my mind. It would be possible, but it would take a longer distance than a regular runway due to lack of friction. (ie a car on ice accelerating vs a car on pavement)

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Airplanepics
Posted 2005-11-29 09:28:50 and read 32767 times.

I think we have another "How many pigeons would it take to lift a 747" thread! Big grin

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Joness0154
Posted 2005-11-29 09:40:12 and read 32767 times.

Think about a plane taking off on ice, with all the wheels locked

The plane would still accelerate, even though the wheels are not moving.

Its a similar situation if you can visualize it.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: RichardPrice
Posted 2005-11-29 09:44:33 and read 32767 times.

Quoting Nighthawk (Reply 5):
The wheels will spin at an infinite speed, but this will not push the aircraft back

The conveyor belt in the question is not designed to push the aircraft back, just match the rotation speed of the wheels.

The wheels on an aircraft are just freewheeling at takeoff, with forward thrust provided by the engines but the traction and forward motion is still a component of the wheel/ground interaction. An aircraft at takeoff is never going faster than the rotation speed of the wheels.

If the conveyor belt exactly cancelled out the motion of the wheels, then as others on this thread have said, there would be no air movement over the wings and thus no lift produced. The engines could be generating 100% thrust but if theres no forward motion because its cancelled out by the moving ground then the aircraft will never get into the air.

Quoting Joness0154 (Reply 6):
I guess you can say the thrust of the A/C is pushing against the air, and would therefore propel it forward, regardless if the conveyor belt was matching the speed of the wheels or not.

However the thrust is provided, the actual motion is set against whatever the aircraft is sitting on at the time. If that 'ground' acts negatively on the forward motion in any way then you reduce the effect of the thrust.

Thust doesnt produce some magical component that makes aircraft fly, it needs to produce forward motion. If that motion is retarded by the ground moving backward at the same speed as the forward motion, then you are never going to get air movement over the wing.

Think of how a tail wind affects an aircraft on takeoff - it reduces the lift produced by the wing and can prove to be costly as now you need to either go faster to lift off, or climb slower. Just think of this that you have a tail wind that ALWAYS matches the speed of the aircraft.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: HAWK21M
Posted 2005-11-29 09:52:39 and read 32767 times.

If you can generate that speed on the Belt & have the Engines at T/O thrust at the same time  Smile
regds
MEL

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Joness0154
Posted 2005-11-29 09:55:20 and read 32767 times.

Quoting RichardPrice (Reply 10):
However the thrust is provided, the actual motion is set against whatever the aircraft is sitting on at the time. If that 'ground' acts negatively on the forward motion in any way then you reduce the effect of the thrust.

Then, by your account, an aircraft would never fly. Thrust acts in relation to the air, not anything on the ground.

Quoting RichardPrice (Reply 10):
An aircraft at takeoff is never going faster than the rotation speed of the wheels.

Again, I would have to disagree. If you held the brakes on ice, and firewalled the throttle, you would still move forward, even though your wheels are not. There are a few situations in which the wheels could be moving slower than the aircraft.

The airplane would move forward in relation to the solid ground even though it is on the conveyor belt. Anyone have a model airplane and a treadmill?

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: RichardPrice
Posted 2005-11-29 09:59:00 and read 32767 times.

Quoting Joness0154 (Reply 9):
Think about a plane taking off on ice, with all the wheels locked

The plane would still accelerate, even though the wheels are not moving.

Its a similar situation if you can visualize it.

Wouldnt work.

ALl of you claiming it will work are overlooking one fairly major point:

The thread starter said that the conveyor belt moved backwards to match the forward speed of the wheels at all times.

This will cancel out all forward motion, totally and utterly. You could stick a million jet engines behind the aircraft, but it must be moving forward to generate lift and if the conveyor belt cancels out that forward motion then the aircraft isnt going anywhere. Until take off, the forward motion is a component of the wheel/ground interaction and since you are removing this component then the aircraft cannot go forward.

Aircraft are just big road vehicles until they move quick enough to generate lift. An aircraft doesnt just leap into the air and its off.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: RichardPrice
Posted 2005-11-29 10:05:23 and read 32767 times.

Quoting Joness0154 (Reply 12):
Then, by your account, an aircraft would never fly. Thrust acts in relation to the air, not anything on the ground.

Thrust acts accordingly to Newtons Third Law of Motion - every action has an equal and opposite reaction. In the case of an aircraft, the reaction of the engines is that of forward motion, against whatever medium it is stationary. But the ground the aircraft is sitting on in this case is NOT stationary, its providing an exactly CANCELLING force pushing the aircraft back.

Quoting Joness0154 (Reply 12):

Again, I would have to disagree. If you held the brakes on ice, and firewalled the throttle, you would still move forward, even though your wheels are not. There are a few situations in which the wheels could be moving slower than the aircraft.

Yes, because ice is not frictionless. What we are talking about here is a PERFECTLY NEGATIVE FRICTIONAL surface, a surface that provides exactly the opposite friction to the wheels and thus an opposite force to the forward thrust of the engines.

To the laws of physics gentlemen, the aircraft will be moving forward at take off speed, but since an opposite force is being exerted on the aircraft, it isnt actually going anywhere.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Nighthawk
Posted 2005-11-29 10:16:26 and read 32767 times.

Quoting RichardPrice (Reply 14):
Thrust acts accordingly to Newtons Third Law of Motion - every action has an equal and opposite reaction. In the case of an aircraft, the reaction of the engines is that of forward motion, against whatever medium it is stationary. But the ground the aircraft is sitting on in this case is NOT stationary, its providing an exactly CANCELLING force pushing the aircraft back.

But the engines are acting on the air, which is stationary, so the engines by newtons theory will still produce forward thrust, moving the aircraft forward relative to the air.

The conveyor belt speed is set to cancel the speed of the wheels turning, not the forward movement of the aircraft. The thrust of the engines will push the aircraft forward, and the conveyor belt will rotate the wheels in the opposite direction. The turning of the wheels will not excert any force on the aircraft, so it will continue to move forward.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Joness0154
Posted 2005-11-29 10:26:40 and read 32767 times.

Quoting RichardPrice (Reply 14):
To the laws of physics gentlemen, the aircraft will be moving forward at take off speed, but since an opposite force is being exerted on the aircraft, it isnt actually going anywhere.

Ahh, yes. Physics. I was actually a physics major for 3 years. There is no opposite force being exerted on the aircraft.

There is no force counteracting the thrust of the aircraft, therefore, the thrust would propel the aircraft forward.

At first I thought of it your way Richard, but after thinking about it for a while and the physics side of things it will be possible for the aircraft to build speed.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: RichardPrice
Posted 2005-11-29 10:31:12 and read 32767 times.

Wheres the lift coming from in order for the aircraft to take off?

Thats all you need to work out.

If the force exterted backward on the aircraft is the same as the force exterted forward by the engines, then theres going to be no lift. Simple as that.

In this case, theres no air movement because the air is technically acting as a tail wind in exactly the same proportion as the forward motion of the aircraft. Thus no lift.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Joness0154
Posted 2005-11-29 10:41:56 and read 32767 times.

I don't think you're seeing the full picture. You have to look at the overall scheme of things.

There is no force being exerted backwards on the plane. None at all. The wheels are not physically connected to the engine, so the speed they move at is irrelevent to the speed of the actual aircraft itself.

Because the wind is calm, we are assuming, the thrust from the engines will work against it, therefore propelling the plane forward in relation to the air. The wheel speed and conveyor speed can totally be neglected. As long as that air is calm, the thrust will propel the plane forward, which will produce airflow over the wings.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: RichardPrice
Posted 2005-11-29 10:43:58 and read 32767 times.

Ok, after all my arguing AGAINST the aircraft taking off, Im going to swallow my pride and change my stance  Smile

The aircraft will take off.

Why?

Start the conveyor moving without the engines on. In a perfect world (since we dont know anything other than the question, we have to assume perfection), the plane will sit still because the wheels will freewheel backward.

The engines add an additional thrust component to the equation, which is not affected by the conveyor. Thus forward motion can be generated and thus also lift.

Apologies  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Joness0154
Posted 2005-11-29 10:47:27 and read 32767 times.

 Smile

I originally saw it like you did, but I changed my mind also.

Its a very confusing problem to envision, but I had to change sides too  Smile

Welcome to the darkside...

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Nighthawk
Posted 2005-11-29 10:47:33 and read 32767 times.

Quoting RichardPrice (Reply 17):
If the force exterted backward on the aircraft is the same as the force exterted forward by the engines, then theres going to be no lift. Simple as that.

In this case, theres no air movement because the air is technically acting as a tail wind in exactly the same proportion as the forward motion of the aircraft. Thus no lift.

How is the air acting as a tailwind? The convayor belt isnt moving the air! Its not even moving the aircraft, its simply spinning the wheels.

It doesnt matter how fast the wheels spin, the aircraft will still move forward as the engines push against the air and generate thrust.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: HT
Posted 2005-11-29 10:50:58 and read 32767 times.

Quoting Sovietjet (Thread starter):
The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.

With this description, the a/c will not move relative to the surrounding. So there will not be any air flowing over the wing thus no lift is produced.

See it the other way ´round:
Your build a really BIG ventilator and place it in front of the a/c.
You apply the brakes of the a/c and then turn on your very big ventilator producing a stable wind at 200 knots. As this head wind is greater than the rotating speed of the aircraft, the wings will produce enough lift to let the a/c take off from the ground - but only as long as it is within the produced currents and the relative airspeed is maintained: This will require the a/c to be kept on a leash, making this a 100 ton-kite ... Big grin
(I hope you got the point ?!).
-HT

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: QFA380
Posted 2005-11-29 11:09:57 and read 32767 times.

Quoting RichardPrice (Reply 13):
Aircraft are just big road vehicles until they move quick enough to generate lift. An aircraft doesnt just leap into the air and its off.

Some can, like helicopters and Harriers.

Just one question, how is a plane supposed to land, if the runway is just one big conveyor belt? Big grin

QFA380

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Sudden
Posted 2005-11-29 11:22:55 and read 32767 times.

Quoting Sovietjet (Thread starter):
as long as a runway

That would not be enough to get up to rotation speed. It would require a rwy that is veeeeery long.  Wink

Aim for the sky!
Sudden

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Nighthawk
Posted 2005-11-29 11:28:30 and read 32767 times.

Quoting Sudden (Reply 24):
That would not be enough to get up to rotation speed. It would require a rwy that is veeeeery long. Wink

Aim for the sky!
Sudden

He didnt specify which runway... If its LCY, good luck, but edwards air force base might just be long enough... Silly

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: PMN
Posted 2005-11-29 12:46:49 and read 32767 times.

Surely the conveyor belt wouldn't really make a difference. The aircraft is moved forward by the engines, not the wheels as in the case of a car. Therefore, as the wheels are free spinning, the thrust of the engines should still move the aircraft forwards, even though the wheels themselves may be spinning at twice the speed the aircraft is physically moving.

Just the way I see it, but I'm more than happy to be proved wrong!

Paul

EDIT: I thought I'd just come up with something almost intelligent to say, but I've forgotten what it was.

[Edited 2005-11-29 12:59:26]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Saintsman
Posted 2005-11-29 12:56:02 and read 32767 times.

Quoting RichardPrice (Reply 19):
Start the conveyor moving without the engines on. In a perfect world (since we dont know anything other than the question, we have to assume perfection), the plane will sit still because the wheels will freewheel backward.

Rubbish. The wheels will stay still and the whole aircraft will move with the conveyor belt.

This is no different to running on a treadmill. In order to stay still you have to run as fast as the tread mill. In the case of our aircraft we are using the thrust of the engines to counteract the speed of the conveyor belt. If you are using the thrust to oppose the speed of the belt the aircraft will stay stationary (Newton). If the aircraft is stationary there will be no lift generated and therefore it will not take off.

End of story.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Joness0154
Posted 2005-11-29 13:00:02 and read 32767 times.

Quoting Saintsman (Reply 27):

You didn't read the original post.

Anyways, in a perfect world physically, when the conveyor belt moved, the wheels would turn (freewheel) and the plane would not go anywhere. Richard is correct.

The plane would take off, btw

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Sudden
Posted 2005-11-29 13:02:03 and read 32767 times.

Quoting Saintsman (Reply 27):
The wheels will stay still and the whole aircraft will move with the conveyor belt

A valid point, and also rather logical.

If no thrust is applied to compensate for the movement of the belt, the A/C would naturally move with the belt.

Aim for the sky!
Sudden

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Alessandro
Posted 2005-11-29 13:06:23 and read 32767 times.

Nicked from AD.com... Wink

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Nighthawk
Posted 2005-11-29 13:12:04 and read 32767 times.

Quoting PMN (Reply 26):
Surely the conveyor belt wouldn't really make a difference. The aircraft is moved forward by the engines, not the wheels as in the case of a car. Therefore, as the wheels are free spinning, the thrust of the engines should still move the aircraft forwards, even though the wheels themselves may be spinning at twice the speed the aircraft is physically moving.

Just the way I see it, but I'm more than happy to be proved wrong!

I agree entirely, the spinning of the wheels is irrelevent, it does not excert any physical force on the aircraft, they mearly spin. The thrust of the engines will produce forward motion which will not be apposed, so the aircraft will move forward and take off.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Nighthawk
Posted 2005-11-29 13:32:24 and read 32767 times.

Quoting Sovietjet (Thread starter):
The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.

I think the logic here is flawed? If the aircraft moves forward down the runway, the wheels rotate clockwise, ie backwards. If the belt moves in the opposite direction of rotation that means the belt is moving forward, carrying the aircraft down the runway!

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Kay
Posted 2005-11-29 13:54:45 and read 32767 times.

Folks,

If it was a free rotating conveyor belt, then yes, we could argue that eventhough it would make it difficult for the plane to move forward, and would slow it down, ultimatly, based on the friction forces of the belt vs the "grip" of the jet engines on the air flow, the plane will slowly move and then build up speed, and maybe possibly take off (a study of friction is needed).


BUT
if the conveyor belt is designed to run exactly at the same varying speed as the wheels intend to, but in the opposite direction, that means that the wheels will never move one inch forward. Therefore full thrust, and all the force you want, will keep the plane in the same place. It will be a question of which has more endurance, the engines, or the conveyor.


Is it so difficult? Am I the smartest here?


Kay

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: A346Dude
Posted 2005-11-29 14:06:24 and read 32767 times.

The answer to the question is, of course, absolutely not. If the speed of the conveyor belt is exactly equal and opposite to the speed of the wheels, as stated in the thread starter, then the aircraft's velocity relative to the air around it is zero the entire time. Thus no lift, and no take-off, period.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: PMN
Posted 2005-11-29 14:16:44 and read 32767 times.

Quoting Kay (Reply 33):
Is it so difficult? Am I the smartest here?

The most patronising perhaps...

Paul

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: A346Dude
Posted 2005-11-29 14:26:05 and read 32767 times.

After considering the problem further, I am going to have to amend my answer. The plane will take off, however the landing roll will be slightly longer than usual due to the friction from the conveyor belt pushing back on the wheels. Clearly, the wheels will be spinning a lot faster than usual on rotation.

BTW, why was this moved from tech-ops? I think it's a totally valid physics question related to aircraft.

[Edited 2005-11-29 14:27:11]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Joness0154
Posted 2005-11-29 14:29:01 and read 32767 times.

Quoting A346Dude (Reply 36):

After considering the problem further, I am going to have to amend my answer. The plane will take off, however the landing roll will be slightly longer than usual due to the friction from the conveyor belt pushing back on the wheels. Clearly, the wheels will be spinning a lot faster than usual on rotation.

See, everyone changes their mind  Smile

People, whether the wheels spin or not, the plane will still move forward, because nothing is hindering it. There is no force to hold the plane back.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Nighthawk
Posted 2005-11-29 14:39:51 and read 32767 times.

actually, I am going to alter my oppinion slightly:

ive posted this to another forum (http://www.gamedev.net/community/forums/topic.asp?topic_id=360975) which has made me realise:

In order to move forward the wheels need to stop gripping the convayor. As long as there is still grip, the wheels will not move forward. However once a certain speed is reached, the wheels will lose friction and start to slide along the conveyor belt, resulting in takeoff.

The question is whether an aircraft is capable of generating enough force to cause loss of friction, and im guessing not.....

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Flyingbabydoc
Posted 2005-11-29 14:45:15 and read 32767 times.

The plane is a BAE Harrier.

It just shifts the vents of the Pegasus engine and it flies away safely...regardless of conveyor belt.

 rotfl 



cheers

Alex

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Joness0154
Posted 2005-11-29 14:47:47 and read 32767 times.

I don't know how to explain this any better than I have.

The wheels are not connected to the motor as in a car.

If you put a car on a treadmill, and stepped on the gas and the conveyor speed matched that of the car, you would go nowhere.

Now, strap a rocket on the back of the car on the conveyor. Fire it up. Although the speed of the car and the conveyor are they same, the car will move forward.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: CHRISBA777ER
Posted 2005-11-29 15:07:49 and read 32767 times.

LOL You lot crease me up.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Kay
Posted 2005-11-29 15:12:35 and read 32767 times.

Hmm,

I will throw in the following:

since the belt is rotating at the same speed as the wheels, then, sorry, but yes, there is a force stopping the aircraft.

Imagine this: the wheels are actually gears, and teh conveyor is striped (metal stripes that the wheels, which are gears, would grab on).


Now strap a rocket to this "plane". imagine actually a piece of wood, with four wheels that are actually gears, and just strap a rocket on it.

Now light up the rocket and watch. the conveyor belt, based on technology that doesn't exist (since it is designed to know waht is the speed of the wheels and turn at the same speed), will do all it can to simply hold the rocket still. Since the wheels are actually metal gears and the belt is striped (metal). It's a huge force, fighting with the rocket. We're not talking about a free turning belt.

In reality, the conveyor belt, or the wheels, will break, and the thing will move. Or, they won't and the rocket will ultimately run out/break.


Now, if the wheels are made of rubber, then in that case, if the engines are powerful enough, they would force the wheels to skid their way to V1. It's really as if you are taking off with break-locked wheels.

So my answer is:

Taking the above into account, then NO

Quoting PMN (Reply 35):
The most patronising perhaps...

Naaah relax. But you're right, here's the missing  


Kay

[Edited 2005-11-29 15:13:47]

[Edited 2005-11-29 15:14:27]

Had to edit to get the font size right, and it's not even right yet

[Edited 2005-11-29 15:16:50]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Nighthawk
Posted 2005-11-29 15:15:10 and read 32767 times.

Quoting Joness0154 (Reply 40):
Now, strap a rocket on the back of the car on the conveyor. Fire it up. Although the speed of the car and the conveyor are they same, the car will move forward.

In order to move forward the tires would need to skid forward along the conveyor belt, losing friction. You would need a rocket powerful enough to break friction and also still provide enough speed once broken to accelerate to liftoff speed.

I dont think a normal jet engine would be up to it.

Its perfectly possible, but the question asks if a plane can do it, to which i believe the answer to be no.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: PMN
Posted 2005-11-29 15:21:50 and read 32767 times.

Quoting Kay (Reply 42):
Naaah relax. But you're right, here's the missing

Ah yes, sorry Kay, I missed the all important smile!  Smile

Paul

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Joness0154
Posted 2005-11-29 15:26:32 and read 32767 times.

I think my brain is about to explode from this topic. I'm done here, its too confusing, and too many variables  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Saintsman
Posted 2005-11-29 15:26:35 and read 32767 times.

Something for you to ponder upon.

If you are doing engine runs on an aircraft, with the brakes on you can go to full power. So, just because the engines are going at full power it doesn't mean you will take off.

The force of the engines are opposed by the force of the brakes. In our example here, the force of the engines are opposed by the conveyor belt. Which means that the aircraft will remain stationary and therefore no lift will be produced......

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: CHRISBA777ER
Posted 2005-11-29 15:46:56 and read 32767 times.

...yes but the brakes will be off otherwise its going backwards!

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SlamClick
Posted 2005-11-29 16:07:49 and read 32767 times.

The plane would not take off but for a reason I've not seen mentioned.

Airplane tires have a speed limit. 225 MPH is a very common speed limit for airliner tires. If we would normally lift off today at 150 MPH then the treadmill would have the wheels spinning at a rotational speed equivalent to 300 MPH and the tires would burst.

Solve the tire speed problem and the plane would take off in completely normal fashion. If you were onboard you would not be able to tell the difference from a routine takeoff.

Those of you who think the treadmill would effect the takeoff run somehow have it in your mind (even if analytically you can state otherwise) that the wheels somehow propel the plane up to takeoff speed. This is simply not true.

Planes take off on floats and skis all the time with no wheels spinning at all. The analogies that various posters tried to make with those things are completely valid. There is no solid connection between the surface of the beltway and the CENTER OF GRAVITY of the airplane. The only physical connection is at the bottom of the tires. Resistance there does not hold back the axles, does not hold back the landing gear legs or trunnions, does not hold back the center of gravity of the airplane - it just makes the wheels spin. If we've solved the tire speed limit then those wheels can spin as fast as they want.

While the tires are spinning at whatever speed they want (around their own axles) the plane accelerates happily down mister runway and takes off as usual and we all get to Fresno on time.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: A346Dude
Posted 2005-11-29 16:08:28 and read 32767 times.

Quoting A346Dude (Reply 36):
The plane will take off, however the landing roll will be slightly longer than usual due to the friction from the conveyor belt pushing back on the wheels.

Whoops, I meant to say take-off roll, not landing roll.

Anyways, this problem is obviously pretty difficult to visualize. I still hold, however, that the force exerted on the plane by the conveyor belt moving backwards is quite small, since the wheels can spin freely. Thus the aircraft will be able to move forwards, relative to the air, and take off - regardless of the speed of its wheels. Make no mistake, however: the plane will travel just as far, and actually a little bit further, in the process of taking off as it would if there was no belt.

[Edited 2005-11-29 16:10:49]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Ariis
Posted 2005-11-29 16:17:25 and read 32767 times.

Quoting Sovietjet (Thread starter):
The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.

This is the key to the answer. Since conveyor belt is somehow designed this way (assumption), the wheels will never move an inch. And the airplane will never take off, regardless of the thrust, airflow, engine, pilot, his girlfriend, anything.

The reason it is not intuitive is that you can never have such conveyor belt for it would have to spin infinitely fast to keep the wheels steady even under infinitely small force trying to push the wheels back or forth.

FAO

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SlamClick
Posted 2005-11-29 16:27:30 and read 32767 times.

Quoting Ariis (Reply 50):
), the wheels will never move an inch.

Ridiculous. You still think the wheels are pushing the airplane. I've moved airplanes with the wheels not turning at all - on ice. The wheel rotational speed has ZERO to do with this.

Hey, since the main tires are bigger than the nose tires, the nose tires are turning faster. Does that mean the nose is moving faster? Is the plane getting longer as it rolls?

[Edited 2005-11-29 16:30:51]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Nighthawk
Posted 2005-11-29 17:08:45 and read 32767 times.

Quoting SlamClick (Reply 51):
Ridiculous. You still think the wheels are pushing the airplane. I've moved airplanes with the wheels not turning at all - on ice. The wheel rotational speed has ZERO to do with this.

the key here being that it was on ice..the wheels were sliding over the surface.

In order to take off friction between the tire and belt must be broken, allowing the aircraft to slide down the conveyor belt. I do not believe the aircraft engines will be powerful enough to achieve this.

Its like taking off with the brakes locked on.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SlamClick
Posted 2005-11-29 17:21:27 and read 32767 times.

Quoting Nighthawk (Reply 52):
In order to take off friction between the tire and belt must be broken

Nonsense. The friction between the wheels and conveyor belt is not even an issue since the tires are free to rotate.

If I clamp a toy car in a vise will you be able to turn the wheels?

If I hold a toy airplane in a level attitude but prevent it from moving and you bring a board up under it and move it fore or aft, would you be able to do that or would the wheels somehow prevent it?

If I take that toy plane and start walking forward and you start pulling the board backward will my toy plane still be moving?

Of course it will.

To take off, a jet needs airflow over the wings. They get this by using powerplants to propel their mass forward through the air. The floatplane and skiplane comparisons are all in the world you need to understand this principle. If wheel rotation was an inherent or required part of the takeoff then float planes and ski planes would not be able to take off. You could mount a Boeing or Airbus on floats or skis.

The jet engines provide a forward force quite independant of the wheels. If this were not true the plane would no longer move forward once the wheels were no longer in contact with the ground.

So if we have jet thrust to move us forward, or if we are towed by a cable (as in glider launches) that is a force that cannot be offset or canceled out just by spinning the tires - in either direction.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: A346Dude
Posted 2005-11-29 17:34:37 and read 32767 times.

Quoting SlamClick (Reply 53):
You could mount a Boeing or Airbus on floats or skis.

Whoa - now THAT would be something to see! Big grin

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Ariis
Posted 2005-11-29 17:51:02 and read 32767 times.

Quoting SlamClick (Reply 51):
Ridiculous. You still think the wheels are pushing the airplane. I've moved airplanes with the wheels not turning at all - on ice. The wheel rotational speed has ZERO to do with this.

Why do you think that I think so? Am I talking about a car or something?

Your comment proves my fault: I have not expressed myself clearly enough.

I was trying to say, that construction of the conveyor belt (as assumed) assures us that the wheels' axes will never move relatively to the surrounding environment.

And ice? What ice?

Quoting SlamClick (Reply 51):
Hey, since the main tires are bigger than the nose tires, the nose tires are turning faster. Does that mean the nose is moving faster? Is the plane getting longer as it rolls?

I don't understand your point here, sorry. Out of context completely, I don't know why you (apparently) address this question to me.

FAO

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SlamClick
Posted 2005-11-29 18:01:05 and read 32767 times.

Quoting Ariis (Reply 55):
Why do you think that I think so?

Because the only way the moving surface could affect the speed of the mass of the airplane is if the wheels were driving the plane.

They are not.

The only effect this conveyor would have would be to make the wheels spin at a higher RPM. Do you think the wings can tell what the wheel RPM is?

One last test: Replace the jet engines with cables and winches producing the same amount of forward impulse. Start pulling the plane forward with the winches and at the same time start pulling the runway/treadmill/conveyor backward. Is it going to stop the plane from moving forward across the earth's surface?

Of course not. The wheels are just going to spin at higher RPM.

Jet engines pushing against the thick atmosphere behind them or cables being pulled by winches, either one will move the airplane forward and the wheels have nothing to do with it.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Kay
Posted 2005-11-29 18:06:05 and read 32767 times.

The way i see it is that the concept of having a conveyor belt designed to spin exactly the same speed as the wheels, is like having a hand come out of the tarmac and hold the gear pillars so that they stay where they are, with huge force. That is why such a conveyor is not intuitive.

Here's another analogy. place a toy plane on that same conveyor, and kick it with your foot with all your force. Will it move? only if the wheels and the conveyor create huge friction. Since the plane is small and your foot is powerful, it will advance half a meter or 1 meter before standing still, but only because the wheels can skid on the conveyor in alot of friction.

And another one: if i stand on a running machine that is designed to roll at whatever speed i try to run. and someone pushes me from my back. If i my back was rigid straight, and if the only way I could advance was through running (not realistic), then no, you can push all you want, you will break your hand (and my back), I will not move!


Kay

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Amy
Posted 2005-11-29 18:08:00 and read 32767 times.

Even if the aircraft was moving forwards at rotation speed, if the conveyor belt was moving backwards I think the gear asssembly would reach maximum rpm before the aircraft could take off.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: JBirdAV8r
Posted 2005-11-29 18:09:06 and read 32767 times.

Quoting RichardPrice (Reply 14):
To the laws of physics gentlemen, the aircraft will be moving forward at take off speed, but since an opposite force is being exerted on the aircraft, it isnt actually going anywhere.



Quoting Joness0154 (Reply 28):
The plane would take off, btw

The sheer fact that this thread has gone on as long as it has is, quite frankly, pretty disturbing. I'd hate to see the high school physics scores of some of you.

No. Assuming calm wind (or any wind with a headwind component less than that required for takeoff), the aircraft will NOT take off.

We'll try a watered-down example.



There are four forces acting on an aircraft: lift, weight, thrust, and drag. Lift counteracts weight and thrust counteracts drag.

If the aircraft is set up as in the initial post:

Quoting Sovietjet (Thread starter):
The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.

then there are only two forces being imparted on the airplane (assuming zero wind) = lift and weight. The weight of the airplane is acting vertically toward the center of the Earth, as it always does. Thrust is being provided by the aircraft's engines, which is being matched exactly by the thrust imparted by the conveyor belt.

To visualize this, imagine a game of tug-of-war between twins who are precisely equal in strength. The rope wouldn't move.

Mathematically, we can illustrate it this way:

Assume the aircraft is producing 2000 N of thrust in a forward direction. The conveyor belt would then be producing 2000 N of thrust in the exact opposite direction. The net thrust would be [2000 + (-2000)] = 0. Therefore the aircraft is stationary relative to the Earth.

The amount of lift produced by a typical aircraft varies directly with the square of the increase in velocity of air flowing over an aircraft's wings. As the aircraft is stationary relative to the Earth, and assuming zero wind, the velocity of the air flowing over the plane's wings would match the speed of the wind: zero. If you were conducting this grand experiment outdoors and the winds were blowing, say, 15 knots straight toward the nose of the plane, the speed of the air over the wings would be 15 knots. Yes, you'd be creating SOME lift, but you'd be creating the exact same amount of lift whether the engines/conveyor belt were producing 1 N in opposite directions or 1,000,000 N in opposite directions. So you could strap on about 48 GE90's to a Cessna 152, pour the coal into 'em, and have the conveyor belt running at about Mach 2--but as long as the net thrust is zero, the plane won't be going anywhere.

Here's a simple trial.

Run on a treadmill.

Do you feel air hitting you in the face as you run faster and faster?

No?

There you have it.

[Edited 2005-11-29 18:10:15]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: A346Dude
Posted 2005-11-29 18:20:01 and read 32767 times.

Quoting JBirdAV8r (Reply 59):
Thrust is being provided by the aircraft's engines, which is being matched exactly by the thrust imparted by the conveyor belt.

No, it isn't! You have made the leap from the conveyor belt moving at a speed that is exactly equal and opposite to the speed of the wheels, to the argument that the conveyor belt exerts an equal and opposite force on the aircraft as that of its engines. This is simply false.

Think of it this way. If the airplane is moving forwards at 150 knots, then, by the definition of the problem, the belt is moving backwards at 150 knots. In this case, it is obvious that the wheels must be spinning as fast as they would be if the airplane were travelling over a normal runway at 300 knots. Clearly, however, the plane is travelling foward at 150 knots with reference to the air, and (depending on exactly what plane we are talking about), can thus take-off. I wouldn't suggest retracting the gear anytime soon though .

I think the key to this problem is this: the fact that the belt spins at a speed equal and opposite to that of the aircraft does not prevent the plane from moving forward. The only effect of the belt is to spin the wheels twice as fast as they would spin if they were on a normal runway - that's it!

The physicists, engineers, and pilots are all agreeing that the plane will take off.

[Edited 2005-11-29 18:21:00]

[Edited 2005-11-29 18:23:23]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Navymidn
Posted 2005-11-29 18:23:08 and read 32767 times.

The plane will not take off. Assuming we are on Earth, and friction is a factor, the wheels will never make forward movement. This is because the conveyor belt is exactly matching the rotational speed. The engines can be running at full power, but the only way for them to create movement on the ground is to translate that through the wheels to the ground. If you draw a free body diagram for this problem, you will find that there are four forces at work: gravity, the normal force, thrust, and the conveyor belt. Thrust, which is causing forward movement, is counteracted by the conveyor belt acting like a brake. But unlike a brake, we assume the conveyor belt can turn infinitely fast. Therefore, there is no amount of thrust that can overcome the force of the conveyor belt.

Think of it like this problem: On a still day, if you put a giant fan on a sailboat and used it on the sail, would the boat move?

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Joness0154
Posted 2005-11-29 18:27:00 and read 32767 times.

Quoting Navymidn (Reply 61):
Think of it like this problem: On a still day, if you put a giant fan on a sailboat and used it on the sail, would the boat move?

Yes, the boat would move. Boat sails work on the principle of lift.

Anyways, I see you're at Purdue....as am I.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Kay
Posted 2005-11-29 18:27:52 and read 32767 times.

Quoting A346Dude (Reply 60):
The physicists, engineers, and pilots are all agreeing that the plane will take off.

Not this engineer.

Quoting A346Dude (Reply 60):
I think the key to this problem is this: the fact that the belt spins at a speed equal and opposite to that of the aircraft does not prevent the plane from moving forward.

A346Dude, you have a huge mistake in your analysis: the belt spins at a speed equal to the *wheels* not equal to the aircraft as you said. And follow your paragraph after making this correction. Yep it falls to pieces. The belt isn't running at 150mph, it is running at 300mph. See, it is matching the rotating speed of the wheels, we don't care about the airplane. And a belt that is running at the same speed as the wheel, is actually as if to say the belt and the wheel are one piece of cement.


After reading this thread, I thought that the physicists and engineers are actually all agreeing that the plane will not take off.



Kay

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SlamClick
Posted 2005-11-29 18:33:20 and read 32767 times.

Quoting JBirdAV8r (Reply 59):
Run on a treadmill.

Hello! Running is an activity where the wheels (your feet) propel the vehicle.

Strap a rocket to your back and get on a treadmill and tell me what happens. Stand on a treadmill with a rope tied around your chest. Have someone yank on that rope. What happened?

[Edited 2005-11-29 18:41:43]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: A346Dude
Posted 2005-11-29 18:38:04 and read 32767 times.

Kay,

I think you've run into a circular argument here. What you are arguing is that the plane could be sitting perfectly stationary, with the conveyor belt and the plane's wheels going at a million miles an hour (each in opposite directions). The question is, if the plane never started moving forward, why did those wheels ever start spinning in the first place?

Please note I have updated my signature to reflect the lunacy of the argument.

[Edited 2005-11-29 18:38:36]

[Edited 2005-11-29 18:39:50]

[Edited 2005-11-29 18:51:28]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Jean Leloup
Posted 2005-11-29 18:38:56 and read 32767 times.

This is definitely the new pigeon thread.

As for my 'vote' (not that it's really up to popular decision, of course), the plane will definitely take off. So far the clearest, most direct explanation is Slamclick's. I suggest you all read it again, slowly!:

Quoting SlamClick (Reply 56):
The only way the moving surface could affect the speed of the mass of the airplane is if the wheels were driving the plane.

They are not.

The only effect this conveyor would have would be to make the wheels spin at a higher RPM. Do you think the wings can tell what the wheel RPM is?

One last test: Replace the jet engines with cables and winches producing the same amount of forward impulse. Start pulling the plane forward with the winches and at the same time start pulling the runway/treadmill/conveyor backward. Is it going to stop the plane from moving forward across the earth's surface?

Of course not. The wheels are just going to spin at higher RPM.

Jet engines pushing against the thick atmosphere behind them or cables being pulled by winches, either one will move the airplane forward and the wheels have nothing to do with it.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SlamClick
Posted 2005-11-29 18:40:00 and read 32767 times.

Quoting Navymidn (Reply 61):
This is because the conveyor belt is exactly matching the rotational speed.

It is absolutely staggering! Educated people who persist in believing that the wheels are somehow driving the airplane.

Don't think of the contact patch, where the tire meets the runway. Think of the center of the axle. That should help.

The spin of the wheels is a really incidental function of the motion of the airplane. Put smaller mainwheels onthe same airplane and they will spin faster. Put bigger wheels on and they will spin slower - expressed in RPM. So if you put small enough wheels the plane would not accelerate?

Guys, seriously you need to scrap all the equations and theoretical stuff and just use common sense.

The engines can make the airplane move forward. They have enough thrust to do that. Does that ability depend in any way on the wheels touching the ground? How about if we do a low pass down the runway. Let's smoke it at 250 knots. Now you start up your treadmill and I'm going to snap the gear down. When the wheels touch the treadmill is the plane going to stop instantly?

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: MiCorazonAzul
Posted 2005-11-29 18:41:13 and read 32767 times.

My physics calculations show that it does take off.........  yes 

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SlamClick
Posted 2005-11-29 18:43:54 and read 32767 times.

Quoting MiCorazonAzul (Reply 68):
My physics calculations show that it does take off.........

"Show your work."
- three generations of physics teachers
 Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: A346Dude
Posted 2005-11-29 18:49:11 and read 32767 times.

Quoting Kay (Reply 63):
The belt isn't running at 150mph, it is running at 300mph. See, it is matching the rotating speed of the wheels, we don't care about the airplane.

That doesn't make sense. Why would the wheels be rotating at 300 knots (/mph - whatever)? The airplane is travelling at 150 knots, so the wheels should be travelling at 150 knots as well (over a normal surface). Simply replace this normal surface with the conveyor belt, and you will see that it travels backwards at 150 knots. The airplane is still free to move forward almost entirely unhindered by the belt, as SlamClick has already illustrated several times.

[Edited 2005-11-29 18:49:44]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: CPH757
Posted 2005-11-29 18:57:28 and read 32767 times.

First I didn't think the plane would take off. Then I understood the point of the free spinning wheels, making it able to take off.

However, one forget the grip factor of the tires. If the engines are turned off, the airplane will move backwards.

Imagine a car in neutral gear setting on a conveyor belt starting to move. Will the car move? Yes it will. The wheels will not start spinning, due to the grip, at least not until the conveyour belt moves very fast.

The question is; if the conveyor belt moves fast enough, is it then possible to overcome the friction that the grip implies?

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Kay
Posted 2005-11-29 19:01:44 and read 32767 times.

The reason is, guys, that a conveyor like this is something that doesn't exist, and cannot be pictured in our heads, really! I think there is a difference between physics and the real world, but still, that conveyor belt is like a sticking the gearstruts in the tarmac 3 feet deep.

Quoting A346Dude (Reply 65):
The question is, if the plane never started moving forward, why did those wheels ever start spinning in the first place?

The process starts with:

1- Huge engines run at increasing thust and want the body of the plane to move forward.
2- The body of the plane is attached to the gear struts that are attached to the small wheels. The struts want to move forward, the mechanism dictates that the wheel must turn.
3- The wheel initiates an epsilon (the smallest amount, 1 millionth of a millimeter) turn, but epsilon turn is counter acted by an epsilonturn backwards from the floor. We have a perfect gear-runway cemented situation. The wheel is absorbing enormous energy from the plane/strut construction.
4- Each gradual increase in the power of the engines creates a situational change, which will initiate the epsilon turn I mentioned above.
5- Until full power, where the system is steady. The plane is still in its place, and the tyres trying to turn. But they do not turn.


Now, I think only an episode of the Mythbusters can make us agree  Wink

Kay

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SlamClick
Posted 2005-11-29 19:07:47 and read 32767 times.

Okay, related question:

A train is going fifty miles per hour.

That means the center of each axle is going 50 MPH. However, the bottom surface of each wheel is not sliding along the rail, but, rather, is rolling along it, so that the point actually in contact is not moving longitudinally with reference to the rail. (othewise each wheel would always be skidding or slipping. So this means that the bottom of each wheel is standing still at the instant it is in contact with the rail.

So if the bottom of the wheel is standing still at any given point, and the axle center is always going 50 MPH, I guess that means that the top of the wheel is going 100 MPH - being twice the distance above the rail surface as the axle center.

So any given point on the surface of the wheel starts at zero, accelerates to 100 MPH and slows back to zero once each revolution.

Okay so far, but there is the flange that extends down inboard of the rail. The tip of that has to be moving backwards all the time!

Discuss it among yourselves. I've already worn out two brains trying to explain this to the guys in the sawmill using Copenhagen cans rolling across the floor as examples.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Kay
Posted 2005-11-29 19:08:34 and read 32767 times.

Quoting A346Dude (Reply 70):
That doesn't make sense. Why would the wheels be rotating at 300 knots (/mph - whatever)? The airplane is travelling at 150 knots, so the wheels should be travelling at 150 knots as well (over a normal surface). Simply replace this normal surface with the conveyor belt, and you will see that it travels backwards at 150 knots. The airplane is still free to move forward almost entirely unhindered by the belt, as SlamClick has already illustrated several times.

No man. You said: 1- plane goes at 150, 2- replace floor with belt moving at 150 backwards, then wheels will run at 300 and plane will take off at 150. The only problem is that the belt is so damn smart it will run at 300 backwards, not 150 backwards! So that the wheels don't move, because the belt is a bastard, the belt is our enemy, that SOB doesn't want the wheels struts to move one inch forward. It has been designed, drawn, built, with this sole purpose......



Kay

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: MD-90
Posted 2005-11-29 19:08:47 and read 32767 times.

Everyone needs to listen to SlamClick, because he's got it right.

What propels an aircraft is AIR, either compressed and heated by a turbine or simply swirled into submission by a propeller. Thrust is not dependent upon what the wheels are doing.

Wheel rotation has nothing to do with it.

It's The Medium, Manfred
http://www.avweb.com/news/columns/191034-1.html

I'm reminded of the New York Times editorial when Robert Goddard's rocket experiments were first being publicized. The author of the editorial said that rockets can't work in space because they have nothing to push against. It was laughably wrong, ignoring one of Sir Isaac's laws of physics that for every action there is an equal and opposite reaction. Here the propeller is pushing against the air, as it does every time an airplane takes off. How fast the airplane is moving over the surface on which its wheels rest is irrelevant; the medium is the magic. On a normal takeoff -- no conveyor involved -- if there is a 20 mph headwind, Manfred and the J-3 will lift off at 45 mph indicated airspeed; but relative to the ground, it is only 25 mph. Should the wind increase to 45 mph and if Manfred can get to the runway, he can take off without rolling an inch. His airspeed is 45 and groundspeed is zero. It is not necessary to have any groundspeed to fly, just airspeed. Conversely, if Manfred has a lot of runway and nothing to hit, and takes off downwind in a 25 mph tailwind, the propeller will have to accelerate the airplane to a zero airspeed, which will be a 25 mph groundspeed, and then on to a 45 mph airspeed, which will have him humming across the ground at 70 mph. The speed over the ground, or a conveyor belt, when an airplane takes off is irrelevant; all that matters is its speed through the air, and unless the pilot sets the brakes, a moving conveyor belt -- under the freely turning wheels -- cannot stop the process of acceleration.


Same reason why pilots don't like to takeoff with a tailwind.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: A346Dude
Posted 2005-11-29 19:09:59 and read 32767 times.

Why must you torment us SlamClick?? We already made it to 70 posts arguing the first question!  Wink

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: A346Dude
Posted 2005-11-29 19:13:15 and read 32767 times.

Quoting Kay (Reply 74):
So that the wheels don't move, because the belt is a bastard, the belt is our enemy, that SOB doesn't want the wheels struts to move one inch forward. It has been designed, drawn, built, with this sole purpose......

No, no, no! It has been designed to rotate backwards at exactly the speed of the wheel! So if you place a wheel spinning at 150 knots on it, it will move backwards at 150 knots.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Fokker Lover
Posted 2005-11-29 19:15:32 and read 32767 times.

I'm amazed that a website for aviation enthusiasts has so many people that don't understand how an airplane works, yet they will argue to their last breath. I've never seen so many systems analysts, and computer programmers argue with real pilots and mechanics. Have you ever flown or taxied an airplane before? Have you ever changed the wheels and brakes to see that they simply rotate, and are connected to nothing? The damn plane will fly, and it doesn't matter what the runway is doing, what it's made out of, what the wheels are doing, or what color the sky is. The damn plane will fly.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SlamClick
Posted 2005-11-29 19:19:19 and read 32767 times.

Quoting Kay (Reply 72):
The plane is still in its place, and the tyres trying to turn. But they do not turn.

The tires are not trying to turn.

IT IS NOT A CAR.

The tires will only turn as a byproduct of the motion of the plane above the surface on which it rests.

cause
A noun
1 causal_agent, cause, causal_agency

any entity that causes events to happen

effect
a phenomenon that follows and is caused by some previous phenomenon;


The effect of the treadmill moving would only be the increase in RPM at which the wheels would spin.

Unless you applied the brakes it would have no effect on the mass of the airplane through its center of gravity. That mass is being accelerated down the runway by tens of thousands of pounds of thrust from jet engines. Just how much drag do you think those wheel bearings cause?

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Kay
Posted 2005-11-29 19:29:20 and read 32767 times.

Quoting A346Dude (Reply 77):
No, no, no!

I don't know why but I had to laugh when I read your reaction  silly 

Quoting SlamClick (Reply 79):
The tires are not trying to turn.

The wheel is trying to turn because that is the only way it knows for moving forward. The wheel is driven to turn mechanically.. Believe me I'll be happy if you turn out to be right.

Kay


PS: Many of us are pilots here, Fokker_lover.. Saying our opinions for the sake of the argument is one thing, causing annoyance is another, that is definitely not the intention...

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: A346Dude
Posted 2005-11-29 19:33:33 and read 32767 times.

Quoting Kay (Reply 80):
I don't know why but I had to laugh when I read your reaction

Hehe, I suppose this discussion has become a little heated. Agree to disagree?

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Fokker Lover
Posted 2005-11-29 19:43:12 and read 32767 times.

Quoting Kay (Reply 42):
We're not talking about a free turning belt.

But we ARE talking about free turning wheels, and that is why your argument is totally and completely wrong. The wheels don't start to turn until the thrust of the engines has already overcome any oposing forces. They could spin backwards all damn day at any speed. It doesn't matter because the engines are going to drag the plane down the runway.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Kay
Posted 2005-11-29 19:43:59 and read 32767 times.

Quoting A346Dude (Reply 81):
Hehe, I suppose this discussion has become a little heated. Agree to disagree?

Yes I agree to disagree on this, after all, very minor thing. Hoping that everyone is cool too...

Kay  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Petertenthije
Posted 2005-11-29 19:48:38 and read 32767 times.

The plane will take of.

The reason for that is that the forward motion is not supplied by the wheels (like a car/train), but by the jet engines. They are not hindered by the conveyor belt. At worst it will give more friction to the plane thereby requiring a longer runway distance and somewhat higher thrust levels.

The wheels will probably spin like crazy, but it will take off.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Fokker Lover
Posted 2005-11-29 19:54:30 and read 32767 times.

Here's a simple experiment that everyone can do. Put a Tonka truck on a treadmill and turn it on. See what happens? The truck moves with the belt and falls off the back. Now, do the same thing and put your hand across the back of the truck to hold it in place. The truck stays in place and the wheels turn. With me so far? Now push it as hard as you can. What happened there?
The truck flew off the front end of the treadmill. That's because your hand is the thrust of the jet engine. It doesn't matter what the wheels or treadmill is doing. THEY ARE NOT CONNECTED.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Nighthawk
Posted 2005-11-29 20:14:28 and read 32767 times.

Heres a thread about this on a physics forum:

http://forum.physorg.com/index.php?showtopic=2417&st=0

16 pages worth, and im guessing an answer was never reached, im off down thepub, i anyone gets bored reading that, heres a google search of even more threads:

http://www.google.co.uk/search?hl=en...aircraft+conveyor+take+off&spell=1

Enjoy  Silly

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Joness0154
Posted 2005-11-29 20:22:18 and read 32767 times.

Mythbusters anyone?  Smile  Smile  Smile

This topic is hilarious now  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: RichardPrice
Posted 2005-11-29 20:42:22 and read 32767 times.

Quoting Fokker Lover (Reply 85):
Here's a simple experiment that everyone can do. Put a Tonka truck on a treadmill and turn it on. See what happens? The truck moves with the belt and falls off the back. Now, do the same thing and put your hand across the back of the truck to hold it in place. The truck stays in place and the wheels turn. With me so far? Now push it as hard as you can. What happened there?
The truck flew off the front end of the treadmill. That's because your hand is the thrust of the jet engine. It doesn't matter what the wheels or treadmill is doing. THEY ARE NOT CONNECTED.

While I am decidedly with the 'it will take off' group, you are making one fatal mistake with this analogy.

The treadmill in the question is 'designed to match exactly the speed of the wheels at any particular time'.

The reason your forward thrust pushes the toy off the treadmill is because its set to go at a constant motion and the forward thrust would accelerate the toy and turn the wheels faster than the treadmill is turning, in the question the threadmill would match the new faster rotation of the wheels and the toy should theoretically remain stationary despite the extra thrust applied.

To induce any forward motion in the aircraft, the wheels would have to turn faster than the treadmill is turning, hence allowing the body of the aircraft to move forward. But the perfect treadmill in the question wont allow the wheels to turn faster than it can, it will always keep up.

How do you induce forward motion in something that cant be moved forward because the ground it is resting on is cancelling whatever forward motion it tries to create?

Yes, the engines arent connected to the ground, yes the wheels are freewheeling, but in the question at the start of the thread the plane needs to take off. To do so, it needs to move forward at a rate where lift is induced over the wings. To do that it needs to rotate the wheels faster than the treadmill is rotating. But it cant do that, because the treadmill is perfect and will match the wheel rotation at all times, so the plane is stuck in one place.

It all comes down to the wording of the question - the treadmill will ALWAYS and PERFECTLY match the speed of the wheels, and thus forward motion of the aircraft is not possible at any speed (since we do not know design limitations here we have to assume perfection on all components for test purposes and thus no failures) because that would require the wheels to rotate at a faster rate than the treadmill will cancel.

I dont reckon the answer to this question is actually possible to deduce outside of a supercomputer lab.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Falcon84
Posted 2005-11-29 20:43:41 and read 32767 times.

Has Anet sunk to THIS?  Silly

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Joness0154
Posted 2005-11-29 20:45:28 and read 32767 times.

The thing is, if the conveyor EXACTLY matched the speed of the wheels, the wheels would not turn, period.

If the conveyor were just a fraction of a second behind the wheel speed, the plane would accelerate and then takeoff

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Kay
Posted 2005-11-29 20:47:14 and read 32767 times.

Quoting Joness0154 (Reply 87):
Mythbusters anyone?

I browsed their whole website, couldn't figure out how to just submit an idea, their forum isn't a formal channel, and there's only an application to implement our own experiment!!! So, who's got a long strip of land and an escalator mehanism???????

Kay

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SlamClick
Posted 2005-11-29 20:48:00 and read 32767 times.

Quoting RichardPrice (Reply 88):
To induce any forward motion in the aircraft, the wheels would have to turn faster than the treadmill is turning

If the wheels are inducing the motion.

They aren't!

All the treadmill can do is increase or decrease the RPM of the wheels. It cannot change the acceleration or speed of the airplane I don't care if it goes a million mile per hour.

The wheels are free to spin to compensate for that motion while the THRUST of the engines accelerates the mass of the airplane, through its center of gravity, down the runway.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Fokker Lover
Posted 2005-11-29 20:59:35 and read 32767 times.

Quoting RichardPrice (Reply 88):
To do that it needs to rotate the wheels faster than the treadmill is rotating. But it cant do that, because the treadmill is perfect and will match the wheel rotation at all times, so the plane is stuck in one place.



Quoting RichardPrice (Reply 88):
the treadmill will ALWAYS and PERFECTLY match the speed of the wheels, and thus forward motion of the aircraft is not possible at any speed

Ok, the conveyor will match the speed of the wheels.
Lock the brakes. The wheels won't turn, the conveyor won't turn. The thrust of the engine will still drag the plane down the runway. I'm telling you here and now, I have done full power engine runs. I have jumped the chocks, and I have skidded the airplane forward with the brakes locked. It doesn't matter what the wheels are doing. The airplane will move.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Joness0154
Posted 2005-11-29 20:59:45 and read 32767 times.

THE VIDEO THAT SOLVES IT ALL!!!

http://videos.streetfire.net/player....E964D9-38DB-4EFD-BE8D-D6BA1A43A06B

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Joness0154
Posted 2005-11-29 21:01:42 and read 32767 times.

and an avweb article

http://www.avweb.com/news/columns/191034-1.html

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Petertenthije
Posted 2005-11-29 21:11:23 and read 32767 times.

Quoting Kay (Reply 91):



Quoting Kay (Reply 91):
Quoting Joness0154 (Reply 87):
Mythbusters anyone?

I browsed their whole website, couldn't figure out how to just submit an idea, their forum isn't a formal channel, and there's only an application to implement our own experiment!!! So, who's got a long strip of land and an escalator mehanism???????

already on their site
http://community.discovery.com/group...ms/a/tpc/f/9701967776/m/7451937218

maybe someone can send them the "flying birds in 747" thread?

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: JBirdAV8r
Posted 2005-11-29 21:35:35 and read 32767 times.

Quoting A346Dude (Reply 70):
That doesn't make sense. Why would the wheels be rotating at 300 knots (/mph - whatever)? The airplane is travelling at 150 knots, so the wheels should be travelling at 150 knots as well (over a normal surface). Simply replace this normal surface with the conveyor belt, and you will see that it travels backwards at 150 knots. The airplane is still free to move forward almost entirely unhindered by the belt, as SlamClick has already illustrated several times.

Upon closer examination, doing a little calculus, and actually playing with a treadmill, you're right. I didn't think it all the way through before posting. You did prove my theory that engineers (even those who are not quite engineers) think they know everything, though  Wink

So much for my 5 on the AP physics test. Damn.

As my flight students will not be taking off from a giant conveyor belt any time soon, I think I'll be fine...

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: MD-90
Posted 2005-11-29 21:37:48 and read 32767 times.

There is a fundamental difference between airspeed and groundspeed...


I think there is an easy way to visualize this. Imagine a gargantuan convey belt that can support a 747. What happens if you turn the conveyor belt on and set it to move at 20 mph? The inertia of the 747 is so great that it probably wouldn't move very much, initially, since there isn't that much friction within the wheel bearings themselves. Eventually, over a period of time, the 747 would accelerate to 20 mph 'airspeed', and 0 mph 'groundspeed'. It would reach a state of equilibrium with the treadmill (presumably), if the resulting aerodynamic drag wasn't too great.

That seems sort of silly, but I think it points out that the conveyor belt doesn't really have much to do with the velocity of the aircraft. It can only affect the plane through minute drag from the wheel bearings. Thus, the plane can easily accelerate, although a 747 would blow its tires before it actually got fast enough to rotate.



Hey, imagine it being 1940, and we're arguing about whether or not a rocket can function in space, since there's nothing to push against. Newton's laws always apply in such situations, even on conveyor belts. Thrust, from a jet engine or a rocket motor, produces and equal and opposite reaction.

[Edited 2005-11-29 21:48:37]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Mir
Posted 2005-11-29 22:05:11 and read 32767 times.

Quoting Falcon84 (Reply 89):
Has Anet sunk to THIS?

Would you like A vs. B? We can do A vs. B.  Smile

-------

After reading the replies in this thread, I would think that in an ideal case (zero friction), the plane would take off normally because of the wheels freewheeling:

Quoting Joness0154 (Reply 28):
Anyways, in a perfect world physically, when the conveyor belt moved, the wheels would turn (freewheel) and the plane would not go anywhere. Richard is correct.

However, once one introduces friction, it would be a lot more difficult, since the fact that the wheels don't want to go anywhere (due to the opposite motion of the treadmill) means that the landing gear legs struts don't want to go anywhere, which means that the plane doesn't want to go anywhere. Taking off would involve dragging the wheels along the conveyer belt all the way to rotation speed (in essence taking off with the parking brake on), and it would take a very long runway/belt to do that if it could be done at all. At least that's how I see it.

-Mir

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: A346Dude
Posted 2005-11-29 22:17:28 and read 32767 times.

Quoting JBirdAV8r (Reply 97):
You did prove my theory that engineers (even those who are not quite engineers) think they know everything, though

Couldn't you at least entertain the possibility that we do? Wink

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Fokker Lover
Posted 2005-11-29 22:19:06 and read 32767 times.

Quoting Mir (Reply 99):
At least that's how I see it.

Well, if that's how you see it, I guess it's settled then.
But then again, maybe you should pay attention to what the professionals have to say, or at least read this article. http://www.avweb.com/news/columns/191034-1.html

In the mean time, I'm going to go get ready for work. That's where I actually work on real airplanes.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Ralgha
Posted 2005-11-29 22:35:20 and read 32767 times.

I'm not reading this whole thread, but everyone who thinks the airplane will not take off has the same basic misunderstanding of airplanes that plagues many pilots.

Groundspeed does not affect how an airplane performs. Airplanes do not depend on the ground to fly in any way.

An airplane's engine provides propulsion by pulling on the AIR, not on the GROUND. The wheels provide very little rotational resistance, which is the only extra resistance the engines will have to overcome. The airplane will have a nearly identical takeoff run regardless of how fast the treadmill is running.

Think of it this way, you're standing on a treadmill wearing roller skates and holding onto a rope that someone else is holding onto in front of the treadmill. The treadmill starts to move. Do you move? No. Now that other person starts to reel in the rope, do you move forward? Yes. Does that other person have to pull harder on the rope to get you to move due to the treadmill rotating? No.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SlamClick
Posted 2005-11-29 22:53:59 and read 32767 times.

Quoting Mir (Reply 99):
I would think that in an ideal case (zero friction), the plane would take off normally because of the wheels freewheeling:

Sorry but this is not Slam's day to cut any slack.

"ideal?" "zero friction?"

Even my puny little A-320 has fifty-four thousand pounds of thrust. Just how much drag do you think there could be in those wheel bearings?

Quoting Mir (Reply 99):
since the fact that the wheels don't want to go anywhere

The wheels pivot around their axles. The axle bone connected to the CFM bone and the wheels will go absolutely anywhere the engines push them. The only thing holding them back is a tiny force at the contact patch on the bottom where they touch the treadmill. If the wheels could not rotate they would start sliding from the thrust like it says here . . .

Quoting Fokker Lover (Reply 93):
I'm telling you here and now, I have done full power engine runs. I have jumped the chocks, and I have skidded the airplane forward with the brakes locked. It doesn't matter what the wheels are doing. The airplane will move.

. . . and shortly after that the rubber would melt and the friction would decrease. But -

The wheels are free to rotate.

Once again, not an opinion here, just my best expression of an obvious truth. The only effect the treadmill would have would be higher RPM for the wheels. Same effect could be had by putting smaller wheels on it. Opposite effect putting larger wheels. The rolling of the wheels is just a byproduct of the takeoff roll, like the changing "Present Position" on the NAV display.

Let's switch the question around. Let's make the treadmill run in the other direction. Let's say that the treadmill will accelerate from 0 to VR in about six thousand feet OF TREADMILL BELT TRAVEL. Now with an endless belt we don't need six thousand linear feet to lay this thing out, just enough to support all the landing gear.

So we park our airplane on the treadmill with the engines idling (so we don't roll of the end of the belt) and we start the treadmill. Now I can sit there and watch some instrument that tells me that I am not at VR and I can shove the throttles up and rotate and take off and fly?

Is that going to work?

Can't work one way without having an effect the opposite way you know.

Sorry to all those who say the plane would not take off, but the rest of us understand, for one thing, how planetary gearsets work. The plane would take off with the wheels spinning madly.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SlamClick
Posted 2005-11-29 22:55:25 and read 32767 times.

Quoting Ralgha (Reply 102):

Well said.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Matt72033
Posted 2005-11-29 23:00:49 and read 32767 times.

ok....ive joined this late, but i thought i'd throw my analogy in for the hell of it! lol

ok.....flat conveyor belt in aircraft terminal.....we've all seen them.
shopping trolley
rope tied to handle of shopping trolley

if i stand at one end of the conveyor, with the belt moving away from me.....no matter what way those wheels are rotating, i can drag the trolley up by the rope

if i hold the rope taught, so that the wheels move the same speed as the belt and then pull the rope, the speed of the wheels will not change whatsoever, but the trolley will move towards me!

Thats how i understand it.........what i fail to understand is the point in this?
surely it has no use? whats the point in wasting our time on this.....when its all purely hypethetical....never gonna happen?!?!?!

EDIT: sorry RALGHA i didnt read your last post, yours an my analogy's appear to be very simmilar!! Great minds think alike! lol

[Edited 2005-11-29 23:04:04]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Matt72033
Posted 2005-11-29 23:07:25 and read 32767 times.

i think someone should start the "do engines push or pull the aircraft" thread now!!  Wink

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: CosmicCruiser
Posted 2005-11-29 23:12:27 and read 32767 times.

I couldn't bring myself to read all these posts so forgive me if I'm repeating something already said. Let's make this real simple...If you're a runner, as I am, when you run down the path do you feel air moving across your body?...YES; when you run on a treadmill do you feel air moving across your body?...NO...that's all folks!

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Matt72033
Posted 2005-11-29 23:14:35 and read 32767 times.

Quoting CosmicCruiser (Reply 107):
I couldn't bring myself to read all these posts so forgive me if I'm repeating something already said. Let's make this real simple...If you're a runner, as I am, when you run down the path do you feel air moving across your body?...YES; when you run on a treadmill do you feel air moving across your body?...NO...that's all folks!

i suggest you read the posts!

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SlamClick
Posted 2005-11-29 23:26:48 and read 32767 times.

Quoting CosmicCruiser (Reply 107):
I couldn't bring myself to read all these posts so forgive me if I'm repeating something already said. Let's make this real simple...If you're a runner, as I am, when you run down the path do you feel air moving across your body?...YES; when you run on a treadmill do you feel air moving across your body?...NO...that's all folks!

Perhaps you should have read some of them.

Quoting SlamClick (Reply 64):
Running is an activity where the wheels (your feet) propel the vehicle.

Your feet propel you when you run. On a treadmill the place where your foot lands on the belt is constantly moving to your rear, so you do not advance as your feet translate beneath your center of gravity.

The wheels do not propel the airplane.
las ruedas no propulsan el aeroplano.
les roues ne propulsent pas l'avion
le rotelle non azionano l'aeroplano
die Räder treiben nicht das Flugzeug an

Sorry if some of those don't really work - blame Babelfish.

So, as has been suggested: Get on the treadmill with rollerskates or a skateboard and let someone pull you with a rope. See if you move.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Ariis
Posted 2005-11-29 23:31:16 and read 32767 times.

Quoting SlamClick (Reply 56):
Is it going to stop the plane from moving forward across the earth's surface?

Of course not. The wheels are just going to spin at higher RPM.

No, they will not spin at higher RPM, read the assumption. The conveyor is never slower!

Having read all the good points by all participants (including SlamClick), one thought comes to my mind.

Why is everyone arguing about the physics and dynamics, giving life-like (good) examples, when on the root of the whole discussion there is one simple UNREAL and UNPHYSICAL assumption: that we have a conveyor belt that at any given moment spins as described (with no latency, ideally spins backward relatively to wheels)? How, having such unreal thing, would anything else behave as we learn in highschools?

I think any physical or scientical explanation is lost on the start here. That might also be the reason why both arguing sides can't convince each other.

Just my two cents.

FAO

[Edited 2005-11-29 23:33:05]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Jean Leloup
Posted 2005-11-29 23:31:29 and read 32767 times.

Quoting CosmicCruiser (Reply 107):
I couldn't bring myself to read all these posts so forgive me if I'm repeating something already said. Let's make this real simple...If you're a runner, as I am, when you run down the path do you feel air moving across your body?...YES; when you run on a treadmill do you feel air moving across your body?...NO...that's all folks!

Unfortunately, airplanes are not runners.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: A346Dude
Posted 2005-11-29 23:38:03 and read 32767 times.

Quoting Ariis (Reply 110):
Why is everyone arguing about the physics and dynamics, giving life-like (good) examples, when on the root of the whole discussion there is one simple UNREAL and UNPHYSICAL assumption: that we have a conveyor belt that at any given moment spins as described (with no latency, ideally spins backward relatively to wheels)?

And furthermore, it has no practical applications since even in the best case scenario it lengthens the take-off roll by a little bit.

I guess it's just fun to argue over theoretical physics!

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SlamClick
Posted 2005-11-29 23:40:40 and read 32767 times.

Quoting A346Dude (Reply 112):
And furthermore, it has no practical applications since even in the best case scenario it lengthens the take-off roll by a little bit.

Ah, but it it does work then we could just reverse the direction of the treadmill and all the airliners would get a free takeoff to start them on their way.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Mir
Posted 2005-11-29 23:41:05 and read 32767 times.

Quoting SlamClick (Reply 103):
Let's switch the question around. Let's make the treadmill run in the other direction. Let's say that the treadmill will accelerate from 0 to VR in about six thousand feet OF TREADMILL BELT TRAVEL. Now with an endless belt we don't need six thousand linear feet to lay this thing out, just enough to support all the landing gear.

So we park our airplane on the treadmill with the engines idling (so we don't roll of the end of the belt) and we start the treadmill. Now I can sit there and watch some instrument that tells me that I am not at VR and I can shove the throttles up and rotate and take off and fly?

Is that going to work?

If you mean that the treadmill would be running in the same direction as the airplane, then the airplane would take off after the 6000' of travel with the engines at idle, the airspeed at whatever Vr is and the wheels not moving at all. Of course, once off the treadmill and in the air, the plane would be coming back down in short order if the engines were kept at idle.

Quoting Fokker Lover (Reply 101):
or at least read this article. http://www.avweb.com/news/columns/19....html

Good article, explains things well. Conceptualizing physics problems can get pretty damn confusing.  embarrassed 

-Mir

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: A346Dude
Posted 2005-11-29 23:42:00 and read 32767 times.

Quoting SlamClick (Reply 113):
Ah, but it it does work then we could just reverse the direction of the treadmill and all the airliners would get a free takeoff to start them on their way.

True enough, but the wheels would of course have to be locked!

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Ariis
Posted 2005-11-29 23:44:05 and read 32767 times.

Quoting A346Dude (Reply 112):
I guess it's just fun to argue over theoretical physics!

I absolutely agree, just in case I sound otherwise. This is a great thread; reminds me of that one about airplane filled with pigeons, the one someone has already mentioned.

FAO

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Ariis
Posted 2005-11-29 23:49:29 and read 32767 times.

Quoting SlamClick (Reply 113):
Ah, but it it does work then we could just reverse the direction of the treadmill and all the airliners would get a free takeoff to start them on their way.

That is known as 'catapult', by the way. You know, Tomcats, Hornets, etc  Smile.

FAO

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Navymidn
Posted 2005-11-29 23:53:59 and read 32767 times.

Okay, now that I am back from class, I will be able to say more on this subject.

Unless the wheels slide down the belt, the aircraft will never take off. If the belt is always turning at the same speed as the wheels, the wheels can never make forward movement. If the wheels can't make forward movement, the aircraft can't make forward movement. In order for forward movement to be made, the wheels will either have to turn faster than the belt, or slide. It does not matter what the propulsion system of the aircraft is, because the wheels, until flight is achieved, hold the aircraft to the Earth.

The examples with the winch, or tow rope, are not valid for this problem, because an outside force anchored to a non-moving surface is providing the force for movement. Plus, if you do it with a cart in an airport, the wheels of your cart will turn faster than the belt. According to this problem, the speed of the belt and wheels is always exactly the same.

For the sailboat question, the answer is the sailboat will make no movement. I have seen this problem demonstrated with an airsled, removing friction from the equation, and the sled made no movement.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SlamClick
Posted 2005-11-29 23:54:43 and read 32767 times.

Quoting Ariis (Reply 117):
That is known as 'catapult', by the way. You know, Tomcats, Hornets, etc

Well there is another argument. Surely a nuclear aircraft carrier has the power to install a treadmill that accelerates from zero to whatever in the same length of time as the cat stroke but in the opposite direction. Would the airplane go anywhere?

I wouldn't stand in front of it!

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Ralgha
Posted 2005-11-30 00:11:04 and read 32767 times.

Quoting Navymidn (Reply 118):
It does not matter what the propulsion system of the aircraft is, because the wheels, until flight is achieved, hold the aircraft to the Earth.

No, they don't. You are not grasping the fact that airplanes do not depend at all on the ground for propulsion or flight.

Quoting Navymidn (Reply 118):
The examples with the winch, or tow rope, are not valid for this problem, because an outside force anchored to a non-moving surface is providing the force for movement.

So have the rope run through a series of pullies that allow you to reel it in yourself.

Here's another example that you might be able to grasp.

Instead of an airplane on the belt, you have a helicopter. You lift up into a hover 6 inches above the belt. All good right? Now you push forward on the cyclic and start moving forward at 5 knots. The belt, in response, starts moving in the opposite direction at 5 knots. Is the helicopter still moving forward at 5 knots? Yes, of course it is.

Now, manipulate the controls of the helicopter so only the tailwheel touches the belt. What happens? Nothing. The helicopter is still moving forward at 5 knots, the belt is going in the opposite direction at 5 knots. The tailwheel is rotating at the equivilant of 10 knots.

Now, drop the rest of the wheels down so the all three wheels are on the belt. What happens? Nothing. The helicopter is still moving forward at 5 knots, the belt is moving the opposite direction at 5 knots, and all three wheels are rotating at the equivilant of 10 knots.

Increase the speed of the belt to 10 knots. What happens? Nothing. The helicopter is still moving forward at 5 knots, the belt opposite at 10 knots, and the wheels are spinning at the equivilant of 15 knots.

Anybody still think the airplane is not going to take off?

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-11-30 00:18:51 and read 32767 times.

I've joined this too late, too. Having rushed through the posts, I was so looking forward to having a go at the conveyor-belt-gripping-the-axles idea, etc., but it seems to have had the kybosh put firmly on it. I had so many quotes ready with the aim of responding "NO!" but, alas, the work was already done. Cheers to those who showed The Way, especially Slamclick, who also managed to entertain as usual.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Fokker Lover
Posted 2005-11-30 00:24:51 and read 32767 times.

Quoting Navymidn (Reply 118):
It does not matter what the propulsion system of the aircraft is, because the wheels, until flight is achieved, hold the aircraft to the Earth.

Listen, Slamclick and I are both telling you the same thing. The wheels and landing gear have one purpose, and one purpose only. That is to keep the belly from scraping the ground, because we like to reuse our airplanes.
There is no other use for a landing gear.

A propeller is a screw that threads itself through the air pulling the airplane behind it. It doesn't matter if the wheels are cement blocks or pizzas, they have no affect on moving the plane.

Wheels spin because the plane is moving. The plane doesn't move because the wheels are spinning.
Which is exactly what you are doing right now, spinning your wheels.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Ariis
Posted 2005-11-30 00:53:57 and read 32767 times.

Quoting Ralgha (Reply 120):
Is the helicopter still moving forward at 5 knots? Yes, of course it is.

It is sliding against the belt (although hovering).

Quoting Fokker Lover (Reply 122):
Wheels spin because the plane is moving. The plane doesn't move because the wheels are spinning.

This is exactly, what, I believe, Navymidn is trying to tell you. If the airplane takes off (as you claim), it will then need to leave its wheels behind, since the wheels can't move (the conveyor won't let them move, unless they slide).

But, as I have repeatedly mentioned, any explanation is right and any result is possible, since we have an impossible situation here (or, unmeetable conditions that are assumed to be met).

FAO

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-11-30 00:56:42 and read 32767 times.

Quoting David L (Reply 121):
I had so many quotes ready with the aim of responding "NO!" but, alas, the work was already done.

Apparently not...

Quoting Navymidn (Reply 118):
If the belt is always turning at the same speed as the wheels, the wheels can never make forward movement.

NO!  Smile

Quoting Sovietjet (Thread starter):
The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.

Wheels (dragged by the aircraft) move at 150 kts in one direction, the belt moves at 150 kts in the opposite direction - groundspeed 300 kts, airspeed 150 kts. If the belt is turning, the wheels must be moving in the opposite direction.

Even if you interpret "moving in the opposite direction of rotation" as meaning that the belt will always move in such a way as to stop the wheels from turning, then with the aircraft moving forward at 150 kts the belt would be moving at 150 kts in the same direction as the aircraft - groundspeed 0 kts, airspeed 150 kts.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: A346Dude
Posted 2005-11-30 00:59:25 and read 32767 times.

Quoting David L (Reply 124):
NO!

You just had to get that out, didn't you.  Wink

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-11-30 01:03:32 and read 32767 times.

Quoting A346Dude (Reply 125):
You just had to get that out, didn't you.

Sure did!

I have to confess it did take a while for my brain to get into gear on this one. Now all I need is for someone to come out with old "everything I say is a lie" routine and I'm done for.  

Edit: Of course, it won't have the same effect if someone just says "David L, everything you say is a lie".

[Edited 2005-11-30 01:05:35]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-11-30 01:15:41 and read 32767 times.

Quoting Matt72033 (Reply 105):
rope tied to handle of shopping trolley

if i stand at one end of the conveyor, with the belt moving away from me.....no matter what way those wheels are rotating, i can drag the trolley up by the rope

if i hold the rope taught, so that the wheels move the same speed as the belt and then pull the rope, the speed of the wheels will not change whatsoever, but the trolley will move towards me!

Misunderstanding of basic shopping trolley physics. The wheels would wobble all over the place. The trolley would do a triple sulko, shoot off to the side and demolish the first stack of tins it could find.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Ralgha
Posted 2005-11-30 01:16:29 and read 32767 times.

Quoting Ariis (Reply 123):
This is exactly, what, I believe, Navymidn is trying to tell you. If the airplane takes off (as you claim), it will then need to leave its wheels behind, since the wheels can't move (the conveyor won't let them move, unless they slide).

You're both trying to explain something that won't happen. The belt can try all it wants to stop the wheels from making forward progress, but it can't do it. All it will do is make the wheels turn faster. Airplane is moving forward at 5 knots, belt goes backwards at 5 knots, wheels are turning at 10 knots. The belt increases to 10 knots to compensate, but now the wheels are rotating at 15 knots. The belt increases again to 15 knots, but now the wheels turn at 20 knots. The belt WILL NEVER CATCH UP, and therein lies the flaw in your reasoning.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SlamClick
Posted 2005-11-30 01:16:52 and read 32767 times.

Let's put to rest a side issue that is not part of the question. That is the "exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation." remark.

As I understand the question here is the situation.

We are taking off to the WEST. (for purpose of illustration) As we accelerate the conveyor belt begins to move EAST. Its speed matches the airplane speed.

It is not moving in the same direction as the plane, but the opposite direction.

The discussion turns on that point.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SlamClick
Posted 2005-11-30 01:19:44 and read 32767 times.

Quoting Navymidn (Reply 118):
If the wheels can't make forward movement, the aircraft can't make forward movement.

That would only be true if the plane was driven through the wheels like a car.
How many times does that have to be said?

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Airfoilsguy
Posted 2005-11-30 01:20:19 and read 32767 times.

In order for the plane to reach takeoff speed it needs to over come the friction of the air and the tires and inertia. The friction that the tires cause is SO small compared to the friction of the air and over coming inertia that it can effectively be ignored in this case. The plane will take off with just a tad bit longer roll out then normal. IE the friction of the tires would be double normal takeoff, and relative to the other forces acting on the plane at that time is tiny

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-11-30 01:24:08 and read 32767 times.

Quoting Airfoilsguy (Reply 131):
In order for the plane to reach takeoff speed it needs to over come the friction of the air and the tires

Strictly speaking, it's the friction of the wheel bearings - tyres are designed to provide as much friction as possible. But your point is valid.  

Edit: I only mention it as there are those who would suggest that the only way the aircraft can take off is if the tyres lose their grip on the conveyer belt.

[Edited 2005-11-30 01:32:51]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SlamClick
Posted 2005-11-30 01:28:06 and read 32767 times.

Quoting David L (Reply 121):
I had so many quotes ready with the aim of responding "NO!" but, alas, the work was already done.

Well bring them on!

Until everyone grasps the simple truth that the airplane will take off unless something stops the motion of the CENTER OF THE AXLE we still have a question before us.

You can run the conveyor belt as fast as you want in either direction, or back and forth. It will have no effect on the mass of an airplane being accelerated through its center of gravity by the engines which have no connection whatever with the ground below. The only thing the conveyor belt would affect would be the RPM and direction of rotation of the tires. I say direction because if you run the belt in the same direction as our takeoff run but faster than we are going, the tires would be rotating "backwards."

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-11-30 01:43:34 and read 32767 times.

Quoting SlamClick (Reply 133):
Well bring them on!

Well, assuming you could find one that behaves, I think the shopping trolley analogy is a good one. The argument that an external force pulling the trolley isn't valid... isn't correct. Applying a force by pulling on a rope is equivalent to applying a force by chucking a fast mass of gas out of the back of the aircraft. "A force" is applied in both cases.

Quoting Kay (Reply 57):
Here's another analogy. place a toy plane on that same conveyor, and kick it with your foot with all your force. Will it move? only if the wheels and the conveyor create huge friction. Since the plane is small and your foot is powerful, it will advance half a meter or 1 meter before standing still, but only because the wheels can skid on the conveyor in alot of friction.

But kicking it with your foot is like applying take-off power on an aircraft and then immediately cutting the power. That wouldn't work on a stationary runway either. The toy plane stops because the force is no longer applied - we're talking about applying take-off power and keeping it there.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Ariis
Posted 2005-11-30 01:44:56 and read 32767 times.

Quoting Ralgha (Reply 128):
Airplane is moving forward at 5 knots, belt goes backwards at 5 knots, wheels are turning at 10 knots.

This is all probably because of incostintency in how one understands the initial post, in terms of the 'design' of the conveyor. As I understand it, when wheels turn to the right, then the conveyor moves to the left to keep the wheels' axles steady. If the author or others mean something different, than we should straighten this first.

Btw, how come wheels turn at 10 knots? Knot is a linear speed (Please read my attempt to explain my point below).

Quoting Ralgha (Reply 128):
The belt increases again to 15 knots, but now the wheels turn at 20 knots. The belt WILL NEVER CATCH UP, and therein lies the flaw in your reasoning.

There is a definite flaw, but it is different in nature than what you say.

If airplane start the engines, it wants to start to move (at, say, 5 knots). The wheels start (or want to start) to rotate (because at that moment the conveyor stands still). But, the belt immediately (literary) starts to move backwards by 5 knots to match the speed of wheels (again, my understanding of the belt's idea). The plane does not give up (because engines still push) and still wants to move ahead and so rotate the wheels against the belt faster (the wheels do not slide). The belt accelerates not to let that happen.

This leads to an infinite loop, when airplane pulls its wheels forward on and on, but the conveyor belt is spinning INFINITELY fast to keep the wheels steady. It does not really matter if the airplane will take off or not, since you have an acceleration of the belt and the wheels from 0 to INIFINITY happening within 0.0000 seconds.

This is the flaw.
FAO

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SlamClick
Posted 2005-11-30 01:52:56 and read 32767 times.

Quoting David L (Reply 134):

I think I don't understand which side of this you are arguing. Let me ask you for a one-word answer.

Would the plane take off?

Yes.
No.

[Edited 2005-11-30 01:54:58]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-11-30 01:54:11 and read 32767 times.

Quoting Ariis (Reply 135):
It does not really matter if the airplane will take off or not, since you have an acceleration of the belt and the wheels from 0 to INIFINITY happening within 0.0000 seconds.

Why should the belt accelerate any faster than the aircraft (or its wheels)? If the belt were spinning infinitely fast, it wouldn't match the speed of the wheels unless the wheels were spinning infinitely fast.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-11-30 01:56:02 and read 32767 times.

Quoting SlamClick (Reply 136):
Would the plane take off?

Yes, definitely. I agree with you so I've goofed somewhere. Let me take another look at what I said.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SlamClick
Posted 2005-11-30 02:03:03 and read 32767 times.

Quoting David L (Reply 121):
I had so many quotes ready with the aim of responding "NO!"

I took this as meaning that your opion was NO it would not take off and you were lining up quotes in support of that.

My dumb.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Ariis
Posted 2005-11-30 02:03:53 and read 32767 times.

Quoting David L (Reply 137):
If the belt were spinning infinitely fast, it wouldn't match the speed of the wheels unless the wheels were spinning infinitely fast.

I said 'the belt and the wheels'.

The wheels are spinning infinitely fast too. The belt is only trying to catch up to the speed of the wheels (which are trying to overcome the speed of the belt). This is a vicious circle.

In order for the plane to take off (or move at all) it would need to spin the wheels faster than infinity (but, on the other hand, the belt will always catch up and compensate).

FAO

Edit: Not that spinning the wheels makes the airplane move, just in case. Spinning the wheels is just an effect of the real cause (the roaring jet engine and good grip of the tyres).

[Edited 2005-11-30 02:07:56]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-11-30 02:05:50 and read 32767 times.

Quoting SlamClick (Reply 136):
think I don't understand which side of this you are arguing.

I was saying that this, which shows how it would take off, is a good analogy:

Quoting Matt72033 (Reply 105):
rope tied to handle of shopping trolley

if i stand at one end of the conveyor, with the belt moving away from me.....no matter what way those wheels are rotating, i can drag the trolley up by the rope

Then I was attempting to show how this, which attempts to show that it wound not take off, is a bad analogy:

Quoting Kay (Reply 57):
The way i see it is that the concept of having a conveyor belt designed to spin exactly the same speed as the wheels, is like having a hand come out of the tarmac and hold the gear pillars so that they stay where they are, with huge force. That is why such a conveyor is not intuitive.

Here's another analogy. place a toy plane on that same conveyor, and kick it with your foot with all your force. Will it move? only if the wheels and the conveyor create huge friction. Since the plane is small and your foot is powerful, it will advance half a meter or 1 meter before standing still, but only because the wheels can skid on the conveyor in alot of friction.

If I've argued for the "opposition", I should probably call it a night.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-11-30 02:11:43 and read 32767 times.

Quoting Ariis (Reply 140):
I said 'the belt and the wheels'.

The wheels are spinning infinitely fast too.

I'm sorry, I just don't see where infinite speed comes into it  Smile. If the aircraft starts to move at 5 kts, the belt will start to move at 5 kts in the opposite direction. The acceleration of the belt will be the same as the acceleration of the aircraft but in the opposite direction.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-11-30 02:15:00 and read 32767 times.

Quoting SlamClick (Reply 139):
My dumb.

Ah, I think I know what the problem was. People tend to fall asleep about halfway through my first sentence! (What you'll actually see is "People tend to... something... something... blah... blah...")

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Mir
Posted 2005-11-30 02:15:41 and read 32767 times.

Quoting Ariis (Reply 135):
This leads to an infinite loop, when airplane pulls its wheels forward on and on, but the conveyor belt is spinning INFINITELY fast to keep the wheels steady. It does not really matter if the airplane will take off or not, since you have an acceleration of the belt and the wheels from 0 to INIFINITY happening within 0.0000 seconds.

I think this is one of the reasons why people are having trouble visualizing this (the other being the difference between the wheels moving the plane and the plane moving the wheels). Since the wheels and the belt would vaporize in short order (before the plane would have reached flying velocity), such a situation is impossible. Or am I missing something?

-Mir

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Fokker Lover
Posted 2005-11-30 02:17:24 and read 32767 times.

Quoting Ariis (Reply 135):
As I understand it, when wheels turn to the right, then the conveyor moves to the left to keep the wheels' axles steady.

Stick with computers. In the real world of aviation the axles are pulled forward by the rest of the plane. The wheels are just along for the ride.
They can spin forward, or backward, or not at all. It doesn't matter. It doesn't matter what the ground under the wheels is doing either. The plane is going to move.

Slamclick... Hold down the fort. Don't let the absurd win. I need to go to work before I blow a gasket. It's my turn to grease the conveyor belt tonight.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Chksix
Posted 2005-11-30 02:18:03 and read 32767 times.

Strange thread.....

I can't see any correlation between the speed of the wheels and getting airborne....
If the conveyor is set to prevent the wheels from rotating, it will just follow with the acceleration until the plane lifts off with stopped wheels.

OTOH if it tries to "push" the plane via the wheels it would cause them to blow up as they reach max rotation speed. The plane would probably do a belly landing as the gear legs got worn down...

The catapults on aircraft carriers are "conveyors" that launch planes from standstill (no wheel rotation) to flight.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Ralgha
Posted 2005-11-30 02:20:51 and read 32767 times.

Quoting Ariis (Reply 135):
Btw, how come wheels turn at 10 knots? Knot is a linear speed (Please read my attempt to explain my point below).

I'm quite aware of that. In my first post I said "equivilant" since the actual RPM will depend on the diameter of the wheel. I assumed people would understand what I meant when I left out that word, but apparently I assumed wrong.

Quoting Ariis (Reply 135):
If airplane start the engines, it wants to start to move (at, say, 5 knots). The wheels start (or want to start) to rotate (because at that moment the conveyor stands still). But, the belt immediately (literary) starts to move backwards by 5 knots to match the speed of wheels (again, my understanding of the belt's idea). The plane does not give up (because engines still push) and still wants to move ahead and so rotate the wheels against the belt faster (the wheels do not slide). The belt accelerates not to let that happen.

You do not understand the reality of the situation. There is NOTHING keeping the airplane from accelerating. The airplane WILL accelerate regardless of the speed of the belt (and the RPMs of the wheels). There is not enough friction in the wheel bearings to hold back TENS OF THOUSANDS of pounds of thrust. If the belt truely tried to match the speed of the wheel (which in knots = d*pi*RPM*60/72913 where d = diameter in inches since you're so hung up on knots being linear), it would continuously accelerate and NEVER catch up to the wheel's "speed". Regardless of the belt accelerating constantly, it will do nothing to stop the airplane from accelerating to takeoff speed and leaving. The underlined fact is your (and probably the majority of people in the world including pilots) misunderstanding.

Last point, before anyone brings up the fact that as the RPMs approach infinity, so does the rotational friction resistance. However, for the belt to be capable of accelerating to infinity, it must be in a perfect physics universe, which has no undesired friction, which means there is no bearing resistance, and no rotational friction.

Regardless, the original post altered the original question which stated that the belt would match the AIRPLANE's speed and not the WHEEL's speed.

[Edited 2005-11-30 02:29:55]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-11-30 02:21:47 and read 32767 times.

No, Slamclick, it was my dumb! I assumed it was my prior post that had clouded the issue. Having read what you actually said, it's clear that my initial "NO!" comment was a bit ambiguous!

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SlamClick
Posted 2005-11-30 02:27:19 and read 32767 times.

Quoting Ariis (Reply 140):
would need to spin the wheels faster

The wheels don't drive the airplane.
The wheels don't drive the airplane.
The wheels don't drive the airplane.
The wheels don't drive the airplane.
The wheels don't drive the airplane.

The error is in your thinking. You are still thinking "car" and you can't solve the problem that way.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Ralgha
Posted 2005-11-30 02:32:43 and read 32767 times.

Quoting SlamClick (Reply 149):
The wheels don't drive the airplane.
The wheels don't drive the airplane.
The wheels don't drive the airplane.
The wheels don't drive the airplane.
The wheels don't drive the airplane.




[Edited 2005-11-30 02:33:17]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-11-30 02:34:10 and read 32767 times.

Quoting Chksix (Reply 146):
I can't see any correlation between the speed of the wheels and getting airborne....
If the conveyor is set to prevent the wheels from rotating, it will just follow with the acceleration until the plane lifts off with stopped wheels.

Yes (I did allude to this interpretation earlier).  Smile

Quoting Ralgha (Reply 147):
Regardless, the original post altered the original question which stated that the belt would match the AIRPLANE's speed and not the WHEEL's speed.

Very true but I don't think either interpretation would stop the aircraft taking off. Either the wheels would be rotating twice as fast or they wouldn't be rotating at all - the aircraft would move at more or less the same airspeed in either case. As has been said, the aircraft doesn't care how fast the wheels are rotating.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SlamClick
Posted 2005-11-30 02:45:35 and read 32767 times.

Quoting David L (Reply 151):
Very true but I don't think either interpretation would stop the aircraft taking off. Either the wheels would be rotating twice as fast or they wouldn't be rotating at all - the aircraft would move at more or less the same airspeed in either case. As has been said, the aircraft doesn't care how fast the wheels are rotating.

Exactly!

I think a person's take on this issue might tell us something about them:

If you get it right the first time you might be a sailor.

If you get it wrong the first time you might be yatchsman.

If you get it after the irrelevancies of your argument are pointed out to you, you are a normal guy.

If you never get it after having all your arguments shredded by cogent example you might be an engineer.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Ralgha
Posted 2005-11-30 02:46:46 and read 32767 times.

Quoting David L (Reply 151):

Very true but I don't think either interpretation would stop the aircraft taking off. Either the wheels would be rotating twice as fast or they wouldn't be rotating at all - the aircraft would move at more or less the same airspeed in either case. As has been said, the aircraft doesn't care how fast the wheels are rotating.

If you factor in bearing friction, you could eventually get to a speed that would prevent the airplane from moving forward since the rotational friction force would eventually become so great that it would take all the engine force to simply keep the airplane from moving backwards with the belt. However, in reality the tires and belt would explode before they could go that fast, so you have to start picking and choosing which laws of physics you follow and which ones you don't.

The original question, belt matching airplane speed, can be posed without any additional assumptions.

[Edited 2005-11-30 02:47:31]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Ralgha
Posted 2005-11-30 02:49:50 and read 32767 times.

Quoting SlamClick (Reply 152):
If you get it after the irrelevancies of your argument are pointed out to you, you are a normal guy.

I must disagree, I've tried to explain it to many "normal" people who were never convinced. Additionally, I am an engineer by education, and I do get it (airplane will take off).

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: PPVRA
Posted 2005-11-30 02:52:38 and read 32767 times.

The wheels would freewheel backwards - cancelling the conveyor belt - even with the engines powered off. Once you turn them on, they will exert forward thrust and move the aircraft forward just like any other vehicle that moves through fluids and require it to generate thrust - boats to aircrafts, including hovercrafts.

The only way it would not move forward would be to place a huge fan in front of it blowing at Mach .XX (the aircrafts max speed). However, it could still take off, vertically.

Cheers

[Edited 2005-11-30 03:15:20]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: A346Dude
Posted 2005-11-30 02:53:00 and read 32767 times.

Quoting SlamClick (Reply 152):
If you never get it after having all your arguments shredded by cogent example you might be an engineer.

Hey, take that back! If you look back in the posts, you will find I understood the question a lot sooner than you did, mister.  Wink

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SlamClick
Posted 2005-11-30 02:53:08 and read 32767 times.

Quoting Ralgha (Reply 154):
Additionally, I am an engineer by education, and I do get it (airplane will take off).

Well, that is why I said "might be." There are three engineers in the world who have common sense.

I wonder who the other two are.
 Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Keta
Posted 2005-11-30 02:54:32 and read 32767 times.

LOL at first when I saw the question I thought "what a silly question, how can there be 100+ posts?" But then I realized that it's not that easy. This is what I got, after discussing with some friends (it doesn't mean much, but they where engineers).

First, the aircraft will accelerate, regardless of the speed of the conveyor. I think SlamClick said it clear, but there are several ways to see it:

The wheels do not cause any force on the airplane, because there is no friction in the axis (well, that's because we suppose it ideal. In the real world there might be a small force, but so small that when thrust is applied to the engines, this force would overtake the friction. Hey, that's actually why planes move forward!). If you isolate the airplane, you will see that no force is pulling from the plane, there is only the thrust pushing. So, it doesn't matter the speed the conveyor has, the airplane will move.

You can visualize it also with a train. If you have a train traveling, with no force acting (forget the friction with the air and the axes) the train will continue moving forever. Now that's the same as if the train was actually still and the railway was moving, like a conveyor (it's just a matter of reference frame). So, no matter the speed of the conveyor, the train will remain steady (remember, that's with no force on it). And now, if you put rockets on the train, it will move forward, again whatever the speed of the conveyor is.

We all got to this conclusion and I think it's quite clear. There are more ways to see it, maybe easier, but well my vocabulary is not very good.  Wink

So, to put it clear, the speed of the conveyor can be whatever you want, slow or fast, but if you apply thrust the airplane will move forward.

**BUT**

Here comes the essence of the question: "The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation"

What does this really mean? You can have two (I think there are no more) understandings:

1) The conveyor is running at the same speed as the airplane. That is, if the airplane is running at 100 knots, the belt will run at 100 knots in the opposite direction. As the airplane accelerates, so will the belt; but, as I said above, that has no effect on the plane's acceleration and therefore the airplane will take off, just like if there was no conveyor belt.

2) And that's my understanding (and my friends' too): the axis of the wheel will remain at rest. That is, by definition the axis will remain still and so will the airplane (since it is attached to the wheels). Remember, by definition the airplane will remain motionless. But if a force is applied the airplane will move. This contradiction means that this situation is IMPOSSIBLE. That is, the question is not correct, the sentence makes no sense.

The definition of "exactly match the speed of the wheels" is the most critical part. Speed relative to what? My understanding is the 2nd mentioned above, but it's actually, and ideally, impossible. Even from a more philosophical point of view, if the airplane (thus the wheels) gains a little speed automatically the belt would go the same speed (in the other direction) but that would mean that the wheels are really not moving i.e. they have no speed... For an airplane case I see it impossible. Everything is different if the object was a car, because wheels do apply force.

So, our final conclusion was that the question is actually wrong and it's impossible such a situation.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Ralgha
Posted 2005-11-30 02:58:08 and read 32767 times.

Which is why SlamClick and I have both stated the correct phrasing of the question.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: A346Dude
Posted 2005-11-30 03:04:30 and read 32767 times.

Keta,

A really good summary of the fundamental problem that I think is responsible for the size of this thread.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Bond007
Posted 2005-11-30 03:05:05 and read 32767 times.

Quoting Ariis (Reply 55):
I was trying to say, that construction of the conveyor belt (as assumed) assures us that the wheels' axes will never move relatively to the surrounding environment.



Quoting Navymidn (Reply 118):
Unless the wheels slide down the belt, the aircraft will never take off. If the belt is always turning at the same speed as the wheels, the wheels can never make forward movement. If the wheels can't make forward movement, the aircraft can't make forward movement. In order for forward movement to be made, the wheels will either have to turn faster than the belt, or slide. It does not matter what the propulsion system of the aircraft is, because the wheels, until flight is achieved, hold the aircraft to the Earth.

That's it right there. The fact that wheels aren't being turned by the engine is irrelevant. The wheels DO need to turn in order to takeoff - and before you say they would start slipping - they would not in this example because the ground underneath is ALWAYS going at the same speed backward as the force is trying to move the plane forward.

I'll repeat the excellent quote:
"If the wheels can't make forward movement, the aircraft can't make forward movement." ...and therefore NO airspeed.


Jimbo

[Edited 2005-11-30 03:13:14]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Ralgha
Posted 2005-11-30 03:10:48 and read 32767 times.

Quoting Bond007 (Reply 161):
"If the wheels can't make forward movement, the aircraft can't make forward movement."

True, however there is nothing preventing the wheels from making forward movement, which is the flaw in your understanding.

Quoting Bond007 (Reply 161):
they would not in this example because the ground underneath is ALWAYS going at the same speed as the force is trying to move the plane forward.

Speed does not equal force.

Argument shredded, next please.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Ralgha
Posted 2005-11-30 03:12:50 and read 32767 times.

Quoting SlamClick (Reply 157):
Well, that is why I said "might be."

Right, I overlooked the "might be" part... banghead 

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Bond007
Posted 2005-11-30 03:15:13 and read 32767 times.

Quoting Ralgha (Reply 162):
True, however there is nothing preventing the wheels from making forward movement, which is the flaw in your understanding.

Yes, the conveyor moving in the opposite direction at the same speed is doing that - that's the whole purpose of this conveyor!


Jimbo

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Ralgha
Posted 2005-11-30 03:20:29 and read 32767 times.

Quoting Bond007 (Reply 164):

Yes, the conveyor moving in the opposite direction at the same speed is doing that - that's the whole purpose of this conveyor!

No, it's not! The wheels are free spinning, that's the whole idea of a wheel, it allows the object to which the wheels are attached (which includes the axel) to move independently of the surface on which the wheels rest.

Allow your argument to rest in peace and move on to the next. Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: PPVRA
Posted 2005-11-30 03:20:52 and read 32767 times.

Wait a second -

The wheels will be spinning fast, really fast. That means there will be less friction that the engines have to overcome, no? Kinda like sliding a block across a rough surface, the faster it moves the less friction is involved.

Wouldn't that make the acceleration more efficient, actually requiring less runway?

Although that may be only to a limited extent....

...?

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Ralgha
Posted 2005-11-30 03:26:23 and read 32767 times.

Quoting PPVRA (Reply 166):
Wait a second -

The wheels will be spinning fast, really fast. That means there will be less friction that the engines have to overcome, no? Kinda like sliding a block across a rough surface, the faster it moves the less friction is involved.

Wouldn't that make the acceleration more efficient, actually requiring less runway?

Although that may be only to a limited extent....

...?

No, there are two types of friction, static and dynamic. Each is a constant, which the dynamic being less (much less often) than static. The friction coefficient of a block at rest is higher than the coefficient of a moving block, which means the frictional force is greater at rest than when moving (at slow speeds). At some speed (depending on the two coefficients) the frictional force of the moving block will again be equal to the resting block. Any higher speed and the moving block friction will be higher.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Bond007
Posted 2005-11-30 03:32:05 and read 32767 times.

Quoting Ralgha (Reply 165):
No, it's not! The wheels are free spinning, that's the whole idea of a wheel, it allows the object to which the wheels are attached (which includes the axel) to move independently of the surface on which the wheels rest.

Hmmmm....so I'm watching from the control tower and for every one foot my airplane moves forward, the runway magically moves backward one foot ....what is the relative motion of the airplane from the standpoint of me in the control tower .... zero forward speed, therfore zero forward airspeed (assuming no wind).


Jimbo

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Jean Leloup
Posted 2005-11-30 03:35:48 and read 32767 times.

Quoting Bond007 (Reply 168):
for every one foot my airplane moves forward, the runway magically moves backward one foot

And this has no effect on the motion of the plane; it simply means that the wheels will be spinning twice as fast.

JL

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Ralgha
Posted 2005-11-30 03:36:42 and read 32767 times.

No, from your vantage point in the tower, the takeoff would look completely normal. The airplane is moving forward just as normal, only the wheels are spinning twice as fast as normal. The wheels cover twice as much ground as normal, but that's the only change, and not one that is noticable from any significant distance.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Turnit56N
Posted 2005-11-30 03:50:12 and read 32767 times.

While I don't want to wade knee-deep into the debate, I'm reminded of the old saying "You know you've landed gear-up when it takes full power to taxi to the gate."

Or in other words....wheels make it easier, but an aircraft moves by thrust nonetheless.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: MD-90
Posted 2005-11-30 03:52:33 and read 32767 times.

Holy cow.

Action = opposite equal reaction
Airspeed does not necessarily = groundspeed

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: No1racer
Posted 2005-11-30 04:00:46 and read 32767 times.

Hey all,
Something that I feel hasn't been brought up is this: We are all here talking about the wheels being free-standing. However, how do planes stop? Honestly? It's not all reverse thurst and slats, it's brakes. The fact is that wheels have air pressure or gas pressure for a good reason. Increase the surface area touching the runway, and you increase the friction associated with the object. There is definate pressure between the wheels and the aircraft, although the wheels don't drive the craft. Here is my theory:

First, the aircraft will remain stationary until a certain thrust is applied to move the aircraft. This will overcome the pressure force of the plane on the air particles and tire.

Secondly, the aircraft will move slowly until the wheels gain enough speed to where air particles actually support the aircraft under the wheels. This phenomenon was seen in the desert when the land speed records were set.

Third, the aircraft will take off normally in relation to the wind speed as lift is caused by air density, speed, and wing surface area along with the fluid flow properties over the wing.

Thanks. Will respond later.

No1racer

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Ralgha
Posted 2005-11-30 04:11:02 and read 32767 times.

Quoting No1racer (Reply 173):
First, the aircraft will remain stationary until a certain thrust is applied to move the aircraft. This will overcome the pressure force of the plane on the air particles and tire.

True, and also not influenced at all by the belt.

Quoting No1racer (Reply 173):
Secondly, the aircraft will move slowly until the wheels gain enough speed to where air particles actually support the aircraft under the wheels. This phenomenon was seen in the desert when the land speed records were set.

Airplanes do not move fast enough when on the ground for this effect to occur (if indeed we are thinking of the same thing).

Quoting No1racer (Reply 173):
Third, the aircraft will take off normally in relation to the wind speed as lift is caused by air density, speed, and wing surface area along with the fluid flow properties over the wing.

Also true, and also not influenced by the belt.

Quoting No1racer (Reply 173):
We are all here talking about the wheels being free-standing. However, how do planes stop? Honestly? It's not all reverse thurst and slats, it's brakes.

Using the brakes is fundementally different than using the engines. The brakes work by pulling on the wheel which in turn pulls on the ground, thus they are dependent on the ground. The engines are in no way dependent on the ground, which is why the belt has no effect on the takeoff of the airplane.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: PPVRA
Posted 2005-11-30 04:15:07 and read 32767 times.

Quoting Ralgha (Reply 167):
At some speed (depending on the two coefficients) the frictional force of the moving block will again be equal to the resting block. Any higher speed and the moving block friction will be higher.

Ok, air friction - right?

Thanks for clearing that up.

Cheers

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SlamClick
Posted 2005-11-30 04:22:02 and read 32767 times.

Quoting Bond007 (Reply 161):
"If the wheels can't make forward movement, the aircraft can't make forward movement." ...and therefore NO airspeed.

Okay if finally occurs to me that there is a language trap in the way the problem is being posed. Let me give you an example:

Three guys check into a hotel. Desk clerk tells them it is $30 for the night. After they go up to their room he realizes he's overcharged them, the room is only $25. So the desk clerk calls the bellhop over, tells him he's overcharged these three guys, gives him five $1 bills and sends him up to give them a refund.

On the way up the bellhop gets thinking: There are three of them and five dollars. You cant divide three into five evenly. So he decides to keep two dollars, gives them three and goes on his way.

So, the guys originally paid $10 each for the room. Then the bellhop gave them each a dollar back. Now they have paid $9 for the room. There are three of them: Three times 9 is 27 plus the two dollars in the bellhop's pocket is $29. What happened to the other dollar.


Think about that problem for a while. I'll be back.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Ralgha
Posted 2005-11-30 04:22:33 and read 32767 times.

No, this is discounting air friction. This is just friction between the block and the surface.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Bond007
Posted 2005-11-30 04:24:38 and read 32767 times.

I guess the clear explanation that I haven't seen (or probably missed), is that a jet engine, or a piston prop even, acts on the air around it, which in this example is no different whether the belt is moving forward or backward, or not at all.

If one turn of a prop can move a Cessna 172 one foot through the air ... it's still gonna move it one foot forward in relation to the air, not the belt (once it's overcome friction etc.), regardless of the ground underneath.

So it will takeoff - just needed some convincing  Smile


Jimbo

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: PPVRA
Posted 2005-11-30 04:33:27 and read 32767 times.

Quoting Ralgha (Reply 177):
No, this is discounting air friction. This is just friction between the block and the surface.

Hmmm... been a while since my last physics class.

So you're saying that after a certain speed the friction with the surface will increase?

I remember form class that friction decreases with an increase in speed... I guess it's not completely true? (discounting air friction)

Or maybe my memory is failing me?

Cheers

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SlamClick
Posted 2005-11-30 04:33:44 and read 32767 times.

Okay, before I get accused of hijacking the thread let me answer:

Quoting SlamClick (Reply 176):
So, the guys originally paid $10 each for the room. Then the bellhop gave them each a dollar back. Now they have paid $9 for the room. There are three of them: Three times 9 is 27 plus the two dollars in the bellhop's pocket is $29. What happened to the other dollar.

The way I phrase that question only makes it more difficult to answer. The trick is whether the numbers you give are inclusive or exclusive or whether you start counting at zero or one. In reality we still have $25 in the cash register, $2 in the bellhop's pocket and $1 each in the hands of the three guests. There is no missing dollar.

Now why do I pose this question?
Well, mostly because it has been my favorite riddle since I was seven years old, but also to illustrate how the language of the question can obscure what is really being asked.

* * *


The conveyor can affect the bottom of the tire only. If no other forces act upon the tire the conveyor will carry it along. If the axle is restrained, as in we have a tug attached that is not on the conveyor, the wheel will spin at a speed determined by the speed of the conveyor. If we apply another force through the axle of the wheel (oh I don't know - let's make it a pair of CFM-56s) the wheel will roll in the direction that those engines are thrusting it. It will roll at a speed determined by the algebraic summing of (a) The actual speed it is moving over the surface of the earth and (b) the speed imparted by the moving surface beneath it.

If the surface beneath is moving forward it will cancel out some of the rotational speed of the tire. If it is moving rearward it will add to it.

Geez, you guys think this simple mechanical transfer question is tough you ought to try to teach how the mirrored-folding-planetary gear with a hydraulic-motor driven ring gear works in your garden-variety CSD!

[Edited 2005-11-30 04:35:33]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Ralgha
Posted 2005-11-30 04:40:36 and read 32767 times.

Quoting Bond007 (Reply 178):
is that a jet engine, or a piston prop even, acts on the air around it

SlamClick and I have said that many times.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Ralgha
Posted 2005-11-30 04:42:19 and read 32767 times.

Quoting PPVRA (Reply 179):
So you're saying that after a certain speed the friction with the surface will increase?

The friction is constant, the force imparted by that friction is dependent on the speed of the object.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Zarniwoop
Posted 2005-11-30 04:42:25 and read 32767 times.

Initially I was convinced that it wouldn't take off but Slamclick changed my mind (check rply 56, cable & winch analogy) & I agree with Bond007, the key point here is that the engines are pulling the aircraft through the air not along the ground so, yes the plane would take off.... the wheels will just spin at a higher RPM as the treadmill speeds up.....

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Bond007
Posted 2005-11-30 04:53:29 and read 32767 times.

Quoting Ralgha (Reply 181):
Quoting Bond007 (Reply 178):
is that a jet engine, or a piston prop even, acts on the air around it

SlamClick and I have said that many times.

Like I said, it wasn't obvious to me anyway. It was said many, many times that the wheels were not 'driving' the aircraft, but not too clear why it would still move forward.

Visualizing a propellor through the air makes it clear to me - hey, just me - I was slow into this thread  Wink


Jimbo

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Navymidn
Posted 2005-11-30 04:57:56 and read 32767 times.

A historic moment: I swallow my pride and admit that the aircraft will take off. After many tests, I have determined that either way the belt is turning, the aircraft will eventually reach flying speed, if the question is explained the way it has been. The way I had been answering it was under the assumption that the conveyor was holding the wheels from movement.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: NAV20
Posted 2005-11-30 06:42:11 and read 32767 times.

When you get to my age the oddest things stir memories of long ago. This time it's a long-lost uncle of mine who was an 'erk' (aircraftsman) during the Battle of Britain, who told me of the horrors of 'Two-six!"

That was the cry when a fitter had finished servicing a Merlin engine and was ready to run it up to test the magnetos. The nearest 'spare' blokes had to drape themselves over the tail while he did that - subjecting themselves to a 250-knot slipstream! There were no reports of any of those Hurricanes taking off, as far as I know. Though apparently it felt like that at times.  Smile

One thing I don't understand about the question is why the conveyor belt has to be 'as long as a runway'? Surely if the aeroplane is not actually going to move relative to the conveyor, the belt doesn't need to be long?

I have to come down on the 'No' side, because I can't see where any wind (and therefore lift) is coming from - even 'relative' wind. Bringing it down to essentials, isn't the situation described analogous to riding an exercise-bike in a gym? Riding a real bike you're conscious of wind, i.e. airflow. Riding an exercise-bike (or running on one of those endless-belt things, for that matter) the thrust you generate is absorbed by the mechanism and you don't actually move relative to the air - hence no wind.

Admittedly, the question stipulates that the conveyor would be mechanically driven - but surely, power-driven or not, it would be moving backwards, not forwards? To me, the aeroplane would remain stationary relative to the surrounding air - and therefore no lift would be generated?

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Jean Leloup
Posted 2005-11-30 06:48:11 and read 32767 times.

NAV20;

You seem not to have read the thread, as the examples you bring up (biking and treadmill) have been brought up and dismissed. I think just about everyone is on the same page now: The airplane will fly. The backwards-moving belt is not able to stop the engines from moving the plane forward, it just makes the wheels spin faster while this happens.

JL

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Sovietjet
Posted 2005-11-30 06:52:14 and read 32767 times.

How can the wheels spin faster than the belt though....if they are both equal the plane will never move no matter how much thrust(assuming no slippage)

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Sovietjet
Posted 2005-11-30 06:53:47 and read 32767 times.

I think the original question was poorly worded IMHO...the problem doesn't specify whether the belt matches the wheel's speed in RPM, tangential speed of a pont on the wheel, groundspeed, etc....

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: NAV20
Posted 2005-11-30 07:08:16 and read 32767 times.

Quoting Jean Leloup (Reply 187):
You seem not to have read the thread

I have read the thread, Jean Leloup. I 'lost it' at the point where people said that 'relative wind' would be generated. An aeroplane (as opposed to a rocket) flies because of the airflow over its wings - no airflow, no flight. And I can't see how having a conveyor to neutralise the effect of engine thrust alters that - the aeroplane is still not moving through the air?

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: BuckFifty
Posted 2005-11-30 07:13:24 and read 32767 times.

The belt matching the speed of the wheels...hmmm.

Okay. Let's think about it this way. Thrust is added. Aircraft has forward momentum. If the aircraft is on a fixed surface, the wheels will reach 150 knots as the aircraft takes off. Okay, done.

If there is a moving surface underneath, the aircraft will still fly relative to the speed of the air. Thus if the wheels are spinning at 150 knots, this conveyor is moving at 150 knots. But if the conveyor is fixed to the ground, it doesn't move. So the aircraft will travel at a speed relative to the air, not the conveyor. But if the whole conveyor assembly is matched to the speed of the aircraft itself (not the wheels), then it's a different story.

So the groundspeed in this instance, minus the conveyor, the aircraft will still take off. With the conveyor, the aircraft will still take off, but the wheels will quickly reach a speed of infinity, since it is an independent variable of the airspeed. So the aircraft will probably not take off, since the tires will blow up, causing the landing struts to dig into this conveyor, and a huge mess will follow.

I suppose this concept of a conveyor matching the speed of the wheels is quite hilarious. If you think of it this way, groundspeed is independent of airspeed. But instead of the usual examples, the moving mass is not the air, but the ground. Does the aircraft need groundspeed to fly? No. But if the conveyor is designed to match the speed of the wheels, they would both spin up rather quickly in exponential terms, since the aircraft will move forward anyway because of thrust. So again, I'm not sure if a conveyor belt can move at lightspeed, but the physics behind that can be quite mind boggling.

Because, afterall, no one has considered ground effect here...

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Jean Leloup
Posted 2005-11-30 07:32:43 and read 32767 times.

Nav20, the conveyor is not neutralising engine thrust. It is not able to do so. The plane will generate lift the old fashioned way, as the engines pull it forward and create that airflow. 'Relative wind' is not an issue here - I don't know what that means, either. The conveyor just makes the wheels spin faster, which is not relevant to the airspeed in this instance.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Matt72033
Posted 2005-11-30 08:09:07 and read 32767 times.

Quoting David L (Reply 127):
Misunderstanding of basic shopping trolley physics. The wheels would wobble all over the place. The trolley would do a triple sulko, shoot off to the side and demolish the first stack of tins it could find

lol you are quite correct, but, my logic was, we're talking about a 10,000 foot or whatever conveyor belt here, if we've got one of those, im sure we can find a shopping trolley that rolls straight  Wink

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: LeanOfPeak
Posted 2005-11-30 08:20:47 and read 32767 times.

Uhhh, folks...

Assuming the tires are not slipping, the have two problems here. The relative motion problem says that wheels turning at x mph on a treadmill turning at -x mph will result in 0 mph groundspeed. It really is that simple.

The "aircraft takeoff" problem says that the motive force is applied by the engines, and there is zero retarding horizontal reaction, at the tires or anywhere else.

The two conditions are incompatible. The inescapable conclusion from the problem is that engine thrust --> Rotation of tires, wheels, and treadmill. It is inherently fallacious, but there is no other way to satisfy the conditions of the problem, so the question, if the tires are not allowed to slip, becomes which of the preconditions one is willing to allow to slip.

In order to maintain zero groundspeed as required by the relative motion problem, any extant engine thrust must be opposed by something...An anchor, a reaction at the tires, /dev/null, it doesn't matter. Since no anchor or gravity well is mentioned in the problem, the appearance ends up being that of a horizontal reaction at the tires.

Rotation of the wheels, tires, and treadmill as a direct result of engine thrust without the motion of the aircraft is an impossibility, so the illusion of the powered-wheels fallacy appears. The conditions could be equally well satisfied by capturing the engine thrust with lossless turbines and routing the energy to lossless electric motors to turn the lossless treadmill while the aircraft is held stationary by an anchor. But that requires considerably more imagination.

The only real-world solution to the problem is if you lock the brakes. Then, the engine thrust is dissipated in the brakes until the point that the tires break loose, at which point any excess goes to moving the aircraft. However, the mere suggestion of a "real-world" application of the conditions of the problem is ludicrous.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Keta
Posted 2005-11-30 09:55:31 and read 32767 times.

Uh you still arguing...

Quoting Sovietjet (Reply 189):
I think the original question was poorly worded IMHO...the problem doesn't specify whether the belt matches the wheel's speed in RPM, tangential speed of a pont on the wheel, groundspeed, etc....

That's the problem.

The answer to the question is not "yes" neither "no".

The way the question is proposed makes it an impossible situation.

If you, by definition, have a belt designed to maintain the center of wheels at rest, then who cares of thrust? By definition it will not move. I mean, it's impossible to apply thrust and have a system with no frictional force (as the axes are) that maintains the aircraft still. It's just that, what answer do you want to an impossible situation?

The only arguable question is "Would an airplane move on a conveyor belt, this at any speed?" This has been discussed, and it seems like most of us agree that yes, it will move.

BTW, isn't this a record for Tech/Ops?? Big grin

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Kay
Posted 2005-11-30 11:05:18 and read 32767 times.

Quoting Keta (Reply 195):
"Would an airplane move on a conveyor belt, this at any speed?" This has been discussed, and it seems like most of us agree that yes, it will move.

Traditional answers from the "it will move" group:

1- a plane is not a car.
2- the wheels can rotate freely.

True, the plane is not a car. It doesn't need the wheels to rotate. On a free rotating conveyor belt, it would take off. On a conveyor belt that moves forward, the plane will take off with the wheels not rotating at all.

The plane does not need the wheels to turn (like a car). But it needs that the wheel assembly moves forward. And the wheel assembly will never move forward which such a smart conveyor belt.

Kay

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Saintsman
Posted 2005-11-30 11:10:50 and read 32767 times.

As others have mentioned, it is an impossible situation.

People have been giving examples to prove their points but none of the examples are comparable with the situation described. You are not comparing like with like which makes the examples invalid.

Therefore although it is theroretically possible for the aircraft to take off, it will not because the situation is impossible.

It's been good fun though  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: VirginFlyer
Posted 2005-11-30 11:31:30 and read 32767 times.

Before starting, I think it is very important to describe the frame of reference here, and the words I am going to use, to avoid any confusion:

A) When I describe the groundspeed of the aircraft, I will be referring to the speed of the aircraft relative to the earth.

B) When I describe the wheelspeed of the aircraft, I will be referring to the speed that the wheels are turning (e.g. when I say wheelspeed of 20kts, that refers to rate of rotation of the wheels that is equal to that produced by the aircraft moving over solid ground at 20kts).

C) When I describe the airspeed, I will be referring to the speed that the aircraft is moving through the air. (As an aside, it is very important to understand (and this is more general than for this) that the aircraft’s MOMENTUM is a function of its ground speed, and its LIFT is a function of its airspeed (hence why a change in wind strength or direction can be so dangerous – an aircraft will continue at the same groundspeed, yet the air speed will have changed).)

D) When I describe the conveyorspeed, I am referring to the speed of the conveyor. We can relate this to the other speeds by the following equation:

Wheelspeed – Conveyorspeed = Groundspeed [Equation 1]

E) We have a conveyor belt. It is fixed the earth. It is set up so that the belt exactly matches the speed of the wheels in the opposite direction. It is VERY IMPORTANT to read the description of the conveyor belt, especially noting that it is slaved to the rotational speed of the wheels (i.e. the wheelspeed), and not to the ground speed of the aircraft.:

Quoting Sovietjet (Thread starter):
The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.

F) We have an aircraft which produces thrust. It is sitting on wheels which are not propelled in any way. The wheels are free to rotate. It is assumed there will be some frictional component in the rotation of the wheels. The aircraft is sitting on the conveyor belt. Initially, we will make the assumption that there is no wheel slippage when full power is applied – that is, the wheels will not slip across the surface they are in contact with when the aircraft’s engine is taken to full thrust. Having run the engine of a Cessna 172N to full power with the brakes applied, I know of at least one aircraft which will not slip under the application of full power.

Now, having defined those, let’s deal with the question at hand.

If the conveyor belt is as described, then whenever the wheels on the aircraft are turning forward, the conveyor belt is running back under them at the same speed. Thus, the wheels turn, but there is no linear motion of the wheel hub. The wheel struts will stay still relative to the fixed ground. So will the aircraft to which the struts are attached. Thus, the ground speed of the aircraft is zero. If there is zero ground speed, and zero wind, then there is zero airspeed. Thus there is no lift. The aircraft will not take off. It doesn't matter how much power you put behind it, if the we have a wheelspeed of x knots, and the surface underneath them is travelling backwards at a speed of x knots (as proposed in the original question), then the aircraft will not move. You can see this from Equation 1. Provided there is no slippage, and the belt matches speed exactly with the wheels, the aircraft will not move.

Now the question should be asked, if everything is still, and it stays still, and the wheels are not powered, what sets the thing moving then? It must be assumed that there is some finite lag between the movement of the wheels, and the movement of the conveyor belt. Thus, when we apply power, the aircraft starts to roll forward before the conveyor can catch it. Thus the wheel speed will be slightly higher than the conveyorspeed. We can see from Equation 1 that the groundspeed will be equal to the difference between the speed at which the wheels are turning, and the speed at which the conveyor is. Due to the lag, the conveyor belt will keep chasing the wheelspeed until it reaches a speed at which the frictional force produced by the wheels is equal to the thrust force produced by the engine (we will call this the critical speed). As the conveyor reaches this speed, the difference between the wheelspeed and the conveyorspeed will reduce. Thus the ground speed will reduce as the conveyor approaches the critical speed. It should be noted that the critical speed will depend on the amount of thrust being produced by the engine – the more thrust, the faster the critical speed. At the critical speed, the entire thrust is required to counter frictional force, and thus there is no net force. With no net force, the aircraft will not accelerate. Because at the critical speed the wheelspeed = conveyorspeed, the groundspeed is equal to zero, and the aircraft will not fly.

Having established that the lag will allow the aircraft to have some groundspeed as the system speeds up, we have to ask ourselves how quickly the system will reach the critical speed. We need to consider the amount of lag between the wheels starting to move, and the conveyor starting to move, as well as what rate of acceleration the conveyor can reach.

If there is enough lag that the wheelspeed becomes sufficiently higher than the conveyorspeed, then the groundspeed (and thus airspeed) could get high enough to allow the aircraft to take off. However, I think you’d agree that to get a 62 knot difference (take-off safety speed of the Cessna 172N) would require quite a significant lag in the machine. I would imagine that a conveyor coupled electronically to an aircraft wheel would not exhibit enough lag to allow the aircraft to reach take-off speed.

If the conveyor is not capable of accelerating as fast as the aircraft, then the difference between the wheelspeed and the conveyorspeed will increase, as above giving us a groundspeed (and thus airspeed) that will be sufficient to allow flight. While this lack of acceleration in the conveyor is conceivable, I think the spirit of the original question is that the conveyor is capable of keeping pace with the acceleration of the aircraft.

So, provided the lag is not excessive, and the conveyor can accelerate at a sufficient rate, and there is no wheel slippage, and the conveyorspeed is slaved to the wheelspeed as was originally proposed, then the aircraft will not be able to achieve a sufficiently high groundspeed to take off. It will simply end up sitting still, with the conveyor running at some stupidly high speed under it, and its wheels turning at the same stupidly high speed. I would hazard a guess that this would lead to a failure of the tires, if not the entire wheel assembly. But, since we are just taking this as a thought experiment, I guess that is beside the point.

Now, we could introduce a slippage – that is, under full thrust, the friction of the tyres against the ground is not enough to prevent the aircraft from moving (as I think has been described above using examples of some jet airliners). In that case, well anything goes. If the wheels continue to rotate at the same speed while slipping, then it would be the same as taking the aircraft out on a runway, locking the brakes on, and powering it up. Perhaps those with experience on these aircraft who have talked about it above would be able to tell us if an aircraft could reach take off speed in such a condition. I would very much doubt it.

Of course, I believe that when a wheel slips, it actually loses a lot of traction, and so it would stop being turned so much by the conveyor. Thus, the wheel’s rotation slows down, and thus the conveyor slows down. Eventually the wheel would grip again, and the process would start from the beginning. As above, I would be very dubious if such a condition would give sufficient groundspeed (and thus airspeed) to allow a take off. I doubt such a situation of wheels locking and unlocking like this has even been tested (though if someone has an example of it, I’d love to hear about it).

So, while slippage may conceivably allow the aircraft to take off, I think it would be very unlikely, not without a sufficiently long strip to allow the slow acceleration, and not without tyres and wheels that could stand such a phenomenal punishment. Then again, this whole scenario is very unlikely, so read into that what you will.

Overall, as explained in this post, I am very firmly of the opinion that the aircraft will not be able to take off in the scenario given by the thread starter.

As a postscript to this, I think it is worthwhile to again point out the major difference between the scenario proposed in this thread’s starter, and the scenario described on the Avweb page - http://www.avweb.com/news/columns/191034-1.html - which states:

Quote:
The conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction)

From reading some posts in this thread, I think this may have caused a bit of confusion. In our example, the conveyorspeed is slaved to the wheelspeed. In the Avweb example, the conveyorspeed is slaved to the groundspeed (which is equal to the airspeed in still air). While our example pretty much prevents movement, the Avweb one doesn’t by any means. It simply means that the wheels will be turning twice as fast as they normally would were the aircraft to be accelerating down a normal runway. This would cause a bit of extra friction on the take-off run, so the acceleration would be slower, and the take-off run would be longer, but the aircraft would be able to take off so long as the wheels could withstand the increased rotational speed. But I must stress that the Avweb example is NOT the example Sovietjet gave.

I hope this crazily long post has given some clarification. If there are any holes in my explanations, I would be very happy to hear them.

V/F

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Ariis
Posted 2005-11-30 12:12:14 and read 32767 times.

Quoting SlamClick (Reply 149):
The wheels don't drive the airplane.

Unless airplane slides, the wheels MUST turn. Do not confuse the cause and the result. The spinning wheels are not the cause, they are the result of the airplane's movement (I've edited my last post, you may have not read it).

This is an ill-conditioned question that was initially asked. Assuming the impossible, there is no reasonable answer nor any explanation to what will happen next. We can use our imagination, draw examples and quote various laws of physics, but they do not apply here. The imaginary conveyor negates them all at the very beginning.

A philosophical question should be asked: Do we really need to know, if the airplane takes off or not?  Wink

FAO

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-11-30 12:23:43 and read 32767 times.

Quoting Ralgha (Reply 153):
If you factor in bearing friction, you could eventually get to a speed that would prevent the airplane from moving forward since the rotational friction force would eventually become so great that it would take all the engine force to simply keep the airplane from moving backwards with the belt. However, in reality the tires and belt would explode before they could go that fast, so you have to start picking and choosing which laws of physics you follow and which ones you don't.

Yes, as Slamclick pointed out, but the aircraft would still accelerate in the desired direction, which is really the point.  Smile

Quoting Bond007 (Reply 161):
I'll repeat the excellent quote:
"If the wheels can't make forward movement, the aircraft can't make forward movement." ...and therefore NO airspeed.

If you mean the wheels as a whole can't move then you'd need something more than a conveyor belt, like a strong chain anchored to the tarmac beyond the belt. A conveyor belt cannot stop the wheel axles from moving. If that is the intended scenario then it's a waste of time considering it.  Smile

Quoting Matt72033 (Reply 193):
if we've got one of those, im sure we can find a shopping trolley that rolls straight

If you find one let me know. I'm fed up getting thrown out of supermarkets for reckless trolley-pushing.  Smile

Quoting LeanOfPeak (Reply 194):
The relative motion problem says that wheels turning at x mph on a treadmill turning at -x mph will result in 0 mph groundspeed. It really is that simple.

... because something must be holding the wheel axles in place, which is not the case with the aircraft on the conveyor belt. Take the same wheels on the same treadmill, moving at x mph, release whatever's holding the wheel axles and start pushing the wheel axles (engine thrust) in the opposite direction to the treadmill.

Quoting Keta (Reply 195):
If you, by definition, have a belt designed to maintain the center of wheels at rest, then who cares of thrust? By definition it will not move. I mean, it's impossible to apply thrust and have a system with no frictional force (as the axes are) that maintains the aircraft still. It's just that, what answer do you want to an impossible situation?

Precisely - a conveyor belt cannot stop the centre of the wheels from moving. We have to assume that the thread starter was incorrectly worded, otherwise the whole discussion is a waste of time.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Sudden
Posted 2005-11-30 13:01:25 and read 32767 times.

Am really surprised that this thread is still alive and kicking! Just as the fact that some folks think the A/C will take off!?

It would rotate, but as we don't have any mph/km per hour of the belt, or if it's a constant speed.

No matter what, the engines of the A/C would still have compensate backwards v forward motion in order to compensate and with that in mind, and what I have stated earlier, it would take rotate, but a normal rwy length would not be sufficient enough.

Aim for the sky!
Sudden

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: NAV20
Posted 2005-11-30 13:47:41 and read 32767 times.

Meeting myself coming back here!

I first of all thought that the aeroplane would stand still. Then I imagined it 'gaining' on the conveyor, and therefore moving.

But let's consider that further. To gain speed over the ground, the wheels have to turn faster. But in this (theoretical) situation, if they turn faster, the conveyor just runs quicker.

Imagine the engines being throttled up a small amount. The aeroplane 'moves' relative to the conveyor, and the wheels turn, but the conveyor exactly negates that movement, no more, no less; and the aeroplane stays still. So now open the throttles wide. The extra thrust 'moves' the aeroplane faster, relatively, but that is translated into the wheels turning faster. Which simply means that the conveyor has to work harder cancelling out the added wheel-speed.

The trick is surely that the wheels can't turn faster than the conveyor runs? That would actually act as a 'brake'. The aeroplane can't 'gain' on the conveyor (i.e. cause the wheels to turn faster than the conveyor runs). So, as far as I can see (theory only), it couldn't and wouldn't move.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Sudden
Posted 2005-11-30 14:04:22 and read 32767 times.

Quoting NAV20 (Reply 202):
The trick is surely that the wheels can't turn faster than the conveyor runs? That would actually act as a 'brake'

Am not totally with you on that one.
The belt would not work as a break as it's just a substitue for the rwy. All it would cause, in my narrow mind, is that the A/C would move slower then if the belt was not moving backwards.

Aim for the sky!
Sudden

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: NAV20
Posted 2005-11-30 14:15:20 and read 32767 times.

Not by any means sure myself, Sudden. But I'm going on the 'terms of reference' as stated in the thread-starter:-

Quoting Sovietjet (Thread starter):
The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.

So more thrust equals more 'speed' - but the speed is 'expressed' by the wheels turning faster, which in turn causes the conveyor to move faster, which......well, I'm sure you get my drift.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: A346Dude
Posted 2005-11-30 14:19:23 and read 32767 times.

It seems like just as everyone has agreed that the aircraft will take off, someone new enters and says it won't.

The point is, even though the conveyor belt is moving at a speed exactly equal and opposite to that of the wheels, it is acting on the bottom of the tires, and not the wheel axle. Thus, it is not pushing against the aircraft, as it would be if it were somehow able to act on the wheel axles, but rather pushing against the bottom of the tires, which causes them to spin. There is no force, other than an insignificant friction force due to the tires spinning twice as fast as normal, that acts against the thrust produced by the engines, and that is why the plane will take off.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Sudden
Posted 2005-11-30 14:35:05 and read 32767 times.

Quoting A346Dude (Reply 205):
is moving at a speed exactly equal and opposite to that of the wheels

This theory works totally the other way around for me. The A/C will not move an inch, and also not rotate as there is no airflow over the wings.

I am sure not an expert of the law of physics, but still!?  Smile

Aim for the sky!
Sudden

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Keta
Posted 2005-11-30 14:36:32 and read 32767 times.

Quoting NAV20 (Reply 202):
The aeroplane 'moves' relative to the conveyor, and the wheels turn, but the conveyor exactly negates that movement, no more, no less

That's where you fail to understand. It's impossible for the belt to negate the movement; when the belt accelerates to catch the wheels' speed, the wheels will also accelerate... you will never have a belt running at the same speed as the wheels, wheels will always go faster. Ohh, I don't know how to explain it better with my English...

Quoting David L (Reply 200):
a conveyor belt cannot stop the centre of the wheels from moving. We have to assume that the thread starter was incorrectly worded, otherwise the whole discussion is a waste of time

That's it. The belt cannot stop the axis. The starting statement is incorrect, it makes no sense. I think you explained it correctly.

Quoting Sudden (Reply 203):
All it would cause, in my narrow mind, is that the A/C would move slower then if the belt was not moving backwards

Actually, since we're assuming no friction on axes, the takeoff length is the same, no matter in what direction the belt moves.

Quoting Saintsman (Reply 197):
It's been good fun though  Smile

I think that's the final conclusion  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Pihero
Posted 2005-11-30 14:38:19 and read 32767 times.

My two cents :
1/ aircraft/wheels/belt do not form a closed system as in the theory, the only force considered comes from the engine thrust, and we might as well consider the couple wheel/belt as a frictionless surface.
2/ left with only one force applied to the aircraft, it is going to accelerate.
Now let's consider the wheels : the tyre speed will be the addition of the instantaneous belt movement and the aircraft forward movement. That means that the tyres will be accelerated faster than the aircraft movement and assuming we will not reach the max design tyre speed, the aircraft will eventually reach its takeoff speed.

Another way of looking into this puzzle is to picture the airplane on skis and start the belt moving backwards. Initially, the aircraft will go backwards but soon the thrust will accelerate the forward movement,stop the backward motion .... it will takeoff.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: CosmicCruiser
Posted 2005-11-30 14:46:25 and read 32767 times.

Quoting Matt72033 (Reply 108):
i suggest you read the posts!



Quoting SlamClick (Reply 109):
Perhaps you should have read some of them.



First let me say I should have stayed out of this. I immediately thought of a runner on a treadmill which lead to my post and later realised the true mechanics.
Secondly, in response to the above remarks I'll just have to say that I am usually paying for PC time when on the road or sitting at home between errands, chores and my kids so I don't take time to read a 150-200 comment post. sorry ...C.C.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Darrenthe747
Posted 2005-11-30 14:54:23 and read 32767 times.

so far i think slamclick has hit the nail on the head. when i first read the original post i was convinced it would not move. my opinion has changed...

i visualized a skateboard on long treadmill, similar to those looong treadmils they have at airports for passengers carrying lots of bags.

so imagine the skateboard on the long treadmil that is NOT moving. turn the treadmil on and watch the skateboard move with the treadmil. for this case let's say the skateboard is moving AWAY from us. now, tie a rope to the end of the sketeboard and hold onto it. what happens? the wheels just spin around underneath the skateboard. it is now not moving away from us nor is it moving toward us. increase the speed of the treadmil. skateboard stays where it is (because i am still holding onto it with the rope). now turn up the speed of the treadmil higher and higher... skateboard stays where it is. now, i start pulling on the rope. the skateboard starts moving towards me. turn the treadmil up to 1000 miles an hour, i slowly pull the rope in and the skateboard will continue to move closer to me.

i think the question becomes confusing because of the fact that we are introducing too many variables... the bottom line question is: can the object accelerate? we proved it can with the skateboard on the rope. so if we can accelerate then we can eventually acheive a high enough headwind to cause lift. remember, nobody cares about the speed of the airplane relative to the track of the conveyor belt. Excellent theoretical physics question. this was one of the most entertaining posts i have ever read on an a.net forum!

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: NAV20
Posted 2005-11-30 15:04:51 and read 32767 times.

Look at it another way, guys. The aeroplane is travelling at 90 knots. The wheels are therefore turning at the appropriate revolutions for that speed. But the 'runway' - the conveyor - is precisely negating the wheel movement - that is, also travelling at 90 knots. Backwards!

Result - the aeroplane stays still. The thrust of the engines is exactly counter-balanced by the power of whatever is driving the conveyor. Nothing happens - apart from a prodigious waste of fuel.........

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Sovietjet
Posted 2005-11-30 15:23:27 and read 32767 times.

the skateboard analogy doesn't work....if you pull on the rope to make it go faster the conveyor will go faster in the opposite direction. Then the skateboard would speed up and so on. The problem is simple, there are no limits....the wheels and conveyor speeds would approach infinity for it to ever move and therefore it is impossible. So IMHO to answer the question...yes it will move and fly if the wheels and conveyor move at infinity....

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Chksix
Posted 2005-11-30 15:28:22 and read 32767 times.

It stands still relative to you if you are also standing on the conveyor.

You, the plane and the belt will be accelerating at the aircraft T/O power acceleration. From your point of view, the plane will magically lift off the belt after doing the normal rotation at 150 kt. Let's say that you are behind a windscreen for this experiment.

If you are sitting in the aiport tower, all you'll see is an aircraft and runway moving together until the plane flies off with stopped wheels.

You can't be serious to argue this fact any longer....  

And I said earlier that if the conveyor/runway is moving opposite the airplane, all that will happen is that the wheels will reach rolling overspeed since the plane will be thundering down the runway in a normal departure.
If the wheels are very strong they will reach a speed of about 300 kt before the plane lifts off.

[Edited 2005-11-30 15:35:23]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Bond007
Posted 2005-11-30 15:54:40 and read 32767 times.

Quoting NAV20 (Reply 211):
Look at it another way, guys. The aeroplane is travelling at 90 knots. The wheels are therefore turning at the appropriate revolutions for that speed. But the 'runway' - the conveyor - is precisely negating the wheel movement - that is, also travelling at 90 knots. Backwards!

That is what I was originally thinking....BUT a jet engine, or propellor, produces forward movement relative to the air around it, NOT relative to the conveyor. If you assume a jet engine running at takeoff power will move the aircraft 10 feet per second forward, then it's going to do that regardless of what's happening to the wheels.

Remember the engine is working relative to the air - not the conveyor.

I'm not sure that the wheels will be going only twice as fast though .... I'm thinking more to infinity, but I'm open on that one  Smile


Jimbo

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SlamClick
Posted 2005-11-30 16:07:24 and read 32767 times.

I have to take my dog to the vet or I would address some of the last twenty or so posts. (My dog understands, by the way, that the plane would take off normally.)

Many of you are hugely over-complicating the question.

Forget friction!

Forget bewildering equations.

Forget infinity. If the plane would normally take off at 150 knots then we need deal with speeds no greater than twice that.

Forget "groundspeed" every use of it I've read on here has been incorrect. It is one of the favorite words of aviation afficionadoes and is one of the most misused words in our language. Groundspeed is only important in navigation.

NAV20 this is for you because you well represent those who think it wouldn't go. Tell me sir, that long lost uncle - when he feared that the Hurricane would take off with him hanging on to the tail; how fast were the wheels spinning at that moment?

Surely if a conveyor could offset the ability to take off then wheelspin must be a cause. Right? No, it was the thrust of the engine and it didn't care a damn what the wheels were doing - it just wanted to pull the plane forward.

Same with our riddle airplane.

Your arguments against, just like all arguments against are predicated on a point of view where the wheels propel the airplane. Others have even been able to articulate that they know that isn't true but they still think and reason as if it is.

The plane in question WILL take off normally. It will do so because it is not being propelled along a moving runway by frictive force, but by utterly independant jet thrust. The wheels are irrelevant. Airplanes do the same stunt uphill on skis and upstream in the river on floats. Wheels are just a frictionproofing layer between the aluminum and the asphalt. And here is the big fact: They are free to spin as fast as they need to because the only thing acting upon them is the retarding force at their circumference at bottom dead center. The axles, at their center on the other hand are bolted to over a hundred thousand horsepower that is moving the airplane through a medium that HAS NOTHING TO DO WITH our runway.

The do not need to break free of anything. They will just roll.

The plane gets going 150 knots forward which means the treadmill is rolling 150 knots astern and the only result is that the wheels are rolling at a speed equivalent to a normal 300 knots. We rotate. We take off. We fly.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-11-30 16:10:50 and read 32767 times.

Quoting Keta (Reply 207):
Quoting Saintsman (Reply 197):
It's been good fun though

I think that's the final conclusion



Quoting Darrenthe747 (Reply 210):
Excellent theoretical physics question. this was one of the most entertaining posts i have ever read on an a.net forum!

Agreed!

Quoting NAV20 (Reply 211):
The aeroplane is travelling at 90 knots. The wheels are therefore turning at the appropriate revolutions for that speed. But the 'runway' - the conveyor - is precisely negating the wheel movement - that is, also travelling at 90 knots. Backwards!

Result - the aeroplane stays still.

NO!  Smile

Quoting SlamClick (Reply 149):
The wheels don't drive the airplane.
The wheels don't drive the airplane.
The wheels don't drive the airplane.
The wheels don't drive the airplane.
The wheels don't drive the airplane.

Look at the skateboard on the moving walkway in Reply 210. Stand just beyond one end of the walkway with a rope attached to the skateboard. Start the walkway moving away from you. You have to apply a small force to hold the skateboard stationary w.r.t. the planet's surface. The trolley is moving at 0 kts, the walkway at, say 2 kts. Now walk away from the walkway at 2 kts while holding the rope. The skateboard is travelling at 2 kts, due to the force (which is now greater than before) which you're applying through the rope, the walkway is travelling at 2 kts in the opposite direction - the conditions have been met. Now, the average speed of the skateboard/walkway combination is 0 kts but the skateboard is still travelling at 2 kts and the walkway is still travelling at 2 kts in the opposite direction.

Now, the wheels are rotating at a rate appropriate for 4 kts so obviously the belt cannot be travelling at the same speed as the wheels are rotating but I think it's been fairly well established that the question was mis-stated and the belt has to equal and oppose the aircraft's speed. The idea that the speed of the belt should match the speed of the tyres just doesn't make sense.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-11-30 16:16:21 and read 32767 times.

Quoting Sovietjet (Reply 212):
The problem is simple, there are no limits....the wheels and conveyor speeds would approach infinity for it to ever move and therefore it is impossible. So IMHO to answer the question...yes it will move and fly if the wheels and conveyor move at infinity....



Quoting David L (Reply 216):
I think it's been fairly well established that the question was mis-stated and the belt has to equal and oppose the aircraft's speed.

I didn't notice that it was you who asked the original question! So, in other words, if you're sticking to your guns about the conditions, the situation could never occur!  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: NAV20
Posted 2005-11-30 16:22:06 and read 32767 times.

Quoting Bond007 (Reply 214):
If you assume a jet engine running at takeoff power will move the aircraft 10 feet per second forward, then it's going to do that regardless of what's happening to the wheels.

I agree, Bond007, that in real life the engines would move the aeroplane, the conveyor wouldn't be able to cope, and the wheels would skid until the tyres ripped off.

But as I emphasised, we're not talking real life; we're talking about a hypothetical/theoretical situation, where the wheels don't skid and the conveyor can cope.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: NAV20
Posted 2005-11-30 16:38:13 and read 32767 times.

Quoting SlamClick (Reply 215):
NAV20 this is for you because you well represent those who think it wouldn't go. Tell me sir, that long lost uncle - when he feared that the Hurricane would take off with him hanging on to the tail; how fast were the wheels spinning at that moment?

Very little to do with the subject, SlamClick, but I also recall him telling us about his squadron being sent to Malta in 1941. They were flown off from a famous carrier called 'Ark Royal'; and the groundcrews were transferred to a ship that was going on to Malta. Luckily as it turned out, Ark Royal was sunk the next day.

On the 'conveyor' principle, though, he once told me how sorry he felt for the Fleet Air Arm pilots in their slow Swordfish biplanes. Normal practice was for the carrier to steam full speed into the wind for 'landing on'. The RAF blokes had a good laugh watching their opposite numbers sending respectful messages to the bridge asking the captain please to slow down; as, often enough, with the ship doing its full 35 knots and the wind blowing hard, the Royal Navy's aircraft couldn't fly fast enough to catch up with the ship to land on it.........

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Bond007
Posted 2005-11-30 16:42:41 and read 32767 times.

Quoting NAV20 (Reply 218):
I agree, Bond007, that in real life the engines would move the aeroplane, the conveyor wouldn't be able to cope, and the wheels would skid until the tyres ripped off.

But as I emphasised, we're not talking real life; we're talking about a hypothetical/theoretical situation, where the wheels don't skid and the conveyor can cope.

The only thing in this scenario that would stop the jet engine from moving the plane forward would be friction between the wheels and the conveyor. Since there isn't any (or very little), it will move forward. Real life or imaginary!


Jimbo

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: LeanOfPeak
Posted 2005-11-30 16:56:39 and read 32767 times.

Quoting David L (Reply 216):
Now, the wheels are rotating at a rate appropriate for 4 kts so obviously the belt cannot be travelling at the same speed as the wheels are rotating but I think it's been fairly well established that the question was mis-stated and the belt has to equal and oppose the aircraft's speed. The idea that the speed of the belt should match the speed of the tyres just doesn't make sense. Smile

And there would be no debate about that question. Absolutely the aircraft would take off as normally.

However, given the question asked, until traction is broken, if the wheels are turning at an equal and opposite rate to the conveyor, this does indeed presuppose that the aircraft is not moving with respect to the ground.

The QUESTION as stated generates the debate because it forces the conclusion that the engine thrust directly has the exclusive result of turning the wheels and the conveyor belt. Those who get wound around the subject at hand being an airplane correctly say there's no horizontal reaction at the wheels, this doesn't work. Those who get wound around the relative motion portion of the original problem say that if the conveyor is preconceived to move opposite the wheel motion, the aircraft never moves.

No matter what the "Yes" crowd has to say, it will fall into one of two categories:
1) The tires are slipping.
2) The tires are not spinning at the same speed as the conveyor belt (Violating one of the major preconditions of the question).

No matter what the "No" crowd has to say, it will fall into one category:
1) That motion CAN NOT be produced by a real aircraft powered solely by the engines.

There is a disconnect between the two generated by the question asked. It is indeed very much like the $30 hotel room.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Chksix
Posted 2005-11-30 17:00:34 and read 32767 times.

Quoting LeanOfPeak (Reply 221):
No matter what the "No" crowd has to say, it will fall into one category:
1) That motion CAN NOT be produced by a real aircraft powered solely by the engines.

So if the tarmac refuses to budge, the aircraft can never start to taxi....

Or a sailboat can't sail if the water don't push it?

Just pulling your leg  Wink

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Sudden
Posted 2005-11-30 17:04:16 and read 32767 times.

I give it a rest here.

Anyone else who got an email from a none A.net member by name Tom LaPrise?
He stated that Pihero had the correct answer.

Aim for the sky!
Sudden

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Bond007
Posted 2005-11-30 17:15:08 and read 32767 times.

Quoting LeanOfPeak (Reply 221):
2) The tires are not spinning at the same speed as the conveyor belt (Violating one of the major preconditions of the question).

So do those who say, yes it will takeoff, agree that the wheels MUST be going faster than the conveyor in order to get forward motion, and therefore takeoff?

If yes, and I guess it's obvious it must, then this is not what was described as the scenario - the conveyor will go the same speed as the wheels and therfore no forward motion.

I was one of those that said yes (and no) by the way!

Jimbo

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: A346Dude
Posted 2005-11-30 17:22:01 and read 32767 times.

Quoting Bond007 (Reply 224):
So do those who say, yes it will takeoff, agree that the wheels MUST be going faster than the conveyor in order to get forward motion, and therefore takeoff?

Absolutely not. As SlamClick has already indicated (I'm sure many more times than he would have liked to!), wheel rotation is not required to get forward motion. Wheel rotation is nothing more than a by-product of forward motion provided by the engines' thrust.

Edit: I will repeat a question I posed earlier in the thread, which I think proves that the idea the plane would not move is false. A few of you are still saying that the plane would be sitting still at the beginning of the belt, with its wheels rotating madly and the belt moving at exactly the same speed in the opposite direction. But, if the plane never moved forward to begin with, why did the wheels ever start spinning in the first place?

[Edited 2005-11-30 17:27:01]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: LeanOfPeak
Posted 2005-11-30 17:26:23 and read 32767 times.

However, getting forward motion without wheel rotation requires that the traction of the tires be broken (NOW we're getting somewhere).

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Joness0154
Posted 2005-11-30 17:36:15 and read 32767 times.

Here's another thing.

Its all in what the conveyor belt is moving in relation to, and whether you measure the conveyor belt speed at the point of the wheels or as a whole.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-11-30 17:54:12 and read 32767 times.

Quoting Bond007 (Reply 224):
So do those who say, yes it will takeoff, agree that the wheels MUST be going faster than the conveyor in order to get forward motion, and therefore takeoff?

If yes, and I guess it's obvious it must, then this is not what was described as the scenario - the conveyor will go the same speed as the wheels and therfore no forward motion.

Assuming we're going with the "silly" version, I see the point about the wheels needing to rotate at half the speed I was thinking of (i.e. slipping) but... unless the brakes are applied (which would "connect" the tyres' rotational speed to the aircraft), that rotational force imparted to the tyres by friction with the belt will not be transmitted to the aircraft since the wheels are free to do their own thing.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Kay
Posted 2005-11-30 19:17:33 and read 32767 times.

Quoting A346Dude (Reply 225):
But, if the plane never moved forward to begin with, why did the wheels ever start spinning in the first place?

They will not spin as fast as you are saying, but only as a result of the wheel struts wanting to move forward, they will want to spin, but the turning conveyor will make the turn of the wheel not successful to dissipate the energy in the wheel strut and so the energy will be dissipated in the axis between the wheel and the strut...

Those who are saying that the wheels are only a by-product must know that us, the "will not move" group reaaally know that. A plane can take off with its wheels not turning at all.

But, just tell me, how will the wheel struts move forward? Or is that also not necessary?


how will the wheel struts move forward?
how will the wheel struts move forward?
how will the wheel struts move forward?
how will the wheel struts move forward?



Imagine your foot was a wheel strut (God forbid Big grin ), and you stepped on that conveyor, how can you move your foot forward? it won't let you!!


All my posts are based on the original statement that the belt will match the speed of the wheel, not the plane.


Kay

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-11-30 19:33:02 and read 32767 times.

Quoting Kay (Reply 229):
Imagine your foot was a wheel strut (God forbid ), and you stepped on that conveyor, how can you move your foot forward? it won't let you!!

The wheel strut is not in contact with the conveyor belt. Imagine your foot was a wheel strut. Now put on a pair of roller skates and put your foot on the conveyor belt.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: LeanOfPeak
Posted 2005-11-30 20:15:32 and read 32767 times.

OK, let's go with the roller-skates analogy.

If you are on a treadmill on roller skates, your roller-skate wheels have a speed with respect to the treadmill and the treadmill has a speed with respect to the ground.

If those two speeds are equal and the wheels are not skidding, you will not move translationally on the treadmill. In order for you to move translationally on the treadmill, your wheels must:

1) Move faster than the treadmill.
2) Move slower than the treadmill.
3) Skid.

It makes no difference where the motive force comes from. You do not have to propel yourself with your legs. If you are on a treadmill with a handrail and pull yourself forward by the handrail (A nice approximation of engine thrust), the wheels of your rollerskates will, while you are moving forward, either go instantaneously faster than the instantaneous speed of the treadmill or skid.

The problem arises when trying to apply the conditions of the problem to a real-world situation.

The problem is NOT APPLICABLE TO a real-world situation.

[Must...Resist...Urge...To duplicate statement...]

The only way to get translational motion from the relative-motion problem where the wheels are constrained to the same speed as the treadmill is:

3) Skid.

The question, as absurdly posed, simplifies to, "Can an airliner take off with the brakes locked?"

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-11-30 20:42:43 and read 32767 times.

Quoting LeanOfPeak (Reply 231):
The question, as absurdly posed, simplifies to, "Can an airliner take off with the brakes locked?"

I "get your drift" but I'm still not convinced that this is the same as having the brakes locked. I'm running out of ways to picture this - as you say, the problem is not applicable to a real-world situation.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: BuckFifty
Posted 2005-11-30 20:46:29 and read 32767 times.

I think people still don't get it, haha. This question, again, is quite farcical. It's akin to the problem of "if a tree falls in the forest, and no one is there to hear it, does it make a sound?"

If they did make a conveyor belt such as the one described, again, the tires will probably burst before anything happens. Let's say that would not be the case, the wheels are made of some exotic metal (no tires), and the bearings are superconducted, the material will probably explode due to centrifugal force.

Okay, so then, if the wheel was to never explode, being able to spin ad infinitum, the aircraft will probably will still take off, with the wheels spinning at a speed so far no human has been able to calculate. The ground effect from such a fast spinning conveyor (as in one spinning at infinite speed) would probably provide sufficient airflow to send the aircraft into the air. However, it will probably stall due to heavy windshear once it leaves the ground effect and into still air, due to excessive windshear.

But such a fast spinning conveyor may produce it's own gravitational field. Anyone care to try?

Aircraft takes off in reference to airspeed, not groundspeed. That is the basic premise of flight. That's it. Other than that, this problem can be discussed to death, but will still have no conclusion. Because I don't think even Einstein can properly explain what would happen when a conveyor belt, moving at the speed of light, will do when it has a plane on top with max takeoff thrust.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: LeanOfPeak
Posted 2005-11-30 20:58:01 and read 32767 times.

DavidL, you are exactly correct. It is not necessary (Or desirable) to lock the wheels. Doing so would flat-spot the tires before blowing them out and maybe coming up with even more dramatic results.

However, in the realm where the bulk of the aircraft's weight is still carried by the wheels, any difference between the thrust required to accelerate an aircraft to 60 mph relative to the conveyor (30 mph airspeed) with wheels rotating at 30 mph and the thrust required to accelerate the aircraft to 30 mph relative to the conveyor (30 mph airspeed) with wheels rotating at 0 mph should be relatively inconsequential in the grand scheme of this as a thought problem (Both require, in simplest terms, overcoming the coefficient of dynamic friction times the normal force), so I merely simplified it down to its most basic form for ease of understanding (Or I thought that was what I did).

[Edited 2005-11-30 21:02:19]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-11-30 21:17:01 and read 32767 times.

Yeah. The bottom line is that the question as stated isn't nearly as interesting to discuss as a lot of us thought it was.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: MD-90
Posted 2005-11-30 22:25:52 and read 32767 times.

Obviously, this is a calculus problem, and there is a limit. I think that this paradox was thought up by someone who accidentally misunderstood the problem, and was thinking of a car, but an airplane was more interesting, since it could actually leave the conveyor belt.

Fundamentally, the problem is flawed. I don't think that conveyor belt cannot keep up with the airplane, since it's reacting to the plane's acceleration (even if it's magically perfectly right....something's still not right).

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-11-30 22:32:58 and read 32767 times.

Quoting MD-90 (Reply 236):
I think that this paradox was thought up by someone who accidentally misunderstood the problem, and was thinking of a car, but an airplane was more interesting, since it could actually leave the conveyor belt.

... or was thinking about a conveyor belt which moves at an equal and opposite speed to that of the aircraft, like all the same discussions elsewhere.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-11-30 22:37:08 and read 32767 times.

Quoting David L (Reply 126):
Now all I need is for someone to come out with old "everything I say is a lie" routine and I'm done for.

Little did I realise at the time how much the question resembles that routine.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: FLY2HMO
Posted 2005-11-30 22:44:30 and read 32767 times.

I would happily volunteer to fly my remote controlled airplane off a conveyor if somebody is willing to build a conveyor for it. Big grin

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: LeanOfPeak
Posted 2005-11-30 22:49:11 and read 32767 times.

Quoting David L (Reply 238):
Quoting David L (Reply 126):
Now all I need is for someone to come out with old "everything I say is a lie" routine and I'm done for.

Little did I realise at the time how much the question resembles that routine. Smile

Also an easy question. The statement is a lie.

The correct half of the reasoning that leads to the impression that this is an unresolvable vicious circle is that the statement can not be true, because, if it were, it would be a lie.

Fortunately, the options are not limited to, "Everything I say is a lie," and "Everything I say is the truth." There is also, "I lie and I tell the truth." This is the only resolution. The statement is a lie because not everything the speaker says is a lie. Just some things. Including this particular one.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-11-30 22:55:45 and read 32767 times.

You've ruined Star Trek.  Sad

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: VSIVARIES
Posted 2005-11-30 22:58:09 and read 32767 times.

This problem has always had a certain condition:

Quoting Sovietjet (Thread starter):
The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.

To consider that condition you have to make a certain number of assumptions mainly dealing with speed and friction. The main ones that conveyor speed and wheel speed it limitless and secondly that conveyor and wheel friction does not exist. This is the only way that this condition could ever exist and is in reality impossible.

If you can envisage what you've now got then the A/C would never move and the conveyor and the wheels would accelerate to infinity, but as defined in the question the AC would not move an inch forward.

Sorry if I've duplicated but I only read to about post 150 and got a bit bored.

Impossible set of conditions laid down in the question. I think this has thrown a few people who have their minds set in the 'real world'.

No disrespect BTW I hang out on another planet half the time  Smile

B/R

P.S. If you include wheel friction then the conveyor would always have to stay slightly ahead of the AC wheels to prevent it from moving and the ultimate load transferred only through wheel bearing friction placed on the AC from the conveyor would need to exceed the ultimate load from the AC engines placed on the surrounding air. Easier to think of an AC engine strapped to a roller skate on a belt that can pass light speed.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Darrenthe747
Posted 2005-11-30 23:04:00 and read 32767 times.

ok guys,

i have been thinking about this ALL DAY!!!!! AAARRGGHHH!!! but here's what I concluded... there is a KEY point to this theoretical problem:

QUOTE FROM THE PROBLEM:

"The conveyer belt is designed to ***exactly match the speed of the wheels at any given time***, moving in the opposite direction of rotation."

This answers everything. The airplane will not move. Once the thrust is enough to make the airplane's wheels exceed the belt speed, the wheels will spin faster than the belt is moving and we have overstepped the boundries of the conditions set forth in the question. My example of the skateboard (reply 210) breaks the fundamental rule set forth in the problem. It is flawed, it is wrong for the conditions set forth in the question.

Many here have argued wheels have nothing to do with it. In the real world, you are right. But for the purposes of this problem, you are wrong. It states CLEARLY that the WHEELS must match the speed of the belt. Read the question VERY carefully.

I'll say it again: Once the thrust from the engines causes the airplane to start moving forward relative to the ground and not the belt... game over. you just broke the key rule. Wheels are EVERYTHING in THIS hypothetical case.

my 2 cents.
darrenthe747

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: LeanOfPeak
Posted 2005-11-30 23:14:42 and read 32767 times.

Quoting David L (Reply 241):
You've ruined Star Trek.  Sad

The only context in which I've heard that problem posed with Star Trek is that there are two tribes on a planet, one 100% truthful and one 100% deceitful, and there are no other differences between them. You are speaking to two individuals and know that one of them is from each tribe. You must determine which is which in one question. What do you ask?

The answer being:






You ask which tribe the other person would say they belong to. The truthful person would tell the truth that the other person would lie and say they are truthful. The liar would lie and say that the truthful person would say they lie. Thus, you've identified both people.

That answer still holds because of the presumption of 100% truth or 100% deceit in this particular context.

Quoting Darrenthe747 (Reply 243):
I'll say it again: Once the thrust from the engines causes the airplane to start moving forward relative to the ground and not the belt... game over. you just broke the key rule. Wheels are EVERYTHING in THIS hypothetical case.

Bingo.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-11-30 23:15:12 and read 32767 times.

Quoting VSIVARIES (Reply 242):
Impossible set of conditions laid down in the question. I think this has thrown a few people who have their minds set in the 'real world'.



Quoting Darrenthe747 (Reply 243):
Many here have argued wheels have nothing to do with it. In the real world, you are right. But for the purposes of this problem, you are wrong. It states CLEARLY that the WHEELS must match the speed of the belt. Read the question VERY carefully.

We're way past that.

Those of us unfortunate enough to have our minds set in the "real world" were clinging to the hope that the question had been incorrectly stated. Once we realised that it hadn't, we said "Who cares? It can't happen - give us a problem which can".  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: VSIVARIES
Posted 2005-11-30 23:32:03 and read 32767 times.

It was a theoretical problem that had some theoretical conditions.
It also has a theoretical answer.
I can't prove the answer either.
Good fun to watch some of a-net's 'greatest minds' go around in circles for a bit though.

B/R

P.S. I'm not a brain surgeon BTW I just enjoy puzzles. I'll do a couple of real mind-fucks in non-av tomorrow.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SlamClick
Posted 2005-11-30 23:34:19 and read 32767 times.

Quoting MD-90 (Reply 236):
Obviously, this is a calculus problem

Obviously you are overthinking it.
It is no such thing unless you want to apply real numbers to it and that is not necessary to find the answer.
It is a logic question and a very simple one.

Airplane on a treadmill running rearward:


Could you pull it off forward with a winch and cable?
Of course you could. If the gear was down the wheels would roll at whatever speed necessary and it would roll right off the end of the treadmill.

How about with a hundred thousand horsepower worth of jet engines? Most bigger airliners have that.

Again, of course it would.


The "no" answers are still being driven by traction issues. It is laughable folks.

Here is an experiment just about any of you can perform: Turn your bicycle upside down and rest it on the seat and handlebars. Have someone turn the pedals to run the rear wheel at any speed you choose.

Take a paint roller to act as a "landing gear" and hold it so that the roller is being spun by the bicycle tire. You will note that it turns many more RPM than the bike tire. I'm sure you can figure out why.

If "NO" is the correct answer
you will be physically unable to move the paint roller "upstream" against the direction of rotation of the tire.

If you are able to move the paint roller just as easily in either direction as I was, you will see that a freewheeling rotor (like an airplane wheel) is able to increase or decrease RPM as needed in response to inputs.

Input no.1 - a rotational velocity applied at the circumference by the contact with the moving belt. It varies with the velocity of the belt in relation to the change of position of the center of the wheel (axle movement in space measured along the axis of the beltway)

Input No.2 - a rotational velocity added or subtracted (depending on direction) by an external force (your arm on the paint roller or a pair of CFM-56s on my Airbus) applied along the gear strut and out to the axle.

Gentlemen, this is stupefyingly simple.

The plane will move because it doesn't need traction and there is no physical restraint of the wheels.

So what if the belt is moving the bottom of the tire rearward. Andy Green went supersonic across the Blackrock Desert with the bottom of his tires

- ready for this?

not moving.

That is right. They were not moving at the bottom unless they were skidding. Airplanes have been taking off for a hundred years with the bottom of the tires not moving!

I'll say that again.

Airplanes have been taking off for a hundred years with the bottom of the tires not moving!

Airplanes have been taking off for a hundred years with the bottom of the tires not moving!

Airplanes have been taking off for a hundred years with the bottom of the tires not moving!

Some of you still think there is some sort of restraint on the wheels.

The only place the belt is touching the wheels is bottom dead center, the contact patch. But wheels, being wheels, are hinged in the middle. They are free to swing around the axle to allow the belt to move on without skidding over the tire surface. This brings the next little bit of tire into contact with the belt. It too is hinged in the middle, at the axle, so it will swing away.

Remember? The brakes are not on during takeoff. The wheels will continue to rotate around their axles as the axles are driven forward by the big howling jet engines.

The plane will take off.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SlamClick
Posted 2005-11-30 23:49:18 and read 32767 times.

Okay, I've been rereading some of the arguments regarding the specific language of the thread-starter question.

When I first read that post I assumed that I was reading a post from a non-English speaking person and that it had translated with a little bit of an accent. But that the problem was, would the plane take off (simplifying here) if there was a treadmill set to run rearwards at a speed equal to the plane's at any moment.

As I re-read it, it sounds more like this: "Given a set of magical conditions that would prevent takeoff - will a plane take off?"

The answer to that one must be no.

Would a plane take off on a rear-running treadmill? Yes, of course.

Moving on now. I'll leave it to someone else to drive the stake into this thing's heart.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SlamClick
Posted 2005-11-30 23:53:23 and read 32767 times.

Besides, I'm not going to waste my five thousandth post on this thread. I feel like I've done that many here alone. Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: VSIVARIES
Posted 2005-11-30 23:56:54 and read 32767 times.

Quoting SlamClick (Reply 247):
The plane will take off.

Slam you are normally right on most things esp. to do with being a pilot.

You are not dealing with a real world situation here. Some of the conditions laid down in the problem are impossible to start with.

One of those is that the wheels of the A/C are unable to move faster than the conveyor belt.

Therein lies the answer however unfathomable it may seem.

I quote from me a bit earlier;

Quoting VSIVARIES (Reply 242):
If you include wheel friction then the conveyor would always have to stay slightly ahead of the AC wheels to prevent it from moving and the ultimate load transferred only through wheel bearing friction placed on the AC from the conveyor would need to exceed the ultimate load from the AC engines placed on the surrounding air. Easier to think of an AC engine strapped to a roller skate on a belt that can pass light speed.

Sounds like bullsh*t but this is not an engineering or technical problem with a real solution, it is a cleverly (possibly inadvertently clever) put together puzzle that only has a theoretical answer.

To get 250 replies in this forum I think was a good one!

Wheel speed = conveyor speed = sorry my friend you're not going anywhere. Although if we forget friction etc. the wheels and the conveyor may go back in time after they pass the speed of light!. Just imagine how that would look Wink

B/R

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: VSIVARIES
Posted 2005-11-30 23:59:53 and read 32767 times.

Just read post 248. Understood.

B/R

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Ralgha
Posted 2005-12-01 00:04:10 and read 32767 times.

Sorry Slam, I've been sleeping and working so I wasn't here to back you up.

To all the people who argue that the question as worded at the beginning of this situation (if possible) would not allow the airplane to take off, you're wrong. You do not understand the concept of infinite speed.

Infinity is not equal to infinity.

First, since we are assuming the conveyor belt can move at infinite speed, we shall also assume that there is no wheel bearing friction. End result, the belt and the wheels will move infinitely fast. The problem with this is that their speeds are still not equal. Infinitely fast does not equal infinitely fast, and regardless of how fast either is turning, they still do not affect the forward acceleration of the airplane. The conveyor belt and the wheels will both be moving infinitely fast, the airplane will still make a normal takeoff run, and fly.

The stake has been driven.

[Edited 2005-12-01 00:06:13]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-12-01 00:04:16 and read 32767 times.

Quoting LeanOfPeak (Reply 244):
The only context in which I've heard that problem posed with Star Trek is that there are two tribes on a planet, one 100% truthful and one 100% deceitful, and there are no other differences between them. You are speaking to two individuals and know that one of them is from each tribe. You must determine which is which in one question. What do you ask?

No, I know the one you mean but this was a different one. I couldn't remember the details but apparently it's from Episode 41, "I, Mudd":

"The landing party (including Spock), then engage in a series of illogical (and moronic) actions in order to confuse and overload the androids. This culminates in the immobilization of Norman himself when Harry tells him "everything I say is a lie." "

http://www.ericweisstein.com/fun/startrek/IMudd.html

Moral: Never trust anyone with pointy ears (especially if they're standing on a magic conveyor belt).

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Keta
Posted 2005-12-01 00:11:05 and read 32767 times.

Quoting Darrenthe747 (Reply 243):
I'll say it again: Once the thrust from the engines causes the airplane to start moving forward relative to the ground and not the belt... game over. you just broke the key rule.

Yes that's what we have been saying since long ago.

Quoting David L (Reply 235):
The bottom line is that the question as stated isn't nearly as interesting to discuss as a lot of us thought it was

 Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: VSIVARIES
Posted 2005-12-01 00:11:48 and read 32767 times.

Quoting Ralgha (Reply 252):
The stake has been driven.

Into your own argument.

Infinity means infinity. Sorry my friend given the condition of the original question (which is actually a puzzle), "The conveyer belt is designed to exactly match the speed of the wheels at any given time".

Then you also are not flying anywhere tonight. Sorry.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: DarkBlue
Posted 2005-12-01 00:26:14 and read 32767 times.

Here's my two cents...

Can the aircraft move down the runway? No. Can it take off? Yes.

Now before you all gasp at this absurd combination of statements, hear me out.

First, no, the aircraft cannot accelerate down the runway.

So, to Slamclick and all others who say that the aircraft will move, sorry I have to disagree. All your examples of treadmills and stationary bikes seem to ignore the fact that the conveyor is allowed to accelerate. As VSIVARIES simply stated, if conveyor speed = wheel speed, then you're not going anywhere.

Now, will it takeoff? Yes, for a completely different reason. Lift can be generated by couette flow. This is moving air induced by the moving conveyor. Since the conveyor speed is matched to the wheel speed, both can accelerate to infinity. Somewhere along the way, the induced couette flow will be great enough to generate lift.

Comments?

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Ralgha
Posted 2005-12-01 00:36:06 and read 32767 times.

Quoting VSIVARIES (Reply 255):
Infinity means infinity.

You do not understand infinity. It is not a quantity, it can not be compared as a quantity.

Infinity > Infinity

Infinity < Infinity

Infinity != Infinity

Infinity == Infinity

All statements are true.

The airplane WILL move forward, there is nothing holding it back, unless you involve magic, which is absurd. Because of this, the conveyor belt will never match the wheel speed regardless of any statements you make. It will not happen. The conveyor will accelerate rapidly and continuously in an attempt to match the wheel speed, but as a result the wheel will accelerate just as rapidly and maintain a the difference. Both the conveyor and the wheels will accelerate to infinity, the airplane will accelerate down the runway (belt) as normal, attain flying speed, and take off.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: FlyMKG
Posted 2005-12-01 00:37:08 and read 32767 times.

Quoting Sovietjet (Thread starter):
The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.

PEOPLE! The question is all relative to the way you read it. You can either picture it as:
(a) the conveyor moves 150 kts, the plane stays put, the casual observer will see the wheels spinning at a RPM that correlates with 150 kts in relation to the conveyor, the plane will not takeoff
or...
(b) the conveyor moves 150 kts, the plane move 150 kts on top of and in the opposite direction of the conveyor, the casual observer will see the wheels spinning at an RPM that correlates with 300 kts but the wheels will only be moving 150 kts in relation to the plane, the plane will take off

Either way a wheel doesn't really have a speed because it isn't really moving; its rotating. The outer edge of the wheel moves faster than the middle of it. Therefore it is impossible to assign a speed to a wheel. It all boils down to this - do the engines produce enough thrust to overcome the speed of the conveyor and still have enough thrust left to accelerate the plane to takeoff speed? The wheels are irrelevant.

I hope I just made sense.  

FlyMKG

[Edited 2005-12-01 00:44:57]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Ralgha
Posted 2005-12-01 00:39:18 and read 32767 times.

Quoting DarkBlue (Reply 256):
So, to Slamclick and all others who say that the aircraft will move, sorry I have to disagree. All your examples of treadmills and stationary bikes seem to ignore the fact that the conveyor is allowed to accelerate. As VSIVARIES simply stated, if conveyor speed = wheel speed, then you're not going anywhere.

Conveyor speed CAN NOT equal wheel speed. The reason for this is that there is nothing holding the airplane back. Nothing counters the engine force. This is fundamentally different than a car, a bike, a person running, or any land based activity.

[Edited 2005-12-01 00:44:29]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Ralgha
Posted 2005-12-01 00:50:28 and read 32767 times.

Quoting FlyMKG (Reply 258):
(a) the conveyor moves 150 kts, the plane stays put, the casual observer will see the wheels spinning at a RPM that correlates with 150 kts, the plane will not takeoff

Unless you put an anchor on the airplane, this situation will not happen.

Quoting FlyMKG (Reply 258):
Either way a wheel doesn't really have a speed because it isn't really moving; its rotating. The outer edge of the wheel moves faster than the middle of it. Therefore it is impossible to assign a speed to a wheel.

Speed is not linear. A wheel's speed is typically measured in RPMs. I made an assumption that people would know this, but I assumed wrong. Search way back up in this thread for my post that gives you a formula to determine the RPM of the wheel given the linear speed of the axel.

Quoting FlyMKG (Reply 258):
It all boils down to this - do the engines produce enough thrust to overcome the speed of the conveyor and still have enough thrust left to accelerate the plane to takeoff speed?

No, it does not boil down to that. You can not compare force and speed.

Quoting FlyMKG (Reply 258):
The wheels are irrelevant.

YES! Exactly! The conveyor belt is also irrelevant.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: A346Dude
Posted 2005-12-01 00:59:31 and read 32767 times.

Quoting Ralgha (Reply 259):
The reason for this is that there is nothing holding the airplane back.

Ralgha is correct, his statement is true and that is why the airplane will take off. The conveyor belt is not holding the airplane back, even though it is spinning backwards at exactly the same speed as the wheels. All it is doing is pushing against the bottom of the wheels, not the wheel axles (and hence the rest of the airplane). The result? The wheels spin. The only force acting on the airplane is a forward force from the engines. The plane takes off.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Darkblue
Posted 2005-12-01 01:19:14 and read 32767 times.

New approach.... change your frame of reference from the ground to the conveyor. If you imagine the conveyor as fixed, and instead of the conveyor moving backwards, everything around it will move forward, including the surrounding air.

So as you accelerate down this fixed conveyor, the air around you also accelerates in the same direction, giving you a tailwind. As you speed up to try to overcome the tailwind, the windspeed increases even more. So you always have zero airspeed, thus no lift.

Please feel free to shoot holes in my line of reasoning.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Flyf15
Posted 2005-12-01 01:46:38 and read 32767 times.

*sigh* I don't have time to read the entire arguements in this thread but it is pretty friggen sad to see that this discussion could go on so long. Of course the plane will take off.

There are an amazing amount of examples to show this. Say you're in a boat travelling upstream being pulled forward by something not connected to the river, such as a rope from shore (just how the engines aren't connected to the treadmill). Even if you increase the river speed accordingly, you're still going to be pulled forward. And, if this were a plane, eventually reach rotation speed.

Heck, say you have a miniture treadmill and a model airplane sitting on it. If you, unconnected to the treadmill, like the engines, push forward on the model, it will move forward... even if the treadmill accelerates accordingly.

Aside from friction in the wheel bearings, and things such as tire speed limits (not the main idea of the problem), there is nothing holding the plane back. It will take off. Guarenteed. I'll bet money on it.



Point being, the conveyor doesn't matter. Its not a relative frame of reference. Its not relative to anything but the speed the tires are rotating. Airplane's wings care about nothing but the frame of reference of the air. Airplane engines care about nothing but the frame of reference of the air. The ground doesn't matter.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Fokker Lover
Posted 2005-12-01 02:31:20 and read 32767 times.

Let's ask ourselves, Why is the belt turning? Because the wheels are turning.
Why are the wheels turning? Because the plane is moving forward.
If the wheels rotate forward one inch, the belt will rotate backwards one inch. Did the belt moving backwards cause the airplane to move backwards? No, it did not. Because the belt is only touching a set of free turning wheels. Therefore the plane will continue to move forward and take off.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: LeanOfPeak
Posted 2005-12-01 02:56:09 and read 32767 times.

If we have a magic conveyor that, as the question was put, always matches exactly the speed of the wheels, the only way to have groundspeed or airspeed is for the tires to skid.

Applying real-world engine thrust or tow ropes or handrails to the situation is irrelevant, because the terms of the question demand zero groundspeed and, accordingly, zero airspeed.

Never mind that there is no horizontal reaction at the tires on a real-world aircraft. The terms of the original question require some sort of opposing horizontal reaction, be it an anchor, an opposing engine thrust, retro-rockets, or somehow diverting the engine thrust to driven wheels in order to be applicable.

The question, as stated, constrains an aircraft with non-skidding wheels to zero groundspeed and zero airspeed and is completely and totally non-applicable to anything even resembling a real-world situation.

Quoting Flyf15 (Reply 263):
Say you're in a boat travelling upstream being pulled forward by something not connected to the river, such as a rope from shore (just how the engines aren't connected to the treadmill). Even if you increase the river speed accordingly, you're still going to be pulled forward.

You have a circular argument. As soon as you give the boat a velocity with respect to the bank, the hull of the boat is being struck by more velocity than that that the river possesses. Vriver + Vboat wrt bank = Vboat wrt river. Which is fine, unless the conditions of the problem assert that the boat hull may never be struck by any velocity other than that equal to the river velocity at that moment.

Quoting Flyf15 (Reply 263):
Heck, say you have a miniture treadmill and a model airplane sitting on it. If you, unconnected to the treadmill, like the engines, push forward on the model, it will move forward... even if the treadmill accelerates accordingly.

Again, if you push forward on the model and it moves forward, the WHEELS are now moving faster than the treadmill for it to have forward velocity with respect to the ground, which breaks the rules in the original question. The original question asked about the wheels, not about the airplane, or you would be fully correct.

Quoting Flyf15 (Reply 263):
it is pretty friggen sad to see that this discussion could go on so long.

Indeed.

Quoting A346Dude (Reply 261):
Ralgha is correct, his statement is true and that is why the airplane will take off. The conveyor belt is not holding the airplane back, even though it is spinning backwards at exactly the same speed as the wheels. All it is doing is pushing against the bottom of the wheels, not the wheel axles (and hence the rest of the airplane). The result? The wheels spin. The only force acting on the airplane is a forward force from the engines. The plane takes off.

All correct, if the aircraft was not constrained to zero groundspeed by the relative motion problem.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Flyf15
Posted 2005-12-01 03:17:30 and read 32767 times.

If you were to push forward on the model on a treadmill, say, make it go 10mph forward. Well, the treadmill then assumes that speed in reverse... Now the wheels are travelling over the ground at 20mph. Accelerate it to rotation speed... say, 150mph. Now the wheels would be spinning at 300mph. Not a problem, the airplane is at rotation speed and can lift off.

For the boat example... it doesn't matter that the boat is being struck by more velocity. Assuming the hull is frictionless (have to for this problem or it can't be determined either way), no extra force is exerted... the boat is not pulled back. In this example, the speed through the water is groundspeed and the speed with respect to the bank is airspeed. Therefore I 100% guarentee to you that the surface and its speed do not matter. It solely matters what the air is doing, and the air is stationary.

This is different than a car on a conveyor. An airplane's forward velocity is provided by its interaction with the air, not the ground. Imagine a fictional L shaped landing gear where the wheels are on the runway but the actual airplane is over stationary ground next to the runway. Exact same problem... nothing has changed. Now if you look only at the airplane and stationary ground, it will behave exactly as expected. You can totally ignore looking at the wheels... they can spin at whatever speed they like, but they have no effect on the airplane's movement.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: LeanOfPeak
Posted 2005-12-01 03:20:08 and read 32767 times.

Quoting Flyf15 (Reply 266):
If you were to push forward on the model on a treadmill, say, make it go 10mph forward. Well, the treadmill then assumes that speed in reverse... Now the wheels are travelling over the ground at 20mph.

...And you've just broken the cardinal rule of the question at hand. You said it yourself. The treadmill is going 10 mph. The wheels are turning 20.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: LeanOfPeak
Posted 2005-12-01 03:22:55 and read 32767 times.

Quoting Flyf15 (Reply 266):
In this example, the speed through the water is groundspeed and the speed with respect to the bank is airspeed.

Nope. The corollary to the question at hand would make the speed through the water the speed with respect to the treadmill and the speed with respect to the bank groundspeed. Which, with no wind, happens to be equal to airspeed. In order for the two situations to be analogous, that's what you get.

Frankly, the analogy makes the situation more complicated, rather than simpler.

[Edited 2005-12-01 03:24:23]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Flyf15
Posted 2005-12-01 03:29:29 and read 32767 times.

Wait wait wait, so the treadmill and wheels have to be moving at the same speed? I guess I must've read this wrong. Then this problem can't exist. There is no way to introduce an initial velocity. The only thing that can happen is that the plane will sit there and the conveyor won't move. Nothing is moving, everything is stationary, and everybody is bored watching it and goes and finds something else to do. Big grin

Then again, some of you may ask... well, the airplane can start to roll forward even the slightest amount. Say, it is able to reach 1mph, then the conveyor must move backwards at 1mph, which means the wheels are now moving at 2mph, which the conveyor much increase its speed to 2mph, and now the wheels are going at 4mph, and so on. In absolutely zero time, the conveyor belt reaches infinite speed and the universe explodes. Too many divide by zeros for my comfort.


So, there are two possible outcomes. The airplane sits there and nothing happens... or the universe collapses. In either case, this problem no meaning, and the airplane cannot do anything... it can't take off and it can't "not" take off - the universe would end well before V1. Big grin



In any case, I appologize, I had thought that I read that the conveyor belt matches the plane's speed, not the wheel's speed. For that version of the problem, I stand by my previous arguements.

[Edited 2005-12-01 03:36:21]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Navymidn
Posted 2005-12-01 03:31:33 and read 32767 times.

Through the fact that I can't seem to get this problem out of my head, I will try to explain why the aircraft will take off.

If the conveyor belt spins to the rear, the belt would have to move twice as fast as the wheel speed to stop forward movement. If they are turning at the same speed, the wheels will just spin twice as fast as the they normally would.

If the belt turns to the front, which is the only way it could to keep the wheels from turning, the belt will carry the aircraft to its takeoff speed. The engines will continue to accelerate the aircraft, so the belt will just turn faster, and the aircraft will fly.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: LeanOfPeak
Posted 2005-12-01 03:42:55 and read 32767 times.

Quoting Flyf15 (Reply 269):
Wait wait wait, so the treadmill and wheels have to be moving at the same speed? I guess I must've read this wrong. Then this problem can't exist. There is no way to introduce an initial velocity. The only thing that can happen is that the plane will sit there and the conveyor won't move. Nothing is moving, everything is stationary, and everybody is bored watching it and goes and finds something else to do. Big grin

Yes, this is exactly what the original post says, and I did say that there is no way for the engines to generate this motion, but granted that, if motion did happen, here is what would result, for the sake of discussion.

Quoting Navymidn (Reply 270):
If the conveyor belt spins to the rear, the belt would have to move twice as fast as the wheel speed to stop forward movement. If they are turning at the same speed, the wheels will just spin twice as fast as the they normally would.

By what analysis do you conclude this. The only thing that "The same speed as the wheels," could reasonably mean is the tangential velocity due to the tires' rotation at the point at which the tires contact the ground. This should be equal to the conveyor belt's speed to maintain zero groundspeed.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Navymidn
Posted 2005-12-01 03:50:47 and read 32767 times.

Okay, I am rethinking what I said about the rear moving belt.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Flyf15
Posted 2005-12-01 04:08:48 and read 32767 times.

Reading through the thread, I see that ralgha has already come to the conclusion I mentioned, with the difference of stating that things still go on as normal when infinities are flying around. If you make this assumption, then yes, the plane takes off and the conveyor cannot match the speed of the wheels, it is impossible.

So, the only way to full satisfy the problem is that the plane, wheels, and conveyor never move. Nothing happens.

Other than that, the plane takes off or the universe collapses, depending on which laws you like to say exist in this problem. There is no intermediate case where things are moving at finite velocities but the plane does not take off. You will never reach a steady state other than 0.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: ScarletHarlot
Posted 2005-12-01 06:02:18 and read 32767 times.

My brain hurts.  covereyes 

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SlamClick
Posted 2005-12-01 07:18:39 and read 32767 times.

Quoting ScarletHarlot (Reply 274):
My brain hurts.

You ain't the lone ranger there lady.

After spending two hours reviewing the posts on here I'd have to say that I haven't felt this foolish since the time I ate three Scotch Bonnet habañero chili peppers to impress a woman I knew to be a lesbian. And this with my wife watching!

Anyway, I just got off the phone.

Thiokol in Utah says we can have a shuttle solid-rocket booster. Talked to a guy who used to build funny cars. He will build a landing gear for the SRB. Still working on the conveyor purveyor! (maybe from a trona mine in Wyoming) Open for suggestions there. Got the Blackrock Desert reserved next dry season. So here's the deal.

We are going to put the SRB-on-wheels on the treadmill on the alkali flat, pointed at the distant mountains. I'm going to light the fuse and run like hell ninety degrees from the longitudinal axis of vehicle and treadmill.

All of you who believe, I mean really believe that the plane would not take off are going to stand at parade rest at the far end of the treadmill and get a really good look at the results of your best reasoning.

Remember - you will be perfectly safe because the wheels on this vehicle are on a backwards-running treadmill.

Looking for sponsorship also, and open for suggestion. Thinking in terms of Lube-a-Boob because no matter what else happens a whole bunch of boobs are going to get greased.

good night all

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: OE-LDA
Posted 2005-12-01 08:42:56 and read 32767 times.

This is the greatest thread I can remember since I am a member here. It is interesting, how complicated one can think about such a simple setup.

The plane will take off. Period.

All that counts for the lift is the airspeed. It does not make a difference, how the plane is connected to the earth. It can be wheels or floats or skis, or just the ground effect. The wing cannot tell the difference and will lift the plane if the airspeed is sufficient. The wheels will either spin faster or slower than they would on a normal runway, depending on the direction the belt is running, but again, the wings will not find out.

Regards, OE-LDA

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Chksix
Posted 2005-12-01 09:51:57 and read 32767 times.

OE-LDA, there's no way to make the "no fly zone" guys to understand LOL Big grin

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Kay
Posted 2005-12-01 10:23:31 and read 32767 times.

Quoting SlamClick (Reply 275):
All of you who believe, I mean really believe that the plane would not take off are going to stand at parade rest at the far end of the treadmill and get a really good look at the results of your best reasoning. Remember - you will be perfectly safe because the wheels on this vehicle are on a backwards-running treadmill.

 rotfl   rotfl   rotfl   rotfl   rotfl 

I'm ready to stand there!!! and take beautiful photos  Smile

Really, I think we can remove the plane and simplify the example.
If I put a wheel on a treadmill that will match the rotating speed of the wheel backwards, can I move the wheel forward (by holding its axle with my hands?)

The answer is no, even if you attach rockets to your arms, the wheel will stay at the same position.


If the answer is yes, then the plane will take off. If the answer is no, it won't. I'm of the "no" camp (clear already, I think  Smile).

Kay

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-12-01 10:28:01 and read 32767 times.

Quoting LeanOfPeak (Reply 267):
...And you've just broken the cardinal rule of the question at hand. You said it yourself. The treadmill is going 10 mph. The wheels are turning 20.

The wheels would have to skid - the question didn't say they couldn't. But where does this magic force come from, the one that causes the wheels to skid? It would normally come from the brakes.

Quoting SlamClick (Reply 275):

That I'd like to see!  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Flyf15
Posted 2005-12-01 10:29:03 and read 32767 times.

Quoting Kay (Reply 278):
The answer is no, even if you attach rockets to your arms, the wheel will stay at the same position.

Uhh, why? It seems like it'd be totally and completely obvious that you wouldn't have any trouble moving the wheel in this situation.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Mighluss
Posted 2005-12-01 10:41:43 and read 32767 times.

I'm sorry, I'm to lazy to read the hole threat, but after reading a few takes, and thinking about it, I say...

If the belt can really match the speed of the wheels at any time, and taking that planes are not wheel driven (the engines push directly the air), I think that the plane will start accelerating, making the belt accelerate, but the plane will keep pushing, making the belt accelerate more and more, until the wheels, or the belt break down!

(It's really funny to think about the image of a plane standing at full thurst, with wheels smoking, and spinning over the belt... for example at 10.000 rpm) Big grin Big grin

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Keta
Posted 2005-12-01 11:07:58 and read 32767 times.

Quoting Kay (Reply 278):
If I put a wheel on a treadmill that will match the rotating speed of the wheel backwards, can I move the wheel forward (by holding its axle with my hands?)

The answer is no, even if you attach rockets to your arms, the wheel will stay at the same position.

You still don't understand... you will never get a treadmill that will match the wheels' speed. No matter the belt, you will move.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: ZSOFN
Posted 2005-12-01 11:10:46 and read 32767 times.

Guys, this is how it works.

OK, some overlooked points.
---------------------------

Firstly the the beginning of the experiment...

So the plane is sitting stationary on the belt, engines off.

Engines started, throttle up, forward thrust induced. Why would this start spinning the wheels in the first place anyway? They would only start moving in the first place if the aircraft begins to roll forward, thus negating anyone's assumptions that the aircraft would remain stationary. The belt is reactionary btw.

The whole experiment is an impossibility i.e. infinite speed would be required in some form. In reality, once an initial forward movement has started, it will continue slowly whilst the rotation rate of the wheels increases rapidly up to a speed at which the tyres explode and cause fire and mass destruction to all.

No that would be "end of story"!

However, if the wheels were modelled in this experiment as frictionless, they would spin up to ridiculous speeds and not impede the forward acceleration of the aircraft, as the thrust produced by the engines only has friction from the air to work against. So it would accelerate normally. However, by rotating time, those wheels will be spinning like madness, and the gyroscopic effect will stop the aircraft from changing direction in any way - including rotation.

So with frictionless wheels the aircraft will move forward at normal speed but never even get the nosewheel off the ground.

Comments, please.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Vikkyvik
Posted 2005-12-01 11:15:43 and read 32767 times.

I guess I kind of agree with both sides (are there still just two?  Smile)

Quoting Ralgha (Reply 259):
Conveyor speed CAN NOT equal wheel speed. The reason for this is that there is nothing holding the airplane back. Nothing counters the engine force. This is fundamentally different than a car, a bike, a person running, or any land based activity.



Quoting Ralgha (Reply 260):
Unless you put an anchor on the airplane, this situation will not happen

Both of the above statements are accurate in the real world. However, the problem posed (like many basic physics problems), cannot actually happen in the real world.

For the question posed (where the conveyor can and does equal the wheel speed), the wheels and the conveyor will indeed accelerate instantaneously and perpetually. The airplane is indeed free to move forward, but it would find it hard to do so while the conveyor is negating the turning of the wheels at an infinite speed. It's completely illogical and impossible, but no less interesting.

In the real world, the conveyor would always be catching up to the wheel speed, and hence the airplane would take off.

In the perfect world of the posed problem, there's no solution because:
the wheels and conveyor would accelerate and continue accelerating in absolutely no time at all. Literally. You actually can't even look past that first instant, because there's no end to the acceleration of the wheels/conveyor that takes place during that time (or lack of time).

If infinite speed could somehow be acheived, then the airplane would start accelerating and would take off. But the situation is a physical impossibility.

At least, that's my take on it.  Smile

~Vik

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: OE-LDA
Posted 2005-12-01 11:21:49 and read 32767 times.

Quoting Chksix (Reply 277):
OE-LDA, there's no way to make the "no fly zone" guys to understand LOL

Incredible, isn't it?

Quoting Mighluss (Reply 281):
, I think that the plane will start accelerating, making the belt accelerate, but the plane will keep pushing, making the belt accelerate more and more, until the wheels, or the belt break down!

Why and how on earth should the plane accelerate the belt? It is the other way round, the belt will accelerate or decelerate the wheels of the plane, but again, that does not make any difference, because all that counts is that the engine accelerates the wing to produce sufficient lift.

This thread is fun, keep it going. Looks like we all will have to meet for the ultimate experiment!

Regards, OE-LDA

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Kay
Posted 2005-12-01 11:50:41 and read 32767 times.

Quoting Flyf15 (Reply 280):
Uhh, why? It seems like it'd be totally and completely obvious that you wouldn't have any trouble moving the wheel in this situation.

Welcome to the irreversible zone of the "It will fly" group.  

Quoting Mighluss (Reply 281):
until the wheels, or the belt break down!

My views exactly.

Quoting Keta (Reply 282):
You still don't understand... you will never get a treadmill that will match the wheels' speed. No matter the belt, you will move.

The treadmill will match the wheels' speed exactly because that's a given. And once you add that given, the plane won't move. It's a theoritcal machine, that can't exist.

Quoting Ralgha (Reply 259):
This is fundamentally different than a car, a bike, a person running, or any land based activity.

I would have to disagree. Taking off is an "air"-based activity, but provided that the land is slippery. Such a conveyor is a stopping force. Just like brakes.







I understand that even if the ground was slippery under these people's feet (above), they STILL would be able to advance. But we're not talking about a slippery floor . If the ground is matching their feet backwards, which is not a slippery floor anymore, they ain't goin' nowhere. So will the plane.



Kay

[Edited 2005-12-01 11:58:39]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Vzlet
Posted 2005-12-01 13:05:31 and read 32767 times.

Another way to look at it. (Please consider these questions as rhetorical!)

A plane intending to do a touch-and-go lands on the conveyor, touching down at 100 mph, a speed the conveyor "instantly" matches. So what's the result?

Does the plane's forward motion (relative to the earth) instantly go to zero while its wheels continue to spin at a 100 mph pace on the treadmill?

Or does the plane continue down the conveyor at a speed 100 mph relative to the earth as the belt speed soars towards infinity as the conveyor futilely chases the now-ever-increasing tire rotational speed?

If the plane continues in motion, will retarding or advancing the throttle affect its speed?

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: NAV20
Posted 2005-12-01 14:10:19 and read 32767 times.

One last try to convert the 'must-flys'.  Smile

You step on a moving walkway and then realise that you're going the wrong way and start walking back. But walking's no use - the walkway is moving you the wrong way at say 3 mph, if you just walk you're only 'standing still'.

So you break into a trot. That's better, you're making 6 mph against the walkway's three. So you're getting back to where you want to be at a net 3 mph.

But now imagine that a house mechanic with an evil sense of humour is watching your performance; and he just happens to have his hand on the speed lever of the walkway. He jacks the walkway speed up to 6 mph and you're 'jogging on the spot' again.

You break into a full run. But the mechanic just grins nastily, and increases the walkway speed just enough to match your running speed exactly...

Now substitute the conveyor for the walkway, the aeroplane for 'your goodself', and the evil genius who thought up this riddle for the mechanic - and you should get the picture  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-12-01 14:59:02 and read 32767 times.

Quoting NAV20 (Reply 288):
You break into a full run. But the mechanic just grins nastily, and increases the walkway speed just enough to match your running speed exactly...

NO!  Smile

You're still thinking about a force being applied by the person/aircraft to the belt. The force of the aircraft's engines is not transmitted to the ground via the wheels.

Quoting SlamClick (Reply 149):
The wheels don't drive the airplane.
The wheels don't drive the airplane.
The wheels don't drive the airplane.
The wheels don't drive the airplane.
The wheels don't drive the airplane.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-12-01 15:07:35 and read 32767 times.

For those who think the force of the belt travelling at the same speed in the opposite direction cancels the aircraft's speed, what do you think will happen if an aircraft, taxying at 20 kts, hits a fly, travelling at 20 kts in the opposite direction? Will the aircraft be stopped? Speed does not equal force. For the full force of the moving belt to be transmitted to the aircraft, the wheels would have to be locked, e.g. by the brakes.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: NAV20
Posted 2005-12-01 15:12:49 and read 32767 times.

David L, please forget the wheels.  Smile

The engine thrust generates forward movement. The conveyor negates it.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Kay
Posted 2005-12-01 15:21:40 and read 32767 times.

Quoting NAV20 (Reply 288):
One last try to convert the 'must-flys'.

I was going to answer you to tell you "you will now be attacked by the 'it will fly' group saying that the wheels of the plane have no role"  Smile


The "It will fly" group must know that WE KNOW that the wheels don't drive the plane, and NAV20's argument isn't what we are saying.

The guy pulling himself on a rope is like the plane trying to take off. It is gripping the rope like the engines grip on the air.

If he is wearing roller blades, and the floor matches the speed of the wheels of his rollers, surprise surprise, he ain't movin'.

Kay

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Keta
Posted 2005-12-01 15:23:12 and read 32767 times.

Quoting NAV20 (Reply 291):

No, the force is not made by the wheels. Your legs yes they do a force. It's different.

Once again...

Quoting SlamClick (Reply 149):
The wheels don't drive the airplane.
The wheels don't drive the airplane.
The wheels don't drive the airplane.
The wheels don't drive the airplane.
The wheels don't drive the airplane.


[Edited 2005-12-01 15:24:40]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: ZSOFN
Posted 2005-12-01 15:26:39 and read 32767 times.

NAV20, you seem to still be thinking in quite simplistic terms. See my post further above about this. Think about it this way:

The analogy of walking on a conveyor belt or driving on a conveyor belt is incorrect, as the force is being channeled through the feet or tyres. In that case, any net motion would indeed not be possible. However if the thrust is coming from a jet engine and is acting AGAINST THE AIR, NOT THE WALKWAY, this thrust WILL propel the aircraft forward whilst the wheels, which for the purposes of this experiment have been agreed to be of zero friction, will spin like mad.

The air is providing the reaction needed to provide propulsion, whereas if the thrust was transmitted through the wheels as with a car, the reaction is negated by the conveyor belt. However the question involves jet engines, not wheel motors.

[Edited 2005-12-01 15:27:33]

[Edited 2005-12-01 15:28:33]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-12-01 15:27:45 and read 32767 times.

Quoting NAV20 (Reply 291):
David L, please forget the wheels.

You cannot forget the wheels - they are crucial!

Stand on a plank of wood while you're wearing rubber-soled shoes. Get someone to lift one end of the plank. Your shoes "stick" to the plank so you don't move. Try the same thing while wearing roller-skates. What happens? With the plank tilted, gravity applies a force which tries to pull you to the lower end of the plank. While wearing the shoes, the plank can be tilted to a point where the force of gravity overcomes the static friction between your shoes and the plank but it takes a much bigger force than it does with wheels. With shoes, you have to overcome the friction between your shoes and the belt (designed to be high), with wheels you only have to overcome the friction in the wheel bearings (designed to be low).

Substitute gravity with engine thrust.

[Edited 2005-12-01 15:38:58]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Fokker Lover
Posted 2005-12-01 15:32:52 and read 32767 times.

Quoting NAV20 (Reply 288):
Now substitute the conveyor for the walkway, the aeroplane for 'your goodself', and the evil genius who thought up this riddle for the mechanic - and you should get the picture

This is where you are making your mistake. You are using the walkway to propel yourself. The airplane is not.

I like to take things apart to see how they work. Let's disassemble everything and identify the different parts. Let's see what they can and cannot do.
To make anything move you have to have a source of power to enact upon it. You also have to have a way to transfer that power. We have two power sources here. The first is the airplane engine. The second is some type of motor to turn the conveyor. I'm going to pick an electric motor because they are very responsive and quiet. Both of these sources are capable of rotating the tires. The airplane engine will push the structure down the runway dragging the landing gear axles in a linear direction. The wheels turn linear energy into rotating energy. Because the wheels are free turning, they can absorb and use energy, but they can't transfer energy. The wheels cannot drive the belt. It has to have it's own energy source.

If you test out the motor on your conveyor, it will spin the belt which will transfer energy to the wheels causing them to rotate. The wheels cannot transfer energy to the axles or to the structure. So the plane sits there with a spinning wheel. The wheels are a black hole for energy. They can use it, but they can't transfer it. The wheels are the only connection between the 2 energy sources. Since the wheels can't transfer that energy, there is no real connection. That means they are both free to do their thing. The airplane engine will push the the plane in a linear direction and fly away. The belt will spin as much or as little as it wants.
The true solution here is the transfer of energy. You believe that the belt is transfering its energy to the entire airplane and holding it in place. That's not possible because the tires are absorbing and using that energy, but not transfering it to the rest of the plane. THE PLANE WILL FLY.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-12-01 15:34:03 and read 32767 times.

Quoting Kay (Reply 292):
I was going to answer you to tell you "you will now be attacked by the 'it will fly' group saying that the wheels of the plane have no role"

They have a role - to isolate the aircraft from the horizontal force from the conveyor belt (excluding wheel bearing friction, which can't be that much of a problem or aircraft would never take of on a normal runway)! The only way the force from the belt can be transmitted to the aircraft is by applying the brakes.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: ZSOFN
Posted 2005-12-01 15:40:52 and read 32767 times.

Quoting ZSOFN (Reply 294):

Further to what I said, I am with the people who say it can fly in general, however something that is yet to be argued over:

Once the plane gets to rotation speed and the wheels are rotating at an "infinite" speed, the wheels themselves will have "infinite" inertia if they have any mass at all. Therefore despite the normally sufficient weight generated by the wings, it will not rotate as the wheels will force the aircraft to plough on horizontally with no allowance for change in direction, be it vertical or otherwise. Thus, it will not fly.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Kay
Posted 2005-12-01 15:41:39 and read 32767 times.

Quoting ZSOFN (Reply 294):
The air is providing the reaction needed to provide propulsion

ZSOFN: The wheels don't drive the plane. Planes can take off with wheels turning at zero speed.

But this plane here won't take off because the wheel struts won't move forward, because the conveyor won't let them.

The plane won't fly.

How many "wheels don't drive the plane" sentences are there by now??  Smile You guys should be looking beyond that obvious trap that few of the "it will not fly" group have fallen into. Most of us haven't, and it's pretty obvious that a car's transmission drives the wheels, unlike a plane... Look at what we're saying. We're saying something else.


Kay

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: ZSOFN
Posted 2005-12-01 15:43:05 and read 32767 times.

Quoting Kay (Reply 299):

Eh? Read what I said again!

And the struts will move forward as I thought we were assuming the wheels to be able to turn without resistance.

[Edited 2005-12-01 15:44:02]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Kay
Posted 2005-12-01 15:48:41 and read 32767 times.

Quoting ZSOFN (Reply 300):
we were assuming the wheels to be able to turn without resistance.

Finally you are looking at it from the right angle: the conveyor is infinite resistance.

Kay

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: ZSOFN
Posted 2005-12-01 15:50:09 and read 32767 times.

HAHA!

I've figured it out...


  • The aircraft applies thrust
  • The aircraft initially remains almost stationary and the wheels spin up rapidly
  • The ground effect caused by the airflow created by the fast-moving conveyor belt causes the plane to lift off and hover close to the belt
  • The aircraft will remain in the air (just floating off the ground) for as long as the belt remains at at least the speed it was travelling at at the point of lift off. In fact, it will gather forward speed in the hover to the extent that it can climb further and further away and be free altogether
  • If the speed of the belt drops, the aircraft will fall back to the ground, after which the whole process starts again.


So either the aircraft bounces repeatedly or it flies.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: ZSOFN
Posted 2005-12-01 15:50:47 and read 32767 times.

Quoting Kay (Reply 301):
Finally you are looking at it from the right angle: the conveyor is infinite resistance.

?

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-12-01 15:51:47 and read 32767 times.

Quoting Kay (Reply 299):
But this plane here won't take off because the wheel struts won't move forward, because the conveyor won't let them.

Go back to the example given earlier. You lift up a bicycle, in the upright position, and carry it along forwards. Now get someone to take a plank of wood, to represent the conveyor belt. Get them to run the plank along the bottom of the wheels in either direction at any speed. Does it stop you moving the bicycle forwards at the same speed? You cannot stop the wheels struts from moving forwards by applying a fore-aft force to the bottom of the wheels.

Quoting Kay (Reply 299):
You guys should be looking beyond that obvious trap that few of the "it will not fly" group have fallen into. Most of us haven't,

Then those comments aren't aimed at you but at those who still are falling into that trap.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Fokker Lover
Posted 2005-12-01 15:52:56 and read 32767 times.

Quoting Kay (Reply 299):
But this plane here won't take off because the wheel struts won't move forward, because the conveyor won't let them.

Tell me exactly how you are keeping the struts from moving forward.
Don't say by spinning the wheels backwards, because you would be wrong.
The only thing the belt can possibly do though is spin the wheels backwards. It has no other affect on the airplane. Where is your transfer of energy??????

[Edited 2005-12-01 16:06:42]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: ZSOFN
Posted 2005-12-01 16:00:38 and read 32767 times.

The only way in which the belt can transfer energy is:

a) into the wheels, and if we're modelling them as frictionless, this energy cannot be transmitted in any way to the rest of the aircraft.

b) by creating an airflow which provides a ground effect to the aircraft which provides lift.

In no other way does the belt make any difference to what would otherwise be a normal takeoff, even though those differences themselves would be significant.

Effectively the takeoff run would be shorter as ground effect assists the lifting process. The only other consideration is inertia of the tyres.


By the by: The belt will reach infinite speed almost instantaneously if the wheels spin without friction. Do we assume the belt itself to have mass? If so, we've got a big problem. By action/reaction aka Newton I, the world would be moving/spinning at an infinite speed too  Wink

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: NAV20
Posted 2005-12-01 16:04:00 and read 32767 times.

Guys, about speed.

Take an aeroplane on the runway (an ORDINARY one!). You can express its speed any number of ways - in knots, in mph, in kph, or as a percentage of the speed of sound - OR by reference to the RPM (revolutions per minute) of the wheels.

But here's the trick. Whichever way you choose to express it, it remains the SAME SPEED.

One among several misleading parts of this (highly entertaining!) puzzle is that it happens to say that the conveyor matches 'the speed of the wheels'. But (I HOPE you will now agree) that is just another way of saying 'the speed of the aeroplane'.

So the conveyor is designed to move at the exact speed of the aeroplane. Never mind that the specific way of measuring that, in the puzzle, is the rpm of the wheels. The conveyor in the puzzle is designed to move backwards as fast as the aeroplane moves forward. And therefore the aeroplane won't move.

[Edited 2005-12-01 16:07:50]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Mighluss
Posted 2005-12-01 16:04:38 and read 32767 times.

Quoting OE-LDA (Reply 285):
Why and how on earth should the plane accelerate the belt?

The plane is not accelerating the belt, the belt is accomoding to the speed of the wheels.

anyway, after seeing that graphic of the man pushing, I start to think that with enough thurst, the plane will start skidding and eventually taking-off (with some sort of hanging rubber and wires as wheels)

Quoting Fokker Lover (Reply 296):
Since the wheels can't transfer that energy, there is no real connection. That means they are both free to do their thing. The airplane engine will push the the plane in a linear direction and fly away. The belt will spin as much or as little as it wants.

I think is the same way I thought at last... (with my simplistic physics)

Another way to see it is:

Lock the brakes, full thurst... the plane will start skidding, but the belt will not move, because wheels won't be turning, and IF the plane is strong and powerfull enough, to achieve take-off speed with wheels locked, it will fly.

Well! let's go back to work, because I'm starting to see to much 0's in the company accounts...

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-12-01 16:15:08 and read 32767 times.

Quoting NAV20 (Reply 307):
The conveyor in the puzzle is designed to move backwards as fast as the aeroplane moves forward. And therefore the aeroplane won't move.

Why not? The wheels isolate the movement of the conveyor belt from the aircraft. You're still thinking of a car. Look again at the bicycle example which I summarised in Reply 304 (I think it was Slamclick's idea).  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: ZSOFN
Posted 2005-12-01 16:17:28 and read 32767 times.

Guys, NAV20 does have a point.

If the plane was to move forward, it would have to mean that the wheels were spinning faster than the conveyor belt was moving. Hence my argument for the hover...

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Saintsman
Posted 2005-12-01 16:19:25 and read 32767 times.

I can believe that this is still going.

Its an impossible situation, therefore all your comparisons and assumptions are invalid.

Because its an impossible situation the AIRCRAFT WILL NOT FLY

People. Just accept it.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Fokker Lover
Posted 2005-12-01 16:22:24 and read 32767 times.

Quoting NAV20 (Reply 307):
So the conveyor is designed to move at the exact speed of the aeroplane. Never mind that the specific way of measuring that, in the puzzle, is the rpm of the wheels. The conveyor in the puzzle is designed to move backwards as fast as the aeroplane moves forward. And therefore the aeroplane won't move.

Yes, it will move!!!!! The conveyor has no way of transferring its energy past the wheels. We have 2 opposing forces here. One is trying to drag the airplane forward. The other is trying to drag the airplane backwards. The belt is failing because it can't get its energy past the spinning wheels. Therefore the 2 opposing forces have NO AFFECT on each other.

Actually the belt is trying to turn the wheels and nothing else. It is succeeding at that. The non-flyers are the ones that want it to drag the plane backwards. That's like pointing the wrong remote control at the tv. I want it to turn on, but it won't.
NAV20 I really have to know. What do you do for a living? What are your real life experiences?

[Edited 2005-12-01 16:32:32]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: ZSOFN
Posted 2005-12-01 16:27:42 and read 32767 times.

Fokker Lover,

Yes, physics would normally dictate that the aircraft would move forward. BUT as one can assimilate from the question, this belt in its nature doesn't hold to the laws of physics. The whole thing's built on an impossible condition that the belt will by nature be moving at the same speed as the tyres.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Bond007
Posted 2005-12-01 16:32:17 and read 32767 times.

Quoting Kay (Reply 57):
Here's another analogy. place a toy plane on that same conveyor, and kick it with your foot with all your force. Will it move? only if the wheels and the conveyor create huge friction. Since the plane is small and your foot is powerful, it will advance half a meter or 1 meter before standing still, but only because the wheels can skid on the conveyor in alot of friction.

This is a perfect example, but you have it completely wrong!

Of course it will move! It's quite the opposite of what you said. It's already been said that friction in the wheel bearings etc is not a factor, so it's as if the toy plan is not attached to the conveyor at all when you kick it.

If there is no friction in the wheel bearings etc, it will take NO more force to kick the toy plan forward, than it did when the conveyor was stopped!

Why will it skid?? The wheels are free to rotate and the conveyor keeps up with them !

You've actually proved why it WILL fly  Smile

Jimbo

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Kay
Posted 2005-12-01 16:34:33 and read 32767 times.

Quoting David L (Reply 304):
You lift up a bicycle, in the upright position, and carry it along forwards. Now get someone to take a plank of wood, to represent the conveyor belt. Get them to run the plank along the bottom of the wheels in either direction at any speed. Does it stop you moving the bicycle forwards at the same speed?

Yes, except by skidding the wheels! If the person with the ply of wood works like this belt, he will counter exactly any movement from the wheels. And the only way you will move the bicycle forward is by skidding the wheels on the plank of wood. You could do that because you are carrying a light bicycle.
A heavy bicycle sitting directly on the wood? still possible, but more difficult.
A plane with its own jet engines? Impossible. Its like applying the brakes.

Quoting Fokker Lover (Reply 305):
Where is your transfer of energy??????

There's a huge engine under that belt. It's a huge energy source. It can stop a plane, if it functions like described!! (match the speed of the wheels).
Dissipation will probably go as heat in the wheels. Into the axle/wheel assembly as heat. Exactly like brakes.


How to envision a wheel moving forward or backward on a belt that will turn in the opposite direction exactly. How?

You can't picture the plane rolling at 100 because of that. If you can, what is happening exacty with the wheels?
The only position where they agree is when the wheels are stopped. and the belt is stopped.


Kay

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-12-01 16:34:35 and read 32767 times.

Quoting Saintsman (Reply 311):
Its an impossible situation, therefore all your comparisons and assumptions are invalid.

I think that's been well established now but there is still an element of the debate about a conveyor belt moving at the same speed as the aircraft but in the opposite direction.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Kay
Posted 2005-12-01 16:42:17 and read 32767 times.

Quoting David L (Reply 316):
I think that's been well established now but there is still an element of the debate about a conveyor belt moving at the same speed as the aircraft but in the opposite direction.

If the belt is moving opposite the aircraft's speed, it will fly. The wheels will just turn double.

Alllllll this argument is about the belt turning opposite the wheels' speed. Was I arguing alone?    


I agree with the person who said it's an impossible situation. The plane running down the runway means the assumption of a "smart belt" is broken, and the plane sitting steady means energy and friction is diffused into the wheel bearings which would in any other situation, slip and turn and do whatever, independantly of the wheels.

Maybe the energy will go to the future?? 

Quoting Bond007 (Reply 314):
Why will it skid?? The wheels are free to rotate and the conveyor keeps up with them !

Bond007, please explain to me how a wheel (regardless of anything else) can move forward on a floor that is matching its turning speed. It can't move forward without skidding.


Kay

[Edited 2005-12-01 16:51:15]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Bond007
Posted 2005-12-01 16:49:15 and read 32767 times.

Ignore my previous post  Smile

I completely understand the theories from the "It will fly" crowd, but there is one thing wrong with the whole theory.

You really have to admit, that if the wheels and the conveyor are going at the SAME speed, the only way the aircraft will move forward is if it skids...there is no other way guys.

As NAV20 says (and others), in order for the aircraft to move forward on the belt, the wheels simply MUST be going faster than the conveyor is ... an impossible situation according to the rules of the game.

So that's it folks ...


Jimbo

[Edited 2005-12-01 16:53:06]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Fokker Lover
Posted 2005-12-01 16:54:51 and read 32767 times.

Quoting Kay (Reply 315):
There's a huge engine under that belt. It's a huge energy source. It can stop a plane,

Stop it how? A free wheeling wheel can do only one thing. That is rotate on its axis. It cannot stop a plane from moving. It can only spin. That's why brakes are added to a wheel. The brakes make the physical connection to exert a force on the rest of the plane. Our plane has no brakes. You have no other physical connection to transfer energy. Your argument is wrong.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Fokker Lover
Posted 2005-12-01 17:00:56 and read 32767 times.

I should have been in bed 3 hours ago. If you guys figure out how to make an airplane stop moving by spinning its wheels backwards wake me up. I plan to go ice skating in Hell.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Bond007
Posted 2005-12-01 17:04:23 and read 32767 times.

Quoting Kay (Reply 317):

I'm with you Kay  Smile

A silly situation, but it cannot happen unless the belt and wheels are going at different speeds.

Example:

Imagine a wheel fixed at 2000 rpm sitting on a treadmill. The treadmill speed is adjusted so that if you hold the spinning wheel on the treadmill, it just rotates and stays in one spot .... the speeds are matched. Agree?

Now push the wheel forward without changing any wheel/treadmill speed. The only way it will move forward is by skidding on the treadmill. Agreed?

It does not matter if this is 2000 rpm or 2,000,000 rpm. As long as the speeds are the same it cannot happen.


Jimbo

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Mighluss
Posted 2005-12-01 17:10:32 and read 32767 times.

Quoting Fokker Lover (Reply 312):
The belt is failing because it can't get its energy past the spinning wheels

Now I caught the idea, if you don't push the brakes, the plane will not skid, but is an impossible anyway, as plane start moving forward, the wheels will accelerate geometricaly towards infinite... HAHAHAHA!

 hot   crazy   banghead   melting   splat 

(what I was thinking when decided to post here... )  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Kay
Posted 2005-12-01 17:33:06 and read 32767 times.

Quoting Bond007 (Reply 321):
As long as the speeds are the same it cannot happen

Bingo Jimbo!

Quoting Fokker Lover (Reply 319):
You have no other physical connection to transfer energy.

Fokker_Lover, I totally agree with you. Totally. But, how can a wheel (regardless of anything else, no plane, nothing) move forward on a floor that is matching its turning speed backwards. It can't move forward without skidding.

Regarding the dissipation of energy, well it's in the wheel assembly, as heat. Unless, since this model can only exist in theory, what if we discovered a dark corner, a lapse of science where the fourth dimension is needed to work, i.e energy goes to the future?





 silly 

Kay

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Keta
Posted 2005-12-01 17:55:17 and read 32767 times.

Quoting David L (Reply 316):
I think that's been well established now but there is still an element of the debate about a conveyor belt moving at the same speed as the aircraft but in the opposite direction

I thought so too...  ill 

Quoting Bond007 (Reply 318):
the wheels simply MUST be going faster than the conveyor is ... an impossible situation according to the rules of the game.

It's the opposite way. The rules of the game are wrong, the first question is based on a false statement. It's impossible that you can follow the rules of the question. In this universe with its four fundamental interactions, 4 (or 11?) dimensions, etc. such a situation is impossible. Maybe if you can have an universe where if you apply thrust the airplane moves back, you can have the conditions of the question.  Wink

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: ZSOFN
Posted 2005-12-01 17:57:04 and read 32767 times.

Is anyone going to read my posts? They seem to be ignored yet I feel they answer these latest questions.

 banghead 

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Bond007
Posted 2005-12-01 18:11:31 and read 32767 times.

Quoting ZSOFN (Reply 313):
Yes, physics would normally dictate that the aircraft would move forward. BUT as one can assimilate from the question, this belt in its nature doesn't hold to the laws of physics. The whole thing's built on an impossible condition that the belt will by nature be moving at the same speed as the tyres.

I read it  Smile

You are correct.

The plane will take off, but only by the wheels going faster than the conveyor. The question says this cannot happen...so question is impossible to answer.


Jimbo

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: ZSOFN
Posted 2005-12-01 18:24:49 and read 32767 times.

Quoting Bond007 (Reply 326):
The plane will take off, but only by the wheels going faster than the conveyor

Well this is my point of contention that no-one's addressing.

With a conveyor belt moving SO fast, the airflow above it that it creates, whilst turbulent, will most definitely be sufficient to create enough ground-effect lift to enable the aircraft to lift off. From that point on it could stay low, and without the bizarre physics constraining the wheels, could slowly accelerate in this stream of moving air until it has enough "natural" airspeed to climb away.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Kay
Posted 2005-12-01 18:46:13 and read 32767 times.

Quoting ZSOFN (Reply 327):
With a conveyor belt moving SO fast,

ZSOFN, you still have to establish that the conveyor will turn so fast. There's no telling if it will turn at all. We have a weird deadlock situation where the conveyor will not allow the wheels to change position. Energy will be dissipated somehow, not sure through rotation of the wheel/belt.

Kay

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: ZSOFN
Posted 2005-12-01 19:07:52 and read 32767 times.

Well the assumptions that lead to the conveyor belt turning so fast:

When the whole system is at rest it is in unstable equilibrium, that is, all it takes is a nudge to get the thing going, after which it perpetuates. Thus, as long as there is a force supplied by the engines nudging the plane forwards constantly, the speed of the belt will increase. Depending on the reaction rate of the belt to to the increase in wheel roll rate, this will happen very rapidly. If the reaction is instantaneous i.e. the belt begins to move backwards INSTANTLY as the wheels start moving forward, the acceleration of the belt would theoretically be infinite.

If the engines stop producing power, the belt will continue to move at a constant velocity, assuming there are no external friction forces.

Meanwhile, the belt's velocity creates an airflow and even though the aircraft's ground speed relative to the earth is zero, its airspeed will become sufficient for the aircraft to lift off vertically and hover. Once the wheels lose contact, the aircraft will now accelerate and rise up further as its airspeed continues to increase. Eventually it will transition into clear normal airspace.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Kay
Posted 2005-12-01 19:26:29 and read 32767 times.

Quoting ZSOFN (Reply 329):
the acceleration of the belt would theoretically be infinite.

hmm. Which means the belt will run at infinite speed. If this inifinite energy indeed creates an airflow, it could just snap the wings of the plane, but before that, it will munch the tires to dust... I think we're getting ahead of ourselves...


This is not a situation where a plane can take off. This should be the consensus. Extraordinary conditions, extraordinary forces etc


Kay

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: ZSOFN
Posted 2005-12-01 19:42:46 and read 32767 times.

Quoting Kay (Reply 330):
This is not a situation where a plane can take off. This should be the consensus. Extraordinary conditions, extraordinary forces etc

I agree 100%!  bigthumbsup 

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-12-01 19:45:04 and read 32767 times.

Quoting Kay (Reply 317):
If the belt is moving opposite the aircraft's speed, it will fly. The wheels will just turn double.

Alllllll this argument is about the belt turning opposite the wheels' speed. Was I arguing alone?

No but, as I stated several times, I've lost interest in the question as stated - it's just too ridiculous to contemplate. However, there is still an "element" ( Smile)which believes that the aircraft would not take of if the belt opposed the aircraft's speed...

Quoting NAV20 (Reply 307):
One among several misleading parts of this (highly entertaining!) puzzle is that it happens to say that the conveyor matches 'the speed of the wheels'. But (I HOPE you will now agree) that is just another way of saying 'the speed of the aeroplane'.



Quoting Kay (Reply 315):
Quoting David L (Reply 304):
You lift up a bicycle, in the upright position, and carry it along forwards. Now get someone to take a plank of wood, to represent the conveyor belt. Get them to run the plank along the bottom of the wheels in either direction at any speed. Does it stop you moving the bicycle forwards at the same speed?

Yes, except by skidding the wheels! If the person with the ply of wood works like this belt, he will counter exactly any movement from the wheels. And the only way you will move the bicycle forward is by skidding the wheels on the plank of wood.

... and that was a response to someone who thinks the aircraft wouldn't even move forward if the belt opposed the aircraft's speed.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: XORDLLAA
Posted 2005-12-01 19:46:26 and read 32767 times.

I cannot believe this thread has almost 900 posts.....

It seems like some of the people don't know what makes an airplane fly, why there are engines and wheels on the aircraft.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-12-01 19:49:54 and read 32767 times.

Quoting Kay (Reply 330):
This is not a situation where a plane can take off. This should be the consensus.

This is a situation which can't happen so whether the plane would take off is irrelevant, undefined.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Bond007
Posted 2005-12-01 20:04:47 and read 32767 times.

Quoting XORDLLAA (Reply 333):
It seems like some of the people don't know what makes an airplane fly, why there are engines and wheels on the aircraft.

So are you saying it'll fly?

Or are you agreeing it's a scenario that could never happen?

I'm sure that most of the people that say it won't fly, and those that say it will, all understand how a plane flies. Under the circumstances of the way the question is described ... it can never happen.

Physics and Aerodynamics says the plane will move forward, get enough lift, and fly ... but not if the wheels are going the same speed as the conveyor ... both cannot happen!!


Jimbo

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: XORDLLAA
Posted 2005-12-01 20:05:47 and read 32767 times.

...and yet, I don't know how to count  Sad I'm sorry, small correction - "over 300 posts"

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Sovietjet
Posted 2005-12-01 20:30:13 and read 32767 times.

What have I created.....

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: XORDLLAA
Posted 2005-12-01 20:31:46 and read 32767 times.

Quoting Bond007 (Reply 335):
So are you saying it'll fly?

Or are you agreeing it's a scenario that could never happen?

Under the circumstances given in thread starter it will NOT fly. It is impossible.

It is like arguing: how far can you jump, while running on gym's equipment...

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: VuelingAirbus
Posted 2005-12-01 21:23:05 and read 32767 times.

can someone here please make a list of people who think it can fly opposed to one with people who say it can't? I will put all people who say it can't fly on my respected user list...

Its like holding a magnet in front of a little metal car which will start moving. Attach the magnet in front of the car with a physical connection and i won't anymore (what a trick). The belt and the plane has the same relationship.

Imagine that the you are on the end of the runway (belt) and your calculated lift of speed is 150 knots. now we start the belt and its set to maintain a speed opposit to the intended flight path with exactly 150 knots. Now you apply thrust and you accelarate to 150 knots and now you cut power on the engines to the point where you would maintain 150 knots. A person standing next to the belt would see you as beeing static at exactly one spot. That ladies and gentlemen was discovered by Einstein already....

So everyone saying it cant fly : Welcome to my respected user list

Everyone saying it will fly : go back to highschool/college and give back your pilot licence if you hold one...

rgds

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Ralgha
Posted 2005-12-01 21:23:06 and read 32767 times.

Quoting ZSOFN (Reply 302):
The ground effect caused by the airflow created by the fast-moving conveyor belt causes the plane to lift off and hover close to the belt

That's not what ground effect is.

Quoting XORDLLAA (Reply 338):
It is like arguing: how far can you jump, while running on gym's equipment...

Invalid comparison. When you jump, you are propelling yourself via the ground. Airplanes do not do this.


Everyone who says it's an impossible situation is correct, but for the wrong reasons. It is impossible because the belt CAN NOT EVER MATCH THE SPEED OF THE WHEEL. The airplane WILL FLY. I've explained why this is the case repeatedly.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Ralgha
Posted 2005-12-01 21:28:54 and read 32767 times.

Quoting VuelingAirbus (Reply 339):
Its like holding a magnet in front of a little metal car which will start moving. Attach the magnet in front of the car with a physical connection and i won't anymore (what a trick). The belt and the plane has the same relationship.

It's not at all like this. If you attach the magnet to the car, it is a rigid attachment. Free spinning wheels DO NOT ATTACH ANYTHING TO ANYTHING ELSE. The wheels are free spinning, you can not transmit force through a free spinning wheel simply by spinning it.

Quoting VuelingAirbus (Reply 339):
Imagine that the you are on the end of the runway (belt) and your calculated lift of speed is 150 knots. now we start the belt and its set to maintain a speed opposit to the intended flight path with exactly 150 knots. Now you apply thrust and you accelarate to 150 knots and now you cut power on the engines to the point where you would maintain 150 knots. A person standing next to the belt would see you as beeing static at exactly one spot. That ladies and gentlemen was discovered by Einstein already....

When you hit 150 knots ground speed and were static to an outside observer, your airspeed would be zero. You would need zero thrust to maintain this condition (discounting bearing friction, add that in and you need minimal thrust to maintain this condition). There is no airplane that takes off at zero thrust. You maintain takeoff power through the roll and after liftoff.

Repeat after me:

Groundspeed is irrelevant
Groundspeed is irrelevant
Groundspeed is irrelevant
Groundspeed is irrelevant
Groundspeed is irrelevant
Groundspeed is irrelevant
Groundspeed is irrelevant
Groundspeed is irrelevant

[Edited 2005-12-01 21:32:26]

[Edited 2005-12-01 21:36:25]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: A346Dude
Posted 2005-12-01 22:06:43 and read 32767 times.

340 posts and counting and yet the "no" crowd still doesn't get it.

The fact that the belt is moving backwards at the same speed that the wheels are moving forwards simply does not mean the plane will not move. Why? The engines are what provide the forward force, not the wheels. The wheels are able to spin freely, which they will as a result of the belt moving backwards. In the meantime, the thrust from the engines, being the sole force acting on the airplane, is free to accelerate it forward.

The crux of this problem is that the belt is not acting against the frame of the airplane. It is simply pushing backwards against the bottom of the tires. The result is this: the bottom of the tires feel a rearward force, the top of the tires feel a forward force, and the wheel axles, which lie at the centre of the tires and are rigidly connected to the rest of the airplane, feel zero force.

Let's sum up the forces. The force exerted by the belt on the wheel axles, and hence the rest of the airplane, is zero - no matter how fast the belt is moving (if, as we must assume, there is no friction at the wheel axles). The forward force exerted by the engines is significant. There is a net unbalanced force acting forwards on the airplane, so by Newton's Second Law the plane must accelerate and take off. It really is that simple, and I am afraid there are some who will simply never get it.

[Edited 2005-12-01 22:10:41]

[Edited 2005-12-01 22:27:14]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Wingscrubber
Posted 2005-12-01 22:08:48 and read 32767 times.

Sod the airplane, but a helicopter on the conveyer belt, that'd bloody well take off. This thread is pointless.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Mdaigle
Posted 2005-12-01 22:14:03 and read 32767 times.

Quoting Sovietjet (Thread starter):
The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation

If this is taken at face value, this just means the belt will stop the wheel from turning and the "ground" will move in the same direction as the aircraft. Since the wheels are freewheeling, it will not help or hinder the aircraft from taking off, just look really bizzare from the tower.

Michel

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Keta
Posted 2005-12-01 22:21:56 and read 32767 times.

Quoting VuelingAirbus (Reply 339):
Imagine that the you are on the end of the runway (belt) and your calculated lift of speed is 150 knots. now we start the belt and its set to maintain a speed opposit to the intended flight path with exactly 150 knots. Now you apply thrust and you accelarate to 150 knots and now you cut power on the engines to the point where you would maintain 150 knots. A person standing next to the belt would see you as beeing static at exactly one spot. That ladies and gentlemen was discovered by Einstein already....

I'm not going to try to explain again why the airplane would move. Read again SlamClick, David L and many other's posts, they're very clear.

In the situation you're saying the airplane will move. The situation of the thread starter is a stupidity.

Quoting VuelingAirbus (Reply 339):
Everyone saying it will fly : go back to highschool/college and give back your pilot licence if you hold one...

Maybe you should go to university then. Is that the physics you are told in a pilot school?

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Wingscrubber
Posted 2005-12-01 23:29:25 and read 32767 times.

Put a helicopter on the conveyer belt, that'll bloody well take off! This thread is so pointless...!

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: OE-LDA
Posted 2005-12-01 23:34:10 and read 32767 times.

Quoting VuelingAirbus (Reply 339):
Everyone saying it will fly : go back to highschool/college and give back your pilot licence if you hold one...

If someone can proof to me that the plane does not take off, then I will voluntarily return my pilots license.

Regards, OE-LDA

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: VuelingAirbus
Posted 2005-12-02 00:04:53 and read 32767 times.

Lets make it easier for everyone. Lets take a cart on ice skates with a big engine on top and if you apply thrust it would move (even if you move the ice surface in the opposite direction). If the cart (plane) would not be on a runway (belt) but on a gear wheel (like in an old watch) you cover a certain amount of gears as you accelerate. Now if you accelerate the gear wheel in the opposite direction the cart (plane) stays static. Thats just the way it is. it stays static and does not lift off.

Quoting Keta (Reply 345):
Maybe you should go to university then. Is that the physics you are told in a pilot school?

Thanx - i did have math and physics as major in college and studied aeronautical engeneering in uni...

rgds

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: DarkBlue
Posted 2005-12-02 00:17:29 and read 32767 times.

Quoting Ralgha (Reply 340):
Everyone who says it's an impossible situation is correct, but for the wrong reasons. It is impossible because the belt CAN NOT EVER MATCH THE SPEED OF THE WHEEL. The airplane WILL FLY. I've explained why this is the case repeatedly.



Quoting A346Dude (Reply 342):
340 posts and counting and yet the "no" crowd still doesn't get it.

Or maybe the "yes" crowd needs to step back and think about what you are saying. All here seem to agree that the question is flawed and is an impossible situation. Then why are you so certain that you are right and we are wrong over an impossible situation?

The reason why is that every single time you say that the aircraft will move down the runway, you are not answering the question but rather changing the question to make it answerable.

The "no" crowd are beyond that point. We recognize that using your adapted question, yes the plane will fly. So please do not belittle us. We just choose to answer the actual question as presented as no, this is not a valid situation, thus the plane will not fly.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: ZSOFN
Posted 2005-12-02 00:17:58 and read 32767 times.

Quoting Ralgha (Reply 340):
That's not what ground effect is.

Ok, but is my point not valid? Would a fast-moving large conveyor belt not create airflow?

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: A346Dude
Posted 2005-12-02 00:19:17 and read 32767 times.

Sorry, but you just proved that the aircraft will take off. Your ice skate analogy is an accurate representation of the problem we are discussing, since the wheels are able to spin freely. The wheels have nothing to do with the forward motion of the aircraft, they are but a mere by-product of that motion, and spinning them in reverse (as the belt does), does not hold the airplane back. You said it yourself, the ice skate, or airplane in the question posed, would move.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: VuelingAirbus
Posted 2005-12-02 00:26:55 and read 32767 times.

To clarify my previous point. The plane does accelarate - and thats out of discusion. So if the plane sits on ice skates it will accelarate depending on the thrust of the engines. However - the plane in our example is on wheels. The wheels need to spin to cover a certain distance (one rotation gives you lets say two meters of distance). So if you turn the surface opposite to the spining of the wheels you coverd a distance on the moving surface BUT NOT relative to your environment. The environment is the air and therefor you dont create lift.

And for the mathmaticians its easy follow too. Accelaration is speedgain per time increment - so if you go infinitive you just multiply by time and you can compare the airplane speed with opposite belt speed. If both speeds are the same but in different directions the plane is static for an observer standing next to the belt. A static aircraft has zero lift and the big conclusion: IT AIN'T GONNA FLY....

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Keta
Posted 2005-12-02 00:38:54 and read 32767 times.

Quoting VuelingAirbus (Reply 348):

Well I can't agree with you. The belt and wheels work the same way the ice does i.e. there is no friction thus no force acting against the airplane.

OK another try...

You have an object on wheels, standing on a belt. Everything is still. Now you accelerate the belt. What happens to the object? It doesn't move. The wheels will start rotating with the same acceleration as the belt, but that does not transmit anything to the airplane since there is no friction on the axes. There is no force acting on the object, therefore it will not move.

Take a paper and put a small car (a toy) on it. Now pull from the paper, and (if you do it quickly) the car doesn't move. You have to pull quickly because we live in a real world and there exists friction on the axes; but if you pull hard you will break the static friction and will only have dynamic friction, which is almost inexistent. In this scenario you can see that the belt means nothing to the object's movement.

If you think it the other way, i.e. you apply a force to the plane (thrust) the airplane will move regardless of what the belt does.

I'm repeating what people have said, but whatever...

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: ZSOFN
Posted 2005-12-02 00:47:30 and read 32767 times.

Whilst I want this thing to fly and generally agree with the theories of those who think it will, these supporters seem to overlook the initial rule and the consequences it has. The conveyor belt is an impossibility, and thus we have to make accommodation for that in our logic.

Whether the plane flies or not, it will not move forwards or backwards relative to the earth until it has left the conveyor belt. Any forward or backward movement could not occur on the belt because the wheels are free spinning and by nature of the puzzle the speeds have to cancel each other out, thus creating zero groundspeed.

IN SHORT, THE ONLY WAY THE AIRCRAFT CAN MOVE ON THE BELT IS IF THE SPEED OF THE BELT NO LONGER EQUALS THAT OF THE WHEELS and this is prohibited by the rules of the puzzle. THIS IS ESSENTIAL! Whilst the aircraft is on the belt, it's going nowhere apart from potentially straight up...

The ONE way that this bird can fly is by rising up and sitting on the airflow created by a fast-moving belt.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: NAV20
Posted 2005-12-02 01:34:47 and read 32767 times.

Because the wheels are causing so many misunderstandings, I thought I might update my walkway analogy to include wheels - by introducing a baggage cart.

You step on a moving walkway, pulling a baggage cart. And then realise that you're going the wrong way and start walking back. But walking's no use - the walkway is moving you the wrong way at say 3 mph, if you just walk you're only 'standing still'.

So you break into a trot, pushing the cart. That's better, you (and the cart) are making 6 mph against the walkway's three. So you're getting back to where you want to be at a net 3 mph.

But now imagine that a house mechanic with an evil sense of humour is watching your performance; and he just happens to have his hand on the speed lever of the walkway. He jacks the walkway speed up to 6 mph and you're 'jogging on the spot' again.

You break into a full run, desperately pushing your cart. But the mechanic just grins nastily, and increases the walkway speed just enough to match your running speed exactly...

So now your legs are the engines, the baggage-cart is the aeroplane (complete with undercarriage), and the mechanic is the conveyor speed sensors.

You (and the cart) are both 'moving' relative to the conveyor-belt. But NOT in relation to anything else. And the wheels make no difference to that situation. Nor are they 'driving' anything.

[Edited 2005-12-02 01:38:11]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-12-02 01:43:39 and read 32767 times.

Quoting ZSOFN (Reply 354):
Whilst I want this thing to fly and generally agree with the theories of those who think it will, these supporters seem to overlook the initial rule and the consequences it has. The conveyor belt is an impossibility,

I don't think that's generally true. Pretty much every "yes" merchant has stated, quite some time ago, that the belt is an impossibility. I, for one, have also stated, several times, that I lost interest in the question once it was established that the wording was as intended, and can only be bothered to discuss with those who think the aircraft would not take off if the belt were to oppose the aircraft's speed.  Smile

Quoting ScarletHarlot (Reply 274):
My brain hurts.

Mine did for a while but it's gone numb now.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: NAV20
Posted 2005-12-02 02:41:06 and read 32767 times.

Oddly enough, I don't think it would be mechanically impossible to set this up (though entirely pointless).

If it was a light aeroplane its maximum speed might be as little as 100 knots. Once it reached that speed (relative to the conveyor-belt) it would go no faster. The conveyor belt, of course, would be moving in the opposite direction, also at 100 knots. From that point on equilibrium would be maintained until either the aeroplane or the conveyor ran out of fuel.

Throughout the exercise, though, the aeroplane's AIRSPEED would remain at zero. So no lift, and no takeoff.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: VuelingAirbus
Posted 2005-12-02 03:08:18 and read 32767 times.

Quoting OE-LDA (Reply 347):
If someone can proof to me that the plane does not take off, then I will voluntarily return my pilots license.

Give me your licence number and I will gladly arrange it for you. Another thought just crossed my mind. Assume the airplane does not lift off. I accelerate the airplane to its maximum speed (lets say 340 kts) and I also leave the engine at a power setting to maintain speed. Now i start moving the ground with 340 kts in the opposite direction. What lift will the wings create? ZERO!!! Why? cause the airplane is static!!!

Oh come on you guys who said yes - just admit you were wrong!

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: A346Dude
Posted 2005-12-02 03:19:44 and read 32767 times.

Your baggage cart analogy is not a fair one, since feet are not wheels. Essentially, your analogy represents the problem posed initially, except that the brakes are locked so the tires cannot rotate. This fundamentally changes the problem. Feet are like wheels that cannot spin, and if the wheels cannot spin, the force from the moving belt can transmit through the rubber in the tires, and apply a rearward force on the wheel axles and the rest of the airplane.

In the problem posed, however, the wheels are free to spin, so the force from the moving belt does not transmit to the wheel axles, nor the rest of the airplane. That is why I continue to contend that the plane will take off - regardless of the speed of the belt. That's right, even if the belt were set to move at the speed of light, if we assume that there is no friction in the wheel bearings, and that the tires don't explode, the airplane will still take off - its wheels rotating at the RPM equivalent to the speed of light plus ~150 knots (the normal rotation speed of the airplane). (Edit: Yes, I realize this is breaking the initial conditions posed, but I am simply using it to illustrate my point)

Quoting DarkBlue (Reply 349):
Or maybe the "yes" crowd needs to step back and think about what you are saying. All here seem to agree that the question is flawed and is an impossible situation. Then why are you so certain that you are right and we are wrong over an impossible situation?

I have been wrong when I was absolutely certain about something enough times before to recognize that I could very well be wrong about this. What I do know is that I have spent a ridiculous amount of time thinking about this problem - especially considering I have a mechanics exam next week on real-world, practical problems, that I could have been studying for instead.

In any case, we've gone back and forth a lot here, and I think that at this point no one is going to change their mind either way, so I'm not sure there's much point in continuing. It's been fun though!

[Edited 2005-12-02 03:25:21]

[Edited 2005-12-02 03:30:06]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Zarniwoop
Posted 2005-12-02 04:38:54 and read 32767 times.

Here goes.....

Taking the "ideal world" case, as some other people have already said, this situation is impossible. Let me explain.
There are 2 completely independant events here:

1: The wheels and the conveyer belt
2: The body of the plane itself and the connected engines

The 2 events are independent because the wheels are connected to the airplane by frictionless bearings (ideal world).

Taking event 1, as the wheels start to spin, the conveyer belt instantly spins at the same speed. In this case the wheel cannot theoretically move relative to the belt.

Taking event 2, the body of the plane is pulled through the air by the engine which is completely independent of what is going on with the ground.

So event 1 says the plane cannot move, event 2 says the plane can move.
So in an "ideal world" case the situation is an impossible one to reconcile and both the "YES" camp and the "NO" camp are both correct and incorrect....

But we do not live in an ideal world!

In reality there would be a delay in the treadmill reacting to the wheels starting to move. The treadmill would also have a max speed so once the power is applied to the engine the plane would start to move, the treadmill would quickly get up to max speed. Once the belt is at its max speed, the wheels can spin quicker than the belt speed so the plane would continue to accelerate and take off. There will be some friction at the bearings but it would be small compared to the force that the engine produces.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Bri2k1
Posted 2005-12-02 05:19:51 and read 32767 times.

Bloody Bollocks!

Quoting Zarniwoop (Reply 360):
the wheel cannot theoretically move relative to the belt

If the wheel rotates about its axis, does it move or not? Answer that question first.

If the wheel rotates about its axis, can its center of rotation translate in a plane? Answer that question second.

Now, if the wheel rotates about its axis, does the aircraft change its position coordinates?

Next, if the wheel's center of rotation translates in a plane, does the aircraft change its position coordinates?

I'd like to encourage the following:

1.) Describe a symptom or condition
2.) Explain a hypothesis that theoretically produces said condition
3.) Propose a test of the hypothesis in step #2
4.) Analytically present the success or failure of the test in step #3

If anyone wishes to demonstrate an alternative method of validating the original question (does anyone remember the original question?) please present it here.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: NAV20
Posted 2005-12-02 05:27:56 and read 32767 times.

Quoting Zarniwoop (Reply 360):
Once the belt is at its max speed, the wheels can spin quicker than the belt speed so the plane would continue to accelerate and take off.

But what if the belt's max. speed is equal to or greater than that of the aeroplane, Zarniwoop - which we must assume from the original question?

I think even using the word 'aeroplane' causes confusion. Until it reaches flying speed it is a 'vehicle' - and as ground-and-gravity-bound as any other vehicle.

Suppose that, instead, it was a jet-propelled car? No-one would expect that to move? Indeed, assuming no friction, you wouldn't even need to posit the conveyor - imagine any jet-propelled vehicle running on rollers?

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Keta
Posted 2005-12-02 10:03:30 and read 32767 times.

Quoting NAV20 (Reply 357):
If it was a light aeroplane its maximum speed might be as little as 100 knots. Once it reached that speed (relative to the conveyor-belt) it would go no faster. The conveyor belt, of course, would be moving in the opposite direction, also at 100 knots. From that point on equilibrium would be maintained until either the aeroplane or the conveyor ran out of fuel.

Please read SlamClick's posts. Don't they make any sense to you?

Quoting VuelingAirbus (Reply 358):
Assume the airplane does not lift off. I accelerate the airplane to its maximum speed (lets say 340 kts) and I also leave the engine at a power setting to maintain speed. Now i start moving the ground with 340 kts in the opposite direction. What lift will the wings create? ZERO!!! Why? cause the airplane is static!!!

Wrong!! First you accelerate the airplane to 340 knots relative to the air or ground, they're both still. So you have an airplane running at 340 knots. Now you start running the belt in the opposite direction. Are you saying the plane's gonna stop? No! No matter what speed you put the belt at, 200 340 or 500 knots, the airplane will continue running at 340 knots relative to the world - just like it was before running the belt. The only thing you will see is the wheels spining faster.

Just curiosity, did you study aeronautical engineering at Madrid?

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: ZSOFN
Posted 2005-12-02 10:21:55 and read 32767 times.

Can we assume any airflow to be created by the moving belt?

By the way conveyor belts are often used in wind tunnels as they can affect airflow quite considerably.

Because of this, if the belt moves fast enough, the aircraft will have an airspeed, even though it will not relative to the ground. No-one seems to be commenting on this. Do I not have a point?

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-12-02 10:41:40 and read 32767 times.

Quoting NAV20 (Reply 355):
You step on a moving walkway, pulling a baggage cart.

Sorry, NAV20 but... NO!  
That represents a car towing a trailer. The motive power is still provided by transmitting force from your feet to the belt. Your feet (car wheels) are driving you.

Quoting NAV20 (Reply 357):
If it was a light aeroplane its maximum speed might be as little as 100 knots. Once it reached that speed (relative to the conveyor-belt) it would go no faster. The conveyor belt, of course, would be moving in the opposite direction, also at 100 knots. From that point on equilibrium would be maintained until either the aeroplane or the conveyor ran out of fuel.

I think you and I are both talking about the belt opposing the aircraft's speed. In that case, the belt's motion would not cancel the aircraft's speed, for all the reasons given above.

Quoting NAV20 (Reply 362):
Suppose that, instead, it was a jet-propelled car? No-one would expect that to move? Indeed, assuming no friction, you wouldn't even need to posit the conveyor

Yes, lots of us would expect it to move, assuming the jet-propelled car was only powered by the jet engine and not by its wheels.

Quoting NAV20 (Reply 362):
imagine any jet-propelled vehicle running on rollers?

Like a jet aircraft?  

[Edited 2005-12-02 10:50:17]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Keta
Posted 2005-12-02 10:53:58 and read 32767 times.

Quoting ZSOFN (Reply 364):
Because of this, if the belt moves fast enough, the aircraft will have an airspeed, even though it will not relative to the ground. No-one seems to be commenting on this. Do I not have a point?

You're right, but I think that's too complicated. I mean, the question and assumptions are very basic, there is no need to consider everything. Are you going to consider too the effect of the moon, or in what position of earth is the aircraft placed? I think it's fair if we think there is no interaction between the belt and the air.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: ZSOFN
Posted 2005-12-02 11:07:44 and read 32767 times.

Quoting Keta (Reply 366):
You're right, but I think that's too complicated. I mean, the question and assumptions are very basic, there is no need to consider everything. Are you going to consider too the effect of the moon, or in what position of earth is the aircraft placed? I think it's fair if we think there is no interaction between the belt and the air.

OK, fair enough. If we forget about airflow generated, then I got to say that the aircraft's going nowhere.

David L, remember the initial conditions of the experiment:

The speed of the conveyor belt must be exactly opposite & equal to that of the wheels at any time. Therefore, BY DEFINITION, the speeds must cancel eachother out.

Thus the only way that the aircraft could move is if the speed of the wheels is no longer equal to the speed of the belt, which we are told is impossible by those same conditions of the experiment. It has to remain that simple.

People, notice that for this experiment, an impossible new DEFINITION is created.

speed of wheels speed of belt (that means "is equivalent to, in all situations"

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Kay
Posted 2005-12-02 11:26:55 and read 32767 times.

Quoting Keta (Reply 363):
Wrong!! First you accelerate the airplane to 340 knots relative to the air or ground, they're both still. So you have an airplane running at 340 knots. Now you start running the belt in the opposite direction. Are you saying the plane's gonna stop? No! No matter what speed you put the belt at, 200 340 or 500 knots, the airplane will continue running at 340 knots relative to the world - just like it was before running the belt. The only thing you will see is the wheels spining faster.

Keta, there are two groups, and I think you are describing the scneario where the belt will match the speed of the plane. In that case yes it will fly.

There's some of us who were arguing about the hypothesis that the belt is running at the speed of the wheels, not the plane. Meaning that the wheel's rotating speed is countered exactly by the belt, not the body of the plane's speed. These are two different scenario.


So here is my vote:

Scenario 1:
If the belt moves backwards matching exactly the rotating speed of the wheels, mathematically the wheels cannot move forward in this space without skidding. Therefore the plane is stuck, an impossible situation with impossible conditions. No take-off.


Scenario 2:
If the belt moves backwards matching exactly the speed of the body of the plane, the plane will take off normally except that its wheels will be spinning exactly double what they are supposed to. Assuming these are slightly better than McLaren's Michelin tires ( Wink), the plane will take off.


Two different scenarios.
People might be arguing against one scenario with people who are arguing for the other scenario.


Kay

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-12-02 11:32:03 and read 32767 times.

Quoting ZSOFN (Reply 367):
David L, remember the initial conditions of the experiment:

The speed of the conveyor belt must be exactly opposite & equal to that of the wheels at any time. Therefore, BY DEFINITION, the speeds must cancel eachother out.

And let me say, yet again, that the question as stated is of no interest to me anymore (it just can't happen) but I'm perfectly happy to continue discussing the "real world" problem of a conveyor belt which opposes the aircraft's speed with anyone who thinks that would stop the aircraft taking off.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Killjoy
Posted 2005-12-02 11:32:10 and read 32767 times.

Quoting VuelingAirbus (Reply 348):
Thanx - i did have math and physics as major in college and studied aeronautical engeneering in uni...

Good luck finding a job and congratulations for not being able to spell your own profession.

I'm not going to take part in the debate as the initial conditions are impossible, but the amount of stupidity and arrogance in this thread is astounding. The argument should be about the rationality of considering massless wheels at infinite speeds around frictionless axles, not about cars vs. airplanes. Christ...

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Keta
Posted 2005-12-02 12:15:16 and read 32767 times.

Quoting Kay (Reply 368):
Scenario 1:
If the belt moves backwards matching exactly the rotating speed of the wheels, mathematically the wheels cannot move forward in this space without skidding. Therefore the plane is stuck, an impossible situation with impossible conditions. No take-off.


Scenario 2:
If the belt moves backwards matching exactly the speed of the body of the plane, the plane will take off normally except that its wheels will be spinning exactly double what they are supposed to. Assuming these are slightly better than McLaren's Michelin tires ( ), the plane will take off.

I fully agree. I know that most of us agree that the initial question is impossible, but there is still people like NAV20 and VuelingAirbus who say that the plane will stop if the belt moves the same speed as the plane (read 358) and are still using the analogy of a person on a treadmill. There is no more arguing with you guys, they're them who I'm trying to convert

[Edited 2005-12-02 12:17:23]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-12-02 12:47:36 and read 32767 times.

Quoting Sovietjet (Thread starter):
hey guys there is a huge debate over at another forum concerning this question...

I wonder if there's another anomaly in the opening thread. Can we have links to the same discussion elsewhere? I can only find discussions about the conveyor belt's speed being equal to and opposite to the aircraft's speed.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: DarkBlue
Posted 2005-12-02 15:44:02 and read 32767 times.

Quoting ZSOFN (Reply 364):
Can we assume any airflow to be created by the moving belt?

By the way conveyor belts are often used in wind tunnels as they can affect airflow quite considerably.

Because of this, if the belt moves fast enough, the aircraft will have an airspeed, even though it will not relative to the ground. No-one seems to be commenting on this. Do I not have a point?

I've mentioned this previously. This is a form of Couette flow.

Quoting Keta (Reply 366):
You're right, but I think that's too complicated. I mean, the question and assumptions are very basic, there is no need to consider everything. Are you going to consider too the effect of the moon, or in what position of earth is the aircraft placed? I think it's fair if we think there is no interaction between the belt and the air.

Nope, not too complicated, it's a very real phenomenon and is fundamental topic in any viscous aerodynamics course. Technically, Couette flow is defined as laminar viscous flow between two plates, one stationary one moving, but in principle the moving flow induced by the conveyor is known as Couette flow.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: VuelingAirbus
Posted 2005-12-02 16:09:05 and read 32767 times.

Quoting Killjoy (Reply 370):
Good luck finding a job and congratulations for not being able to spell your own profession

Well - what can I say except that it was a typo and I am not an English native speaker. I didn't know I have to pass a grammar exam to be able to post in that forum. Other than that I have a job as captain on A320 based in BCN and I quite like it. So i appreciate your concerns but they are not needed nor welcome.

RGDS

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: VuelingAirbus
Posted 2005-12-02 16:25:55 and read 32767 times.

Quoting Keta (Reply 363):
Just curiosity, did you study aeronautical engineering at Madrid?

No - at the technical university in Munich

Lets brin in another example. I put a plane on the belt with parking brake on. I let the belt run (lets say 150 kts). So the plane on the belt stays at its position and moves with 150 kts backwards for an observer (lets say on the tower). Now I release the brakes and apply thrust. The plane starts moving and will accelerate. When it reaches 150 kts (messured on the wheels cause it will have no airflow) it will look static for the observer on the tower. So there isn't any airflow whatsoever which could create lift over the wings. If the belt keeps its speed I continue to accelerate and I will reach 150 kts in respect to the wind (so I could fly) and 300 kts towards the ground (in that case belt). Lets assume I start accelerating the belt with the same rate the plane accelerates after reaching 150 kts the plane will increase speed towards the ground BUT remain static for the observer in the tower...

Come on guys - noone can argue with that! (and of course we live in a ideal world where the belt doesn't create any kind of wind and so on)

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-12-02 18:23:05 and read 32767 times.

Quoting VuelingAirbus (Reply 375):
Lets assume I start accelerating the belt with the same rate the plane accelerates after reaching 150 kts the plane will increase speed towards the ground BUT remain static for the observer in the tower...

Come on guys - noone can argue with that!

Oh yes they can (well, it is panto season)!  

I assume you're adressing the "conveyor belt matches aircraft speed" situation. How does the belt transfer its motion to the aircraft through unbraked wheels? Have a look at the example here:

Quoting David L (Reply 304):

If you're addressing the "conveyor belt matches wheel speed" situation, then ignore me - I'm done with that one.  

[Edited 2005-12-02 18:25:35]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-12-02 18:33:40 and read 32767 times.

Quoting VuelingAirbus (Reply 375):
I put a plane on the belt with parking brake on. I let the belt run (lets say 150 kts). So the plane on the belt stays at its position and moves with 150 kts backwards for an observer (lets say on the tower). Now I release the brakes and apply thrust.

OK, I misunderstood - so you're starting with an airspeed of -150 kts? That's not supposed to happen - the belt is supposed to match and oppose the speed the whole time, not just after the aircraft has reached a backward airspeed of 150 kts! With a starting airspeed of -150 kts it's going to take longer to reach +150 kts airspeed.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Killjoy
Posted 2005-12-02 20:22:58 and read 32767 times.

Quoting DarkBlue (Reply 373):
Nope, not too complicated, it's a very real phenomenon and is fundamental topic in any viscous aerodynamics course. Technically, Couette flow is defined as laminar viscous flow between two plates, one stationary one moving, but in principle the moving flow induced by the conveyor is known as Couette flow.

I'm sure you're right, but clearly the problem didn't have this in mind. Perhaps it would be better to ignore the wings and just ask whether the plane will move in relation to an observer standing on a stationary surface.

Quoting VuelingAirbus (Reply 374):
Well - what can I say except that it was a typo and I am not an English native speaker. I didn't know I have to pass a grammar exam to be able to post in that forum. Other than that I have a job as captain on A320 based in BCN and I quite like it. So i appreciate your concerns but they are not needed nor welcome.

The insult was there to imply that you're not really an engineer, not just to be an ass about spelling.

Quoting VuelingAirbus (Reply 375):
Lets brin in another example. I put a plane on the belt with parking brake on. I let the belt run (lets say 150 kts). So the plane on the belt stays at its position and moves with 150 kts backwards for an observer (lets say on the tower). Now I release the brakes and apply thrust. The plane starts moving and will accelerate. When it reaches 150 kts (messured on the wheels cause it will have no airflow) it will look static for the observer on the tower. So there isn't any airflow whatsoever which could create lift over the wings. If the belt keeps its speed I continue to accelerate and I will reach 150 kts in respect to the wind (so I could fly) and 300 kts towards the ground (in that case belt). Lets assume I start accelerating the belt with the same rate the plane accelerates after reaching 150 kts the plane will increase speed towards the ground BUT remain static for the observer in the tower...

Come on guys - noone can argue with that!

Your example isn't equivalent to the problem. If you're really an engineer, you should be able to see that.

The issue in the riddle is that it creates an impossible situation where any movement immediately accelerates the wheels and belt to infinity. Several people have explained this. Depending on how we want to treat the issue, the plane will either take off or make the universe explode.

In your example, you tie belt velocity to plane velocity. This means that the wheels will simply spin or skid faster. The plane will accelerate relative to the air, and take off.

If you persist in your reasoning, please explain how you think the belt is able to transfer enough force to the plane through the wheels to counter the thrust of, say, a GE90. The brakes are of course not engaged.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Lehpron
Posted 2005-12-02 21:44:19 and read 32767 times.

Quoting Sovietjet (Thread starter):
Imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
There is no wind.
Can the plane take off?

No, the wind speed the wings see would be zero, hence no lift. So what if the engines are on full countering the rolling drag of a conveyer belt, doesn't mean it will fly. When cars are tested for smog, they run their wheels on rollers just to run up the engine; is the car dong 45 miles an hour really? Will it hit a wall and crash? of course not, it aint moving, less the roller suddenly stops. Then there is a conversion from stored knetic energy to applied energy.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: VuelingAirbus
Posted 2005-12-02 22:00:48 and read 32767 times.

Maybe I am missing something. In normal conditions I am taking off with a certain acceleration rate it takes me about 30 seconds and i cover 2000 meters to reach 150 kts relative to the sourounding air. Now I am taking off again but this time the ground was moving 2000 meters in the opposite direction so after 30 seconds I coverd a ground distance of 2000 meters and I have a ground speed of 150 kts BUT what a suprise I am still in the same spot where I started relative to the air.

Quoting Killjoy (Reply 378):
If you persist in your reasoning, please explain how you think the belt is able to transfer enough force to the plane through the wheels to counter the thrust of, say, a GE90. The brakes are of course not engaged.

My powerfull GE90 did accelerate me - they have done an excellent job. I have done 2000 meters within 30 seconds, but to lift off I need lift and since I am still in the same spot I doubt that there is messurable airflow over the wings let alone say enough to lift me up.

So what am I or you missing? My point is that if you mount the engine on a hover craft you would accelerate. If the plane is on ice skate it would also accelerate but not on wheels.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-12-02 22:05:17 and read 32767 times.

Quoting Lehpron (Reply 379):
No, the wind speed the wings see would be zero, hence no lift. So what if the engines are on full countering the rolling drag of a conveyer belt, doesn't mean it will fly. When cars are tested for smog, they run their wheels on rollers just to run up the engine; is the car dong 45 miles an hour really? Will it hit a wall and crash? of course not, it aint moving, less the roller suddenly stops. Then there is a conversion from stored knetic energy to applied energy.

Oh dear! Be honest - you haven't read the posts in between, have you?  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-12-02 22:15:29 and read 32767 times.

Quoting VuelingAirbus (Reply 380):
If the plane is on ice skate it would also accelerate but not on wheels.

Ha! If the plane is on skates it would also accelerate. So, it's summer and you want to "ice-skate" on concrete. What do you do? You take off your ice-skates and put on a pair of... roller-skates! Unpowered wheels! What does an aircraft have between it and the runway? Unpowered wheels!

Lehpron: if this isn't the most quoted post on a.net it soon will be!

Quoting SlamClick (Reply 149):
The wheels don't drive the airplane.
The wheels don't drive the airplane.
The wheels don't drive the airplane.
The wheels don't drive the airplane.
The wheels don't drive the airplane.

The error is in your thinking. You are still thinking "car" and you can't solve the problem that way.


[Edited 2005-12-02 22:20:33]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: VuelingAirbus
Posted 2005-12-02 23:36:01 and read 32767 times.

Quoting David L (Reply 382):
Unpowered wheels! What does an aircraft have between it and the runway? Unpowered wheels!

You would be right if the belt would match the speed of the aircraft. If the aircraft moves 150 kts in one direction but the belt with 150 kts in the other direction the tire speed would be 300 kts and the airplane would lift off.

But the original quote is the following:


Quoting Sovietjet (Thread starter):
The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation

So speed is distance coverd divided by the time needed. Mathmatically i can multiply time (cause belt and tire speed are matching) and I compare distance by distance. Meaning after 30 seconds of acceleration plane moves 1500 meters in one direction and belt 1500 meters in the other direction but since plane sits on the belt I MUST conclude that it didn't move compared to the surrounding air. PROVE ME WRONG!!!

The way the question is phrased by the threat starter the plane can't take off. If you would like to see what scientist say visit the following link:

http://forum.physorg.com/index.php?showtopic=2417&st=0

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-12-02 23:56:02 and read 32767 times.

Quoting VuelingAirbus (Reply 383):
But the original quote is the following:

OK, we can stop right there because:

Quoting David L (Reply 376):
If you're addressing the "conveyor belt matches wheel speed" situation, then ignore me - I'm done with that one.

 Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: VuelingAirbus
Posted 2005-12-03 00:05:44 and read 32767 times.

No matter what source of power I have or whether the wheels are free spining or not. The plane taxies with 20 kts and the belt underneath moves with 20 kts in the opposite direction means I stand still to an observer on the tower. As long as the belt adjusts to the wheel speed. I increase power and accelerate to 100 kts and keep it and the belt does the same so the result for the observer on the tower is that I still stand still. My tire speed is only 100 kts in that case!!! YES - messured on the wheels I have 100 kts ground speed compared to the belt surface. However - If the plane has a speed of 100 kts (compared to an observer on the tower) and the belt runs with 100 kts in the opposite direction than I have a resulting tire speed of 200 kts for the observer on the tower. That is in contradiction to the initial post. Because in order to lift of I need 150 kts and if the belt matches that speed I have a tire speed of 300 kts (since the belt is supposed to move in the opposite direction). But the initial post clearly says that the belt matches wheel speed and not the airplane speed. Everyone who says the plane lifts off please read the initial post carefully again!!!



Quoting Sovietjet (Thread starter):
Imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
There is no wind.
Can the plane take off?

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-12-03 00:52:52 and read 32767 times.

Quoting VuelingAirbus (Reply 385):

You're preaching to the converted. We all know what the question is. We all know it's not a real world situation. Most of us have been saying so for over a hundred posts now. However, a lot of us originally thought that the question had been mis-stated; a) because we assumed it was a real world problem and b) because it was presented as something being discussed elsewhere. Since we couldn't find it elsewhere, we went with the discussion which was taking place elsewhere. When we found out that the question hadn't been mis-stated (ages ago) some of us (e.g. me) lost interest in the question and continued to debate the real world problem where the belt travels at the same speed as the aircraft but in the opposite direction. Others chose to continue debating the actual question. There is no need to restate the question.

It's a long thread but it's a bit unfair for anyone joining it late to just ignore a couple of hundred responses and go straight from the question to a reply without bothering so see what's already been said.

Quoting David L (Reply 376):
I assume you're addressing the "conveyor belt matches aircraft speed" situation. How does the belt transfer its motion to the aircraft through unbraked wheels? Have a look at the example here:

Quoting David L (Reply 304):


If you're addressing the "conveyor belt matches wheel speed" situation, then ignore me - I'm done with that one.



Quoting David L (Reply 384):
Quoting VuelingAirbus (Reply 383):
But the original quote is the following:

OK, we can stop right there because:

Quoting David L (Reply 376):
If you're addressing the "conveyor belt matches wheel speed" situation, then ignore me - I'm done with that one.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Fokker Lover
Posted 2005-12-03 00:56:09 and read 32767 times.

Quoting VuelingAirbus (Reply 383):
moving in the opposite direction of rotation


You do realise that for any wheel to turn, the contact surface MUST move in the opposite direction. For every action there is an equal and opposite reaction.
We have two actions here. We also have two reactions. They are completely separate, and have no affect on each other. We are talking about cause and affect. One action is the engine thrusting against the airplane. The reaction is the airplane moving. The other action is the conveyor moving under the wheels. The reaction is the wheel moving in the opposite direction the belt is moving. (It doesn't matter which direction you wish to move the belt.)
The action of the belt is not physically capable of having any affect on the airplane itself. I could also build this model to physically prove it to you.
Two hydraulic actuators mounted horizontally. We would mount a wheel on one and drive the belt with the other. The timing would be perfect. We would paint a witness stripe at the 12:00 position of the wheel for reference. In one diection the stripe would move like a clock. In the other direction the stripe would stay at the top position, yet the whole wheel and axle assembly would still move in a lateral direction.

Quoting Killjoy (Reply 378):
I have a job as captain on A320

VuelingAirbus
About 20 years ago I worked with another mechanic. He wasn't very good at being a mechanic. All thumbs and no ability to see how things work. He quit to become a pilot. Today, he too is an A320 captain. I hear he's pretty good at that.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Aloges
Posted 2005-12-03 01:19:36 and read 32767 times.

Has anyone yet mentioned the bleedin' conveyor belt can't even move as fast as the wheels spin?  duck 

If the belt tries to make the wheels spin slower, it will actually make them move faster because reletively speaking they'll have to travel an ever greater distance in case they want to remain firmly attached to the airplane; which I assume they do. The only things that you'd have to worry about are a) the structural integrity of the gear because it might not be made to resist speeds as high as that and b) the turbulent airflow created by the belt zipping along at hypersonic speed.

To sum up, the plane will take of if the tyres don't blow first. And I don't wanna know what'd happen to the plane if it were decelerated like that.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: 727EMflyer
Posted 2005-12-03 10:05:37 and read 32767 times.

Many sorrys, I couldn't possibly read the whole thread, but I see there are still some Yes'er's out there.

The definitive answer is no, the plane can not fly. The truth is revealed by one word that I didn't see in the 200 ish posts I read:

I
N
E
R
T
I
A

Somebody claimed to be a physics major and said yes inspite of this very basic principle! Shame! I'm surprised you made it three years!

Quoting Joness0154 (Reply 28):
Anyways, in a perfect world physically, when the conveyor belt moved, the wheels would turn (freewheel) and the plane would not go anywhere.

You and those in agreement, see the following, courtesy of dictionary.com

in·er·tia (-nûrsh)
n.

1. Physics. The tendency of a body to resist acceleration; the tendency of a body at rest to remain at rest or of a body in straight line motion to stay in motion in a straight line unless acted on by an outside force.

In true application, the engines must generate thrust at a greater force than the inertia generated by the weight of the aircraft. When that happens, the aircraft moves forward, it's weight supported by the wheels until enough lift is generated to move the support to the wings. In your statement, the inertia of the aircraft is pushing on the conveyor belt, and nothing is pushing on the aircraft. Therefore the conveyor belt will move the aircraft, not spin the tires.

Back to the scenario, we must combine the two statements above: first, we imagine the engines producing thrust, overcoming the inertia of the weight, causing the aircraft's center of gravity to surge forward. For the wheels to continue supporting the weight, they must also move. However, simultaneously, the conveyor belt begins to move. This time, the thrust of the engines couneracts the inertia of the plane, allowing the conveyor belt to spin the tires.... hence the movement required for the wheels to continue supporting the aircarft.

Simple physics put into practice if anyone can explain physical law that say's otherwise, well, you can't.

Quoting OE-LDA (Reply 347):
If someone can proof to me that the plane does not take off, then I will voluntarily return my pilots license.

Send it to me and I'll shred it for you!

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-12-03 11:24:17 and read 32767 times.

Quoting Aloges (Reply 388):
Has anyone yet mentioned the bleedin' conveyor belt can't even move as fast as the wheels spin?

Yes, just about everybody.  Smile

Quoting 727EMflyer (Reply 389):
I
N
E
R
T
I
A

Somebody claimed to be a physics major and said yes inspite of this very basic principle! Shame! I'm surprised you made it three years!

An aircraft at rest has inertia. You're saying the engines can't overcome that? How do planes move? The point is that the conveyor belt will have a hard time transferring it's energy to a 250 ton aircraft through free-spinning wheels, other than by the small amount of friction in the axles. But real world aircraft manage to overcome that friction every day.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-12-03 12:07:42 and read 32767 times.

Quoting 727EMflyer (Reply 389):

My apologies - another read tells me you're not saying that but I still don't see how the belt can "drag" the aircraft backwards unless it's allowed to do so before the aircraft starts to move, i.e. while there's still static friction in the axles and slowly enough not to break it.

And just a reminder, I left the surreal "belt-opposing-wheel-speed" discussion ages ago.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: 727EMflyer
Posted 2005-12-03 12:24:15 and read 32767 times.

DavidL,
Yes the engines overcome the inertia, like I said, and the airplane moves. However in our scenario with the conveyor belt, the conveyor belt physically MUST crate the same energy as the engines. The given in the scenario is that the conveyor belt and the wheels are moving at the same speed, so if the wheels turn faster as a result of increased thrust, then the conveyor belt moves faster thus cancelling the thrust, thus cancelling the overcoming of inertia, thus the plane sits still and the wheels spin like there's no tomorrow.

Heres a simple equation as backup to my statement.
Mass= 1
Thrust = 1.1
Tire circumference = 1

where accleration (units per second squared) = force / mass
and inertia of a body at rest = mass (note, until the wings begin producing lift there is 0 vertical acceleration, therefore the aircrafte is always "at rest" as far as long as the full weight is on the wheels.)
and rate = distace / time

Therefore acceleration = 1.1 unit/second sq. and the rate of rotation of the tires after 1 second = 1.1 units/second

Per the givens in the scenario the conveyor belt must move at the same rate as the tires in the opposite direction, and per good ole Newton, the conveyor belt places an equal inertia on the airplane as the airplane puts on it.

Therefore
conveyor rate = -1.1 units/second
conveyor inertia = 1

and therefore
the acceleration, or force, applied to the aircraft by the conveyor = -1.1

thus the total force applied to the airplane = 0 and that means inertia is never overcome in our scenario.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-12-03 12:33:48 and read 32767 times.

Quoting 727EMflyer (Reply 392):
The given in the scenario is that the conveyor belt and the wheels are moving at the same speed,

Ah, OK. I'll but out of that one. As I said, I just don't care about the "magic" conveyor belt anymore. I just feel a need to continue the discussion with those who think an aircraft wouldn't get any airspeed if the belt were moving at the same speed as the aircraft but in the opposite direction.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Killjoy
Posted 2005-12-03 13:12:22 and read 32767 times.

Quoting Fokker Lover (Reply 387):
Quoting Killjoy (Reply 378):
I have a job as captain on A320

Note to everyone: He's actually quoting VuelingAirbus.

Quoting 727EMflyer (Reply 389):
In true application, the engines must generate thrust at a greater force than the inertia generated by the weight of the aircraft.

The engines must generate enough thrust to overcome drag and the wings enough lift to overcome gravity. Inertia is related to *mass*, not weight, and isn't something to be countered in this sense. A small force will still cause a small acceleration.

Quoting 727EMflyer (Reply 389):
When that happens, the aircraft moves forward, it's weight supported by the wheels until enough lift is generated to move the support to the wings. In your statement, the inertia of the aircraft is pushing on the conveyor belt, and nothing is pushing on the aircraft. Therefore the conveyor belt will move the aircraft, not spin the tires.

The vertical forces will cause a certain amount of static friction between the wheels and the surface/axle. This is what will determine whether they spin or not. Do you understand what you're talking about? You did not address this point in your reply to David's criticism.

Quoting 727EMflyer (Reply 392):
The given in the scenario is that the conveyor belt and the wheels are moving at the same speed

David already said he left that discussion.

Quoting 727EMflyer (Reply 392):
if the wheels turn faster as a result of increased thrust, then the conveyor belt moves faster thus cancelling the thrust

How do you intend to move this force from the wheels to the aircraft?

Quoting 727EMflyer (Reply 392):
Per the givens in the scenario the conveyor belt must move at the same rate as the tires in the opposite direction, and per good ole Newton, the conveyor belt places an equal inertia on the airplane as the airplane puts on it.

Are you comparing horizontal forces with vertical ones?

[Edited 2005-12-03 13:35:27]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: VuelingAirbus
Posted 2005-12-03 13:44:23 and read 32767 times.

Quoting David L (Reply 393):
As I said, I just don't care about the "magic" conveyor belt anymore. I just feel a need to continue the discussion with those who think an aircraft wouldn't get any airspeed if the belt were moving at the same speed as the aircraft but in the opposite direction.

Why? Because 727EMflyer made a valid point? Its totally out of the question that the airplane would pick up speed and take off normally with just double the tire speed if the belt matches the aircraft speed. But that was not the original post. In an ideal world it wouldn't take off. Throughout my studies we always assumed either no drag, no friction, a complete vacuume, totally clean burning processes to get or apply a formula.

HOWEVER - since you are so keen on the real world I can tell you that the plane could not take off either. On the Airbus I have a VR of 150 kts and if the belt matches that speed so the resulting tire speed is 300 kts. The max tire speed is 195 kts so about 50 kts (messured in airplane speed in respect to the air) before I could lift off. ALL tires would blow up and after that the structure of the gear might fail and the plane lands with its belly on the belt and will definatly not get airborne anymore...

So where do you live now? In the real world or in an ideal world. It wouldn't take off in the real or in the ideal world!!!

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Jamesbuk
Posted 2005-12-03 13:48:03 and read 32767 times.

ill try and simplify this for everyone
ok on the ground the aircraft acts like a car as it isnt in the air as such so that would mean the aircraft would have to keep accelarating but as the belt is matching the aircrafts wheel speed the aircraft is not progressing anywhere and only the wheels turn so there is no air flowing over the wings to take off
its like when your on a tread mill ( running machine) you dont progress anywhere as you are matching the speed of the conveyer belt

i hope this cleared it up guys
--james--

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: 727EMflyer
Posted 2005-12-03 14:08:39 and read 32767 times.

Killjoy,
1. If an aircraft starts it's take-off run, it is not yet flying, therefore drag has not yet become a major player. I think in this scenario we can assume the airplane didn't taxi onto the conveyor and immediately advance to take-off thrust... so drag = 0! The only force initially acting on the plane is inertia.

2. Yes, but that same static friction will apply to the bearings and then to the strut... and it is all equal to the static friction that the strut applies to the bearing which is applied to the wheel which is applied to the conveyor.

3. Yes, but note my disclaimer... I jumped here very late and appologized. Regardless the discussion is going around in circles. Also I privately appologized for any misunderstandings to david.
But since you mention it, take the wheels out of my equations and the numbers still zero out. I'll show you if you want.

4. See no. 2, and in case you hadn't noticed, the wheels are part of the aircraft. Something I seem to be understanding that I think others miss, is that this fanciful scenario depends on inter-related actions, not a series of events. Yes, if the aircraft was already moving, THEN the conveyor started we would soon have a flying airplane.

5. No. Actually your question causes me to clarify a statement after a fact-check. Inertia is solely related to mass, so, there isn't really a vertical component here. Rather than saying the conveyor applies equal inertia I should friction forces applied by the airplane and by the conveyor to the ccontact point of the airplane and the conveyor are equal when the airplane is at rest. Further, since the conveyor must move at the same speed (as whatever....) the friction will remain equal throughout acceleration, and the airplane's inertia will still never be overcome.

Cheers!

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-12-03 15:36:20 and read 32767 times.

Quoting VuelingAirbus (Reply 395):
Its totally out of the question that the airplane would pick up speed and take off normally with just double the tire speed if the belt matches the aircraft speed.



Quoting VuelingAirbus (Reply 395):
The max tire speed is 195 kts so about 50 kts (messured in airplane speed in respect to the air) before I could lift off. ALL tires would blow up and after that the structure of the gear might fail and the plane lands with its belly on the belt and will definatly not get airborne anymore...

Again... oh dear! This was introduced 347 posts ago! We agreed to forget that for the sake of discussion because there were people claiming that the aircraft could not move forwards because of the belt opposing the aircraft's speed. Those of us saying the aircraft would accelerate forwards under those conditions knew the tyres couldn't take it and said so. OK, forward acceleration and take-off have been interchanged but, had you read the discussion, you would have a better understanding of why people are saying what they are.  Smile

Quoting SlamClick (Reply 48):
The plane would not take off but for a reason I've not seen mentioned.

Airplane tires have a speed limit. 225 MPH is a very common speed limit for airliner tires. If we would normally lift off today at 150 MPH then the treadmill would have the wheels spinning at a rotational speed equivalent to 300 MPH and the tires would burst.



Quoting Killjoy (Reply 394):
David already said he left that discussion.

Thank you, Killjoy! Someone is actually reading the posts before commenting.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-12-03 15:41:58 and read 32767 times.

Quoting VuelingAirbus (Reply 395):
since you are so keen on the real world

Aren't you?  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-12-03 15:50:15 and read 32767 times.

Quoting Jamesbuk (Reply 396):
ok on the ground the aircraft acts like a car as it isnt in the air as such



Quoting Jamesbuk (Reply 396):
its like when your on a tread mill ( running machine) you dont progress anywhere as you are matching the speed of the conveyer belt

No. You haven't read the posts either, have you?  Smile

Quoting SlamClick (Reply 149):
The wheels don't drive the airplane.
The wheels don't drive the airplane.
The wheels don't drive the airplane.
The wheels don't drive the airplane.
The wheels don't drive the airplane.

The error is in your thinking. You are still thinking "car" and you can't solve the problem that way.

How many times is that now?

Quoting Jamesbuk (Reply 396):
i hope this cleared it up guys

Not even slightly.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-12-03 16:06:27 and read 32767 times.

Quoting 727EMflyer (Reply 397):
Also I privately appologized for any misunderstandings to david.

Indeed - no problem! But the problem is that a lot of things which have already been covered are being brought up again and again and some of those releate to either scenario.

See Reply 396 for an example.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Jamesbuk
Posted 2005-12-03 16:38:00 and read 32767 times.

Quoting David L (Reply 400):
David L

yeh your right i didnt read all the post as it wouldve taken me hours lol so i only said what i thought was correct and so i guess there was a misunderstanding on my part

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2005-12-03 18:08:36 and read 32767 times.

Quoting Jamesbuk (Reply 402):

No problem but you have to understand that after 4 days of explaining why an aircraft on a conveyor belt is different from a car on a conveyor belt, it's become a bit like a red rag to a bull for some of us.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: CCA
Posted 2005-12-03 23:45:19 and read 32767 times.

I've tried to read most of the post and got through almost all.

If you want to be true to the question you have to pick whether you want the "speed" to be in RPM or Velocity.

Quoting Sovietjet (Thread starter):
conveyer belt is designed to exactly match the speed of the wheels

If you're and RPM guy then forget starting engines, the RPMs can only match at 0 aircraft velocity (otherwise the tyres will skid) the wheels and conveyor can do whatever they like, as soon as the aircraft moves, the wheels RPM will be greater than the conveyor and the game is over, as long as the aircraft moves over the conveyor it's RPM will be different, it is impossible to maintain the same or equally accelerating RPMs if the axle moves over the conveyor, it MUST increase RPM over the conveyor and therefore fail the question or skid.

So RPM guys CANNOT take off as they can't move without skidding or being unfaithful to the question.

If you are a velocity guy then as said in previous posts aircraft and wheels move 50kts WEST conveyor moves 50kts EAST, RPM equal to 100kts.

The question said speed so unless it's defined as rotational or vectored there will always be two sides.

RPM = CANNOT FLY (as they arn't allowed to move)
Velocity = FLY

QED

Edit Grammar

[Edited 2005-12-03 23:56:09]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Pihero
Posted 2005-12-04 00:07:13 and read 32767 times.

Ok, I'll try one last time .

Just imagine a 4x4 on the ice of a frozen lake.
Its wheels are spinning and its make no headway whatsoever as the surface is too slippery.

Now, its driver being a clever chap hooks the front cable too a nearby pine tree, starts the winch motor. Whether the wheels are spinning or not, won't we agree that the car will move forward due to the traction produced by the winch motor ?(if only because the cable will get shorter)
The problem with that image is that the winch will have a finite speed therefore the car will not accelerate much.

Now, let's replace the winch system with a big weight that attaches to the cable which goes around a pully,and over a cliff. By law of physics, the falling speed of the weight will accelerate and so will the car. With a pair of wings, it could even fly !
One could even imagine the driver's son (quite an enthusiast ) inside the 4x4 still revving the engine to max and the odometer would read a phenomenal speed That has nothing in common with the car actual speed on that frozen lake.

In our initial puzzle, the combination of weight/pully/cable is just a force that acts on the car as a whole exactly equivalent to the thrust produced by the aircraft engine

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: ATCme
Posted 2005-12-27 01:48:38 and read 32767 times.

So are most of us in agreement, "The plane could take off."? I for one say, YES.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Sovietjet
Posted 2005-12-27 03:35:58 and read 32767 times.

I still say no. Although I see the other side of the story the way I understood the question I say no. I see why some people say yes though and it really depends on the way you interpret the question.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: HAWK21M
Posted 2005-12-27 10:14:29 and read 32767 times.

Quoting ATCme (Reply 406):
So are most of us in agreement, "The plane could take off."? I for one say, YES.

I'd say yes too.Now why dont we start a Part II of this thread.People are getting old waiting for the Thread to appear.  biggrin 
regds
MEL

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Abbs380
Posted 2005-12-28 11:59:54 and read 32767 times.

I have read most of this, I think. And I believe the a/c could take off. But the other night my brain was so frazzeled thinking about it that I had to ignore the computer and take a break to watch some TV.
It just so happened that there was an early WWII movie playing. In the movie the male hero -Joe- was saying goodby to his girlfriend. They were at the railroad station waiting for his train to depart, Joe was going off for Army pilot training. As go time approached Joe got on the train and was talking to Sue from the door way, soon the train began to move forward so Sue began to walk along with the train, still talking to Joe while assuring him that she would wait for him. As the train began to pick up speed Sue had to walk faster. Joe, realizing that Sue would soon reach the end of the platform, and not wanting to loose sight of his true love, began to walk towards the rear of the train. As the train continued to accelerate Sue did reach the end of the platform and had to stop, however Joe continued to run toward the rear of the train and therefore could still see her face to face.

After that scene I must have fallen asleep and began to dream. In my dream Joe came out of the rear passenger car onto a flat car. A quick look showed Joe there were at least 20 more flat cars attached to the train behind the last passenger car, and lo and behold there was a Piper J3 cub sitting on that first flat car with its engine idling. Since Joe had a civillian pilot ticket and he knew that this train only traveled in a straight line EAST & WEST there would be no breaks between the cars, he could easily take off on the back of the train.

Joe jumped into the J3 and closed the lower door. Not wanting to loose sight of his beloved he pushed up the throttle and taxied back towards the train station. As he got in line with Sue who was still on the end of the platform he noticed the engine didnt sound quite right but dismissed it. Soon the train began to accelerate more rapidly so Joe, not wanting to loose sight of Sue, added power to stay just in front of her. The powerfull steam locomotive now began to pull harder and harder, as it did Joe had to add more throttle to the Continental A-65 powering the J3. Soon the little Piper was bouncing up and down quite hard against the train's flat cars as it rolled at 70 mph relative to the train, but Sue was always right there in the side window.

Now the only problem is: Joe ran off the end of the last flat car at about 80 mph, and having no lift he smacked into the railroad tracks with full power on and the thing exploded in a huge fireball.
Then I woke up.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Chksix
Posted 2005-12-29 18:04:20 and read 32767 times.

Joe wouldn't have to add power to stay besides Sue since he had to be at zero speed. The only thing to compensate for would be the slight rolling resistance against the wagontops.

(Then there would be no fireball since the plane would just hop off the last car onto the rails and be at a standstill  Wink )

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Bri2k1
Posted 2005-12-29 19:02:37 and read 32767 times.

I'm glad we've reached the conclusion the plane would fly. It appears to be sound.

The only other comment I must make is to those who wish to say that because a wheel rotates, it has no speed. This is not correct. There are two types of motion: rotation, and translation. If the wheel is at a standstill, and locked there, it can still be dragged (drug?) against the pavement, so it has translation, but not rotation. Or it can have both, in the normal rolling wheel example. They can even be in opposite directions, like the 4x4 on the frozen lake example. Or, more dangerously, skidding sideways through a turn in a sports car; the tires translate sideways even though they're rotating forwards.

And, don't just limit your thinking to the center of the wheel translating (the hub). Sure, the hub will translate. But, every other point on that wheel will translate, as well. This entire problem is a good illustration of the concept of "inertial frame of reference," which is cruciual to understanding Newton's laws. Motion must be relative to something. Some would say to a fixed point, which is the easy way to analyze it. But not all coordinate systems are fixed, and it is valid to define motion with respect to a moving coordinate system, or a moving frame of reference (like the boxcars above; the plane is stationary with respect to the platform, but it moves with respect to the train, and that motion can be analyzed).

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: GRZ-AIR
Posted 2005-12-29 20:28:33 and read 32767 times.

YES, it can TAKE OFF!

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Jetmatt777
Posted 2007-05-20 07:55:36 and read 32767 times.

I'm sending this to the Mythbusters. Really, I am.

-Matt

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: ThirtyEcho
Posted 2007-05-20 10:01:19 and read 32767 times.

This has to be the STUPIDEST thread ever on A.net.

This doesn't belong in Tech/Ops: it belongs in a shredder or a looney bin.

Suggesting deletion.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SilverComet
Posted 2007-05-20 16:58:40 and read 32767 times.

Thought this thread had been locked a long time ago. If it isn't, then it should be. Not suggesting deletion though, as it is interesting to see how some people put forward (somewhat valid) arguments to support their theories. Starts off being something fun to think about, then it becomes more of an intellectual adventure into the realm of mechanics, and ends up being just plain exasperating. I stopped reading at about #300. Changed my mind three times along the way.

BTW the plane won't fly.  Big grin  duck 

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: BoeingOnFinal
Posted 2007-05-20 17:45:36 and read 32767 times.

The ground has nothing to do with the thrust being created to create lift. All it does, is increase or decrease friction which will have an certain amount of effect on the thrust.

So if the conveyor belt moved backwards at the same speed as the ground speed would be with fixed runway, the aircraft would take off with twice the rotation speed on the wheels (not counting that the friction would have some effect on gaining thrust).

Think of it this way: Runway is fixed. Vr is 120 KTS.
If you have a headwind of 120 KTS, you would have to spool up the engines so you have equal thrust to match headwind, then you could take off with no runway used.
If you have 120 KTS tailwind, you would have to accelerate to 120 KTS, which would make TAS 0 KTS, then accelerate to Vr which is 120 KTS. Ground speed is then 240 KTS which would represent the question. Except in the question we have zero wind and runway moving opposite direction at 120 KTS upon Vr.


Very simply put. But I say the plane would take off with increased rotation of the wheels, GS relative to belt is double but GS relative to the ground around is the same as TAS if no wind is taking place.

This is also considering that the conveyor belt does not cause air to move, which is most likely will on a very thin layer depending on fricion of the belt.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Flexo
Posted 2007-05-20 18:18:54 and read 32767 times.

What all of you who vote for a possible take off are forgetting is that the fictional conveyor belt will match ANY speed the tires will accelerate to. So there is no way the aircraft will even move an inch forward.

Consider this: On a regular surface our fictional aircraft would need 1% of its engine power to overcome friction of the wheels and start moving. So if the power is set to less than 1% it would decelerate and if it is set higher it would accelerate.
So imagine the flight crew in the plane on top of the conveyor belt sets the power to 2%. The wheels and the conveyor belt would start moving until the increasing friction in the wheels is high enough to cancel the forward motion by the 2% power of the engines.
Now they increase power even further, but the effect will remain the same. No matter how high they set the power, the wheels on the conveyor belt will always move just fast enough to cancel all the power applied by the throttle.

Of course real wheels and a real conveyor belt could never be constructed to endure the speeds they would have to turn with to neutralize a jet engine but we are talking about fictional wheels that could theoretically spin at an infinite speed and thus produce infinite friction!

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Keta
Posted 2007-05-20 19:46:10 and read 32767 times.

Who brought this up again??  bigthumbsup  I can't believe it's been more than a year since we almost went mad, and it's still kicking!  eek 

Anyway, I suggest blocking this thread. It's too long, if somebody still wants to debate, it would be better to start another one. I'm against deletion though, it's an interesting read.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Sovietjet
Posted 2007-05-20 19:56:02 and read 32767 times.

Wow I can't believe my thread is still going lol. Either way, I talked to both my dynamics teacher and my aerodynamics teacher about this some time ago and they both said there's two possible answers depending on the interpretation. The question itself is sort of a paradox because of the way it's stated and because of the little detail given(specifically detail about friction).

1) It will take off when the friction is overcome, essentially the plane will "drag" off the conveyor belt. Since the belt matches the rotational speed of the wheel, there is no other way for it to translate but to be dragged along with the wheels spinning. It doesn't mean the wheels will be spinning twice as fast, becuase we don't know any friction details.

2) It will not takeoff, and the conveyor belt and wheels will reach an infinite speed within an infinitecimal amount of time. In this case, frictional force also reaches infinity.

Again, read the question carefully before you try answering it, you will see what I mean. Try looking at both sides of the answer and yo uwill see that both are technically correct depending on how you interpret the question and what initial assumptions are made. Also, while I do realize that the engines and wheels are not connected in any way, it doesn't matter since the belt is made to match the speed of the wheels.

So yes it will fly and no it won't.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SilverComet
Posted 2007-05-20 20:53:40 and read 32767 times.

Quoting BoeingOnFinal (Reply 415):
So if the conveyor belt moved backwards at the same speed as the ground speed would be with fixed runway, the aircraft would take off with twice the rotation speed on the wheels

You didn't understand the question properly. Had it been stated differenty i.e. the conveyor belt is designed to match any forward speed of the aircraft, then you would have been right. The aircraft would take off at its usual air/ground speed of (say) 120 kts, the belt would be moving backwards at 120 kts and the wheels would be 'seeing' 240 kts.

It's all in the wording.

Quoting Sovietjet (Thread starter):
The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.

The key word here is wheels. But I doubt Sovietjet used that word intentionally in an attempt to drive us all mad Big grin.

Anyhoo I suggest locking of the thread. The subject has been discussed to death and whatever new argument anyone wants to put forward/debate/lose friends over has probably already been covered in one of the 400 replies above. Enough is enough. Let's not revive this monster.

Now if the aircraft had been made out of chickenwire...  Yeah sure

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Starlionblue
Posted 2007-05-20 21:10:22 and read 32767 times.

When I saw this thread again I wanted to go out back and shoot the computer.  Wink

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Chksix
Posted 2007-05-20 23:00:11 and read 32767 times.

LOL This is fun! Big grin

I've posted before in this thread but....

If the belt is designed to match the rotation of the wheels it means that the belt will start to move forward as soon as the plane applies thrust.
The wheels will be stopped but the plane and belt will accelerate together towards Vr.

A belt that tries to cancel the movement of the plane will have to spin fast enough to shred the tyres, wheels, landing gear etc to try to achieve enough friction to stop the plane rolling forward. I think the plane will fly before that happens though...

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Jutes85
Posted 2007-05-21 04:32:07 and read 32767 times.

http://static.flickr.com/51/108912900_a39b18c749_o.gif

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Starlionblue
Posted 2007-05-21 06:17:33 and read 32767 times.

That's hilarious Jutes85. Especially the owls! Big grin

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SilverComet
Posted 2007-05-21 06:40:25 and read 32767 times.

Quoting Jutes85 (Reply 422):

LOL nice one.

But still wrong, assuming the conditions laid down in the original post.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SilverComet
Posted 2007-05-21 07:01:41 and read 32767 times.

AAARGH!! I hate this thread.  banghead 

Changed my mind again. The conveyor will NEVER be able to match the speed of the wheels, even in theory.

Friction between the belt and the wheels means that the wheels will always be spinning faster than the belt. As soon as the wheels start to spin, the belt tries to 'match' the speed of the wheels. It will move backwards faster, and because it is in contact with the wheels the latter will also spin faster by the same amount. The belt will try to match this new speed etc etc and it never ends.

In theory, the plane takes off because at some point friction between the wheels and the the belt is lost.

In reality, the plane would never take off. The tyres would have burst long before.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: CptSpeaking
Posted 2007-05-21 07:06:37 and read 32767 times.

In this situation, because there is no wind, the airspeed and the groundspeed are ALWAYS going to be equal. No matter how much thrust you have, you have to generate some sort of airspeed to lift off. Because the two are always equal, you can substitute and say that you have to generate some sort of GROUNDSPEED to take off. If the conveyor belt is always spinning at the exact same speed as the wheels, it is cancelling any sort of groundspeed, because if the plane starts to move forward, then the wheels are moving faster than the conveyor belt REGARDLESS of whether the engines and wheels are connected or not, and that cannot happen in the scenario, disregarding any shredding speeds and friction nonsense. So my position is that, NO, it wouldn’t be able to take off.

Your CptSpeaking

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: BoeingOnFinal
Posted 2007-05-21 07:24:51 and read 32767 times.

Ok, I mistook the explanation on how the conveyor belt is working. So it actually cancels the speed of the wheels?

So, unless you have an enormous amount of friction to the belt so it moves masses of air above it's surface, the plane will actually take off easier. Cause you cancel the friction of the wheels. So you build up air speed easier. And that is what creates lift.

Quoting CptSpeaking (Reply 426):
the airspeed and the groundspeed are ALWAYS going to be equal.

It is, but it still is GS taking place. Cause the belt is only as wide and as long as the RUNWAY. So GS is calculated to fixed terrain around, which also is where the wind is calculated. So, compared to the terrain around the belt, and also the air, everything will be the same as if no conveyor belt takes place.
Exept, as I said, if the belt moves masses of air. Then the masses would be as wind compared to fixed terrain around it. THEN we can start talking about the aircraft not taking off.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Mir
Posted 2007-05-21 10:04:54 and read 32767 times.

I was going to write a big detailed post about why the plane will take off, but then I decided that it would be less painful to beat myself over the head with a rubber truncheon.

All I'll say is this: the whole scenario is technically impossible because of the infinite velocities involved, but if you look at it from the standpoint of what is generating force in which direction (i.e. up, down, forwards and backwards), it becomes very clear.

Quoting Jutes85 (Reply 422):



-Mir

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Sovietjet
Posted 2007-05-21 10:12:57 and read 32767 times.

Jutes85 - Lol so I see you're also on NASIOC....

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: MD-90
Posted 2007-05-21 11:56:47 and read 32767 times.

lol, Jutes should make a graphic that shows the wheels exploding (which, according to the paradox that is this question, is what I believe would happen long before the aircaft ever got going fast enough to take off. Unless it the aircraft in question is a Super Cub and it's a windy day. In Alaska. Yeah.)

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Jetlagged
Posted 2007-05-21 13:08:43 and read 32767 times.

Quoting Sovietjet (Reply 418):
Again, read the question carefully before you try answering it, you will see what I mean. Try looking at both sides of the answer and yo uwill see that both are technically correct depending on how you interpret the question and what initial assumptions are made. Also, while I do realize that the engines and wheels are not connected in any way, it doesn't matter since the belt is made to match the speed of the wheels.

So yes it will fly and no it won't.

Both your dynamics teacher and aerodynamics teacher should go back to school. They appear to be confusing static friction with rolling friction. There is no paradox in the question and the friction between the wheels and the conveyor belt is irrelevant. The only drag imparted by the belt to the aircraft through the wheels is rolling friction, due to the deformation of the tyre as it rolls and any friction in the wheel bearings. Both forces independent of the belt surface coefficient of friction.

There will be more rolling friction because the wheels spin faster. However, the thrust from the engines is orders of magnitude greater than rolling friction. You can safely ignore it even if the wheels were running twice as fast due to the belt moving.

Because the wheels are not driven, there is no way the infinite velocity outcome could occur. As for the aircraft "dragging itself off" there is hardly any extra drag to overcome.

The only paradox is that some people can't stop themselves thinking that somehow the backwards movement of the belt must slow the aircraft significantly. That would only happen if the wheel brakes were applied.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: CptSpeaking
Posted 2007-05-21 14:21:37 and read 32767 times.

Reminds me of something else...can this guy generate any forward airspeed and if he had wings, take off?



Your CptSpeaking

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: BoeingOnFinal
Posted 2007-05-21 15:29:44 and read 32767 times.

Quoting CptSpeaking (Reply 432):
Reminds me of something else...can this guy generate any forward airspeed and if he had wings, take off?

No, because he gains speed by moving on the ground. And if an aircraft gained speed to take off by turning the wheel like a car, it wouldn't take off either. But it doesn't work like that, does it?

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-05-21 15:34:05 and read 32767 times.

Quoting CptSpeaking (Reply 432):
Reminds me of something else...can this guy generate any forward airspeed and if he had wings, take off?

No!  banghead  That's the fundamental mistake made so many times in this thread.  biggrin 

The animal propels the the wheel by applying a rearward force to the bottom of the wheel with its feet. The aircraft's wheels do not apply the driving force to the runway. Stick a rocket pack on the animal's back and castors on its feet and then we can compare.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Moriarty
Posted 2007-05-21 15:46:33 and read 32767 times.

If you start running on a treadmill, will the wind blow in your hair just because you're running?

My answer would be: no, it won't (unless you put a fan in front or something like that).

I guess if you want to get an airplane airborne without moving it forward in relation to the ground you have to put a fan in front of the aircraft and make sure the fan is big and powerful enough to generate wind that provides lift... once the aircraft is airborne you better wind up the engines as it eventually will rise above the stream generated by the fan.

A thing that comes in mind is the tv-series Knight Rider. They often "launch" (and retreives) the Kit car into a moving trailer...

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-05-21 15:50:56 and read 32767 times.

Quoting Moriarty (Reply 435):
If you start running on a treadmill, will the wind blow in your hair just because you're running?

"Running on a treadmill" is not the same thing! The wheels do not propel the aircraft!

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: MidEx216
Posted 2007-05-21 16:26:22 and read 32767 times.

Quoting Nighthawk (Reply 5):

I was gonna say no at first, but Nighthawk is right. Though the conveyor belt is moving in the opposite direction, that's not going to slow the plane down at all because the engines don't power the wheels, they push air. So the wheels will just be moving that much faster once it gets up to takeoff speed

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: CptSpeaking
Posted 2007-05-21 16:41:28 and read 32767 times.

Quoting David L (Reply 434):
No!    That's the fundamental mistake made so many times in this thread.   

I understand the difference in propulsion...my point in that is, if your speed over the ground (not the belt or in my example, wheel) remains at ZERO, then you'll have NO AIRSPEED.

Maybe another try...

Imagine this: The belt moves underneath the airplane without the airplane's thrust pushing back. Imagine standing still on a moving walkway. So now, the plane is moving backwards in relation to the ground. Also, negative airspeed. In order to counteract this, the pilot advances the throttles and now the airplane is stationary over the ground, not the belt. The belt is trying to move the aircraft backwards, but the thrust is counteracting that. Now, the belt speeds up and the aircraft is moving backwards over the ground again, and also again, negative airspeed. If it were the opposite, and the throttles were advanced with the same belt speed, creating a positive difference between the wheels and belt, then the aircraft would start to accelerate over the ground and also start to gain positive airspeed, which will be the reason it takes off. As I said before, because there is no wind, the TAS and GS are always gong to be equal!! Therefore, if you can never generate any GS (again, not belt speed, but what the belt is fixed to), then you're never going to generate any airspeed. The wheels and belt can spin as fast as they want, but if they are the same, as in the scenario given, then the plane is going nowhere.

Another example..let me know if it is flawed...  Smile
You're riding in a car and you toss a ball in the air. Even though in relation to the ground moving beneath you, that ball may be moving 70mph, it doesn't move laterally in relation to you because you are going the same speed. Get where I'm going with that?

Your CptSpeaking

[Edited 2007-05-21 16:57:44]

[Edited 2007-05-21 17:04:31]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Jetlagged
Posted 2007-05-21 16:51:53 and read 32767 times.

Quoting Moriarty (Reply 435):
I guess if you want to get an airplane airborne without moving it forward in relation to the ground you have to put a fan in front of the aircraft and make sure the fan is big and powerful enough to generate wind that provides lift

Another false assumption: that the aircraft will not move forward relative to the ground because the belt is running backwards. In fact the conveyor belt has no effect, whether it runs backwards or forwards.

The speed of the belt under the aircraft wheels is completely irrelevant to the question of whether the aircraft can gain airspeed (which of course it can). We are talking jet propulsion here, not traction. Anyone who has inflated a balloon and let it go will know it doesn't need ground friction to react against to make it move.

Ah, you say, but what if the engines were at idle and the conveyor starts to move backwards, won't the aircraft move with it? Well yes it will, but only because the rolling friction is just enough to maintain it stationary. As soon as takeoff thrust is applied there is a large force acting on the aircraft which is independent of wheelspeed, ground speed, etc. and much greater than rolling friction.

This question is not the same as a car on a rolling road, a hamster in a treadmill, or David Hasselhof driving up the ramp of a moving truck come to that. If you still don't get it don't feel too bad, because Sovietjet's teachers didn't get it either.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Mir
Posted 2007-05-21 17:11:34 and read 32767 times.

Quoting CptSpeaking (Reply 438):
Imagine this: The belt moves underneath the airplane without the airplane's thrust pushing back. Imagine standing still on a moving walkway. So now, the plane is moving backwards in relation to the ground.

If one were to disregard friction between the wheels and the axles, the airplane wouldn't move at all - the wheels would just spin. Since there is friction, however, you will get some motion of the airplane. However, unless the belt gets up to speed very slowly, so that the acceleration does not exceed what the friction is capable of relaying to the airplane, the speed at which the airplane moves will be less than that of the belt, and the wheels will spin forward. If the belt moves too fast alltogether, you'll get something like this:



-Mir

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Jetlagged
Posted 2007-05-21 17:31:07 and read 32767 times.

Quoting CptSpeaking (Reply 438):
The belt moves underneath the airplane without the airplane's thrust pushing back. Imagine standing still on a moving walkway. So now, the plane is moving backwards in relation to the ground. Also, negative airspeed. In order to counteract this, the pilot advances the throttles and now the airplane is stationary over the ground, not the belt. The belt is trying to move the aircraft backwards, but the thrust is counteracting that. Now, the belt speeds up and the aircraft is moving backwards over the ground again, and also again, negative airspeed.

True, but you are only allowing the pilot to add just enough thrust to counter the increased rolling friction. If the pilot sets takeoff thrust, rolling friction is only a tiny fraction of that. So the aircraft accelerates w.r.t the ground and the air. The belt runs as programmed but even with the increased wheel speed rolling friction remains very much less than thrust.

If you take the opposite view and the belt runs forwards to keep up with the wheels, rolling friction is zero and takeoff is even easier.

Quoting CptSpeaking (Reply 438):
You're riding in a car and you toss a ball in the air. Even though in relation to the ground moving beneath you, that ball may be moving 70mph, it doesn't move laterally in relation to you because you are going the same speed. Get where I'm going with that?

Is the car a Birdcage Maserati? Big grin

All the belt does is vary the wheel speed, and so the rolling friction. It has no significant effect on ground speed or airspeed because rolling friction is always very small compared to thrust.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: CptSpeaking
Posted 2007-05-21 17:33:25 and read 32767 times.

Quoting Jetlagged (Reply 439):
Ah, you say, but what if the engines were at idle and the conveyor starts to move backwards, won't the aircraft move with it? Well yes it will, but only because the rolling friction is just enough to maintain it stationary. As soon as takeoff thrust is applied there is a large force acting on the aircraft which is independent of wheelspeed, ground speed, etc. and much greater than rolling friction.

No Wind: TAS=GS.

If you generate TAS, regardless of the type of propulsion...it doesn't matter, then you have generated GS.

If you've generated GS, then the wheels are moving faster than the belt, and according to the scenario, that cannot happen!!

Quoting Mir (Reply 440):
However, unless the belt gets up to speed very slowly, so that the acceleration does not exceed what the friction is capable of relaying to the airplane, the speed at which the airplane moves will be less than that of the belt, and the wheels will spin forward. If the belt moves too fast alltogether, you'll get something like this:

I don't care about any friction, or if dynamic friction is less than static friction. A rolling wheel is maintaining static friction anyway! It would have to be sliding for there to be dynamic friction...I learned that years ago in High School physics! I understand what you're getting at with the concept of starting the belt slowly or suddenly, but if the wheels don't move, the belt won't move according to the scenario. Also, with the tablecloth example, the plates (or cloth, depending on how you look at it) are sliding, not rolling. Again, I understand what you're getting at, but it isn't consistent with the scenario. Put a ball on there and then you've got something.

Your CptSpeaking

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Bond007
Posted 2007-05-21 17:33:39 and read 32767 times.

Simple question .... and no it's not completely obvious in the original question.

Quoting Sovietjet (Thread starter):
The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.

what does this mean??

That the aircraft does not move in relation to the air surrounding it ... or to an observer watching alongside??

An aircraft can only fly if there is air passing over the lift surfaces ... it doesn't matter how fast the wheels are going, or the conveyor belt, if the end result is an aircraft still in relation to the air.

I guess I'm missing something.... and sure to be corrected.

Jimbo

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: DarkBlue
Posted 2007-05-21 17:36:18 and read 32767 times.

Uggh, not this again.

Quoting MidEx216 (Reply 437):
So the wheels will just be moving that much faster once it gets up to takeoff speed



Quoting Jetlagged (Reply 439):
The speed of the belt under the aircraft wheels is completely irrelevant to the question of whether the aircraft can gain airspeed (which of course it can).



Quoting Jetlagged (Reply 431):
The only paradox is that some people can't stop themselves thinking that somehow the backwards movement of the belt must slow the aircraft significantly.

Please reread the question. The question is over-constrained. Everytime you state that the wheels are irrelevant you are choosing to ignore the actual question. Yes, that makes it an easy question to answer, but you have failed to answer the question that was asked.

Please ponder these three things:

1.) You need airspeed to create lift, correct?
2.) Assuming no wind, ground speed = airspeed, correct?
3.) Question states that conveyor belt is designed to match wheel speed - this means ground speed = 0, correct?

If you choose to introduce a scenario where the tire burst and stop rotating, fine, I agree. The aircraft will move. However, this wasn't part of the question.

Quoting Jetlagged (Reply 439):
If you still don't get it don't feel too bad, because Sovietjet's teachers didn't get it either.

If you still don't get it don't feel too bad. I tried explaining to my cats this morning, and i don't think they understood it either. They just gave me a blank stare.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Zeke
Posted 2007-05-21 17:42:53 and read 32767 times.

This is a very simple problem in physics ....

The problem is F=ma,

where
F is the thrust - (rolling resistance + aerodynamic drag)
m is the mass of the aircraft
a is the acceleration of the aircraft

The rolling resistance depends on a number of factors, the main ones would be the type of surface, mud, tall grass, short grass, hard surface, ice, so with a piper cub taking off on a muddy runway takes more runway than taking off on ice, no brainer. The rolling resistance also reduces as a function of the weight being reduced by lift form the wings, faster we go less rolling resistance (the reason we have spoilers for landing, increase the weight on wheels to increase braking efficency). So the only thing we have to keep in mind is that if out Conveyor Belt is made from mud, it would take more distance than a Conveyor Belt made from ice, no brainer.

But in any case, when thrust is larger than the rolling resistance and drag, the aircraft will accelerate, how fast it accelerates depends on the the magnitude of excess thrust, and the mass of the aircraft (a=F/m), but it will accelerate. An extreme example of this would be forgetting to release your brakes when being launched off a carrier, the force of the catapult and the thrust is so great the aircraft will still accelerate, but not as well as it should.

An aircraft can accelerate with the wheels going slower, faster, or not moving at all as would be the case with a headwind, tailwind, or with the brakes set when on ice, or in this case at the same speed as being on a Conveyor Belt, it is absolutely irrelevant, it is a red herring.

To take off, an aircraft needs to accelerate, and will do so any time the excess thrust is greater than the rolling resistance, it is the same simple means we use to move from a stationary position.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: CptSpeaking
Posted 2007-05-21 17:43:38 and read 32767 times.

Quoting DarkBlue (Reply 444):

LOL thank you ...finally somebody who thinks the same...

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Jetlagged
Posted 2007-05-21 18:16:44 and read 32767 times.

Quoting DarkBlue (Reply 444):

1.) You need airspeed to create lift, correct?
2.) Assuming no wind, ground speed = airspeed, correct?
3.) Question states that conveyor belt is designed to match wheel speed - this means ground speed = 0, correct?

1. Yes
2. Yes
3. NO. Belt speed is not ground speed, nor does it cancel out ground speed. Whatever the conveyor belt does the aircraft accelerates. It does not matter what the belt speed is. The belt has no effect on the aircraft.

The big fallacy here is that the conveyor belt somehow affects ground speed.

The question is supposed to make you think the belt will cancel out the aircraft motion by somehow dragging it backwards. People think of rolling roads, treadmills, etc and think yes of course groundspeed will be zero, so airspeed is too. Rolling roads and treadmills work using tractive friction. The aircraft moves by jet propulsion. No comparison.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Vzlet
Posted 2007-05-21 18:29:23 and read 32767 times.



Let's make the conveyor belt a tabletop model. Then take an American Junior Classics Hornet balsa plane and place it on the belt, keeping your hand on it. After a deep breath, attempt to move the plane forward. Will the belt's instant response prevent you from doing so, or will you be able to move the plane the length of the belt pretty much at will?

I can't picture that the belt (no matter how fast it goes, and it "can't" go any faster than the plane's speed) could keep anyone from walking the plane the length of the table. A full size plane on such a magic conveyor belt would replace your hand with a jet engine or propellor, and the result would be the same.

(Apologies if someone has already stated a similar analogy somewhere in this thread.)

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: MD-90
Posted 2007-05-21 18:47:57 and read 32767 times.

Quoting Jetlagged (Reply 431):
There is no paradox in the question and the friction between the wheels and the conveyor belt is irrelevant.

There is a paradox, mainly because I think whoever worded the initial question was thinking in terms of the wheels propelling the aircraft like a car.

Quoting Jetlagged (Reply 431):
You can safely ignore it even if the wheels were running twice as fast due to the belt moving.

The problem is that the wheels aren't moving merely twice as fast as the belt. The problem states that the velocity of the wheel rotation and the conveyor belt match...which would very quickly cause the tires to overheat and explode. Unless, I suppose, the conveyor belt had a maximum speed that it could attain and that was a limiting factor...

Quoting Sovietjet (Thread starter):
The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.

Any thrust would cause movement which would cause the wheels and conveyor belt to accelerate to a theoretical infinite velocity (as the conveyor belt tried to "exactly match" the speed of the wheels)

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: CptSpeaking
Posted 2007-05-21 19:03:38 and read 32767 times.

Quoting Vzlet (Reply 448):
Let's make the conveyor belt a tabletop model. Then take an American Junior Classics Hornet balsa plane and place it on the belt, keeping your hand on it. After a deep breath, attempt to move the plane forward. Will the belt's instant response prevent you from doing so, or will you be able to move the plane the length of the belt pretty much at will?

I can't picture that the belt (no matter how fast it goes, and it "can't" go any faster than the plane's speed) could keep anyone from walking the plane the length of the table.

This would be correct if you were talking relative lateral velocities. The wheels and the belt underneath would be moving laterally at the same velocity, otherwise the wheels would be sliding.

I'm starting to see how we are both correct. DarkBlue and I are thinking that the rotational velocity of the wheels and the belt always match, which would cause zero groundspeed and therefore no airspeed. JetLagged and Vzlet are thinking of it as the lateral velocity between the wheel and the top side of the belt. If you think of it that way, then yes the airplane will gain airspeed and groundspeed and lift off.

I've been thinking of it as rotational speed, which would make us correct.

The question doesn't specify which one, so yes AND no. Make sense?

Your CptSpeaking

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Keta
Posted 2007-05-21 19:12:15 and read 32767 times.

Quoting Zeke (Reply 445):

Well said, I agree.  yes 

There's simply no way that the belt will stop the aircraft. No matter how fast it moves, it exerts no force (or a very little one even assuming friction) to the plane.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Starlionblue
Posted 2007-05-21 19:14:38 and read 32767 times.

Quoting Mir (Reply 428):
I was going to write a big detailed post about why the plane will take off, but then I decided that it would be less painful to beat myself over the head with a rubber truncheon.

Second that.

Quoting David L (Reply 434):
Stick a rocket pack on the animal's back and castors on its feet and then we can compare.

How about in the ass? Centerline thrust is better.

Quoting Moriarty (Reply 435):
I guess if you want to get an airplane airborne without moving it forward in relation to the ground you have to put a fan in front of the aircraft and make sure the fan is big and powerful enough to generate wind that provides lift... once the aircraft is airborne you better wind up the engines as it eventually will rise above the stream generated by the fan.

That's a bit like what aircraft carriers do. You gain a good 20-30 knots depending on cruising speed, meaning takeoff speeds relative to the deck can be lower.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Chksix
Posted 2007-05-21 19:44:14 and read 32767 times.

Cancelling wheel speed means to me that the belt moves forward when the wheel tries to roll when thrust is applied.

That means wheel rpm will be zero, belt speed will thus be aircraft speed and the plane will lift off with the wheels stopped.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Bond007
Posted 2007-05-21 19:53:25 and read 32767 times.

Quoting MD-90 (Reply 449):
There is a paradox, mainly because I think whoever worded the initial question was thinking in terms of the wheels propelling the aircraft like a car.

Well, I think it worded that way exactly to get the hundreds of anwers that this thread has got.

Quoting MD-90 (Reply 449):
Any thrust would cause movement which would cause the wheels and conveyor belt to accelerate to a theoretical infinite velocity (as the conveyor belt tried to "exactly match" the speed of the wheels)

Yes, that it the answer.

Quoting CptSpeaking (Reply 450):
I'm starting to see how we are both correct. DarkBlue and I are thinking that the rotational velocity of the wheels and the belt always match, which would cause zero groundspeed and therefore no airspeed.

Not sure ... see above ... the belt would never to be able to match the speed of the wheels, because the aircraft simply MUST move forward ... there is no way to stop that.

Jimbo

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Starlionblue
Posted 2007-05-21 20:37:12 and read 32767 times.

Can we enter a feature request for a little whiteboard in the posting window?  Wink

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Sovietjet
Posted 2007-05-21 20:40:03 and read 32767 times.

Quoting Keta (Reply 451):
I'm starting to see how we are both correct. DarkBlue and I are thinking that the rotational velocity of the wheels and the belt always match, which would cause zero groundspeed and therefore no airspeed. JetLagged and Vzlet are thinking of it as the lateral velocity between the wheel and the top side of the belt. If you think of it that way, then yes the airplane will gain airspeed and groundspeed and lift off.

I've been thinking of it as rotational speed, which would make us correct.

The question doesn't specify which one, so yes AND no. Make sense?

Exactly. There's two interpretations and therefore two answers.

Quoting Jetlagged (Reply 431):
Both your dynamics teacher and aerodynamics teacher should go back to school.

They aren't wrong, as I said they came up with two answers based on two interpretations of the problem.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Bond007
Posted 2007-05-21 20:50:14 and read 32767 times.

Quoting Sovietjet (Reply 456):
Exactly. There's two interpretations and therefore two answers.

Not sure that affects the outcome though .. since regardless of what the belt does (or can do), all rules of physics say that the aircraft WILL move forward right?

So the belt will never be able to match the speed of the wheels .... whatever that means. Assuming that the belt can do this, starts to break the laws of physics.


Jimbo

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Mir
Posted 2007-05-21 20:54:44 and read 32767 times.

Quoting MD-90 (Reply 449):
Any thrust would cause movement which would cause the wheels and conveyor belt to accelerate to a theoretical infinite velocity (as the conveyor belt tried to "exactly match" the speed of the wheels)

 checkmark 

Quoting Bond007 (Reply 454):
the belt would never to be able to match the speed of the wheels, because the aircraft simply MUST move forward ... there is no way to stop that.

 checkmark 

-Mir

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Starlionblue
Posted 2007-05-21 20:56:43 and read 32767 times.

Quoting Mir (Reply 458):

-Mir

I thought you gave up on this one. How is the truncheoning going?  Wink

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Flexo
Posted 2007-05-21 21:19:14 and read 32767 times.

Even the tiniest amount of friction on the wheels, insignificant it may seem, becomes a great force once it is multiplied by infinity. As the hypothetical conveyor belt can turn at an infinite speed, the force exerted by friction on the wheels will reach a point where it is greater than any engine thrust.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Starlionblue
Posted 2007-05-21 21:27:11 and read 32767 times.

Quoting Flexo (Reply 460):
Even the tiniest amount of friction on the wheels, insignificant it may seem, becomes a great force once it is multiplied by infinity.

It will in fact be infinite if multiplied by infinity.  Wink

For any of this stuff to work, we have to assume zero friction. Then again, since we are dealing in theoreticals...

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: DarkBlue
Posted 2007-05-21 21:28:23 and read 32767 times.

Quoting Jetlagged (Reply 447):
The big fallacy here is that the conveyor belt somehow affects ground speed.

The question is supposed to make you think the belt will cancel out the aircraft motion by somehow dragging it backwards. People think of rolling roads, treadmills, etc and think yes of course groundspeed will be zero, so airspeed is too. Rolling roads and treadmills work using tractive friction. The aircraft moves by jet propulsion. No comparison.

Ah, see that's where the confusion is. We in the "no fly" group understand (at least most of us do) that there is no physical or mechanical constraint of the wheels & conveyor belt. If you have enough thrust, the aircraft will move. However, based on the way the question is stated it's completely a theoretical constraint.

Anyone here familiar with Zeno's paradox?

"In a race, the quickest runner can never overtake the slowest, since the pursuer must first reach the point whence the pursued started, so that the slower must always hold a lead".

Now, we all know that in real life this paradox falls apart just as our plane on a conveyor belt problem. Yes, physics will rule and quicker runner will overtake the slower runner, just as the aircraft will overtake the conveyor belt. However, the issue in both cases is the wording of the question. Each one is a paradox.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Keta
Posted 2007-05-21 21:56:57 and read 32767 times.

Quoting Sovietjet (Reply 456):
Quoting Keta (Reply 451):
I'm starting to see how we are both correct. DarkBlue and I are thinking that the rotational velocity of the wheels and the belt always match, which would cause zero groundspeed and therefore no airspeed. JetLagged and Vzlet are thinking of it as the lateral velocity between the wheel and the top side of the belt. If you think of it that way, then yes the airplane will gain airspeed and groundspeed and lift off.

I've been thinking of it as rotational speed, which would make us correct.

The question doesn't specify which one, so yes AND no. Make sense?

Exactly. There's two interpretations and therefore two answers.

I'd just like to note that I didn't say that  Wink

Quoting Flexo (Reply 460):
As the hypothetical conveyor belt can turn at an infinite speed, the force exerted by friction on the wheels will reach a point where it is greater than any engine thrust.

The friction between the conveyor and the wheels is independent on the velocity, it just depends on the normal force applied (i.e. the weight).

For more info on rolling resistance, see http://en.wikipedia.org/wiki/Rolling_resistance

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Jetlagged
Posted 2007-05-21 22:05:38 and read 32767 times.

Quoting DarkBlue (Reply 462):
However, based on the way the question is stated it's completely a theoretical constraint.

Explain please, because the wording seems very clear to me. I see no artificial contraint, just a theoretical belt working in a specified way. Any paradox in this question is in the eye of the "no fly" beholder, based on a misunderstanding of how the belt must affect the wheels and so the aircraft.

Quoting DarkBlue (Reply 462):
Anyone here familiar with Zeno's paradox?

"In a race, the quickest runner can never overtake the slowest, since the pursuer must first reach the point whence the pursued started, so that the slower must always hold a lead".

Zeno knows full well swift Achilles can beat the slow Tortoise, but creates an argument which makes it appear logically impossible. However, the argument's logic requires Achilles to be slowing down each step. So Achilles is not allowed to use his superior speed. As Achilles is not slowing down, he overtakes the Tortoise as common sense dictates. Archimedes disproved the paradox mathematically, but you can also show it by plotting speed v time.

The problem with the futile conveyor belt question is that the alleged constraint is not a logical one, as in Zeno's paradox, but a false perception only seen by the non flyers. I can see the paradox in Zeno's argument, though it can be mathematically disproven, but there is no paradox in this thread.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Moriarty
Posted 2007-05-21 22:48:29 and read 32767 times.

This thread is hilarious! A model is the way to go!  Smile

Quoting Mir (Reply 440):
Quoting Moriarty (Reply 435):
I guess if you want to get an airplane airborne without moving it forward in relation to the ground you have to put a fan in front of the aircraft and make sure the fan is big and powerful enough to generate wind that provides lift

Another false assumption: that the aircraft will not move forward relative to the ground because the belt is running backwards. In fact the conveyor belt has no effect, whether it runs backwards or forwards.

Ok, I will move back from assumptions to speculations then!  Wink Seriously, it makes sense when I think of it. I was a little bit eager to contribute. However, I am very satisfied to add David Hasselhoff to the discussion.

With that I will rest my case and let the people that actually *know* something speak.

Never the less, I love the thread!

Cheers!

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: MD-90
Posted 2007-05-21 22:57:55 and read 32767 times.

Ain't no tires ever built that have an infinite speed rating. Well, maybe on one or two ricers that drive pimped Civics, but they don't count.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Starlionblue
Posted 2007-05-21 23:04:14 and read 32767 times.

Quoting Moriarty (Reply 465):
However, I am very satisfied to add David Hasselhoff to the discussion.

The song "Hot Shot City" was especially good.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: DarkBlue
Posted 2007-05-21 23:45:46 and read 32767 times.

Quoting Jetlagged (Reply 464):
Explain please, because the wording seems very clear to me. I see no artificial contraint, just a theoretical belt working in a specified way. Any paradox in this question is in the eye of the "no fly" beholder, based on a misunderstanding of how the belt must affect the wheels and so the aircraft.

The theoretical constraint I see is the part of the question that says:

Quoting Sovietjet (Thread starter):
The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.

As I've said many times before, I understand that wheels are not a physical constraint on the aircraft. However, when I read the above statement and you tell me that the aircraft will move forward regardless of what the wheels are doing, then I see this as a violation of the problem statement.

Yes, if the pilot holds the brakes so that the wheels don't turn, the conveyor will not move and yes the aircraft can be dragged until it has enough airspeed to takeoff. But in the theoretical world where wheels and conveyors are able to rotate freely to infinity I still see a constraint.

Please, just show me an example of how the aircraft can move forward and have the matching speeds for the wheels and conveyor belt. If your example introduces anything into the problem where the wheels are dragging across the conveyor, i see this as a "real world" situation and don't believe this belongs in my "theoretical world".

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Jutes85
Posted 2007-05-22 00:32:24 and read 32767 times.

Quoting Sovietjet (Reply 429):
Jutes85 - Lol so I see you're also on NASIOC....

haha, OT ftw. which reminds me:



Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Zeke
Posted 2007-05-22 01:10:08 and read 32767 times.

Quoting DarkBlue (Reply 468):

Please, just show me an example of how the aircraft can move forward and have the matching speeds for the wheels and conveyor belt. If your example introduces anything into the problem where the wheels are dragging across the conveyor, i see this as a "real world" situation and don't believe this belongs in my "theoretical world".

Any traction powered machine is an example, be it a tank, snow mobile, escevator, the wheels on the tank move at the same speed at the track (the conveyor belt), and it still can move forward and backwards.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Mir
Posted 2007-05-22 06:17:30 and read 32767 times.

Quoting Starlionblue (Reply 459):
I thought you gave up on this one. How is the truncheoning going?

I guess I discovered my masochistic side. The truncheoning is going bludgeoningly.  Wink

-Mir

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SilverComet
Posted 2007-05-22 18:51:28 and read 32767 times.

Quoting Mir (Reply 458):

 checkmark   checkmark   checkmark 

I used to be in the 'no fly zone'. Then I saw the light.

The truth of the matter is all the occupants of said aircraft get off the plane and everyone goes home and the flight is cancelled cos they're all sick of waiting while you lot decide upon their fate.

nuff said. back to my fcom's.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Iwok
Posted 2007-05-23 10:50:02 and read 32767 times.

Quoting Kay (Reply 32):
if the conveyor belt is designed to run exactly at the same varying speed as the wheels intend to, but in the opposite direction, that means that the wheels will never move one inch forward



Quoting Ariis (Reply 49):
This is the key to the answer. Since conveyor belt is somehow designed this way (assumption), the wheels will never move an inch. And the airplane will never take off, regardless of the thrust, airflow, engine, pilot, his girlfriend, anything.



Quoting Starlionblue (Reply 461):
For any of this stuff to work, we have to assume zero friction. Then again, since we are dealing in theoreticals...

There are two solutions to this problem. Since we are assuming that the conveyor belt would have the same speed as the wheels (but in opposite directions) and we are also assuming zero friction we have the following.

1) the speed of the wheels reaches infinity and the plane takes off
but we know infinity is never reached so the plane never takes off
2) the wheels never rotate at all are in fact stopped by the brakes.
the plane will not take off

The reason is: if the belt in increasing in speed as thrust is applied, one would think that as the plane starts to move, the coveyor will inrease its speed which means the plane stops moving; and therefore there is no net thrust on the plane, which means that the engines are not applying any thrust (even though we know they are). Therefore this problem only has a solution at zero and infinity.

In-between thrusts of zero and infinity this problem has not possible solution. In otherwords, it is physcially impossible for the wheels to spin if there is no thrust on the plane, or infinite thrust.

In other words, even if you put a GE115 onto a Cesna and put it on this conveyor belt, it is physically impossible for the plane wheels to start at zero speed and then increase by any amount without the plane moving relative so some fixed point on the ground. Therefore the postulated question is impossible.

This was fun  bigthumbsup 

iwok

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: FutureUALpilot
Posted 2007-05-23 16:03:56 and read 32767 times.

It will takeoff. Ignoring the wheel bearing limits, etc, the thrust comes from the engines. If the aircraft had powered wheels, it would not move. However, because the thrust comes from the engines, and the wheels freely rotate, it will roll, and eventually takeoff, but the wheels will be spinning at very high rpm.

My roommate used this example: An RC car on a treadmill. You turn the treadmill, and car on, and start driving. As you drive, the treadmill matches the speed of the car. You then use your hand, and push the car up the treadmill (engines of the aircraft). The car moves, but the wheels turn at a faster rate...but the car is still accelerating.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: CptSpeaking
Posted 2007-05-23 16:04:04 and read 32767 times.

Quoting Iwok (Reply 473):

That was my point to begin with...as long as that belt is moving at the same speed as the wheels, you're not getting any groundspeed and therefore no airspeed.

The point others are making is that no matter what speed you reach or how fast you reach that speed, the wheel will always be going the same relative speed as the belt. In this frame of mind, the belt never moves and the scenario is still accurate...it just depends on how you interpret the question. To demonstrate this, pick a roadsign in front of you while you are driving. No matter how fast you are going, or if you accelerate or not, that sign is always "coming at you" exactly as fast as you are going at it.

The difference between the two interpretations is whether you think the belt actually spins the opposite direction or remains stationary. In either one, when the plane moves, the belt is "moving" in relation to the plane. So both answers are correct, and which one you choose depends on which interpretation you are biased towards.

Glad you had fun with this though...me too!  Smile

Your CptSpeaking

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Yellowstone
Posted 2007-05-23 16:30:27 and read 32767 times.

Quoting DarkBlue (Reply 468):
But in the theoretical world where wheels and conveyors are able to rotate freely to infinity I still see a constraint.

Please, just show me an example of how the aircraft can move forward and have the matching speeds for the wheels and conveyor belt.

If a plane is taking off a normal runway, it is traveling at a finite forward velocity, and its wheels (with radius r) have a finite rotational velocity (call it w). If the plane is taking off on a conveyor belt that is set to move at a finite speed v, its wheels will have a rotational velocity equal to w plus v/r, where everything is finite. We have established that in this paradoxical case, the wheels and conveyor belt will both accelerate to an infinite velocity (v=infinity) as soon as the plane starts trying to move. But if we substitute that into our expression for the rotational velocity of the wheels, we get (w+infinity/r), which is still equal to infinity. Therefore, regardless of the finite forward velocity of the airframe, the wheels and the belt will be moving at the same speed. Adding finite quantities to infinite quantities gives an infinity of equal cardinality.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Keta
Posted 2007-05-23 17:45:37 and read 32767 times.

Quoting CptSpeaking (Reply 475):
That was my point to begin with...as long as that belt is moving at the same speed as the wheels, you're not getting any groundspeed and therefore no airspeed.

I think you're still confused... there is no way that the belt will move at the same speed as the wheels, the plane will always run faster than the belt. As some stated, in order to be the same, the belt must run at infinite speed as quick as the plane applies thrust... but you still get two infinities, one being a little bigger than the other. That is, for me it makes no difference in what direction and at what speed the belt runs, the plane will move forward.  spin 

Quoting CptSpeaking (Reply 475):
The point others are making is that no matter what speed you reach or how fast you reach that speed, the wheel will always be going the same relative speed as the belt. In this frame of mind, the belt never moves and the scenario is still accurate...it just depends on how you interpret the question. To demonstrate this, pick a roadsign in front of you while you are driving. No matter how fast you are going, or if you accelerate or not, that sign is always "coming at you" exactly as fast as you are going at it.

The same relative speed, relative to what? The belt never moves? I think I don't get you  boggled 

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Bond007
Posted 2007-05-23 18:03:44 and read 32767 times.

Quoting CptSpeaking (Reply 475):
That was my point to begin with...as long as that belt is moving at the same speed as the wheels, you're not getting any groundspeed and therefore no airspeed.

It really needs no interpretation of the question, since you are immediately breaking a basic rule of physics if you say the aircraft never moves forward.

Physics 101 will tell you that the plane MUST move forward in relation to the air ... regardless of what happens to the wheels and the belt.

As long as the wheels are free running and friction isn't an issue ... it WILL move forward. Any other part of the scenario where belt matches wheel speed is irrelevant after that.


Jimbo

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SilverComet
Posted 2007-05-23 19:13:04 and read 32767 times.

Quoting Bond007 (Reply 478):
Physics 101 will tell you that the plane MUST move forward in relation to the air ... regardless of what happens to the wheels and the belt.

That's another fine way of putting it in simple terms. Newton's 3rd law says if you push back on the air, the air pushes you forward relative to it, c'est obligé! Wheel bearing friction being ignored, you therefore have airspeed.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: BoeingOnFinal
Posted 2007-05-23 19:15:05 and read 32767 times.

Quoting CptSpeaking (Reply 475):
That was my point to begin with...as long as that belt is moving at the same speed as the wheels, you're not getting any ground speed and therefore no airspeed.

I have to agree with others here, this is not correct. It would only apply if the belt moved the mass of air above it along at the same speed as the belt. Then airspeed = ground speed. But since the air above the belt is relative to the ground around the belt rather than the belt itself, it will gain air speed.

IF, and only IF, the belt where to move in the opposite direction of the plane and create enough friction in the landing gear to stop the plane from accelerating, it would be able to hold til aircraft still. But only until Thrust > Friction, and that wouldn't take long since the wheels are running free.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: DarkBlue
Posted 2007-05-23 19:43:28 and read 32767 times.

Quoting Bond007 (Reply 478):
Physics 101 will tell you that the plane MUST move forward in relation to the air ... regardless of what happens to the wheels and the belt.

As long as the wheels are free running and friction isn't an issue ... it WILL move forward. Any other part of the scenario where belt matches wheel speed is irrelevant after that.



Quoting SilverComet (Reply 479):
That's another fine way of putting it in simple terms. Newton's 3rd law says if you push back on the air, the air pushes you forward relative to it, c'est obligé! Wheel bearing friction being ignored, you therefore have airspeed.

The point that CptSpeaking and I are trying to make is not if in a realistic scenario would the aircraft move -- we understand that wheels are irrelevant and yes the aircraft will move. The point we are trying to make is that the question is invalid. The aircraft cannot move and have matching wheel and conveyor speeds.

So to answer the original question, NO, this cannot occur. If you say yes, then congratulations, you've answered a different question.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Bond007
Posted 2007-05-23 19:55:30 and read 32767 times.

Quoting DarkBlue (Reply 481):
So to answer the original question, NO, this cannot occur.

Congratulations.. you've also answered a different question  Smile

The question was "can the plane take off?" .

You say "No", but also "yes the aircraft will move." .... so the answer to the question is actually YES.

Jimbo

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: DarkBlue
Posted 2007-05-23 21:22:31 and read 32767 times.

Quoting Bond007 (Reply 482):
Congratulations.. you've also answered a different question

The question was "can the plane take off?" .

You say "No", but also "yes the aircraft will move." .... so the answer to the question is actually YES.

Ha, yeah okay, fine. How's this: No, the scenario of an aircraft that takes off and has wheels matched to the conveyor cannot occur.

The only reason why I said "yes the aircraft will move" is because I agree that the aircraft will move (or takeoff) regardless of the wheels. My point is that this is not a valid answer to the question. I am not saying "No, the aircraft will not move", my answer to the question is "No, this scenario cannot occur".

I suppose you could twist my answer into, "No, an aircraft that has wheels matched to a conveyor cannot takeoff", or "No, an aircraft that does take off cannot have wheels matched to a conveyor". Either way my answer is no.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-05-23 22:34:41 and read 32767 times.

Quoting DarkBlue (Reply 481):
The point we are trying to make is that the question is invalid

I agree wholeheartedly with that...

Quoting David L (Reply 216):
if you're sticking to your guns about the conditions, the situation could never occur!



Quoting David L (Reply 333):
This is a situation which can't happen so whether the plane would take off is irrelevant, undefined.

 Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Bhill
Posted 2007-05-24 00:14:02 and read 32767 times.

Sounds like a dynomometer to me...the wheels spin and the car does not move forward...

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-05-24 01:20:33 and read 32767 times.

Quoting Bhill (Reply 485):
Sounds like a dynomometer to me...the wheels spin and the car does not move forward...

Aaargh! Aircraft wheels do not drive the aircraft!

Even in this magical scenario, the belt is not being driven by the aircraft wheels (unlike a dynamometer), it's "magically" matching the wheel speed by some unkown means. If you put a real aircraft on to a real dynamometer and applied take-off power, it would shoot off into the distance before the dynamometer had time to react. It would be just like pushing a car off a dynamometer with the gearbox in neutral and the brakes off.

 biggrin 

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: CptSpeaking
Posted 2007-05-24 06:44:34 and read 32767 times.

Quoting Bond007 (Reply 478):
It really needs no interpretation of the question, since you are immediately breaking a basic rule of physics if you say the aircraft never moves forward.

Good grief...it is a hypothetical question. The question is impossible in the real world, but given the variables we were given, NOT ADDING ANYTHING, there are two answers, both correct.

Quoting Keta (Reply 477):
I think you're still confused... there is no way that the belt will move at the same speed as the wheels, the plane will always run faster than the belt.

Nope...I'm definitely not confused...you're ignoring the last part of the question here, which, if you interpret it as the belt "spins" the same speed as the wheels, says that the wheels cannot spin faster. I don't care what you do to it, they are always going to spin the same. The key word in the question is "if". This would never or could never be duplicated...we are talking hypotheticals.

And to answer your other question...relative to the belt. My example of the roadsign should clear that up. Pick a point on the belt and you would be moving towards it exactly as fast as it is "moving towards you", regardless of how fast you or the belt are going. Another example with airplanes would be intercepts. Your closing velicoty on the other airplane is exactly the same as his closing velocity on you, regardless of speed or acceleration. This is why the answer can be yes as well as no while still staying within the guidelines the question gives. Clear as mud?

Quoting BoeingOnFinal (Reply 480):
It would only apply if the belt moved the mass of air above it along at the same speed as the belt. Then airspeed = ground speed. But since the air above the belt is relative to the ground around the belt rather than the belt itself, it will gain air speed.

By stating that the wheels will always spin faster than the belt, you are also ignoring the last part of the original question, unless of course you interpret it as I wrote in a previous response.

[Edited 2007-05-24 06:52:48]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Iwok
Posted 2007-05-24 07:21:45 and read 32767 times.

Quoting CptSpeaking (Reply 475):
That was my point to begin with...as long as that belt is moving at the same speed as the wheels, you're not getting any groundspeed and therefore no airspeed.

Right, BUT we are assuming that the plane is starting from a zero thrust situation at the end of the conveyor belt.

Quoting CptSpeaking (Reply 487):
Good grief...it is a hypothetical question. The question is impossible in the real world, but given the variables we were given, NOT ADDING ANYTHING, there are two answers, both correct.

Correct the two answers are: Zero and Infinity.

I can't beleive I have been thinking about this all day  bigthumbsup 

iwok

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: CptSpeaking
Posted 2007-05-24 12:43:21 and read 32767 times.

Quoting Iwok (Reply 488):
Right, BUT we are assuming that the plane is starting from a zero thrust situation at the end of the conveyor belt.

Yes, BUT the question doesn't say otherwise, and this is what a normal takeoff consists of. Any other situation (i.e. rolling start, touch and go, etc.) would be a different question and a [possibly] different answer...

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: StealthZ
Posted 2007-05-24 14:04:51 and read 32767 times.

Quoting Iwok (Reply 488):
groundspeed and therefore no airspeed.

Groundspeed has NOTHING to do with airspeed!!!

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: DarkBlue
Posted 2007-05-24 15:09:22 and read 32767 times.

Quoting StealthZ (Reply 490):
Groundspeed has NOTHING to do with airspeed!!!

Please read the original question.

Quoting Sovietjet (Thread starter):
There is no wind.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Bond007
Posted 2007-05-24 16:26:53 and read 32767 times.

Quoting Iwok (Reply 488):
Correct the two answers are: Zero and Infinity.

To what question again ???

"can the plane take off??"

The answer isn't 'zero and infinity' !


Jimbo

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Keta
Posted 2007-05-24 17:37:53 and read 32767 times.

Quoting CptSpeaking (Reply 487):
Good grief...it is a hypothetical question. The question is impossible in the real world, but given the variables we were given, NOT ADDING ANYTHING, there are two answers, both correct.

OK, now I see what you mean. You're right, but that's like answering "if something stands still, it stands still"

I don't like quoting myself, but anyway, here's an extract from my first post:

Quoting Keta (Reply 157):
So, our final conclusion was that the question is actually wrong and it's impossible such a situation.

What I, and I think many people too, tried to explain throughout this thread was "the scenario is impossible. Why? Because the plane must move". I was trying to convince people who said "it's physically possible for the speeds to be the same", which is not. This is a physics question, remember. I see you're not part of that group.  Wink

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SilverComet
Posted 2007-05-24 17:53:18 and read 32767 times.

And to think that all this has been unleashed becasue the OP didn't choose the right words...

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: CptSpeaking
Posted 2007-05-24 19:06:19 and read 32767 times.

Quoting Keta (Reply 493):
You're right, but that's like answering "if something stands still, it stands still

I guess that one is too hard for some here then also, eh?  Wink

Quoting Keta (Reply 493):
"the scenario is impossible. Why? Because the plane must move"

Agreed...

Your CptSpeaking

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Jetlagged
Posted 2007-05-24 19:14:59 and read 32767 times.

I see a pattern here. People dive into the thread and initially say the aircraft cannot move because the ground under it is moving backwards at exactly the same speed. Then the penny drops and they realise that this is irrelevant. The trap that somehow the ground moving slows the aircraft down is tempting, even to people who should know better. To keep the thread going a hardcore of the "no fly group" stands guard to keep pointing out that the question is flawed but if you follow its logic the aircraft can't move. There is a name for people like this on forums.

This whole argument is a circular as the wheels. The "no fly group" will never give up because they know how provocative their argument is.

The no fly group say that if the wheel can't move, neither can the aircraft. But then they get confused whether the wheel can't move relative to the ground or the belt. They try to have it both ways at once.

Basically the current no fly argument is that the only way the constraints of the question can be satisfied is if the aircraft doesn't move. Therefore the aircraft cannot move. This is of course a logical nonsense. You cannot prove an impossibility by simply saying some absurd pre-condition requires it.

It doesn't matter though, illogical logic aside. There is no way that the belt can transmit a linear force to the aircraft unless the brakes are applied. Then the question is reduced to "Can the aircraft take off with the brakes on?" So in any universe where the Laws of Physics apply the aircraft will take off. In some parallel universe where logic is stood on it's head who knows. That was not the question.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: DarkBlue
Posted 2007-05-24 23:10:40 and read 32767 times.

Quoting Jetlagged (Reply 496):
Basically the current no fly argument is that the only way the constraints of the question can be satisfied is if the aircraft doesn't move. Therefore the aircraft cannot move. This is of course a logical nonsense. You cannot prove an impossibility by simply saying some absurd pre-condition requires it.

Yes, I am obviously confused.    I interpreted the question as: Can an aircraft take off and have wheels matched to a conveyor? I still say, no, this scenario cannot happen.

Can we all just agree that there are different ways to interpret a question? You've chosen to answer the physics of a question, I've chosen to answer the logic of a question.

I'm sure you would say it is a physics question, so a physics answer is appropriate. However, I would argue that it's actually a theoretical question (come on, a conveyor the size of a runway that can move as fast as an aircraft?), so to answer a theoretical question, I choose to give the logic answer.

I see from your profile that you are a flight sim engineer, so you certainly are familiar with Boolean functions used in logic tests. If the Boolean "AND" connects two incompatible "IF" statements, isn't the answer to this logic test "FALSE"?

Now here's the real question.... why hasn't a mod locked this thread? You'd think after ~500 posts stating the same thing over and over someone would put an end to this agony!

[Edited 2007-05-24 23:12:06]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: StealthZ
Posted 2007-05-25 00:44:16 and read 32767 times.

Quoting DarkBlue (Reply 491):
Please read the original question.

You are right there in the no fly group aren't you.
I read the question and the activity of the wheels, rotating forward or backward or locked has nothing to do with the plane moving forward relative to the still air the OP mentioned. If it has enough thrust to overcome whatever friction is in the "system" it will move forward and will gain airspeed even if the "ground" underneath it is stationary.

Not sure how many times this needs to be explained.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Starlionblue
Posted 2007-05-25 02:39:39 and read 32767 times.

Quoting DarkBlue (Reply 497):

Now here's the real question.... why hasn't a mod locked this thread? You'd think after ~500 posts stating the same thing over and over someone would put an end to this agony!

Because it's still entertaining. I would think they're placing bets by now. Big grin

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SlamClick
Posted 2007-05-25 03:47:13 and read 32767 times.

To whom will go the dubious honor of the 500th post?

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Starlionblue
Posted 2007-05-25 05:24:18 and read 32767 times.

Quoting SlamClick (Reply 500):
To whom will go the dubious honor of the 500th post?

 rotfl   rotfl   rotfl   rotfl   rotfl 


Well well well... Look what the treadmill dragged in...

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Iwok
Posted 2007-05-25 06:08:45 and read 32767 times.

This is a really cool thread so please dont delete is mods  Smile

Quoting StealthZ (Reply 490):
Groundspeed has NOTHING to do with airspeed!!!

Well, if the headwind=0, then groundspeed is equal to airspeed, which is what was stated in the OP.

Quoting CptSpeaking (Reply 489):
es, BUT the question doesn't say otherwise, and this is what a normal takeoff consists of. Any other situation (i.e. rolling start, touch and go, etc.) would be a different question and a [possibly] different answer...



Quoting Bond007 (Reply 492):
can the plane take off??"

The answer isn't 'zero and infinity' !

The plane can take off if it has infinite thrust, otherwise it cannot because its air speed is zero...

Quoting StealthZ (Reply 498):
You are right there in the no fly group aren't you.
I read the question and the activity of the wheels, rotating forward or backward or locked has nothing to do with the plane moving forward relative to the still air the OP mentioned. If it has enough thrust to overcome whatever friction is in the "system" it will move forward and will gain airspeed even if the "ground" underneath it is stationary.

Not sure how many times this needs to be explained.

Again... its impossible for any thrust to be acting on the plane if its speed relative to a fixed point is zero, under the proposed zero air speed scenario. Friction is irrelevant to this discussion.

Its like saying something like this " if my bicycle had 4-wheels it would be a car".

I think this is the longest thread I have ever seen  eyepopping 
iwok

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Chksix
Posted 2007-05-25 09:40:11 and read 32767 times.

There are longer ones  Wink

Why are the "No Fly Zone" so hung up on the wheels?

I'll reiterate my point:
The belt is computer controlled to make sure it will move when a point on the airplane tyre is starting to move.

So the pilot adds power, the plane starts to roll.
The point on the tyre is now starting to move backwards and upwards so the belt is now compensating by "scrolling" forward to bring the point forward and down again.
Repeat this as the plane accellerates forward, the belt accellerates.

After a while, both belt and plane are doing enough knots to allow the plane to fly off. To an observer on the belt near the tyre it will look like the plane lifted off vertically.
To an observer on the ground beside the contraption it will look like a normal takeoff, just with stopped wheels.

What's so hard to understand??

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SilverComet
Posted 2007-05-25 10:32:06 and read 32767 times.

Quoting SlamClick (Reply 500):

LOL trust SlamClick to chime in at the right time.. glad to know he's still watching over us helpless Tech/Oppers.

Quoting Chksix (Reply 503):
What's so hard to understand??

What's so hard to understand is how you haven't understood yet (from the ~500 posts above) that the belt is actually moving backwards to try to prevent the plane from moving forward.  Wink I think the majority of posters here would instantly agree that the plane would take off if it were the other way round (but then again, this is Tech/Ops).  Yeah sure

Besides, even if the OP meant that the belt moved forward as you say, the magic of this place is that a very simple question can somehow be turned into something very complicated and be debated endlessly.. It no longer matters what Sovietjet meant initially. What's now being questioned is "can the plane take off with the conveyor belt moving backwards at the same speed as the wheels moving forward?"

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: XT6Wagon
Posted 2007-05-25 11:04:08 and read 32767 times.

Uhm, Its very simple. The conveyor belt/treadmill has NO effect on the airplane other than a highly likely overspeeding of the tires and hub bearings. The plane doesn't apply its force through the wheels. So while a car on a belt system like this is unable to move at all, an airplane doesn't apply its power in this manner.

Look at it this way, the landing gear and wheels are only there to reduce the drag on takeoff. Its kinda a bitch to take off with the fuselage and/or wings dragging on the ground, so you prop it up and stuff wheels or skis under it. Floats if you plan on water landings are also a nice idea.

So yes, a plane on a treadmill will take a bit longer to take off since it would have to overcome the extra friction from the higher wheel speeds. One might note that a 5-10K foot long treadmill would be just a bit epic of an engineering project so...

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: ThirtyEcho
Posted 2007-05-25 12:39:38 and read 32767 times.

This thread is for all of you folks who think that the airplane is steered on the ground with the yoke and that the floor pedals are gas and brake.

If an airplane is carrying 200,000 pounds of ducks and all of the startled ducks take flight when the engines spool up for takeoff, does the airplane take off more quickly?

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-05-25 12:43:11 and read 32767 times.

Quoting SilverComet (Reply 504):
What's so hard to understand is how you haven't understood yet (from the ~500 posts above) that the belt is actually moving backwards to try to prevent the plane from moving forward.

But the best the belt can do is to go so fast in the opposite direction that friction in the wheel bearings and between the tyres and the belt becomes infinite. After the wheels have locked, in order to provide infinite friction between the tyres and the belt (otherwise you'd effectively have an aircraft taking off with the brakes on, which would just take longer) it would still need to be moving infinitely fast, so would no longer be matching wheel speed. It can't happen within the conditions stated.

Quoting XT6Wagon (Reply 505):
The plane doesn't apply its force through the wheels. So while a car on a belt system like this is unable to move at all, an airplane doesn't apply its power in this manner.

Something that over half of the thread contributors haven't grasped.  Smile

Here's a reminder of why so many of us continued to debate the "classic" question, where the belt moves at the same speed as the aircraft but in the opposite direction ...

Quoting Sovietjet (Thread starter):
there is a huge debate over at another forum concerning this question...

Since we couldn't find any discussion elsewhere about the magical belt in the OP, some of us assumed the OP was referring to the "classic" question and debated that, which, I thought, was an interesting discussion. Others chose to debate a situation that could not happen.  Smile

Quoting SlamClick (Reply 500):

A rare but strategically placed post from the Master.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: StealthZ
Posted 2007-05-25 13:10:26 and read 32767 times.

Quoting Iwok (Reply 502):
Again... its impossible for any thrust to be acting on the plane if its speed relative to a fixed point is zero, under the proposed zero air speed scenario.

I think others above have answered this but it's speed relative to a fixed point is NOT zero, it's speed relative to a point on the conveyor belt may be zero but that by definition is NOT a fixed point

Quoting Chksix (Reply 503):
After a while, both belt and plane are doing enough knots to allow the plane to fly off. To an observer on the belt near the tyre it will look like the plane lifted off vertically.
To an observer on the ground beside the contraption it will look like a normal takeoff, just with stopped wheels.

Perfect..

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: DarkBlue
Posted 2007-05-25 15:33:42 and read 32767 times.

Quoting David L (Reply 507):
Here's a reminder of why so many of us continued to debate the "classic" question, where the belt moves at the same speed as the aircraft but in the opposite direction ...



Quoting StealthZ (Reply 508):
Quoting Chksix (Reply 503):
After a while, both belt and plane are doing enough knots to allow the plane to fly off. To an observer on the belt near the tyre it will look like the plane lifted off vertically.
To an observer on the ground beside the contraption it will look like a normal takeoff, just with stopped wheels.

Perfect..

Perfect? Ehhhh really? Now it seems to me it's the fly group that is confused. You now have the conveyor running in the opposite direction!

With the exception of the guy who wanders in here for the first time and falls into the "car or person on a treadmill" mistake, most of the "no fly" group understands that wheels are irrelevant to the physics of the problem. It's the logic of the problem that we question.

Quoting David L (Reply 507):
so would no longer be matching wheel speed. It can't happen within the conditions stated.

Aha! A "fly group" member that agrees with me! David L and I now agree that an aircraft cannot take off "within the conditions stated". All others agreed?

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SilverComet
Posted 2007-05-25 15:43:48 and read 32767 times.

Quoting David L (Reply 507):
Here's a reminder of why so many of us continued to debate the "classic" question, where the belt moves at the same speed as the aircraft but in the opposite direction ...

Quoting Sovietjet (Thread starter):
there is a huge debate over at another forum concerning this question...

Exactly!
However, the answer to the 'classic' situation is quite straightforward and the thead would have died pretty quickly. Tech/Ops, true to its nature, has turned a poor choice of words into a 508 (and still counting)-post long thread that's still going after one and a half years.

Quoting David L (Reply 507):
friction in the wheel bearings and between the tyres and the belt becomes infinite

I don't think any surface can provide infinite friction. I seem to recall from high school mechanics that the maximum friction that the surface can offer is equal to a certain coefficient times the normal reaction. What I expect to happen in that theoretical scenario is that the belt, in its attempt to match the speed of the wheels, reaches a speed where it can no longer provide enough friction to keep the wheels in place. The wheels start slipping down the belt and the plane takes off, c.f. #425.

But then again, as you say, "I may be wrong.."  Wink.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: BoeingOnFinal
Posted 2007-05-25 16:14:26 and read 32767 times.

Quoting DarkBlue (Reply 509):

Aha! A "fly group" member that agrees with me! David L and I now agree that an aircraft cannot take off "within the conditions stated". All others agreed?

I might get in on that as well. I say that the aircraft will take off if such a scenario is created, because there is no way that a conveyor belt can hold the aircraft still. But under the condition stated;

Quoting Sovietjet (Thread starter):
The conveyor belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.

it is not possible. I actually think that the question is confusing, because how do you match the speed? As the wheels start to move when you apply thrust, the belt would have to move in the opposite direction. So, this would be increasing the wheel's speed as well. So it would have to go faster to match the speed, thus increasing wheel speed even more.

So if the question was formulated: "The conveyor belt is designed to exactly match the speed of the wheels with reference to surrounding terrain at any given time, moving in the opposite direction of rotation." Because then the wheel speed would just double. And in this scenario the aircraft would definately take off.

It could also be formulated: "The conveyer belt is designed to speed up backwards giving enough resistance in the wheels to stop the aircraft from accelerating." This would actually give an opposite reaction to thrust, which would hold the aircraft still even though it is moving masses of air. Although it can be discussed if this scenario is actually possible, because the friction in the wheels is not much compared to thrust created.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: CptSpeaking
Posted 2007-05-25 16:29:04 and read 32767 times.

Quoting XT6Wagon (Reply 505):
Uhm, Its very simple. The conveyor belt/treadmill has NO effect on the airplane other than a highly likely overspeeding of the tires and hub bearings. The plane doesn't apply its force through the wheels. So while a car on a belt system like this is unable to move at all, an airplane doesn't apply its power in this manner.

But if it does move forward, then as I've stated dozens of times before, the question has been ignored. Work with what you've got!

Quoting ThirtyEcho (Reply 506):
This thread is for all of you folks who think that the airplane is steered on the ground with the yoke and that the floor pedals are gas and brake.

If an airplane is carrying 200,000 pounds of ducks and all of the startled ducks take flight when the engines spool up for takeoff, does the airplane take off more quickly?

Don't get us going again already...we have to turn you to the dark side on this one first  Wink

Quoting David L (Reply 507):
But the best the belt can do is to go so fast in the opposite direction that friction in the wheel bearings and between the tyres and the belt becomes infinite. After the wheels have locked, in order to provide infinite friction between the tyres and the belt (otherwise you'd effectively have an aircraft taking off with the brakes on, which would just take longer) it would still need to be moving infinitely fast, so would no longer be matching wheel speed.

You're adding things to the question again that AREN'T THERE!!! If the wheels are skidding, then fine, I agree, it will take off. But that isnt in the question, you're making that up.

Quoting David L (Reply 507):
It can't happen within the conditions stated.

With the conditions stated in the OP, both can happen base in how you interpret it. With your added conditions, then you're correct, but that isn't what we're discussing.

Quoting David L (Reply 507):
Something that over half of the thread contributors haven't grasped.

Ad hominem there because it isn't true about the ones actually defending the point...

Quoting StealthZ (Reply 508):
I think others above have answered this but it's speed relative to a fixed point is NOT zero, it's speed relative to a point on the conveyor belt may be zero but that by definition is NOT a fixed point

Think a car on a dyno...before you start up, I KNOW the difference in propulsion, but given the conditions of the question, it really doesn't matter. It will never work in real life, but that's not what we're discussing now, are we?

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: StealthZ
Posted 2007-05-25 16:38:45 and read 32767 times.

Quoting CptSpeaking (Reply 512):
Think a car on a dyno...before you start up, I KNOW the difference in propulsion, but given the conditions of the question, it really doesn't matter. It will never work in real life, but that's not what we're discussing now, are we?

This has nothing to do with a car on a dyno.
The car on a dyno, the wheels drive the dyno, this does not happen with an aircraft with reactive propulsion systems!

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: DarkBlue
Posted 2007-05-25 17:08:13 and read 32767 times.

Quoting StealthZ (Reply 513):
Quoting CptSpeaking (Reply 512):
Think a car on a dyno...before you start up, I KNOW the difference in propulsion, but given the conditions of the question, it really doesn't matter. It will never work in real life, but that's not what we're discussing now, are we?

This has nothing to do with a car on a dyno.
The car on a dyno, the wheels drive the dyno, this does not happen with an aircraft with reactive propulsion systems!

Ah, but see that's exactly the point. Realistically, yes this has nothing to do with a car on a dyno, yeah yeah, wheels are irrelevant i think someone once pointed out.  wink  However, because of how this particular question is phrased, the conditions require the same thing as a car on a dyno.

Any wagers on how soon we reach 1000 posts?

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: SilverComet
Posted 2007-05-25 17:49:05 and read 32767 times.

Quoting DarkBlue (Reply 514):
Any wagers on how soon we reach 1000 posts?

I say about 40-50 days.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-05-25 19:16:20 and read 32767 times.

Quoting DarkBlue (Reply 509):
Aha! A "fly group" member that agrees with me! David L and I now agree that an aircraft cannot take off "within the conditions stated". All others agreed?

I didn't say that. I said the situation couldn't happen. I don't agree at all. I only said, explicitly, that the aircraft would fly in the classic scenario. In any case, that statement is as contradictory as the question - How can I be in the "fly group" and agree that it wouldn't fly?  

Quoting SilverComet (Reply 510):
I don't think any surface can provide infinite friction.

True but I said the only way the belt could prevent the aircraft from taking off is if it caused infinite friction. If the belt produced "a little friction" or "a lot of friction", it would still be overcome by the thrust of the engines... eventually. Unless you introduce some magic force, of course.

Quoting CptSpeaking (Reply 512):
You're adding things to the question again that AREN'T THERE!!! If the wheels are skidding, then fine, I agree, it will take off. But that isnt in the question, you're making that up.

I didn't say it was in the question. I said it was the best the belt could do... under any circumstances. I was not making anything up.

Quoting CptSpeaking (Reply 512):
Quoting David L (Reply 507):
Something that over half of the thread contributors haven't grasped.

Ad hominem there because it isn't true about the ones actually defending the point...

Ad hominem? I'm talking about those who compared it to a car driving or a person running on a moving converyor belt. Did you assume I was referring to anyone in the "would not fly" group?

Quoting DarkBlue (Reply 514):
However, because of how this particular question is phrased, the conditions require the same thing as a car on a dyno.

The conditions require an aircraft with powered wheels and no jet engines?

Quoting CptSpeaking (Reply 512):
But if it does move forward, then as I've stated dozens of times before, the question has been ignored.

And that's what it all boils down to. If you really want to argue the question as stated, you have to accept a magical force that "somehow" allows the conveyor belt to stop the aircraft from gaining airspeed. In other words, if the belt could "somehow" stop the aircraft from gaining airspeed, would it stop the aircraft from gaining airspeed? A real conveyor belt could not deliver much horizontal force to the rest of the aircraft through the unpowered wheels of a real aircraft so it wouldn't care what was happening to the wheels.

Quoting SilverComet (Reply 510):
However, the answer to the 'classic' situation is quite straightforward and the thead would have died pretty quickly.

I think quite a few kept the debate going in addressing the "person running on a treadmill" misunderstanding.  

[Edited 2007-05-25 19:18:53]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: DarkBlue
Posted 2007-05-25 20:24:35 and read 32767 times.

Quoting David L (Reply 516):
I didn't say that. I said the situation couldn't happen. I don't agree at all. I only said, explicitly, that the aircraft would fly in the classic scenario.



Quoting David L (Reply 507):
Since we couldn't find any discussion elsewhere about the magical belt in the OP, some of us assumed the OP was referring to the "classic" question and debated that, which, I thought, was an interesting discussion. Others chose to debate a situation that could not happen.

Now we're getting somewhere. I'll agree with you that for the "classic" question the aircraft will fly, however the question stated was not the "classic" question even though as you said, many have assumed it was and this is what you have chosen to debate. On the other hand, I have chosen to debate the situation that could not happen only because this is the literal interpretation of the question.

Quoting David L (Reply 516):
In any case, that statement is as contradictory as the question - How can I be in the "fly group" and agree that it wouldn't fly?

Hah, good point.  Smile

Quoting David L (Reply 516):
The conditions require an aircraft with powered wheels and no jet engines?

Nope, not at all. Please don't be silly. The comparison is that a car on a dyno has matching speeds just as this imaginary conveyor defined by the OP requires matching speeds. A real aircraft certainly does not require anything of the wheels.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Chksix
Posted 2007-05-25 20:33:30 and read 32767 times.

By "matching the wheel speed" is see a solution where the belt is trying to keep the wheel rpm at zero.

To read the function the opposite way it should be stated like: "matching the speed of the plane or LG strut" and then it would mean just a doubling of the rpm.

A plane lifting off at 120 kt would be rolling on the belt at 240 kt. I think that's well below tyre shredding speed....

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-05-25 20:44:36 and read 32767 times.

Quoting DarkBlue (Reply 517):
I'll agree with you that for the "classic" question the aircraft will fly, however the question stated was not the "classic" question even though as you said, many have assumed it was and this is what you have chosen to debate. On the other hand, I have chosen to debate the situation that could not happen only because this is the literal interpretation of the question

Once the reality of the thread dawned, I and several others explicitly said we were happy to continue the "classic" debate but that the question as stated probably belonged in Non-aviation. As you say, it could not happen so why debate what "would" happen? It would be better for philosophers to discuss in Non-aviation how much the aircraft to "wanted" to fly and how much the conveyor belt didn't "want" it to fly.  Smile

Quoting DarkBlue (Reply 517):
Nope, not at all. Please don't be silly. The comparison is that a car on a dyno has matching speeds just as this imaginary conveyor defined by the OP requires matching speeds. A real aircraft certainly does not require anything of the wheels.

But it's "silly" to have an aircraft behaving like a car on a dyno. It's not aviation, let alone Tech/Ops.

I still believe the wording in the OP was initially a mistake and that the "classic" question was intended but I can't prove it.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Chksix
Posted 2007-05-25 20:57:20 and read 32767 times.

No matter how many times I read the question I understand it as the wheels will be stopped from rolling by the belt.

My jet is taking off from left to right on this belt, it's tyres will roll clockwise, the belt will "match" the clockwise rotation by making the wheels rotate counterclockwise. In this case the speed is matched perfectly so the CCW rotation will just make the wheel stop rotating.
The belt top surface in this situation (as stated in the OP) is moving from left to right in the picture.

I actually made illustrations long ago with vectors and stuff drawn in but they are gone now  Sad

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-05-25 21:23:22 and read 32767 times.

Quoting Chksix (Reply 520):
No matter how many times I read the question I understand it as the wheels will be stopped from rolling by the belt.

That was another option entertained by those of us who wanted to deal with real physics, i.e. something that could actually happen. Alas, we were effectively told to "get lost".  Smile

Quoting Chksix (Reply 520):
My jet is taking off from left to right on this belt, it's tyres will roll clockwise, the belt will "match" the clockwise rotation by making the wheels rotate counterclockwise. In this case the speed is matched perfectly so the CCW rotation will just make the wheel stop rotating.

And, as you said, you'd have an aircraft taking off at normal airspeed with the wheels not rotating. But no, that's not what the conditions state... apparently.  confused 

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Keta
Posted 2007-05-25 21:23:22 and read 32767 times.

Quoting David L (Reply 519):

Pretty much what I think  Smile

Quoting Chksix (Reply 520):
The belt top surface in this situation (as stated in the OP) is moving from left to right in the picture.

I understand it the other way, that the belt tries to stop the wheels relative to the surrounding, not just stop rotating.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-05-25 21:45:29 and read 32767 times.

Quoting Keta (Reply 522):
Pretty much what I think

I know, I think I borrowed this...

Quoting David L (Reply 516):
In other words, if the belt could "somehow" stop the aircraft from gaining airspeed, would it stop the aircraft from gaining airspeed?

from something you said earlier... like, about a year ago.  Smile

Quoting Keta (Reply 522):
I understand it the other way, that the belt tries to stop the wheels relative to the surrounding, not just stop rotating.

I think there are 4 possibilities:

1. Belt opposes aircraft's ground-speed, not its "belt-speed" ("classic" scenario)
2. Belt stops tyres rotating ("as stated" version 1),
3. Magic force stops the aircraft from gaining air/groundspeed ("as stated" version 2)
4. The question was never asked in the first place and what are you talking about?

Results:

1. Aircraft takes off at normal air/groundspeed but wheels rotate twice as fast as normal,
2. Aircraft takes off at normal air/groundspeed but wheels not rotating,
3. Twighlight zone with magic force - belt changes colour, slips in and out of focus, we all feel one with the universe. "But what happens to the plane?"... "I don't care, look at all the colours!"...
4. With hindsight, I'd like to believe this was the case.

 biggrin 

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Starlionblue
Posted 2007-05-25 21:49:38 and read 32767 times.

Quoting Chksix (Reply 503):
So the pilot adds power, the plane starts to roll.
The point on the tyre is now starting to move backwards and upwards so the belt is now compensating by "scrolling" forward to bring the point forward and down again.
Repeat this as the plane accellerates forward, the belt accellerates.

After a while, both belt and plane are doing enough knots to allow the plane to fly off. To an observer on the belt near the tyre it will look like the plane lifted off vertically.
To an observer on the ground beside the contraption it will look like a normal takeoff, just with stopped wheels.

What's so hard to understand??

Correct.

Quoting SilverComet (Reply 504):
What's so hard to understand is how you haven't understood yet (from the ~500 posts above) that the belt is actually moving backwards to try to prevent the plane from moving forward. Wink I think the majority of posters here would instantly agree that the plane would take off if it were the other way round (but then again, this is Tech/Ops).

The belt cannot prevent the plane from moving forward since it has no effect on the engines.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-05-25 22:16:54 and read 32767 times.

Quoting Starlionblue (Reply 524):
The belt cannot prevent the plane from moving forward since it has no effect on the engines.

Unless you believe the conditions determine that "it just does - we don't know how or why or what other aspects of the universe need to be changed to let it happen but it just does".  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Jetlagged
Posted 2007-05-25 22:42:42 and read 32767 times.

Quoting DarkBlue (Reply 497):
I see from your profile that you are a flight sim engineer, so you certainly are familiar with Boolean functions used in logic tests. If the Boolean "AND" connects two incompatible "IF" statements, isn't the answer to this logic test "FALSE"?

I studied logic at school too. Two statements which can never be true at the same time will always produce a false result when ANDed, but that is not what is happening.

You say that the conditions require the belt speed to be matched to the wheels. Then you say this can only happen when groundspeed is zero, and conclude that the condition requires that the aircraft can't take off. All I can see is the condition demands the belt attempt to keep up with the wheels, it doesn't demand that the aircraft must behave in such a way as to make this happen. The fact that the only way this is satisfied is if ground speed is zero does not logically mean the aircraft cannot take off. THE AIRCRAFT IS NOT CONSTRAINED BY THE QUESTION. Aircraft speed determines belt speed but not vice versa. The no fly logic is flawed.

It reminds me of the biologist who experimented with fleas. He trained a flea to jump on command. It learned to jump every time. Then he removed the flea's legs and commanded it to jump. The flea did not jump, so the biologist concluded that a flea's ears must be on its legs. Logical but flawed.

Logic and physics are compatible, even in this case. Paradoxes are just that, a way of logically stating a problem so that it appears to contradict normality. The key word here is appears. Even Zeno didn't believe his paradoxes proved anything, they just posed apparent logical problems to be explained. This question is not in that league as paradoxes go.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Mrocktor
Posted 2007-05-25 22:52:47 and read 32767 times.

Quoting Starlionblue (Reply 524):
The belt cannot prevent the plane from moving forward since it has no effect on the engines.

The belt can prevent the plane from moving forward if it generates enough friction on the wheels to balance out the thrust. Yes, the treadmill would have to be moving at a ridiculous speed, the wheels would be spinning at a million RPM, tires would burst, it is not a realistic scenario. But a treadmill does create a force on the aircraft that is contrary to motion and proportional to the relative speed between aircraft and treadmill surface.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Keta
Posted 2007-05-25 23:42:36 and read 32767 times.

Quoting Mrocktor (Reply 527):
But a treadmill does create a force on the aircraft that is contrary to motion and proportional to the relative speed between aircraft and treadmill surface

I've seen many times this statement, and I'm going to discuss only this . Rolling friction is not dependent on the speed, it only depends on the normal force (i.e. weight-lift-whatever). No matter what are the relative speeds between a wheel and whatever it's rolling over, the force is always the same.

The speeds would only have an effect on the power or dissipated energy, which would make the tires and pavement get hotter, but I don't think it's a good idea adding more heat to the discussion.

Edit: For more info on Rolling Friction, you can visit Wikipedia or other pages, like this one I found with Google.

[Edited 2007-05-25 23:51:49]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-05-26 00:12:06 and read 32767 times.

Quoting Keta (Reply 528):
Rolling friction is not dependent on the speed, it only depends on the normal force (i.e. weight-lift-whatever). No matter what are the relative speeds between a wheel and whatever it's rolling over, the force is always the same.

Ah, yes, you're right. I got carried away pointing out that only "ridiculous" forces could allow the belt to prevent a jet aircraft from moving forwards at all. I'd stopped caring where those ridiculous, "alleged" forces were supposed to come from.  Smile

Quoting Mrocktor (Reply 527):
Yes, the treadmill would have to be moving at a ridiculous speed, the wheels would be spinning at a million RPM, tires would burst, it is not a realistic scenario.

As a concession, those who insisted on pondering the imponderable were "allowed" to assume the tyres wouldn't burst.  Smile

Quoting Mrocktor (Reply 527):
But a treadmill does create a force on the aircraft that is contrary to motion and proportional to the relative speed between aircraft and treadmill surface.

Yes, but, without the magic force that "by definition" stops the plane from gaining airspeed, a real conveyor belt wouldn't be able to exert enough force through real aircraft wheels to stop a real aircraft from gaining airspeed, unless it was extremely light and extremely low powered. I think the belt would have a hard time preventing a lightly loaded 757 from gaining airspeed.  Smile

Quoting Keta (Reply 528):
but I don't think it's a good idea adding more heat to the discussion.

Oh dear...  biggrin 

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Starlionblue
Posted 2007-05-26 03:49:48 and read 32767 times.

Quoting Mrocktor (Reply 527):
The belt can prevent the plane from moving forward if it generates enough friction on the wheels to balance out the thrust. Yes, the treadmill would have to be moving at a ridiculous speed, the wheels would be spinning at a million RPM, tires would burst, it is not a realistic scenario. But a treadmill does create a force on the aircraft that is contrary to motion and proportional to the relative speed between aircraft and treadmill surface.

Well, ok. But I doubt the original poster intended to include friction.

Quoting David L (Reply 525):
Quoting Starlionblue (Reply 524):
The belt cannot prevent the plane from moving forward since it has no effect on the engines.

Unless you believe the conditions determine that "it just does - we don't know how or why or what other aspects of the universe need to be changed to let it happen but it just does".

I seriously doubt that "it just does" holds up to some solid scientific experiments. I will take a page from Carl Sagan's book and keep my Baloney Detector handy. I suspect it would get a reading.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Iwok
Posted 2007-05-26 08:16:59 and read 32767 times.

Quoting Chksix (Reply 503):
So the pilot adds power, the plane starts to roll.

Impossible scenario with this miraculous treadmill.

Quoting ThirtyEcho (Reply 506):
This thread is for all of you folks who think that the airplane is steered on the ground with the yoke and that the floor pedals are gas and brake.

 rotfl 

Quoting XT6Wagon (Reply 505):
Look at it this way, the landing gear and wheels are only there to reduce the drag on takeoff. Its kinda a bitch to take off with the fuselage and/or wings dragging on the ground, so you prop it up and stuff wheels or skis under it. Floats if you plan on water landings are also a nice idea.

As I said, I belive the question is an impossibility.

Reviewing the original OP again, it does not state that the wheels are at rest when this sequence starts. So, lets assume the wheels are rotating at an equivalent 250kts and the belt at an equivalent -250kts; the plane will not take off as we all know.

So the original question could also be interpreted as: is it possible for the plane to accelerate relative to a fixed point on the ground?; and the answer is NO. This is the pertinent question, because we all know that ground speed (because there is no wind as per the OP) is required to get the plane in the air.

Fun thread.  Smile

iwok

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: StealthZ
Posted 2007-05-26 08:39:47 and read 32767 times.

Quoting Iwok (Reply 531):
is it possible for the plane to accelerate relative to a fixed point on the ground?; and the answer is NO.

why is it not possible?

Some people seem to forget that the only purpose of the undercarraige on an aeroplane is to hold it off the ground, they only have wheels because it makes it easier for the tug driver to push them around, the takeoff performance of an aircraft at no time depends on the rotational speed or direction of the wheels. If the conveyor belt runs in a direction that increases the rotational speed or a direction that reduces the wheel speed... doesn't matter as long as the thrust of the engines moves the plane forward at sufficient speed it will fly.

If an aircraft depended on the rotation of the wheels to fly then there would be no floatplanes, flyingboats or ski equiped planes.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: XT6Wagon
Posted 2007-05-26 09:12:41 and read 32767 times.

The speed of the treadmill is near on meaningless. The direction of the treadmills movement is of some importance since if it operates at a direction not in line with the direction the tires point it will have an effect on airspeed.

Now in reality you would very much find that if you somehow made a 1.5 mile long treadmill capible of supporting an aircraft, that it would be unable to safely take off (jets anyhoo) with it opposing your takeoff. This has nothing to do with the physics of flight, but rather very real realities about wheel speed limits. Its not a good plan if you have a 130mph take off speed and your wheels are doing 260mph while rated for only 170mph or so. Tire bursts and frozen bearings at near take off speeds could quickly boil down to wreckage being chucked off the back of the treadmill.

I think people need to really work it out in thier minds just how meaningless a treadmill is to a plane as far as highschool physics questions like this go. If you ignore the friction, and operational limits on the speed of the wheels/tires then the plane could be taking off on anything without much concern. The engines apply thier power to the air. The wings generate lift by the flow of air past the wing. The landing gear simply reduces the friction with whatever surface the plane is on.

Oh and if the treadmill operates at an angle to the direction the plane is pointing, then it rather acts like a cross wind since thats what will in effect happen. The plane moves sideways through still air. So yes, given that planes can't take off in severe crosswinds, it is possible for a treadmill to prevent a plane from taking off. However given the size of the required treadmill to prove it, don't expect more than computer simulation of it. Though it would be cool to see a couple square miles of treadmill with a 737 trying to take off on it.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-05-26 11:41:24 and read 32767 times.

Quoting Starlionblue (Reply 530):
I seriously doubt that "it just does" holds up to some solid scientific experiments.

You know that and I know that but that's what some people are saying.  Smile

Quoting Starlionblue (Reply 530):
I will take a page from Carl Sagan's book and keep my Baloney Detector handy. I suspect it would get a reading.

 checkmark 

Quoting Iwok (Reply 531):
So, lets assume the wheels are rotating at an equivalent 250kts and the belt at an equivalent -250kts; the plane will not take off as we all know.

Are you saying that the aircraft can't accelerate to gain airspeed because the wheels are already rotating at take-off speed? I really hope not because that simply isn't the case. The aircraft doesn't care how quickly or slowly the wheels are turning - the wheels are only there to isolate the rest of the aircraft from the friction of the ground.

In the scenario you describe, the aircraft would only need to produce a small amount of thrust to counteract the friction so would be able to apply more thrust. The aircraft doesn't feel the full force of the conveyor belt moving at 250 kts because the wheels isolate almost all of it from the aircraft. The full force applied by the conveyor belt is felt by the bottom of the wheels but, instead of transferring that energy to the aircraft, it causes the bottom of the wheel to move in one direction and the top of the wheel to move in the opposite direction. Friction in the wheel bearings transfer only a very small part of that force to the aircraft.

I suspect another misunderstanding is that some believe the force of the conveyor belt on the aircraft is the same as it would be if applied to the nose instead of the bottom of the wheels. It's not - the wheels won't let most of it reach the rest of the aircraft.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Iwok
Posted 2007-05-26 19:18:42 and read 32767 times.

Quoting David L (Reply 534):
Are you saying that the aircraft can't accelerate to gain airspeed because the wheels are already rotating at take-off speed?

What I am saying that that, irregardless of the initial wheel/conveyor belt speed: it is impossible for the plane to achieve an air speed greater than zero.

The amount of thrust available, wheel speed limits etc.. It is impossible for the plane to accelerate.

Its the mysterious backwards treadmilll that prevents this.

-iwok

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Mir
Posted 2007-05-26 19:45:43 and read 32767 times.

Quoting DarkBlue (Reply 491):
Quoting StealthZ (Reply 490):
Groundspeed has NOTHING to do with airspeed!!!

Please read the original question.

Quoting Sovietjet (Thread starter):
There is no wind.

Groundspeed STILL has nothing to do with airspeed in this case, because the ground (treadmill) is moving. You could spin the treadmill at 200kts, which would spin the wheels at 200kts in the opposite direction, but the plane wouldn't go anywhere relative to the air (or to the non-treadmill ground). So forget about groundspeed relative to the treadmill - it doesn't matter.

Quoting Iwok (Reply 531):
So the original question could also be interpreted as: is it possible for the plane to accelerate relative to a fixed point on the ground?; and the answer is NO.

So those big jet engines are doing nothing?

-Mir

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-05-26 21:40:16 and read 32767 times.

Quoting Iwok (Reply 535):
Its the mysterious backwards treadmilll that prevents this.

Ah, OK. If you're in the group that interprets the problem as meaning "whatever happens and however illogical and unrealistic it may be, the aircraft is not allowed to gain airspeed", then I see what you mean. However, with such an arbitrary constraint like that, for no good reason, it's really no wonder so many wanted to pursue more realistic and logical interpretations... which actually do warrant discussion.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: MidEx216
Posted 2007-05-26 22:27:31 and read 32767 times.

Quoting DarkBlue (Reply 444):
Everytime you state that the wheels are irrelevant you are choosing to ignore the actual question. Yes, that makes it an easy question to answer, but you have failed to answer the question that was asked.

Not quite, because even though the belt is matching the wheel speed, that doesn't mean it undoes the affect of the engines any. Once it starts getting up to speed, the engines are going to take effect on the air, no the ground, so the plane will start moving forward, and the belt will match that speed. But the engines are still pushing, so the plane is still gonna move forward, which means the belt will just keep moving faster and faster (essentially in an endless cycle) with no real affect on the aircraft. I think the only problem here is the structural limitation, that after a while, the wheels will just be spinning too fast, and (I believe) blow out the tires.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: CptSpeaking
Posted 2007-05-26 22:30:53 and read 32767 times.

Quoting Mir (Reply 536):
Groundspeed STILL has nothing to do with airspeed in this case, because the ground (treadmill) is moving.

Groundspeed has everything to do with it because in a no wind situation, as the OP specifies, Groundspeed will always equal true airspeed.

Quoting David L (Reply 537):
Ah, OK. If you're in the group that interprets the problem as meaning "whatever happens and however illogical and unrealistic it may be, the aircraft is not allowed to gain airspeed", then I see what you mean. However, with such an arbitrary constraint like that, for no good reason, it's really no wonder so many wanted to pursue more realistic and logical interpretations... which actually do warrant discussion.

Holy Smokes!!! You finally understand what we're talking about!!!! Woohoo!!!  Smile

So now you see that we've been talking about the question asked, and you're talking about another, "classic" version of the question...I agree that there is no real good reason to be discussing this hypothetical, but thats the beauty of this place  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-05-26 22:47:35 and read 32767 times.

Quoting CptSpeaking (Reply 539):
Holy Smokes!!! You finally understand what we're talking about!!!! Woohoo!!!

So now you see that we've been talking about the question asked, and you're talking about another, "classic" version of the question

No, I don't "finally understand". I've been differentiating between the "classic" version, other feasible interpretations and the version "as stated" from waaay back in the early days of the thread, almost a year and a half ago. I don't know how many times I need to say so!  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Jetlagged
Posted 2007-05-27 00:15:19 and read 32767 times.

Quoting Keta (Reply 528):
Rolling friction is not dependent on the speed, it only depends on the normal force (i.e. weight-lift-whatever). No matter what are the relative speeds between a wheel and whatever it's rolling over, the force is always the same.

Actually it's static friction that is related to normal reaction. Rolling friction is partially speed dependent. However, rolling friction is a relatively small force. If it wasn't, aircraft would not need brakes to stop.

People do not seem to understand how friction works but go on about the belt exerting frictional forces on the aircraft.

Quoting Iwok (Reply 535):
What I am saying that that, irregardless of the initial wheel/conveyor belt speed: it is impossible for the plane to achieve an air speed greater than zero.

Regardless of the fact that irregardless is not a word.... now we've been switched from logic back to the very simplistic arguments presented when the thread started.  banghead 

The question is designed to make you think of rolling roads, treadmills etc. The intelligent belt is added only to make the wheels rotate as if it was a rolling road and so apparently cancel the groundspeed. Because the aircraft's thrust is relative to the air, it is unaffected by the belt. Friction is irrelevant, the aircraft could be on racing slick tyres and still take-off.

Quoting CptSpeaking (Reply 539):
Groundspeed has everything to do with it because in a no wind situation, as the OP specifies, Groundspeed will always equal true airspeed.

Round and round it goes: of course ground speed and airspeed are the same, but that is speed relative to the ground, not speed relative to the belt. Ground speed increases, whatever happens with the belt. The question only says how the belt behaves, it does not mention that the laws of physics must change to allow this to happen. To infer that is illogical. You no fly guys are all too happy to change your frame of reference as it suits you.

There is nothing in the question, as ambiguously stated as it is, which constrains aircraft behaviour or implies that it is in some way affected by the belt. The wheels are free to roll over the belt. How fast they roll depends on ground speed and belt speed, but roll they will. How you interpret "wheel speed" determines what belt speed will be, but does not affect ground speed.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-05-27 00:46:22 and read 32767 times.

Quoting Jetlagged (Reply 541):
The intelligent belt is added only to make the wheels rotate as if it was a rolling road and so apparently cancel the groundspeed.

I agree but, according to some, the conditions in the thread starter dictate that the belt "somehow" prevents the aircraft from gaining airspeed... therefore, end of an extremely pointless discussion.

What you're describing is option 2 below...

Quoting David L (Reply 523):
I think there are 4 possibilities:

1. Belt opposes aircraft's ground-speed, not its "belt-speed" ("classic" scenario)
2. Belt stops tyres rotating ("as stated" version 1),
3. Magic force stops the aircraft from gaining air/groundspeed ("as stated" version 2)
4. The question was never asked in the first place and what are you talking about?

Results:

1. Aircraft takes off at normal air/groundspeed but wheels rotate twice as fast as normal,
2. Aircraft takes off at normal air/groundspeed but wheels not rotating,
3. Twighlight zone with magic force - belt changes colour, slips in and out of focus, we all feel one with the universe. "But what happens to the plane?"... "I don't care, look at all the colours!"...
4. With hindsight, I'd like to believe this was the case.

Many others, basically those in the "will fly" group (including me in at least 17 posts I can find), explicitly stated that options 1 and 2 were worth discussing, and continued to do so, while option 3 was impossible, illogical, etc.

It would have been a lot simpler if those who wanted to "debate" option 3 had taken it to Non-aviation, where it belongs.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Mir
Posted 2007-05-27 00:47:29 and read 32767 times.

Quoting CptSpeaking (Reply 539):
Groundspeed has everything to do with it because in a no wind situation, as the OP specifies, Groundspeed will always equal true airspeed.

In relation to what? If you're talking about the belt, then it doesn't matter, for reasons that I've described. If you're talking about the ground, then it does.

But the speed of the wheels in this case does not equal groundspeed, and does not equal airspeed.

-Mir

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-05-27 01:07:52 and read 32767 times.

Quoting Mir (Reply 543):
In relation to what? If you're talking about the belt, then it doesn't matter, for reasons that I've described. If you're talking about the ground, then it does.

But the speed of the wheels in this case does not equal groundspeed, and does not equal airspeed.

Except that CptSpeaking is arguing option 3, the impossible, illogical, non-aviation one so he doesn't need to "explain" why the aircraft can't gain airspeed. He interprets the OP as dictating that it can't gain airspeed - it doesn't matter how or why.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: StealthZ
Posted 2007-05-27 01:19:46 and read 32767 times.

Quoting Mir (Reply 543):
But the speed of the wheels in this case does not equal groundspeed, and does not equal airspeed.

Can you not see that it does not matter what the wheels rotation is relative to, the only important thing is the airspeed and I see nothing in this question that will prevent the aircraft gaining airspeed.
(apart from the structural limitations of tyres and bearings etc but the question dispenses with those issues regarding the construction of this mythical conveyor belt so perhaps we can as well.)

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Keta
Posted 2007-05-27 02:18:02 and read 32767 times.

Quoting Jetlagged (Reply 541):
Actually it's static friction that is related to normal reaction. Rolling friction is partially speed dependent. However, rolling friction is a relatively small force. If it wasn't, aircraft would not need brakes to stop.

I'm interested in seeing that. All sources I have seen say that rolling friction is just a kind of static friction, related to the deformation of the objects, and is only dependent on the normal force. More explicitly, I saw everywhere F = Crr·N, where Crr is the coefficient of rolling friction, a constant for two given materials (well, I have seen that it may change with temperature, but whatever). Really, it would make sense to me that rolling friction was speed dependent, but it's not what I've seen.

I think you misunderstood me, I was precisely saying that the friction between the wheels and the belt would be very small. Even at infinite speed, the friction would be the same. That is, very little.

Quoting David L (Reply 542):
Many others, basically those in the "will fly" group (including me in at least 17 posts I can find), explicitly stated that options 1 and 2 were worth discussing, and continued to do so, while option 3 was impossible, illogical, etc.

It would have been a lot simpler if those who wanted to "debate" option 3 had taken it to Non-aviation, where it belongs.

Sometimes I wish option 4 was the true one  wink 

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-05-27 02:37:09 and read 32767 times.

Quoting Mir (Reply 543):
Quoting CptSpeaking (Reply 539):
Groundspeed has everything to do with it because in a no wind situation, as the OP specifies, Groundspeed will always equal true airspeed.

In relation to what? If you're talking about the belt, then it doesn't matter, for reasons that I've described. If you're talking about the ground, then it does.

But the speed of the wheels in this case does not equal groundspeed, and does not equal airspeed.

I'm not quite sure what's happening there but let's just make sure we agree there's a ground speed, an air speed and a belt speed (aircraft speed relative to the belt). Confusing ground speed with belt speed has happened more than once in this thread.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: CptSpeaking
Posted 2007-05-27 03:37:07 and read 32767 times.

Quoting David L (Reply 544):
Except that CptSpeaking is arguing option 3, the impossible, illogical, non-aviation one so he doesn't need to "explain" why the aircraft can't gain airspeed. He interprets the OP as dictating that it can't gain airspeed - it doesn't matter how or why.  

Good grief...I suppose as long as you know what I'm talking about, then I'm done with it. I'll be happy to agree with you with your "option" 1 and 2 though...

Not sure how it isn't aviation related though, and even though it is impossible, given the conditions stated it isn't illogical...

Whatever...I'm done with the impossible situation now, so bring on something else  Smile You're going on the RU for at least being civil about it and explaining a position you don't agree with and most don't understand...

Your CptSpeaking

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Jetlagged
Posted 2007-05-27 03:55:52 and read 32767 times.

Quoting Keta (Reply 546):
I'm interested in seeing that. All sources I have seen say that rolling friction is just a kind of static friction, related to the deformation of the objects, and is only dependent on the normal force. More explicitly, I saw everywhere F = Crr·N, where Crr is the coefficient of rolling friction, a constant for two given materials

Clearly the faster the tyre rolls the more power is consumed deforming the tyres. The coefficient of rolling friction may be speed independent but the drag force produced will increase with speed.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Iwok
Posted 2007-05-27 04:41:37 and read 32767 times.

Quoting Jetlagged (Reply 541):
Regardless of the fact that irregardless is not a word....

Well, its is indeed a word: disputed, but a bonafide word listed in dictionaries.

Dictionary.com Unabridged (v 1.1) - Cite This Source
ir·re·gard·less [ir-i-gahrd-lis] Pronunciation Key - Show IPA Pronunciation
–adverb Nonstandard.
regardless.
[Origin: 1910–15; ir-2 (prob. after irrespective) + regardless]

—Usage note Irregardless is considered nonstandard because of the two negative elements ir- and -less. It was probably formed on the analogy of such words as irrespective, irrelevant, and irreparable. Those who use it, including on occasion educated speakers, may do so from a desire to add emphasis. Irregardless first appeared in the early 20th century and was perhaps popularized by its use in a comic radio program of the 1930s.


iwok

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: NWADC9
Posted 2007-05-27 05:38:45 and read 32767 times.

Quoting Sovietjet (Thread starter):
Posted Tue Nov 29 2005



Quoting Iwok (Reply 550):
Reply 550, posted Sat May 26 2007

This is a really old thread...

Besides, like what the others said, a plane can't take off a conveyer belt matching the plane's speed (or attempt of) because the wheels are there basically to support the stupid thing on the ground. Planes move by pushing air no matter how high or low they are. The wheels are just there. That being said, planes need wind to blow on 'em. Now, if you have a 170kt wind blowing on your 747 on the conveyer belt, then yes, it could take off. So, if there's no wind blowing to match the speed, no it won't take off. If there is, then yes, most likely.

[Edited 2007-05-27 05:39:48]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: BoeingOnFinal
Posted 2007-05-27 10:39:42 and read 32767 times.

Quoting NWADC9 (Reply 551):
Planes move by pushing air no matter how high or low they are. The wheels are just there. That being said, planes need wind to blow on 'em.

Ohh, so that is what all the delays are about? Lack of wind?

Some say that because the belt is designed to cancel any forward thrust by increasing backwards rotation to increase friction in the wheels, the aircraft will not move. This is hypothetically correct. Because Thrust > Friction to accelerate. But if Thrust < or = Friction, it will not move.

But since this is impossible because of the lack of friction without slamming the brakes, Thrust will overcome friction and the aircraft will accelerate.
Relative to surrounding ground and airmass, the aircraft will gain both groundspeed and airspeed. It will then take off, regardless of what the speed of the wheels are (this obviously is affected by the belt).

One realistic event that could happen if such a scenario where to be created, is that the wheels exceeds it's maximum rotation speed and get damaged in some way.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Keta
Posted 2007-05-27 12:08:08 and read 32767 times.

Quoting Jetlagged (Reply 549):
Clearly the faster the tyre rolls the more power is consumed deforming the tyres. The coefficient of rolling friction may be speed independent but the drag force produced will increase with speed.

Please read reply 528 and tell me what's wrong there. The force is always the same, it's just the rolling friction coefficient times the normal force. Nothing else, it's that easy. It's a constant, no matter the speed. BTW, when the aircraft speeds up, the wing will start creating lift, so the normal force will be less, ans so will be the friction. Anyway we can forget about this.

I did say that the power will increase with the speed, but that doesn't mean that the force would be higher too. Power is force times speed, so if force is constant power will rise. But that only has effect on how hot the wheels and the pavement get. It has nothing to do with force balance.

I say again, this is what I've read. I will be pleased to see any source stating what you say.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Mir
Posted 2007-05-27 13:05:20 and read 32767 times.

Quoting StealthZ (Reply 545):
Can you not see that it does not matter what the wheels rotation is relative to, the only important thing is the airspeed and I see nothing in this question that will prevent the aircraft gaining airspeed.

I do see that, and I agree!

Quoting NWADC9 (Reply 551):
Besides, like what the others said, a plane can't take off a conveyer belt matching the plane's speed (or attempt of) because the wheels are there basically to support the stupid thing on the ground. Planes move by pushing air no matter how high or low they are. The wheels are just there.

The reasons you mention are the very ones that explain why the plane CAN take off.

Quoting David L (Reply 547):
I'm not quite sure what's happening there but let's just make sure we agree there's a ground speed, an air speed and a belt speed (aircraft speed relative to the belt).

 yes 

-Mir

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Starglider
Posted 2007-05-27 13:23:40 and read 32767 times.

The plane will fly, regardless of rotating wheels and contra-rotating conveyor belt.

Jet engines produce thrust against the surrounding air, they are action/reaction engines, regardless what the wheels do, the thrust will push against the air and the aircraft will take off.

Put another way, if the conveyor belt and aircraft were placed in a vertical plane, would the counter rotating conveyor belt prevent the airplane from crashing to the ground? That would be the first anti-gravity machine that actually worked.

In this case thrust and gravity have the same effect on acceleration of the aircraft. The airplane will accelerate, with or without a conveyor belt.

Regards,
Starglider

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-05-27 14:42:53 and read 32767 times.

Quoting NWADC9 (Reply 551):
That being said, planes need wind to blow on 'em. Now, if you have a 170kt wind blowing on your 747 on the conveyer belt, then yes, it could take off. So, if there's no wind blowing to match the speed, no it won't take off.

Aircraft take off in still air all the time. I assume you're talking about option 1, belt opposing aircraft's ground speed. In option 1, you just end up with the aircraft taking off at normal airspeed, normal groundspeed (if there's no wind) and a speed relative to the belt equal to twice the air/ground speed (ground speed is the aircraft's speed relative to the ground next to the belt, not the belt itself). The belt moving at, for example, 170 kts backwards does not cancel the thrust of the aircraft! The wheels prevent almost all of the horizontal force from reaching the aircraft - it's not the same as applying the opposing force to the nose.

Quoting CptSpeaking (Reply 548):
Not sure how it isn't aviation related though, and even though it is impossible, given the conditions stated it isn't illogical...

It's the inexplicable "mysterious element", preventing the aircraft from gaining airspeed contrary to any real physics, that makes me think option 3 is a language/philosophy question, not a a physics/aviation question.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: DarkBlue
Posted 2007-05-27 16:38:51 and read 32767 times.

Quoting David L (Reply 519):
Once the reality of the thread dawned, I and several others explicitly said we were happy to continue the "classic" debate but that the question as stated probably belonged in Non-aviation.

I apologize, I missed this part of the conversation.

Quoting CptSpeaking (Reply 548):
You're going on the RU for at least being civil about it and explaining a position you don't agree with and most don't understand...

Agreed, of the "fly group" David L has done the best job of showing that he understands both sides and doesn't treat the "no fly group" like idiots. Unlike many, he actually gets it.

Quoting David L (Reply 523):
I think there are 4 possibilities:

1. Belt opposes aircraft's ground-speed, not its "belt-speed" ("classic" scenario)
2. Belt stops tyres rotating ("as stated" version 1),
3. Magic force stops the aircraft from gaining air/groundspeed ("as stated" version 2)
4. The question was never asked in the first place and what are you talking about?

Results:

1. Aircraft takes off at normal air/groundspeed but wheels rotate twice as fast as normal,
2. Aircraft takes off at normal air/groundspeed but wheels not rotating,
3. Twighlight zone with magic force - belt changes colour, slips in and out of focus, we all feel one with the universe. "But what happens to the plane?"... "I don't care, look at all the colours!"...
4. With hindsight, I'd like to believe this was the case.

I suggest before anyone posts their argument, please identify which option you interpret the question as. I have always interpreted the OP literally, and have interpreted it as option 3 (without the changing colors  Smile). To me, how the magic force happens is irrelevant - i see a constraint in the wording of the OP, so that's all I have to go on regardless if it's realistic or not.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-05-27 17:45:02 and read 32767 times.

Quoting DarkBlue (Reply 557):
I apologize, I missed this part of the conversation.

It was bound to happen from time to time - it's like three independent football matches taking place on the same field, using the same lines and goals, and trying to work out who's in your game and who's in one of the other games.  crazy 

Quoting DarkBlue (Reply 557):
and doesn't treat the "no fly group" like idiots

Not strictly true  Sad . I did become a bit exasperated at the number of people who likened an aircraft on a conveyor belt to a car or a person - unless they explicitly said it's not right but it has to be assumed for option 3 (and I didn't always notice that qualifier). I don't think they're idiots but it is a fundamental point that needs to be understood, especially for options 1 and 2.  hissyfit 

Quoting DarkBlue (Reply 557):
Quoting CptSpeaking (Reply 548):
You're going on the RU for at least being civil about it and explaining a position you don't agree with and most don't understand...

Agreed

Thank you very much - I'm humbled.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Mrocktor
Posted 2007-05-28 15:08:55 and read 32767 times.

Quoting Keta (Reply 528):
Rolling friction is not dependent on the speed, it only depends on the normal force (i.e. weight-lift-whatever). No matter what are the relative speeds between a wheel and whatever it's rolling over, the force is always the same.

Thats right. Too long since high school, too long thinking about aerodynamics. Mea culpa.

Quoting David L (Reply 529):
a real conveyor belt wouldn't be able to exert enough force through real aircraft wheels to stop a real aircraft from gaining airspeed, unless it was extremely light and extremely low powered

Thats for sure, one way or the other.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Aviateur
Posted 2007-05-29 00:48:22 and read 32767 times.

I wrote and published an entire article on this topic, if you'd like to read it. The piece ran some months ago on Salon.com. It was inspired by David Pogue's New York Times.com column. Yes, the plane will fly, but mostly it's a moot point....


You can read the analysis here:

http://www.salon.com/tech/col/smith/2007/01/05/askthepilot215/


- PS

[Edited 2007-05-29 00:51:51]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-05-29 15:36:34 and read 32767 times.

Quoting Aviateur (Reply 560):
You can read the analysis here:

http://www.salon.com/tech/col/smith/...t215/

"For good measure, the treadmill itself, as described, could never be built. It can't "exactly match the speed of the wheels," because the wheels will turn at the speed of the treadmill plus the speed of the plane relative to the ground. "

Exactly! At least, not without the aforementioned element of "magic".  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: ANITIX87
Posted 2007-05-29 16:35:50 and read 32767 times.

I cannot believe we're this far into the thread and it's still going. The plane would not fly.

If you put it down on a conveyor belt, with ALL its weight on the belt, then, we agree, the wheels would have the same contact with the belt as they would with a real runway.

The assertion some of you make, that if the belt moved, the wheels would spin freely, is completely bogus. That means the belt has frictionless friction. And we can't use ice as an relative example, because on ice, the plane would skid, not roll. If the plane on the belt doesn't move backwards that means that there is no static friction. This is impossible. You cannot have a complete lack of static friction in the real world. If you turned on a conveyor belt, the entire aircraft would move with the belt, and the wheels wouldn't spin.

Now, we also all must agree that just because the belt is moving, that doesn't mean the air above the belt is moving. This is a fact. Therefore, there is no air to flow over the wings. Now, if we add thrust, the plane will want to move forward, relative to its previous position. That means, as the belt is moving backwards, the aircraft will start to thrust more, and the wheels will start to spin faster and faster until they match the speed of the belt and the plane stops moving backwards. If the belt were remaining at a constant speed, then, yes the plane would begin to move forward and eventually lift off. The opening poster specifically said the belt is designed to match the speed of the wheels at all times. That means the aircraft's wheels will NEVER exceed the backwards motion of the belt, and the plane will remain forever stationary. Since there is no air flow due to the belt's movement, there is no air flow over the wings, and it will not lift off. Ever. Period. Think of someone on a treadmill on a skateboard. If they don't hold onto anything, they will be thrown backwards as the treadmill speeds up. And don't anyone try and say otherwise. I've found out the hard way. If, however, they hold onto the bar at the front, they will be able to stay on, their wheels matching the treadmill's speed. That force, of the skateboarder holding onto the bar, pulls him forward, and is identical to the thrust provided to the aircraft by its engines!!! As the readmill goes faster and faster, the boarder must use more and more force to hold himself on. Again, an identical situation. And I know someone will say, "Yes, Antonis, but if the skateboarder is holding himself steady, and then adds more force, he will start to move forward." Yes, this is true. However, once again, I bring you to what the opening poster said. The belt matches the speed of the wheels at all times. If the treadmill were like the conveyor belt, as the boarder added more force, his wheels would speed up, but SO WOULD THE TREADMILL, so that he still wouldn't move forward.

TIS

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: ANITIX87
Posted 2007-05-29 16:47:36 and read 32767 times.

Oops. I just spoke with a colleague of mine.

I was half-wrong. If the belt moves without any thrust to the aircraft, then the plane will remain stationary. However, my colleague and I both agree that the plane will not take off in a realistic setting, because the plane will not be able to gain any speed. As soon as thrust is added, the speed of the wheels depends on the entire aircraft's movement, and not on the movement of the belt. Therefore, since the aircraft is moving at a speed identical to that of the belt (since that is how the belt is designed) the wheels will spin but the aircraft will not movie.

TIS

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-05-29 19:10:20 and read 32767 times.

Quoting ANITIX87 (Reply 563):
However, my colleague and I both agree that the plane will not take off in a realistic setting, because the plane will not be able to gain any speed.

Is this Option 1, Option 2 or the magical Option 3 you're talking about?

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Bond007
Posted 2007-05-29 19:18:24 and read 32767 times.

Quoting ANITIX87 (Reply 563):
However, my colleague and I both agree that the plane will not take off in a realistic setting, because the plane will not be able to gain any speed.

No, quite the opposite. In a 'realistic' setting ... i.e. real physics and no 'magic' ... the plan WILL fly, and it WILL gain airspeed.

In a 'non-realistic' setting, where the belt somehow spins the same speed as the wheels .... then the answer in 'non-realistic'.


Jimbo

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-05-29 19:34:42 and read 32767 times.

Quoting Bond007 (Reply 565):
No, quite the opposite. In a 'realistic' setting ... i.e. real physics and no 'magic' ... the plan WILL fly, and it WILL gain airspeed.

Assuming Option 1 or Option 2, this part bothers me:

Quoting ANITIX87 (Reply 563):
As soon as thrust is added, the speed of the wheels depends on the entire aircraft's movement

The speed of rotation of the wheels does not depend solely on the speed of the aircraft, it depends on the difference between the speed of the aircraft relative to the ground and the speed of the surface it's moving on relative to the ground. The only reason wheel speed matches aircraft ground speed on a real runway is that the runway has a speed of zero relative to the ground.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: ANITIX87
Posted 2007-05-29 21:25:26 and read 32767 times.

Quoting David L (Reply 566):
The speed of rotation of the wheels does not depend solely on the speed of the aircraft, it depends on the difference between the speed of the aircraft relative to the ground and the speed of the surface it's moving on relative to the ground. The only reason wheel speed matches aircraft ground speed on a real runway is that the runway has a speed of zero relative to the ground.

After more discussion with my colleagues, many of which disagree with one opinion or the other, I'm starting to see why my opinion is wrong, and why the idea that the plane will accelerate could be correct. I don't think there's a way to know unless someone tries it, either with a scale model or a full-size one. I'm going to suggest it to MythBusters, haha.

TIS

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Starlionblue
Posted 2007-05-29 21:41:40 and read 32767 times.

Quoting ANITIX87 (Reply 567):
I'm going to suggest it to MythBusters, haha.

Well, they already did the "pigeons flying in a truck" myth.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-05-29 21:51:11 and read 32767 times.

Quoting ANITIX87 (Reply 567):
I'm going to suggest it to MythBusters, haha.

As long as you only suggest Option 1 and/or Option 2.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: DarkBlue
Posted 2007-05-29 21:58:53 and read 32767 times.

Quoting David L (Reply 569):
Quoting ANITIX87 (Reply 567):
I'm going to suggest it to MythBusters, haha.

As long as you only suggest Option 1 and/or Option 2.

Actually it might be quite humurous to see them try to build the imaginary Option 3 machine. I'm sure there's someplace on ebay where you purchase some "magic force".

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: ANITIX87
Posted 2007-05-29 22:10:43 and read 32767 times.

Quoting David L (Reply 569):
As long as you only suggest Option 1 and/or Option 2.

I just basically wrote what the opening poster wrote, I'll leave the theories up to them, so as not to bias their opinionis.

TIS

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-05-29 22:23:52 and read 32767 times.

Quoting DarkBlue (Reply 570):
Actually it might be quite humurous to see them try to build the imaginary Option 3 machine.

Well... yeah, I suppose... kind of...  smile 

Quoting DarkBlue (Reply 570):
I'm sure there's someplace on ebay where you purchase some "magic force".

A former work colleague of mine managed to burn his back quite badly in an alcohol-related T-shirt incident. When he checked the label to see what it was made of, it said (something like) 30% cotton, 65% polyester and 5% Magic. He never did find out what "Magic" is but he's convinced it isn't good for you.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Starlionblue
Posted 2007-05-29 22:38:43 and read 32767 times.

Quoting DarkBlue (Reply 570):
I'm sure there's someplace on ebay where you purchase some "magic force".

They try that kind of stuff on a regular basis. My favorites are the personal flying machine (plans on eBay) and all the 100mpg devices.


BTW the Mythbuster forum members seem similar to a.nutters. They are apparently STILL arguing about the pigeons. The physics of the problem are quite straightforward. Even if 100 pigeons in a truck lift off at the same time, the weight of the truck will remain the same. Mythbusters proved this experimentally of course.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Sovietjet
Posted 2007-05-29 23:21:54 and read 32767 times.

I can't believe this is still going, and that Mythbusters havent tried this yet. Although it would be kind of hard to realistically pull off a conveyor belt exactly matching wheel speed.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Bond007
Posted 2007-05-30 00:25:19 and read 32767 times.

Quoting ANITIX87 (Reply 567):
I don't think there's a way to know unless someone tries it,

There is a simple way, and it's called basic physics ... which says the plane will move. Anything else means breaking these fundamental rules.


Jimbo

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: DL787932ER
Posted 2007-05-30 05:21:01 and read 32767 times.

Okay, I've read the entire thread (there are some hours of my life I'll never get back) and after much thought, I can picture each of the two questions posed. Here is my attempt to discuss each of the two cases, with the results of each based on logics and physics, along with a couple "modifications" of the second case that I think many of the posters here are considering (whether they realize it or not).

The first case is that in which the conveyor matches the speed of the airplane. I think most if not all posters have realized that in this case, the airplane will take off. As the engines accelerate the airplane to any given speed, the belt accelerates in the opposite direction, but even with rolling resistance and friction between the wheels and axles, the airplane will accelerate and take off normally. The airplane will lift off at speed x (say, 130 knots), the belt will be moving backwards at 130 knots, and the wheels will be spinning at 260 knots, but assuming the wheels and tires can take that speed the airplane will lift off at its normal airspeed, while the wheels turn at twice that airspeed.

The second case, which I think is actually less interesting for reasons explained below, is the one in which the conveyor matches the speed of the wheels. The reason this case is actually uninteresting is that the only way it can actually exist is if neither the wheels nor the conveyor ever move. Picture the situation: the airplane is stopped on the conveyor, with brakes off. The pilot advances the throttles, the airplane begins to roll, and - oops! - we've violated the rule. As soon as the airplane moves the first inch, its wheels have gone at a different speed from the conveyor. The key is that the wheels are not directly powered. The only way they can spin is if they are pulled forward by the airplane as its engines throttle up. If we permit the engines to pull the plane forward even a fraction of a millimeter, we have moved the wheels at a different speed from the conveyor belt and thus violated our "rule". Since we are not allowing the engines to move the plane forward, then the wheels will never move, the conveyor will never move, and the plane will never take off.

The problem is that the conveyor cannot prevent the airplane from moving forward. As in case 1, no matter how fast it spins, it will just spin the wheels backwards and the plane will accelerate normally; furthermore, in this case we cannot move the wheels at all without immediately breaking our rule. But because our rule is that the conveyor speed must match the wheel speed, then we must implement that rule externally. In other words, we must force the airplane not to provide enough forward thrust to move itself forward with respect to the ground. So the result of the rule that the conveyor must move at the speed of the wheels is that we cannot provide enough thrust to overcome the rolling resistance of the tires or the air resistance of the atmosphere.

Therefore, the second case is simply "will the airplane take off on the conveyor if we cannot apply enough thrust to move the airplane?" Obviously, the answer is "no" - in fact, we no longer even need the airplane. An airplane on a regular runway would never take off if we were prevented from applying enough thrust to move it. This is why I think this case is less interesting - it boils down to an application of rules that effectively prevent any takeoff under any circumstances, with or without a conveyor.

Now, for the two sub-scenarios that I think some posters have been discussing unintentionally. First, what if we let the airplane "cheat" - give it, say, a three second head start before the conveyor starts up. The airplane goes to takeoff thrust, accelerates up to one knot in those three seconds, and then the conveyor starts. As soon as it starts spinning, it accelerates to one knot backwards - which spins the wheels up to two knots, causing the conveyor to accelerate to two knots, spinning the wheels up to three knots, causing the conveyor to accelerate to three knots, etc. The conveyor will instantaneously reach infinite speeds and still will never catch up to the wheels, which will instantaneously travel at the speed of the conveyor plus one knot for the airplane's forward speed. We have to decide whether this experiment gives us control only of the conveyor or whether we have control over the airplane as well. If we can only control the conveyor, we reach the same solution as in case 1 - no matter how fast the conveyor turns backwards, it only accelerates the wheels an equal amount, and since the airplane itself neither drives nor is driven by the wheels, it will accelerate independently of the wheels until it takes off with the wheels spinning at (takeoff speed + conveyor speed). If we can control the airplane itself, we can reduce throttles until the airplane no longer moves, and eventually friction will slow it that one knot so it is no longer moving with respect to the ground. We are then back at a version of case 2, where the conveyor is spinning the wheels at an arbitrary speed and the airplane is not moving - but it is not moving because our rules say that it cannot apply enough thrust to move.

The second sub-scenario is one in which the conveyor can control the speed of the system independently of the wheels. In other words, the conveyor does not respond to the wheels, but merely accelerates independently. In this case, in an ideal system with no friction, the conveyor can move at any speed it wants, forwards or backwards, and it will not move the airplane, but simply spin the wheels underneath. If we have control over the throttles, we can again accelerate down the conveyor and take off normally, with the wheel speed being equal to takeoff speed plus conveyor speed. In a real system, with effects of friction and air resistance, moving the conveyor will move the airplane, but the force exerted on the airplane through the rolling resistance of the tires and the friction of the axles is minute compared to the force that the engines can exert. So once again we are back to deciding whether we can control the engines independently (in which case, we can still take off, regardless of what the conveyor does) or whether we are arbitrarily limited to providing just enough engine power to overcome the friction in the wheel axles (in which case we can't take off, just as we can't take off from a normal runway if we can't apply enough thrust to overcome friction and air resistance).

So the two cases really just boil down to whether our experiment controls the conveyor or whether we can also control the airplane. If we can only control the conveyor, than all we can do is match the airplane speed - it is impossible to match the wheel speed because the conveyor is spinning the wheels and the airplane's forward motion (caused by the engine thrust) is also spinning the wheels. If we can also control the airplane, then of course we can prevent takeoff, because this rule requires that we limit thrust to less than the amount required to move the airplane forward (whether or not it is on a conveyor). Thus the question is not whether we are matching the speed of the airplane or the speed of the wheels - it is whether we can provide enough thrust to move the airplane forward (in which case we cannot match conveyor speed to wheel speed, and we will take off) or whether we are not permitted to do so (in which case we can match conveyor speed to wheel speed - they both must equal zero - and we cannot take off because we cannot apply any thrust).

I hope this explanation adequately covers each of the two possibilities and that it makes a modicum of sense. I'm not arrogant enough to believe it'll end the debate, but if you believe I've made an error or can think of a scenario I haven't covered, please let me know and I'll do my best to align my thoughts with yours.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-05-30 12:07:47 and read 32767 times.

Quoting DL787932ER (Reply 576):
This is why I think this case is less interesting - it boils down to an application of rules that effectively prevent any takeoff under any circumstances, with or without a conveyor.

Correct - that would be Option 3.

Quoting DL787932ER (Reply 576):
I hope this explanation adequately covers each of the two possibilities and that it makes a modicum of sense.

Yes, excellent summary... except we've been discussing 3 scenarios (Option 4 doesn't really warrant discussion).

Quoting David L (Reply 523):
I think there are 4 possibilities:

1. Belt opposes aircraft's ground-speed, not its "belt-speed" ("classic" scenario)
2. Belt stops tyres rotating ("as stated" version 1),
3. Magic force stops the aircraft from gaining air/groundspeed ("as stated" version 2)
4. The question was never asked in the first place and what are you talking about?

Results:

1. Aircraft takes off at normal air/groundspeed but wheels rotate twice as fast as normal,
2. Aircraft takes off at normal air/groundspeed but wheels not rotating,
3. Twighlight zone with magic force - belt changes colour, slips in and out of focus, we all feel one with the universe. "But what happens to the plane?"... "I don't care, look at all the colours!"...
4. With hindsight, I'd like to believe this was the case.

Excellent post, though.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Access-Air
Posted 2007-05-30 17:33:54 and read 32767 times.

I cant believe that ANCFlyer has not locked this thread. I think its abssurd that I am even responding to this but its like saying If a rocket was launched from a really fast moving Escalator.


Access-Air

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Starlionblue
Posted 2007-05-30 17:49:54 and read 32767 times.

Quoting Access-Air (Reply 578):
I cant believe that ANCFlyer has not locked this thread. I think its abssurd that I am even responding to this but its like saying If a rocket was launched from a really fast moving Escalator.

He can't lock it now. You just added new data! AAAAAAAAAAAAH!  Wink

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Iwok
Posted 2007-05-31 07:38:35 and read 32767 times.

Quoting Access-Air (Reply 578):
I cant believe that ANCFlyer has not locked this thread. I think its abssurd that I am even responding to this but its like saying If a rocket was launched from a really fast moving Escalator.Access-Air

MODS!!! Do not delete this thread. The same mystical force that propells the treadmill is propelling this thread on to infinity...  pray 

This is a great thread. I must admit when I first read the OP I was convinced the plane would take off and was in the process of formulating a response highlighting why everyone was wrong. However reading a little more and it became clear and I have see the light.

I think DavidL has taken this thread to a new ambit.  bigthumbsup 

iwok

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Jetlagged
Posted 2007-06-01 02:08:45 and read 32767 times.

Don't delete it, just lock it now the no fly arguments are becoming even more circular than the conveyor belt. Then A.netters in years to come can see the legendary conveyor belt thread in all it's glory with no risk of accidentally starting it running again.

Quoting Iwok (Reply 580):
I think DavidL has taken this thread to a new ambit.

What's an ambit? Is it a zero g gambit?

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Superstring
Posted 2007-06-02 09:05:59 and read 32767 times.

Well this topic is one of the longest I have ever seen, however I think to discuss also very "strange" questions is never wrong: It is illuminating because even if the question is completely absurd, well there should be one guy out there answering it:

Quoting DL787932ER (Reply 576):
Okay, I've read the entire thread (there are some hours of my life I'll never get back) and after much thought, I can picture each of the two questions posed. Here is my attempt to discuss each of the two cases, with the results of each based on logics and physics, along with a couple "modifications" of the second case that I think many of the posters here are considering (whether they realize it or not)..........

Well who should want to add something to this great summary?
(Will we break 600? Would be great a 1K thread...)

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-06-02 14:50:28 and read 32767 times.

Quoting Access-Air (Reply 578):
I cant believe that ANCFlyer has not locked this thread. I think its abssurd that I am even responding to this but its like saying If a rocket was launched from a really fast moving Escalator.

I think the number of responses indicates the level of interest. Somebody might want clarification of a point they don't quite get. Do we really want them to start a new thread and go through the whole thing again?  crazy 

Nobody's forced to read the thread.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Airfoilsguy
Posted 2007-06-02 15:46:16 and read 32767 times.

I dub this the new SUPER THREAD!!!!

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Vikkyvik
Posted 2007-06-02 16:41:27 and read 32767 times.

Quoting Jetlagged (Reply 581):

What's an ambit? Is it a zero g gambit?

 Wow!




 faint 


I think that needs to be nominated for the Tech/Ops "Quote of the Week"?

Quoting SlamClick (Reply 500):
To whom will go the dubious honor of the 500th post?

Well, I'm glad there was at least ONE decent reason for me to re-open this thread.... Wink

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Starlionblue
Posted 2007-06-03 03:27:23 and read 32767 times.

Quoting Vikkyvik (Reply 585):
Quoting Jetlagged (Reply 581):

What's an ambit? Is it a zero g gambit?

Wow!




faint


I think that needs to be nominated for the Tech/Ops "Quote of the Week"?

 rotfl 

Quoting Airfoilsguy (Reply 584):
I dub this the new SUPER THREAD!!!!

What, not Peter Max?  Wink

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Iwok
Posted 2007-06-05 06:37:27 and read 32767 times.

Quoting Jetlagged (Reply 581):
What's an ambit? Is it a zero g gambit?

AMBIT: noun; "the scope, extents or bounds of something"

It seems $5 words are required in this thread.

So; what is consensus here? It appears to me that most people agree that the plane would forever sit in one stationary location with niether ground, treadmill or airspeed and that the brakes must be fully actuated. But I just though of something which debunks this theory; and soundly!!!  stirthepot 

One particular twist that no one has considered, and I think I have found the solution....

As the engine thrust is increased, we all agree that the plane cannot move.... However there will be a sligh shear stress between the engine thrust parallel to the treadmill. The treadmill will then start to roll; but the airspeed will still remain at zero. After some time the treadmill speed will be very high, but there will still be no airspeed.

Hence: its not possible that the following two scenarios are possible:
1) the plane sits wtih brakes on and has all three velocities = 0 (wind, ground and treadmill)
2) the wheel spin like mad as the engines thrust continues, but airspeed stays at ZERO.

So there you have it. I think I have finally solved the problem.  bigthumbsup 

iwok

A

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: StealthZ
Posted 2007-06-05 09:17:57 and read 32767 times.

Quoting Iwok (Reply 587):
As the engine thrust is increased, we all agree that the plane cannot move....

No we DO NOT... no one has put forward a half plausible argument why the plane will not move forward relative to a point on the ground or in the still air.

When will you folks accept that the wheels and the conveyor belt have NOTHING to do with the forward motion and therefore airspeed of the aircraft?

Cheers

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: HAWK21M
Posted 2007-06-05 10:29:15 and read 32767 times.

Quoting StealthZ (Reply 588):
When will you folks accept that the wheels and the conveyor belt have NOTHING to do with the forward motion and therefore airspeed of the aircraft

Thats true.
Increasing the Relative Airflow past the Aircrat would.
regds
MEL

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-06-05 13:21:07 and read 32767 times.

Quoting StealthZ (Reply 588):
No we DO NOT... no one has put forward a half plausible argument why the plane will not move forward relative to a point on the ground or in the still air.

Given that Option 3 is not a real world scenario, I think it's fair to say the aircraft cannot move simply because of the wording of the problem. There's no physical reason it can't move in Option 3 - it just can't, for some unspecified reason. Options 1 and 2 can be rationalised in physical terms, Option 3 can only be rationalised in philosophical terms.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Bond007
Posted 2007-06-05 14:14:30 and read 32767 times.

Quoting David L (Reply 590):
Given that Option 3 is not a real world scenario, I think it's fair to say the aircraft cannot move simply because of the wording of the problem. There's no physical reason it can't move in Option 3 - it just can't, for some unspecified reason. Options 1 and 2 can be rationalised in physical terms, Option 3 can only be rationalised in philosophical terms.

Whatever your options are ... if you believe in physics ... the aircraft will move. It's simple.

Jimbo

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Jetlagged
Posted 2007-06-05 14:41:26 and read 32767 times.

Quoting Iwok (Reply 587):
AMBIT: noun; "the scope, extents or bounds of something"

Apologies for my sense of humour, Iwok.

Quoting Iwok (Reply 587):
As the engine thrust is increased, we all agree that the plane cannot move....

For anyone who doesn't know what the phrase "begging the question" really means, this is a classic example. No, we are not all agreed.

For my part I don't accept David's Option 3 is even mandated by the question. Re-read it please. All the question says is that the conveyor will match wheel speed. This does not mean that groundspeed must remain zero. Philosophically or otherwise. What people are doing is taking their instinctive or desired result (no fly) and deciding how this can come about. That kind of logic proves nothing. The cart is before the horse.

It all depends on how you define wheel speed:

(a) If it's the rotational velocity about the axle, then how do you define the rotational velocity of the belt in order to match it? The belt has a linear speed. So really this option can be discounted.

(b) If it's the linear velocity of the wheel axle, this is the same as aircraft speed and so either Option 1 or 2 can apply, Option 3 not being necessary.

(c) If it's the tangential velocity of the wheel, then at which point in the circumference? Wheel speed could be anything from 0 to twice the aircraft speed, depending which reference point you choose.

(i) If at the point of contact with the belt, then this will always be zero if the belt is stationary, whatever the aircraft ground speed. (So the belt does not need to move to satisfy the condition).

(ii) If you choose a reference point at the axle, then the condition is easy to satisfy, the belt will always drive to make the wheel speed zero (Option 2). Any other motion cannot equalise the speeds without some skidding occurring or unless static friction is zero. Note in this case, the speed of the belt relative to the reference point is zero, equal to the wheel speed. You can't choose one reference point for the wheel and another for the belt (basic mistake some people are making).

None of these choices requires an Option 3 to satisfy the condition.

Now for the logical bit. How the belt is driven is not specified, so we can assume perfect speed matching can be achieved by the control system, however we have defined wheel speed and the reference point. The belt has no effect on the aircraft motion, whatever it's speed. So logically it doesn't matter what the speed of the belt is. The aircraft can and will accelerate.

The important thing is to choose a frame of reference and stick to it. Then there is no confusion.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-06-05 15:30:56 and read 32767 times.

Quoting Jetlagged (Reply 592):
For my part I don't accept David's Option 3 is even mandated by the question.

Neither do I, it's not my option!

I was simply clarifying that some people were pursuing that option and it was getting interwoven and confused with discussions about the "real world" Options 1 and 2. I said many times that I was only interested in discussing Options 1 and 2 and that Option 3 was a philosophical question and of no interest to me.  Smile

Quoting Bond007 (Reply 591):
Whatever your options are ... if you believe in physics ... the aircraft will move. It's simple.

Option 3 cannot happen - that is what's simple about it.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Jetlagged
Posted 2007-06-05 15:52:41 and read 32767 times.

Quoting David L (Reply 593):
Option 3 cannot happen - that is what's simple about it.

More than that, it is not even necessary to fully comply with the conditions of the OP. It is a fantasy for those who want to be able to find a way that the aircraft won't fly. What better way than to have an "implied" clause stating the brakes must be applied.  Smile

I realise it isn't "your" option, of course.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: DarkBlue
Posted 2007-06-05 16:19:45 and read 32767 times.

Quoting Jetlagged (Reply 592):
You can't choose one reference point for the wheel and another for the belt

Uhhh, why not? That's the fundamental idea behind calculating the speed of a person walking on a train. A train is traveling at 100 km/hr in one direction (referenced to the ground), while a person walks on the train in the opposite direction at 5 km/hr. How fast is the person traveling with respect to the ground?

That's the entire point of option 3. If I set up the conveyor at 10 meters/sec, this means that 10 meters of belt will pass a stationary ground reference point every second. Correct? If I roll a wheel down the conveyor at 10 meters/sec (axle of wheel with respect to the conveyor) this means that the axle will pass 10 meters of belt every second. Correct? Since the question states that velocities are in the opposite direction the speed of the wheel (axle) with respect to the ground is zero.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-06-05 16:32:22 and read 32767 times.

Quoting Jetlagged (Reply 594):
I realise it isn't "your" option, of course.

Well, I suppose I did name it.

Quoting Jetlagged (Reply 592):
What people are doing is taking their instinctive or desired result (no fly) and deciding how this can come about.

I think that's true in some cases but I also think there are others who do interpret the wording of the problem as dictating that the aircraft can't move. I never considered that possibility long enough as I just didn't see the point. If the aircraft cannot move, it cannot fly... it doesn't matter how or why, except that it would require something extra that is not stated in the conditions.

I also believe there are still a few who think the aircraft cannot take-off in Options 1 or 2, in spite of the explanations to the contrary.  Sad

Quoting Jetlagged (Reply 592):
It all depends on how you define wheel speed:

And that, of course, is how all the confusion began in the first place!

I still think the wording of the problem was accidental and was intended to describe Option 1 or, possibly, Option 2.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: DarkBlue
Posted 2007-06-05 16:38:07 and read 32767 times.

Quoting Jetlagged (Reply 496):
To keep the thread going a hardcore of the "no fly group" stands guard to keep pointing out that the question is flawed but if you follow its logic the aircraft can't move.



Quoting Jetlagged (Reply 592):
What people are doing is taking their instinctive or desired result (no fly) and deciding how this can come about. That kind of logic proves nothing. The cart is before the horse.



Quoting Jetlagged (Reply 594):
It is a fantasy for those who want to be able to find a way that the aircraft won't fly.

Your comments go both ways.... people who instinctively jumped into the "fly group" at some point realize that the there is a problem with the wording of the question. This attitude seems to be more, "my answer is correct" and to make this work, therefore, "the question must be wrong".

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: DarkBlue
Posted 2007-06-05 16:45:48 and read 32767 times.

Quoting David L (Reply 596):
I think that's true in some cases but I also think there are others who do interpret the wording of the problem as dictating that the aircraft can't move. I never considered that possibility long enough as I just didn't see the point. If the aircraft cannot move, it cannot fly... it doesn't matter how or why, except that it would require something extra that is not stated in the conditions.

  

Quoting David L (Reply 596):
I also believe there are still a few who think the aircraft cannot take-off in Options 1 or 2, in spite of the explanations to the contrary.

Yes, unfortunately that makes a debate much more difficult when people throw in incorrect assumptions and inappropriate analogies.

Quoting David L (Reply 596):
And that, of course, is how all the confusion began in the first place!

I still think the wording of the problem was accidental and was intended to describe Option 1 or, possibly, Option 2.

I still smile when I picture your football analogy of this debate   :

Quoting David L (Reply 558):
It was bound to happen from time to time - it's like three independent football matches taking place on the same field, using the same lines and goals, and trying to work out who's in your game and who's in one of the other games



:edit for spelling

[Edited 2007-06-05 16:46:57]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-06-05 17:53:54 and read 32767 times.

Quoting DarkBlue (Reply 598):
I still smile when I picture your football analogy of this debate

I have nightmares about that football match! I wish I hadn't thought of it.  crazy 

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Jetlagged
Posted 2007-06-05 19:18:56 and read 32767 times.

600 up, doesn't time fly!

[Edited 2007-06-05 19:19:32]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Iwok
Posted 2007-06-05 21:04:02 and read 32767 times.

Quoting StealthZ (Reply 588):
No we DO NOT... no one has put forward a half plausible argument why the plane will not move forward relative to a point on the ground or in the still air.

When will you folks accept that the wheels and the conveyor belt have NOTHING to do with the forward motion and therefore airspeed of the aircraft?

Well its tricky and tied to the magical treadmill which is designed to rotate in the same rate but NEGATIVE to the wheels. Hence the plane cannot move relative to the ground.

iwok

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Jetlagged
Posted 2007-06-05 21:31:42 and read 32767 times.

Quoting DarkBlue (Reply 595):
Uhhh, why not?

Because if you want to match the speeds they must have the same frame of reference, otherwise the whole thing is meaningless. Hang on a minute, the whole thing is meaningless.....

Quoting DarkBlue (Reply 595):
That's the entire point of option 3. If I set up the conveyor at 10 meters/sec, this means that 10 meters of belt will pass a stationary ground reference point every second. Correct? If I roll a wheel down the conveyor at 10 meters/sec (axle of wheel with respect to the conveyor) this means that the axle will pass 10 meters of belt every second. Correct? Since the question states that velocities are in the opposite direction the speed of the wheel (axle) with respect to the ground is zero.

If that's option 3, then the aircraft flies as well. Sounds more like option 1 to me.

One last try: Simply because it is impossible for belt speed (rel to ground) = -wheel speed (rel to belt) when ground speed is non zero does not mean that ground speed must remain zero, logically or philosophically. Because you insist on defining belt speed as the relative speed between the belt and the wheel the condition becomes impossible to achieve unless aircraft groundspeed is zero. Defining all the speeds relative to the same frame of reference makes all the problems disappear and there is no need for any magical force to restrain the aircraft. That is where the paradox is: in the varying frames of reference.

It must be stressed (again) that whatever the belt speed is the aircraft will accelerate.

Quoting Iwok (Reply 601):
Well its tricky and tied to the magical treadmill which is designed to rotate in the same rate but NEGATIVE to the wheels. Hence the plane cannot move relative to the ground.

Are you one of those who think option 1 and 2 also are no fly results? Because that's what this sounds like. Treadmills have nothing to do with this as has been explained ad infinitum.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: DarkBlue
Posted 2007-06-05 22:30:26 and read 32767 times.

Quoting Jetlagged (Reply 602):
Because if you want to match the speeds they must have the same frame of reference

No they don't. If I'm on a train that's crawling along at 5km/hr (relative to the ground) and I walk backwards at 5 km/hr (relative to the train), I should be able to look out the windows and see that the trees outside are stationary relative to me. I now have matching speeds using different frames of reference.

Quoting Jetlagged (Reply 602):
Because you insist on defining belt speed as the relative speed between the belt and the wheel

No, i define belt speed as the relative speed between belt and ground.

Quoting Jetlagged (Reply 602):
Defining all the speeds relative to the same frame of reference makes all the problems disappear and there is no need for any magical force to restrain the aircraft. That is where the paradox is: in the varying frames of reference.

And now you have Option 1 or 2 and the aircraft flies. However, using the same frame of reference is not how I interpreted the "matching speeds" in the OP. I interpreted these speeds referenced in the same way as my train example above.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Starlionblue
Posted 2007-06-05 22:30:54 and read 32767 times.

Quoting Jetlagged (Reply 602):
Hang on a minute, the whole thing is meaningless.....

602 posts later... Big grin

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Jetlagged
Posted 2007-06-05 23:23:13 and read 32767 times.

Quoting DarkBlue (Reply 603):
No, i define belt speed as the relative speed between belt and ground.

Right, but you also said the belt would move (backwards) at the same speed as the relative speed between it and the wheels. So the magnitude of belt speed is equal to the relative speed between wheel and belt. That can only happen if the wheel is stationary relative to the ground.

Quoting DarkBlue (Reply 603):
I should be able to look out the windows and see that the trees outside are stationary relative to me. I now have matching speeds using different frames of reference.

In the train example, the train is equivalent to the belt right? OK, though you are doing the speed matching using the ground as a reference, not your speed relative to the train. Anyway, so if you start walking faster the train travels faster to compensate and you remain stationary relative to the trees? I'm sorry, but this is the treadmill analogy. If you were to be on a skateboard, with a rocket pack, the train would not be able to stop you accelerating forward relative to the ground. As soon as you pick up some groundspeed the train would never be able to match your speed relative to it.

If you forget relative speed, and match everything to a ground reference the condition on the OP can be satisfied (options 1 or 2).

Even accepting your definitions, it does not mean that groundspeed is constrained to zero, all it means is that the speed condition is impossible to satisfy. This can be shown mathematically, does not involve infinite speeds and for both these reasons is preferable to the magical force theory of non-flying.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: DarkBlue
Posted 2007-06-06 14:58:25 and read 32767 times.

Quoting Jetlagged (Reply 605):
Right, but you also said the belt would move (backwards) at the same speed as the relative speed between it and the wheels. So the magnitude of belt speed is equal to the relative speed between wheel and belt. That can only happen if the wheel is stationary relative to the ground.

   Yep, agreed. Although I consider this more of the result of the relationship of speeds and not the definition. I define the belt speed relative to the ground.

Quoting Jetlagged (Reply 605):
In the train example, the train is equivalent to the belt right? OK, though you are doing the speed matching using the ground as a reference, not your speed relative to the train. Anyway, so if you start walking faster the train travels faster to compensate and you remain stationary relative to the trees? I'm sorry, but this is the treadmill analogy. If you were to be on a skateboard, with a rocket pack, the train would not be able to stop you accelerating forward relative to the ground. As soon as you pick up some groundspeed the train would never be able to match your speed relative to it.

Yes, this is the treadmill example and the aircraft will move. I only bring it up because it best describes my interpretation of the relationship of speeds. No need to explain to me the forces involved. I agree that a skateboard with a rocket pack on a train is the same as our aircraft on a conveyor and you will accelerate. Please read my train post as an explanation for my interpretation of relative speeds only.

:edit for spelling

[Edited 2007-06-06 15:00:07]

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-06-06 17:36:52 and read 32767 times.

Quoting DarkBlue (Reply 603):
Quoting Jetlagged (Reply 602):
Because you insist on defining belt speed as the relative speed between the belt and the wheel

No, i define belt speed as the relative speed between belt and ground.

 crazy  Another example of where confusion can arise. Just for the record, I was considering the various speeds thus:

Ground speed = aircraft speed relative to the ground (as in real life)
Air speed = aircraft speed relative to the air (as in real life)
Belt speed = aircraft speed relative to a point on the belt's surface (to be consistent with the real life speed definitions)

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: DarkBlue
Posted 2007-06-08 22:50:13 and read 32767 times.

Must... not... let... thread... die...


It's been more than 2 days since the last post, soooooo, let's change the question....  Smile How about the aircraft is sitting on a gigantic hamster wheel, say, 100meters in diameter. No wheel constraint (thank goodness) and the imaginary magic stopping force has been replaced by gravity.

So what happens? Aircraft rolls up one side of the hamster wheel until the vertical component of thrust equals the weight of the aircraft? Since the aircraft doesn't have the opportunity to gain any significant speed, lift is insignificant.

So here's the tough question, does the hamster wheel turn? The motto of this thread has become "wheels don't make the aircraft move!", which applied to this means that unlike a hamster that pushes against the wheel to make it turn, a rolling aircraft will not turn the hamster wheel. Hamster wheel is stationary.

Thoughts?

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Starlionblue
Posted 2007-06-08 23:01:25 and read 32767 times.

Quoting DarkBlue (Reply 608):
So here's the tough question, does the hamster wheel turn? The motto of this thread has become "wheels don't make the aircraft move!", which applied to this means that unlike a hamster that pushes against the wheel to make it turn, a rolling aircraft will not turn the hamster wheel. Hamster wheel is stationary.

You answered your own question.  Wink

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Mir
Posted 2007-06-09 12:52:45 and read 32767 times.

Quoting DarkBlue (Reply 608):
So here's the tough question, does the hamster wheel turn? The motto of this thread has become "wheels don't make the aircraft move!", which applied to this means that unlike a hamster that pushes against the wheel to make it turn, a rolling aircraft will not turn the hamster wheel. Hamster wheel is stationary.

I was going to respond to that, but I think it might be more productive to post this:



Breakfast time....  yummy 

-Mir

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-06-09 15:42:47 and read 32767 times.

Quoting DarkBlue (Reply 608):
So here's the tough question, does the hamster wheel turn?

Hmm... well, in this case, due to the curvature of the hamster wheel, the aircraft will transfer a slightly larger (but still quite small) tangential force to the hamster wheel.

Without using any physics at all (for the time being, anyway  crazy  ), my hunch is a complex series of ever increasing oscillations of both the wheel and the aircraft around the wheel's axle, as the aircraft's thrust, weight and momentum battle it out. If the aircraft's thrust is sufficient, you might end up with a stable situation where the aircraft moves around the e wheel at a constant RPM. Friction in the aircraft's wheel bearings would eventually drag the hamster wheel's speed up towards the aircraft's speed. However, it wouldn't reach the aircraft's speed due to the turning ability of the aircraft's wheels.

Quoting Mir (Reply 610):

Actually, I think you might be right.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Vatveng
Posted 2007-06-10 05:50:32 and read 32767 times.

Wow... how have I missed this thread all this time???


I think some poster about 200+ posts back started on the right track to an explanation...

Here's the scenario. Engines spool up, exerting force on the plane and pushing it forward. As a result, the wheels begin to turn. Magic conveyor belt immediately matches wheel speed in the opposite direction, which causes wheels to spin faster, which causes belt to immediately match wheel speed in the opposite direction, which causes wheels to spin faster, which causes belt to immediately match wheel speed in the opposite direction, which causes wheels to spin faster, which causes belt to immediately match wheel speed in the opposite direction, which causes wheels to spin faster, which causes belt to immediately match wheel speed in the opposite direction, which causes wheels to spin faster......

Which means that as soon as engine power begins to push the plane forward enough to turn the wheels, both wheels and conveyor belt immediately accelerate to infinite speed. Which is, of course, impossible. But so is a magic conveyor belt.

So the answer to "will the plane take off" in my opinion depends ENTIRELY ON THE SKILL OF THE PILOT. Because realistically, the wheels will fail mechanically before reaching infinite speed (the question mentions nothing about magic wheels that can withstand infinite acceleration, so we must assume that everything about the airplane in question is factory standard), causing the magic conveyor belt to stop because it is no longer counteracting the motion of the wheels because they no longer function as wheels (and likely no longer even exist). At this point the engines are pushing the plane forward on what's left of the landing gear. Depending on pilot skill, the plane will either take off, or it will skid along the magic conveyor belt (which no longer cares because there are no wheels to turn anymore) until it crashes into something.

Thanks for giving me something to fry a few brains at work... the best way to get people to leave me alone is to confuse them into silence Big grin I'll be able to milk this for weeks  bouncy 

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-06-10 11:57:08 and read 32767 times.

Quoting David L (Reply 611):
If the aircraft's thrust is sufficient, you might end up with a stable situation where the aircraft moves around the e wheel at a constant RPM

On reflection, it's quite likely that there would come a time when the aircraft would find itself going past the vertical with insufficient energy to continue and would thus fall off and land in a crumpled mess at the bottom of the wheel.

Any volunteers?

Quoting David L (Reply 611):
around the e wheel

An "e wheel"?  embarrassed 

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Mir
Posted 2007-06-10 12:44:36 and read 32767 times.

Quoting David L (Reply 613):
On reflection, it's quite likely that there would come a time when the aircraft would find itself going past the vertical with insufficient energy to continue and would thus fall off and land in a crumpled mess at the bottom of the wheel.

Not if it were going fast enough.

-Mir

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-06-10 16:37:07 and read 32767 times.

Quoting Mir (Reply 614):
Not if it were going fast enough.

I think I covered that in my first attempt. However, that ignored the fact that early on it would quite possibly find itself having gone 1/3 of the way round the circle but without enough energy to complete the loop. You'd have to bypass that part to get to the looping the loop part.

You'd have to have a situation where on Attempt n it didn't make it past 1/4 of the way round so would roll back down the wheel, and on Attempt n+1 it made it all the way round in one go. If it could accelerate fast enough, it might make it on the first attempt but how "real" would such an aircraft be, bearing in mind that the wheel diameter is 100m? A very powerful model, perhaps.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: DarkBlue
Posted 2007-06-10 18:52:16 and read 32767 times.

Quoting David L (Reply 615):
how "real" would such an aircraft be, bearing in mind that the wheel diameter is 100m? A very powerful model, perhaps.

Yeah, I suppose any aircraft with a thrust-to-weight ratio greater than 1.0 would be able to accomplish the full circle. When it comes to high thrust-to-weight ratios, the F-15 certainly comes to mind. No one said the aircraft had to be a commercial airliner, correct?

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: HAWK21M
Posted 2007-06-10 18:55:04 and read 32767 times.

Quoting Vatveng (Reply 612):


So the answer to "will the plane take off" in my opinion depends ENTIRELY ON THE SKILL OF THE PILOT

You are forgetting about relative Airflow over the Control surfaces.
regds
MEL

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Vatveng
Posted 2007-06-10 23:58:24 and read 32767 times.

Quoting HAWK21M (Reply 617):
Quoting Vatveng (Reply 612):


So the answer to "will the plane take off" in my opinion depends ENTIRELY ON THE SKILL OF THE PILOT

You are forgetting about relative Airflow over the Control surfaces.

Did you stop reading at this line? There's going to be plenty of airflow over the control surfaces once the plane starts skidding on the bloody stumps that used to be the landing gear.

Maybe you thought I was talking about the hamster wheel thing?? which I wasn't.

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-06-11 00:45:33 and read 32767 times.

Quoting DarkBlue (Reply 616):
Yeah, I suppose any aircraft with a thrust-to-weight ratio greater than 1.0 would be able to accomplish the full circle.

But a thrust to weight ratio of "just over"1 would only allow it to crawl up to the vertical. Once it got past the vertical, the thrust would no longer be applied vertically so gravity could start to take over. I think the thrust to weight ratio would have to be significantly larger than 1.

Quoting Vatveng (Reply 612):

I think some of the Option 3 philosophers might disagree.  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: Keta
Posted 2007-06-11 03:33:36 and read 32767 times.

Quoting David L (Reply 613):
On reflection, it's quite likely that there would come a time when the aircraft would find itself going past the vertical with insufficient energy to continue and would thus fall off and land in a crumpled mess at the bottom of the wheel.



Quoting David L (Reply 619):
But a thrust to weight ratio of "just over"1 would only allow it to crawl up to the vertical. Once it got past the vertical, the thrust would no longer be applied vertically so gravity could start to take over. I think the thrust to weight ratio would have to be significantly larger than 1.

Doing some calculations, I ended up wit the different situations the aircraft would face, quite interesting ones. I assumed that the airplane starts at the lower point, from rest, and accelerates using its thrust, which I assumed to be constant. This simplified things a bit  Smile Depending on the thrust to weight ratio (a on the following), different things happen:


  • If the ratio is small (think about 0.3 or so) the airplane will start to run. At first it will accelerate, but there will be a point where the weight is just too much for the thrust. At this point, the plane would face a backwards acceleration, which would make it slow down. It would continue running until it stopped, then it would start going down, and the same would happen, but on the opposite direction.

    This way the plane would be in an infinite repeating movement, going up until the stopping point, then down to the starting point, then up again, then down, then up...

    This movement would happen for a between 0 and 0.7246 (not included), and the movement would be between angles 0 and the angle that satisfies cosθ + aθ = 1 (not an easy equation to solve, huh?). For example, for a=0, the angle would be zero (zero thrust, no movement), and for a=0.7245, the maximum angle would be 133.562 degrees (note, quite well passed 90º. That's the stopping point, so it would fall down. This will come later)

  • In the case of a being equal to 0.7246, the aircraft would asymptotically move to the 133.562 degrees point. It would need an infinite time to reach there (but don't worry, it would have fallen down much earlier).

  • If a was bigger than 0.7246, the aircraft would never achieve 0 speed again: it would continue to run for ever. Each revolution, there would be a point of minimum speed (at θ=arcsina) , but not zero; once passed, the aircraft would accelerate again, and each turn it would be going faster than the previous one.

  • If a was bigger than one, there wouldn't be a minimum speed point: the aircraft would be always accelerating. Which makes sense, because if the thrust is stronger than the weight, the aircraft will always be able to have a forward force and thus forward acceleration.


This was considering that the wheel exerts a normal force on the plane which could act in either the inside or the outside direction of the circle. But we have an airplane running inside a wheel, so the normal force can only act in one direction. At the point where the normal force equals to zero, the airplane would fall off the wheel. So, we need to know what is the thrust to weight ratio that makes the normal force be positive during all the revolution.


  • At first, it could look like the minimum normal force happens at the upper point (θ=180º), but this is not a normal pendulum. The normal force has a minimum at another point, and we have to make this minimum to be bigger than zero. This led to a=0.88. If a was bigger, the normal force would never be zero, and the plane would never fall off. Also, note that this is the situation in which the plane would never stop. If less, the aircraft would fall.

  • If a is smaller than 0.88, the normal force does have a zero. But this is only interesting if the airplane's stopping point is passed 90º. If the point is below 90º, obviously it can't fall off (the equations also show that the stopping point is reached before the normal force equals to zero). But if the point is above 90º, the speed wouldn't be enough to get a positive normal force. The 90º point happens for a=0.636. Note that in this situation the aircraft would be going up to the vertical, then down to the start, then up...


Conclusion: If the thrust to weight ratio is less than 0.636, the aircraft will be doing an infinite repeating movement, with the maximum at the vertical line. For a between 0.636 and the 0.8807, the aircraft would fall off at a point between 90º and 143.35º. For a bigger than 0.8807, the aircraft would neither fall down nor stop: it would have a continuous movement, faster each revolution.

It could seem strange that the thrust to weight ratio doesn't have to be higher than 1 in order to not falling down. But the wheel acts as the wings of an airplane, to some extent at least. They both produce a force which is normal to the movement, and we know that an airplane doesn't need a 1:1 thrust to weight ratio to make a loop.

I have not taken into account the lift the wings would generate when gaining speed. If somebody wants to...  Smile

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: HAWK21M
Posted 2007-06-11 09:49:52 and read 32767 times.

Quoting Vatveng (Reply 618):
There's going to be plenty of airflow over the control surfaces once the plane starts skidding on the bloody stumps that used to be the landing gear.

And the pitch down moment,since the wheels are not present.
regds
MEL

Topic: RE: If A Plane Took Off A Conveyor Belt...
Username: David L
Posted 2007-06-11 13:23:21 and read 32767 times.

Quoting Keta (Reply 620):

Excellent. I certainly wasn't about to do any calculations!  

Quoting Keta (Reply 620):
It could seem strange that the thrust to weight ratio doesn't have to be higher than 1 in order to not falling down.

On further reflection, as I was trying to get to sleep, I hadn't taken into account the speed, hence momentum, already achieved by the time the aircraft reaches the Questionable Zone. The ratio of 1:1 is only relevant if the aircraft is barely crawling towards the vertical. My earlier