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sovietjet
Topic Author
Posts: 2678
Joined: Sat Mar 15, 2003 12:32 am

### If A Plane Took Off A Conveyor Belt...

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.

BA
Posts: 10514
Joined: Fri May 19, 2000 11:06 am

### RE: If A Plane Took Off A Conveyor Belt...

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.
"Generosity is giving more than you can, and pride is taking less than you need." - Khalil Gibran

joness0154
Posts: 650
Joined: Sat Nov 19, 2005 12:56 am

### RE: If A Plane Took Off A Conveyor Belt...

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.
I don't have an attitude problem. You have a perception problem

BA
Posts: 10514
Joined: Fri May 19, 2000 11:06 am

### RE: If A Plane Took Off A Conveyor Belt...

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]
"Generosity is giving more than you can, and pride is taking less than you need." - Khalil Gibran

diamond
Posts: 3000
Joined: Sun Apr 11, 2004 8:01 am

### RE: If A Plane Took Off A Conveyor Belt...

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.
Blank.

nighthawk
Posts: 4879
Joined: Sun Sep 16, 2001 2:33 am

### RE: If A Plane Took Off A Conveyor Belt...

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!

joness0154
Posts: 650
Joined: Sat Nov 19, 2005 12:56 am

### RE: If A Plane Took Off A Conveyor Belt...

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)
I don't have an attitude problem. You have a perception problem

Airplanepics
Posts: 2606
Joined: Fri Jun 27, 2003 4:12 am

### RE: If A Plane Took Off A Conveyor Belt...

I think we have another "How many pigeons would it take to lift a 747" thread!
Simon - London-Aviation.com

joness0154
Posts: 650
Joined: Sat Nov 19, 2005 12:56 am

### RE: If A Plane Took Off A Conveyor Belt...

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.
I don't have an attitude problem. You have a perception problem

RichardPrice
Posts: 4474
Joined: Sat Apr 23, 2005 5:12 am

### RE: If A Plane Took Off A Conveyor Belt...

 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.

HAWK21M
Posts: 30067
Joined: Fri Jan 05, 2001 10:05 pm

### RE: If A Plane Took Off A Conveyor Belt...

If you can generate that speed on the Belt & have the Engines at T/O thrust at the same time
regds
MEL
I may not win often, but I damn well never lose!!!

joness0154
Posts: 650
Joined: Sat Nov 19, 2005 12:56 am

### RE: If A Plane Took Off A Conveyor Belt...

 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?
I don't have an attitude problem. You have a perception problem

RichardPrice
Posts: 4474
Joined: Sat Apr 23, 2005 5:12 am

### RE: If A Plane Took Off A Conveyor Belt...

 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.

RichardPrice
Posts: 4474
Joined: Sat Apr 23, 2005 5:12 am

### RE: If A Plane Took Off A Conveyor Belt...

 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.

nighthawk
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Joined: Sun Sep 16, 2001 2:33 am

### RE: If A Plane Took Off A Conveyor Belt...

 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.

joness0154
Posts: 650
Joined: Sat Nov 19, 2005 12:56 am

### RE: If A Plane Took Off A Conveyor Belt...

 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.
I don't have an attitude problem. You have a perception problem

RichardPrice
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Joined: Sat Apr 23, 2005 5:12 am

### RE: If A Plane Took Off A Conveyor Belt...

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.

joness0154
Posts: 650
Joined: Sat Nov 19, 2005 12:56 am

### RE: If A Plane Took Off A Conveyor Belt...

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.
I don't have an attitude problem. You have a perception problem

RichardPrice
Posts: 4474
Joined: Sat Apr 23, 2005 5:12 am

### RE: If A Plane Took Off A Conveyor Belt...

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

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

joness0154
Posts: 650
Joined: Sat Nov 19, 2005 12:56 am

### RE: If A Plane Took Off A Conveyor Belt...

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

Welcome to the darkside...
I don't have an attitude problem. You have a perception problem

nighthawk
Posts: 4879
Joined: Sun Sep 16, 2001 2:33 am

### RE: If A Plane Took Off A Conveyor Belt...

 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.

HT
Posts: 5864
Joined: Sat May 07, 2005 6:20 am

### RE: If A Plane Took Off A Conveyor Belt...

 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 ...
(I hope you got the point ?!).
-HT
Carpe diem ! Life is too short to waste your time ! Keep in mind, that today is the first day of the rest of your life !

QFA380
Posts: 2012
Joined: Fri Jul 22, 2005 4:38 pm

### RE: If A Plane Took Off A Conveyor Belt...

 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?

QFA380

sudden
Posts: 3936
Joined: Fri Jul 13, 2001 5:20 pm

### RE: If A Plane Took Off A Conveyor Belt...

 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.

Aim for the sky!
Sudden
When in doubt, flat out!

nighthawk
Posts: 4879
Joined: Sun Sep 16, 2001 2:33 am

### RE: If A Plane Took Off A Conveyor Belt...

 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...

PMN
Posts: 547
Joined: Sun Mar 06, 2005 7:44 am

### RE: If A Plane Took Off A Conveyor Belt...

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]
Edith in his bed, a plane in the rain is humming, the wires in the walls are humming some song - some mysterious song

saintsman
Posts: 2037
Joined: Fri Mar 15, 2002 12:34 am

### RE: If A Plane Took Off A Conveyor Belt...

 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.

joness0154
Posts: 650
Joined: Sat Nov 19, 2005 12:56 am

### RE: If A Plane Took Off A Conveyor Belt...

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
I don't have an attitude problem. You have a perception problem

sudden
Posts: 3936
Joined: Fri Jul 13, 2001 5:20 pm

### RE: If A Plane Took Off A Conveyor Belt...

 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
When in doubt, flat out!

Alessandro
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### RE: If A Plane Took Off A Conveyor Belt...

From New Yorqatar to Califarbia...

nighthawk
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Joined: Sun Sep 16, 2001 2:33 am

### RE: If A Plane Took Off A Conveyor Belt...

 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.

nighthawk
Posts: 4879
Joined: Sun Sep 16, 2001 2:33 am

### RE: If A Plane Took Off A Conveyor Belt...

 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!

Kay
Posts: 1797
Joined: Fri Mar 01, 2002 3:41 pm

### RE: If A Plane Took Off A Conveyor Belt...

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

A346Dude
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Joined: Tue Nov 30, 2004 11:23 am

### RE: If A Plane Took Off A Conveyor Belt...

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.
You know the gear is up and locked when it takes full throttle to taxi to the terminal.

PMN
Posts: 547
Joined: Sun Mar 06, 2005 7:44 am

### RE: If A Plane Took Off A Conveyor Belt...

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

The most patronising perhaps...

Paul
Edith in his bed, a plane in the rain is humming, the wires in the walls are humming some song - some mysterious song

A346Dude
Posts: 1161
Joined: Tue Nov 30, 2004 11:23 am

### RE: If A Plane Took Off A Conveyor Belt...

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]
You know the gear is up and locked when it takes full throttle to taxi to the terminal.

joness0154
Posts: 650
Joined: Sat Nov 19, 2005 12:56 am

### RE: If A Plane Took Off A Conveyor Belt...

 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

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.
I don't have an attitude problem. You have a perception problem

nighthawk
Posts: 4879
Joined: Sun Sep 16, 2001 2:33 am

### RE: If A Plane Took Off A Conveyor Belt...

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.....

flyingbabydoc
Posts: 1059
Joined: Wed Oct 19, 2005 6:12 pm

### RE: If A Plane Took Off A Conveyor Belt...

The plane is a BAE Harrier.

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

cheers

Alex
Marriage is the art of turning a lover into a relative

joness0154
Posts: 650
Joined: Sat Nov 19, 2005 12:56 am

### RE: If A Plane Took Off A Conveyor Belt...

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.
I don't have an attitude problem. You have a perception problem

CHRISBA777ER
Posts: 3715
Joined: Thu Mar 01, 2001 12:12 pm

### RE: If A Plane Took Off A Conveyor Belt...

LOL You lot crease me up.
What do you mean you dont have any bourbon? Do you know how far it is to Houston? What kind of airline is this???

Kay
Posts: 1797
Joined: Fri Mar 01, 2002 3:41 pm

### RE: If A Plane Took Off A Conveyor Belt...

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.

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]

nighthawk
Posts: 4879
Joined: Sun Sep 16, 2001 2:33 am

### RE: If A Plane Took Off A Conveyor Belt...

 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.

PMN
Posts: 547
Joined: Sun Mar 06, 2005 7:44 am

### RE: If A Plane Took Off A Conveyor Belt...

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

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

Paul
Edith in his bed, a plane in the rain is humming, the wires in the walls are humming some song - some mysterious song

joness0154
Posts: 650
Joined: Sat Nov 19, 2005 12:56 am

### RE: If A Plane Took Off A Conveyor Belt...

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

I don't have an attitude problem. You have a perception problem

saintsman
Posts: 2037
Joined: Fri Mar 15, 2002 12:34 am

### RE: If A Plane Took Off A Conveyor Belt...

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......

CHRISBA777ER
Posts: 3715
Joined: Thu Mar 01, 2001 12:12 pm

### RE: If A Plane Took Off A Conveyor Belt...

...yes but the brakes will be off otherwise its going backwards!
What do you mean you dont have any bourbon? Do you know how far it is to Houston? What kind of airline is this???

SlamClick
Posts: 9576
Joined: Sun Nov 23, 2003 7:09 am

### RE: If A Plane Took Off A Conveyor Belt...

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.
Happiness is not seeing another trite Ste. Maarten photo all week long.

A346Dude
Posts: 1161
Joined: Tue Nov 30, 2004 11:23 am

### RE: If A Plane Took Off A Conveyor Belt...

 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]
You know the gear is up and locked when it takes full throttle to taxi to the terminal.

ariis
Posts: 390
Joined: Wed Sep 22, 2004 8:04 am

### RE: If A Plane Took Off A Conveyor Belt...

 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.

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