Off topic for sure, but potentially a great debate question.

Discussion in 'Off-Topic Chat' started by TheMusicMan, Mar 30, 2010.

  1. TheMusicMan

    TheMusicMan tMP Founder Staff Member

    Imagine a plane is sitting on a massive conveyor belt, as wide and as long as a runway, and intends to take off. The speed of the conveyor belt is designed to exactly match the speed of the wheels of the plane at any given time, and moving in the opposite direction of rotation.

    Can the plane take off?
  2. _si

    _si Member

    nope, or if yes only for a second. It has no thrust from the enigines, so if it did lift , as soon as it left the belt it would drop again (i think)
  3. FlugelD

    FlugelD Member

    If I understand the statement correctly, the plane won't actually move - and if the plane isn't moving, the wings can't generate lift, so the plane can't take off.
  4. _si

    _si Member

    if the belt is moving at the same speed as the plane would travel normally at take off, and its outside (?) then the plane is moving through the air at the correct speed for takeoff, thewind under the wings would try to lift the plane? the problem would occur the second it did leave the belt, as no thrust would mean it would just come straight down again, ???? lol
  5. DublinBass

    DublinBass Supporting Member

    Wheels don't make a plane take off, the props or engine does.
  6. Anno Draconis

    Anno Draconis Well-Known Member


    Lift is generated by air moving faster over the top of the wing than the bottom. Props or jets are there to move the plane forward through the air fast enough to generate lift. If the conveyor is exactly counteracting the movement of the wheels then the plane remains stationary relative to the air around it and no lift can be generated.
  7. KMJ Recordings

    KMJ Recordings Supporting Member

    Where are the physics people to explain Bernoulli when you need them?
  8. WoodenFlugel

    WoodenFlugel Moderator Staff Member

    Try this ;-):'s_principle

    But to put it simply a plane needs to move through the air at a given speed for it to take off/fly. The wings are designed such that the path over them is longer than the path under them so as the plane moves forward a low pressure zone is created above the wing and [effectively] a high pressure zone created below, giving lift, which causes the plane to take off.

    So if I understand the question correctly the plane will not move (assuming the conveyor prevents forward motion) so therefore will not take off.

    EDIT: Hang on - the question says the conveyor runs in the opposite direction to the wheels' rotation. For it to prevent any forward movement it would have to run in the same direction so in theory the plane will be propelled forward at twice the normal speed of any given engine speed. It would still need to meet the correct velocity for it to take off, but it would be using less engine speed, so it would leave the ground but wouldn't have enough engine speed to maintain velocity and therefore would slow and eventually descend. I think.

    The other thing to take into consideration is prop planes usually relay on the air-flow over the wings caused by the propeller to give them extra lift...
  9. WoodenFlugel

    WoodenFlugel Moderator Staff Member

    BTW: The above is posted after a long day and a couple of pints so its probably total bunkum. I hope I still have enough knowledge of basic physics to understand the question properly :)
  10. brassneck

    brassneck Active Member

    Part of what you are stating is relevant to solving the problem ... :cool:
  11. WoodenFlugel

    WoodenFlugel Moderator Staff Member

    Ahh right having untied the lager induced knots in my mind I was forgetting the obvious. The wheels themselves are not powered and the engines (either jet or propeller flavored) work by pushing the plane through the air, not along the ground. So, yeah the conveyor can go at any speed in any direction and it would have no effect - the plane still takes off.

    Simples. :)

    Right I can go to bed now...;)
  12. brassneck

    brassneck Active Member

    :clap: ... simply put, but the plane does take off!
  13. brassneck

    brassneck Active Member

    Planes, whether jet or propeller, work by pulling themselves through the air. The rotation of their tires results from this forward movement, and has no bearing on the behavior of a plane during takeoff. The only difference between a regular plane and one on a conveyor belt is that the conveyor belt plane's wheels will spin twice as fast during takeoff.
  14. TheMusicMan

    TheMusicMan tMP Founder Staff Member

    I agree with this Tom/Ian - that the propulsion is against the air, as the wheels are free standing and are not connected to any of the 'drive' of the plane (compared to a car, for example) but... the wheels can't go at twice the speed of the conveyor, as the conveyor is designed to match the speed of the planes wheels at any point.

    That's the bit I don't understand.

    The whole question in my opinion, is fundamentally flawed - based on the laws of physics i.e. that you could never ever get a conveyor to match exactly the speed of the free running wheels of a plane resting on it. That is the impossible bit.
  15. KMJ Recordings

    KMJ Recordings Supporting Member

    Is it a homework question?
  16. lynchie

    lynchie Active Member

    I've heard this question before, and I still don't like it. Essentially the answer is supposed to be that, because the wheels aren't propelling the plane, it will take off anyway. In "reality", the rotational speed of the wheels will quickly tend toward infinity, therefore causing the bearings to explode, the wheels to fall off, and the plane to flop on the ground.
  17. Thirteen Ball

    Thirteen Ball Active Member

  18. WoodenFlugel

    WoodenFlugel Moderator Staff Member

    But then the plane would already be in the air so no problem - until it comes to landing. But if the conveyor were long enough you could speed match it to the landing speed of the plane allowing a smooth 'belly-landing', before bringing belt, plane and passengers to a gradual and safe halt. ;)

    I think I've just designed the perfect ultra-safe runway! Anyone got the number of the patents office?
  19. Darth_Tuba

    Darth_Tuba Active Member

  20. Thirteen Ball

    Thirteen Ball Active Member

    The point about wording is a good one. Another way it was put to me was - "ok, what about if there's a big fan in front of the plane that blows backwards towards it at 100mph?"

    My response was "The already have those. They call them propellors....."

    It's the same, the plane still takes off - because it's the speed of the air over the wing that generates lift. Hence why planes always take off into a headwind, and have an 'airspeed' indicator because it's that which affects stalling speed, not the plane's speed relative to the earth.

    OK, as soon as the plane escapes the influence of said "big fan" it's instantly in trouble because the realative airspeed will drop right off! But so long as you relate everything to the concept of airspeed, rather than treating 'speed' as an absolute, the physics still works, so the plane still flies.
    Last edited: Mar 31, 2010

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