Sovietjet From Bulgaria, joined Mar 2003, 1530 posts, RR: 8 Posted (2 years 7 months 1 week 2 days 8 hours ago) 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.
Joness0154 From United States, joined Nov 2005, 667 posts, RR: 1 Reply 2, posted (2 years 7 months 1 week 2 days 8 hours ago) 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.
I don't have an attitude problem. You have a perception problem
BA From United States, joined May 2000, 10118 posts, RR: 60 Reply 3, posted (2 years 7 months 1 week 2 days 8 hours ago) 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]
"Generosity is giving more than you can, and pride is taking less than you need." - Khalil Gibran
Nighthawk From United Kingdom (Scotland), joined Sep 2001, 3604 posts, RR: 26 Reply 5, posted (2 years 7 months 1 week 2 days 6 hours ago) 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!
"Thats why you need people like me in the secret service" Random Drunk. Manchester. 01/08
Joness0154 From United States, joined Nov 2005, 667 posts, RR: 1 Reply 6, posted (2 years 7 months 1 week 2 days 6 hours ago) 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)
I don't have an attitude problem. You have a perception problem
Airplanepics From United Kingdom (England), joined Jun 2003, 2693 posts, RR: 42 Reply 7, posted (2 years 7 months 1 week 2 days 6 hours ago) and read 32767 times:
I think we have another "How many pigeons would it take to lift a 747" thread!
RichardPrice From , joined today!, posts, RR: Reply 9, posted (2 years 7 months 1 week 2 days 6 hours ago) 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.
Joness0154 From United States, joined Nov 2005, 667 posts, RR: 1 Reply 11, posted (2 years 7 months 1 week 2 days 5 hours ago) 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?
I don't have an attitude problem. You have a perception problem
RichardPrice From , joined today!, posts, RR: Reply 12, posted (2 years 7 months 1 week 2 days 5 hours ago) 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.
RichardPrice From , joined today!, posts, RR: Reply 13, posted (2 years 7 months 1 week 2 days 5 hours ago) 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.
Nighthawk From United Kingdom (Scotland), joined Sep 2001, 3604 posts, RR: 26 Reply 14, posted (2 years 7 months 1 week 2 days 5 hours ago) 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.
"Thats why you need people like me in the secret service" Random Drunk. Manchester. 01/08
Joness0154 From United States, joined Nov 2005, 667 posts, RR: 1 Reply 15, posted (2 years 7 months 1 week 2 days 5 hours ago) 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.
I don't have an attitude problem. You have a perception problem
RichardPrice From , joined today!, posts, RR: Reply 16, posted (2 years 7 months 1 week 2 days 5 hours ago) 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.
Joness0154 From United States, joined Nov 2005, 667 posts, RR: 1 Reply 17, posted (2 years 7 months 1 week 2 days 5 hours ago) 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.
I don't have an attitude problem. You have a perception problem
RichardPrice From , joined today!, posts, RR: Reply 18, posted (2 years 7 months 1 week 2 days 5 hours ago) and read 32767 times:
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.
Nighthawk From United Kingdom (Scotland), joined Sep 2001, 3604 posts, RR: 26 Reply 20, posted (2 years 7 months 1 week 2 days 5 hours ago) 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.
"Thats why you need people like me in the secret service" Random Drunk. Manchester. 01/08
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