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Thread: Stupid Question

  1. #1
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    Stupid Question

    I've got this off another forum, one,I believe, of many where it is being discussed. You wouldn't believe the debate that has resulted from a simple question.

    I'm sure that the enough knowledgeable, intelligent members of this forum will know the correct answer.....go on prove me wrong!

    The question is:

    A plane is standing on runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in opposite direction).

    The question is:

    Will the plane take off or not? Will it be able to run up and take off?


  2. #2
    Vulture of The Western World Eric's Avatar
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    Re: Stupid Question

    I will hazard an answer and say no.

    Because in order to fly, an airplane needs lift - and in order to get thenecessary lift, it must be traveling through the air at "x" speed.

    In your example, the plane is actually static (not moving) relative to the air around it, so there is no airflow over the wings, hence no lift. Hence no flying.

    Or am I looking silly now?

  3. #3
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    Re: Stupid Question

    The plane's direction is a vector. The conveyor's direction is a vector.

    You sum the vectors. If the sum is > than speed required for takeoff, then the arifoil will have enough lift.

  4. #4
    DonTom
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    Re: Stupid Question

    Will it be able to run up and take off?

    Of course not. Turn the plane around and then take off using very little energy.

    -Don-


  5. #5
    mrblanche
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    Re: Stupid Question

    Quote Originally Posted by Dave Brand
    A plane is standing on runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in opposite direction).

    The question is:

    Will the plane take off or not? Will it be able to run up and take off?

    And here's a simple answer. The conveyor belt is the ground. An airplane flies by airspeed, not groundspeed.

    But you would have an infinite feed back loop, here. The plane's engines act on the air, not the ground. So it would accelerate to it's take-off speed, while the conveyor would be trying to counteract that speed merely with wheel spped.

  6. #6
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    Re: Stupid Question

    Quote Originally Posted by Dave Brand

    The question is:

    Will the plane take off or not? Will it be able to run up and take off?

    Run up, yes, at least in the sense that term is used in US aviation jargon.

    For take off there must be some lift generated and if I understand the question there is no relative movement to the surrounding air so no take off. What will the A/S indicator say? An airplane flies on IAS.

    What about a helicopter on a turn table which I refuse to contemplate?

  7. #7
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    Re: Stupid Question

    >>An airplane flies on IAS.<<

    Not all of them do-- The Harrier for one needs no IAS to take off.

  8. #8
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    Re: Stupid Question

    Quote Originally Posted by Jim Rose
    >>An airplane flies on IAS.<<

    Not all of them do-- The Harrier for one needs no IAS to take off.
    As does that thing with the motors on the wing tips which kills Marines. I could quibble about whether any of that is flight but being a kind and gentle soul will not.


  9. #9
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    Re: Stupid Question

    Quote Originally Posted by jdm
    For take off there must be some lift generated and if I understand the question there is no relative movement to the surrounding air so no take off. What will the A/S indicator say? An airplane flies on IAS.
    There needs to be acceleration - the plane must accelerate to the lift off velocity. If the 'runway' belt moves in an opposite direction, with the same acceleration, there will no net gain in velocity, thus the plane will not experience lift. If the plane is stationary - relative to the earth, it cannot fly.

    Think about it... what would happen when the plane rotated? He would have to have instant acceleration to his normal lift off velocity.

    It's a really dumb problem.

  10. #10
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    Re: Stupid Question

    >>but being a kind and gentle soul will not.<<

    About as kind and gentle as a prison camp guard, huh?

  11. #11
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    Re: Stupid Question

    Quote Originally Posted by mrblanche
    And here's a simple answer. The conveyor belt is the ground. An airplane flies by airspeed, not groundspeed.

    But you would have an infinite feed back loop, here. The plane's engines act on the air, not the ground. So it would accelerate to it's take-off speed, while the conveyor would be trying to counteract that speed merely with wheel spped.
    Exactly!

    ...and for the doubters, read the question carefully. The key phrase is: 'the plane moves'! Another key fact is that plane wheels are free-running, so there is no way any force can be transmitted from the conveyor to the plane; in the context of flight what is happening to the wheels is irrelevant.

  12. #12
    DonTom
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    Re: Stupid Question

    so there is no way any force can be transmitted from the conveyor to the plane;

    Cute trick question. We forgot the wheels are in neutral.

    -Dummy Don

  13. #13
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    Re: Stupid Question

    Quote Originally Posted by Jim Rose
    >>but being a kind and gentle soul will not.<<

    About as kind and gentle as a prison camp guard, huh?
    Sometimes a lot nicer.

  14. #14
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    Re: Stupid Question

    Quote Originally Posted by Dave Brand


    Exactly!

    ...and for the doubters, read the question carefully. The key phrase is: 'the plane moves'! Another key fact is that plane wheels are free-running, so there is no way any force can be transmitted from the conveyor to the plane; in the context of flight what is happening to the wheels is irrelevant.
    The question is ambiguous. What we're talking about here is relativity, the speed of the leading edge of the wing in relation to the air surrounding it. Or, which is the much the same thing, the speed on the airspeed indicator.

  15. #15
    DonTom
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    Re: Stupid Question

    Another key fact is that plane wheels are free-running,

    They wheels would rotate at twice the speed, but the plane wouldn't notice the difference because internally, the wheel connect to nothing. I think more about motorcycles than planes, that's why I got it wrong!

    Now if it were a rice rocket motorcycle instad of a plane, it would not be going anywhere. A safe way to test a motorcycle at 300 MPH as it doesn't even move!
    -Don-

  16. #16
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    Re: Stupid Question

    Quote Originally Posted by DonTom
    Now if it were a rice rocket motorcycle instad of a plane, it would not be going anywhere. A safe way to test a motorcycle at 300 MPH as it doesn't even move!
    A company in the UK is advertising in the classic car magazines a set of rollers which can be put under the driven wheels of a car to allow it to be 'driven' without going anywhere!

  17. #17
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    Re: Stupid Question

    Quote Originally Posted by jdm
    The question is ambiguous. What we're talking about here is relativity, the speed of the leading edge of the wing in relation to the air surrounding it. Or, which is the much the same thing, the speed on the airspeed indicator.
    It is rather badly written. Whether by design or not, I'm not sure. However, as, by the nature of its propulsion system the plane can only move itself relative to the surrounding air, airspeed is implicit in the statement that it moves.

  18. #18
    TC
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    Re: Stupid Question

    Quote Originally Posted by Dave Brand
    I've got this off another forum, one,I believe, of many where it is being discussed. You wouldn't believe the debate that has resulted from a simple question.

    I'm sure that the enough knowledgeable, intelligent members of this forum will know the correct answer.....go on prove me wrong!

    The question is:

    A plane is standing on runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in opposite direction).

    The question is:

    Will the plane take off or not? Will it be able to run up and take off?

    No, it won't take off because it is not moving through the air which it needs to do to provide the necessary lift.

  19. #19
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    Re: Stupid Question

    Quote Originally Posted by TC
    No, it won't take off because it is not moving through the air which it needs to do to provide the necessary lift.
    This is what I said earlier, the plane's engines supply thrust to move it forward, the forward thrust provides a constant force to accelerate the plane to take off velocity.

    For pratical purposes the air speed and ground speed are the same, until the plane lifts off, then the ground speed and air speed can vary. (yes I know there can be a headwind)

    Here's the thought experiment - Let's assume the plane needs an air speed of 150 knots, in order to get enough lift to take off. So ... when the conveyor belt runs at 150 knots, the "ground stationary" plane will become airbore? At what velocity? - it has to be 150 knots, if not it will not fly. The plane would have to experience an instant acceleration to 150 knots. That's not possible.

    Trust me, my answer, and logic are correct! But it's a dumb question.

  20. #20
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    Re: Stupid Question

    >>But it's a dumb question.<<

    I noticed that-- there is no reason for it. There is no situation I can think of where this could happen.

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