Plane on a coneyor belt
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You can't keep a powered plane in one place by pulling the ground from under it....even at a matched speed.
Even the pilot himself thinks he's going to sit there like a brick.
http://www.youtube.com/watch?v=YORCk1BN7QY&feature=related -
Well that answers that.
Next!
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you know. .. Dave Barry had the theory that flight as in airplane flight was all a myth. .. and was really jsut an example of mass-hallucination. . .his thinking was that an airplane is much heavier than say . .. a piano . .. and yet when he and a bunch of his friends pushed a piano down the runway they couldnt get it to take off, so logic being what it is, a plane can't fly either.
Hmmm.
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ok. so one half of us approached the whole experiment theoretically (with a high speed conveyor that uses the minimal frictional resistance of the tyres to keep the plane in place)
and the other half expected the plane to actually move forward (because an ordinary conveyor can't put up enough speed)...next challenge!
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You asked for it: imagine the same situation again (plane on a conveyor belt) except this time the conveyor belt matches the speed of the planes wheel rather than the speed of the plane itself.
Can it take off?
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yes!
As it uses its jets to propel forward, even if it had it's brakes on the jet would take off.
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well, because the wheels have no motor, the speed of the wheels is defined by the speed the conveyor has to put up to keep the plane (that is trying to move forward) in place...
this time the plane does not take off
and now it is official, I completely misunderstood the first question
[Edit] understood the first question like the second one is meant...[] -
Im with jakob on this one, although i'll see what weird and wonderful explanations everyone comes up with before offering my own interpretation.
Jakob, me too. it took me a good half a day (before posting this thread) to understand what was happening properly.
And just so we're clear about the question, when i say wheel speed i mean the speed at which the wheel is rotating, rather than the speed at which it is moving forward (or not.)
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I fail to see how this is a different question? We've established that the conveyor belt can move however fast it wants. The rotational speed of the wheels has nothing to do with the plane moving forward, as long as the brakes aren't on and there's nothing physically holding the plane to the ground.
So the plane still takes off.
Chris
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@chris fullmer said:
I fail to see how this is a different question? We've established that the conveyor belt can move however fast it wants. The rotational speed of the wheels has nothing to do with the plane moving forward, as long as the brakes aren't on and there's nothing physically holding the plane to the ground.
So the plane still takes off.
Chris
i agree
pav
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I also agree .... however! see if brakes stop an F16 from taking off.
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Ok, well just focus on the system of the wheel for the moment.
For the plane to take off it must have forward velocity relative to the ground (assuming its a still day.) If the plane has forwad velocity so do the wheels...
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but the conveyor belt moving backwards doesn't push backwards at all on the plane. The wheels are free spinning, they will just spin faster to accommodate the conveyor belt while the plane moves forwards.
Here's what I think it would take to stall the plane on a conveyor belt:
The conveyor belt would have to move backwards at the speed at which the propellers/turbine are capable of moving air through them. So if the propeller speeds up air to 1000mph (or km/h), then the conveyor would have to move the plane backwards at that rate, so that their would be air rushing backwards into the prop/turbine at the same speed at which it could be sucked in. That seems more like it. So the conveyor has to move at the speed of the air rushing through the propeller.
Chris
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But the wheels would have to be able to spin infinitely fast to accommodate the plane taking off and the conveyor belt, and infinity=impossible.
To work it trough with numbers: the plane starts moving at 1 mph, thus the wheels are also turning at 1 mph, the conveyor belt spins to accommodate this meaning that the wheel is then doing 2 mph (aeroplane speed+conveyor belt.) the conveyor belt then has to speed up again to match the wheel speed which gives a wheel speed of 4 mph. You can keep going in this fashion for as long as you like, there is no end to the sequence.
The root of the problem lies in that for the plane to take off the wheel must have forward velocity, but while the conveyor belt is there the wheels cant have forward velocity.
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the wheel doesnt have its own velocity, it has spin. It is always at the same velocity as the plane. So as soon as the conveyor goes backwards, the whold plane does, until the plane kicks on its engines and pulls itself forward through the air. At that time, the wheels spin faster only because their speed is equal to the speed of the conveyor, plus the speed of the plane. The velocity is the same as the plane's though. the conveyor belt can't hold up their velocity, it only affects their spin.
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My above reasoning works if you use spin instead of velocity.
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Remus,
You are still presupposing that the backward motion of the belt can exert a significant effect on a plane powering forward...enough to cancel out any forward motion...it can't. You might as well try to lasso the plane with a smoke ring.
Everyone would agree that the plane could not take off if its groundspeed remained at zero, but that's the essence of the problem...it won't.
You could see that happening in the Mythbusters clip. It wasn't the case that the plane was travelling as fast as the car and then some...enough extra to get some lift. They were both travelling at the same speed; but the belt can't hold the plane motionless any more than it could if it were being towed by a rocket. The wheels are irrelevant; spinning or otherwise. They do little more than keep the aircraft level. It would work exactly the same if the plane was on skids and the belt had a low coefficient of friction.Think of it this way. A car wheel in high revs on a muddy surface throws a ton of mud backwards. It is exerting a very substantial force in the opposite direction to its motion. A plane wheel in high revs on the same surface does absolutely nothing other than roll over it. The only force it is exerting is downwards, until it reaches takeoff speed. The reverse is also true...if the aircraft wheel exerts no lateral force on the ground, then the ground exerts no opposing lateral force on the wheel; certainly not enough to hold it stationary, however fast it travels. It's a completely false proposition.
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I'm afraid my example is the only way in which this would work. Logic it through and see if I missed anything. The conveyor belt has to move backwards at the speed at which air could be sucked in to the propellers, thereby canceling out the propellers ability to work, so the plane would sit still with its engines burning full speed. The wheels would therefore not be spinning at all and the conveyor would in fact be moving at its own speed, plus the speed (zero) of the wheels).
Chris
though once the plane is moving that fast backwards, you have to ignore all the wind coming backwards at it. I don't know if its possible to ignore that though.
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Chris, I don't understand what you mean. what has the conveyor to do with wind speed?
but to the idea that the plane can't be stopped by the conveyor belt:
you allways asume, that the wheels have no frictional resistance whatsoever. but this is wrong. if that were true, you only needed to speed up a car once and it would roll on forever (almost a perpeto mobile).
but no matter how good the bearings are, wheels will allways have some tiny amount of frictional resistance.
I have no idea about physics or maths. but asume that 1/1.000 of the rotation energy of the wheel is transformed into warmth by this resistance.
that only means, that the conveyor has to rotate 1.000 x faster than the plane is pushing forward.so you simply need an incredibly fast conveyor to stop the plane from moving forward - therefore stopping it from taking off.
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Jakob,
But that wasn't the question. Certanly if you had a conveyor capable of infinite speed it could exert enough drag on the wheels to stop the plane advancing. You could achieve the same thing by chaining the undercarriage to a fixed object.
The question was can you stop the plane by having the conveyor go backwards at the same speed as the plane's forward motion. I quote: "This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction)." The answer is no. The moments of force just don't add up.I think the main problem is that it's not a very scientific question. It talks about the plane's speed. What speed?...groundspeed or airspeed?...an aircraft only has those two. Yet the question involves some weird speed relative to to a fixed point on a moving surface. That sends the whole thing into a circular argument, because that speed is, itself, influenced by the speed of the conveyor. That's probably why the debate has gone on for 5 pages. It falls into the realm of "How long is a piece of string?"
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