A plane on a conveyor belt

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iBot said:
"A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. 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). Can the plane take off?"
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Does the plane speed refer to the speed of the plane relative to the ground or does it refer to the speed at which the wheels are turning?

Clearly, if it refers to the speed of the plane relative to the ground, then if the treadmill is moving then the plane must be moving relative to the ground, and hence relative to the air, so there is lift.

If it refers to the speed at which the wheels are turning then I agree with Eriedor (page 3 maybe?), that such a scenario is not possible.
 
If the belt is moving at the same speed as the plane, then YES
If the belt is moving at the same speed as the wheels, then YES
/close thread
 
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Tefal said:
If the planes engines are on full why isn't the plane moving :confused:
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Because these new fangled aircraft have a brilliant new invention known as "brakes".



Bes said:
If the belt is moving at the same speed as the plane, then YES
If the belt is moving at the same speed as the wheels, then NO
/close thread
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Wrong, it will take off regardless of what the belt is doing. FACT.
 
Scuzi said:
Because these new fangled aircraft have a brilliant new invention known as "brakes".

Wrong, it will take off regardless of what the belt is doing. FACT.
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I agree that it will take off regardless of what the belt is doing. However, I do not agree with Bes saying "If the belt is moving at the same speed as the wheels, then YES." The reason I disagree with this, is that it is an impossible situation to have a treadmill such that the belt moves at the same speed as the wheels. My reasoning is the same as Eriedor's: Suppose that the situation of the speed of the treadmill being the same as the plane's wheels is possible. The thrust of the plane will cause it to move forwards - I think we all agree with this. But the moving forwards means that it has a relative velocity with the treadmill, and therefore the wheels must be moving faster than the treadmill. This contradicts the fact that the wheels are moving at the same speed as the treadmill, so this situation is impossible.

Anyway, I think I'm being a bit pedantic. Never mind :p
 
iBot said:
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).
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The conveyor belt matches the speed of the wheels NOT the plane
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This is the real root of the problem people have with this question.

The second quote is the real world answer to the problem and yes the plane will take off.

However if you take the original question in a literal form and ignore the wheels etc and imagine a conveyor belt that tracked the forward (air)speed of the aircraft meaning the aircraft is then stationary in relation to the ground then no it won't. The question was, I think, originally posed by someone with little or no grasp of physics.

(A similar thread on a physics forum ended up at hundreds of pages btw :p )

~S
 
Sp!ke said:
However if you take the original question in a literal form and ignore the wheels etc and imagine a conveyor belt that tracked the forward (air)speed of the aircraft meaning the aircraft is then stationary in relation to the ground then no it won't.
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Yes it would. It doesn't matter how fast the conveyor is moving, it'll still take off.
 
Zefan said:
Yes it would. It doesn't matter how fast the conveyor is moving, it'll still take off.
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Yes I agree, in the real world. But the point I was trying to make was that I think whoever originally posed the question didn't understand it and was trying to say that the aircraft will remain stationary whilst happily ignoring the laws of physics.

It's the question that's the problem, not the answer.

(I know it takes off although it took me a few minutes to work it out beyond doubt when I first saw the question)

/edit Actually scrap that, on reflection it's the reader of the question that doesn't understand it, perhaps the guy was a genius after all.
 
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Am I missing something here? In order for a fixed-wing aircraft to take off, air must pass over the wings at a particular speed, or faster. Let's say that for a nominal aircraft in a nominal geometry, takeoff speed in still air is 120 knots. That means the plane will take off when air passes over and under the wing at 120 knots. If the plane stays in same place on the ground as measured by (say) GPS, then the only way it is going to fly is if there is a 120 knot head wind. If it moved forward at 60 knots and there was a 60 knot headwind it would fly. If the plane move backwards at 30 knots, but there was a 150 knot headwind it would fly.

But if it stayed in place with respect to the earth - which is what the question seems to imply in its badly worded way - and there's a headwind of less than 120 knots, no, it won't fly. It's airspeed that matters - ground speed is irrelevant.


M
 
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