Logic puzzle finally solved...or is it?

Discussion in 'Off-topic' started by Fallschirmjäger, Sep 27, 2010.

  1. Fallschirmjäger

    Fallschirmjäger I watch the watchers

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    “Imagine a plane is sitting on a massive conveyor belt, as wide and as long as a runway. The conveyer belt is designed to exactly match the speed of the wheels, moving in the opposite direction. Can the plane take off?
    Many say no, because the plane will not move relative the the ground and air, and thus, very little air will flow over the wings. However, other people are convinced that since the wheels of a plane are free spinning, and not powered by the engines, and the engines provide thrust against the air, that somehow that makes a difference and air will flow over the wing.â€

    So.........

    They decided to try out the theory in New Zealand. I present - - -
    [​IMG]
     

  2. gunsmoker

    gunsmoker Lawyer and Gun Activist

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    I would like to see this airplane-on-treadmill thing tested in real life.
    It could be done with a radio-controlled airplane on a big treadmill.
    Some R/C aircraft are made to fly at pretty low speeds-- maybe 15 mph?
    I wonder if a treadmill made for runners would go that fast?

    P.S. My guess is that the aircraft "would" accellerate up the treadmill. It would move in relation to the ground and air. The treadmill could not hold the aircraft in place just by unreeling the pavement underneath it. At least not if the wheels were truly friction-free and if you didn't consider momentum from the mass of the aircraft and wheels.

    Imagine you hold in your hand a toy aircraft with free-spinning wheels.

    You set the treadmill to 10 mph. and press the airplane down on the belt.
    You hold the airplane steady, using your arm muscles as an outside force unrelated to the wheels, just as a jet's engines create thrust unrelated to the free-spinning wheels. The wheels roll at 10 mph, and the belt runs at 10 mph, and your hand is exerting a certain amount of force to hold the aircraft in place. If the wheels would spin freely with no friction, no drag, very little force would be used to hold the aircraft "steady" in the middle of the treadmill.

    Can you accellerate the plane by pushing it higher up the treadmill with your hand? Sure.
    Could the operator of the treadmill stop you from moving the aircraft forward by increasing the speed of the belt? No. If belt moves faster, the wheels spin faster, but that doesn't mean the body of the aircraft is affected. Your hand pushes the toy to the front of the treadmill just like the jet engines on a real plane would, even if the runway is rushing toward it at ever-faster speed.

    So I see the aircraft taking off more or less normally even as the belt/ runway spins like crazy underneath it, and the belt/ runway had better have a mile and a half of weight-bearing surface to let a commercial passenger jet take off from it.
     
  3. Hughduffel

    Hughduffel Member

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    I really don't understand how people are still confused by this.
     
  4. ber950

    ber950 Active Member

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    Mythbusters did an episode on this. Both the scale model and the full size airplane took off.

    http://mythbustersresults.com/episode97
     
  5. Mrs_Esterhouse

    Mrs_Esterhouse Swollen Member

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    :ditto:
     
  6. jsmn4vu

    jsmn4vu New Member

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    Buried somewhere in their subconscious is the notion that wheels are what propel vehicles along a surface, so they have a hard time taking the wheels out of the equation in the case of an aircraft, even when they logically know better.
     
  7. ClayD

    ClayD New Member

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    I thought those birds were extinct, guess I was wrong. (wouldnt be the first time) :lol:
     
  8. gunsmoker

    gunsmoker Lawyer and Gun Activist

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    On the Mythbusters website it looks like about half the posters who left messages said the test was flawed and an "accurate" test would have shown the aircraft could not move forward on the conveyor belt at all. Even a self-proclaimed pilot said this.
    One guy said a better test would be to tie the tail of the aircraft to an anchor point, so it could not move forward!!

    That would be like "testing" whether a sprinter runs faster when being threatened by a man with a gun or does he run faster after being shot by the guy with the gun?
     
  9. livesounder

    livesounder New Member

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    I believe that's a Kiwi.

    New Zealand
    New Zealander
    Kiwi (slang for a new Zealander)
     
  10. Fallschirmjäger

    Fallschirmjäger I watch the watchers

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    I'll :ditto: your ditto.

    As the tractive force is not the wheels against the conveyor belt but the propeller against the air it makes not a whit's difference whether it is wheels, skids, or floats.

    Factor in a headwind that matches the aircraft's forward motion and it's another story, though.
     
  11. CountryGun

    CountryGun New Member

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    Lift is created by air moving over the wings. No engine, nor propeller in the world can create the lift with prop-blast alone. The plane must move forward, with sufficient speed, so as to create lift. It's foolish to believe otherwise. Even in the Mythbuster segment, the plane must gain air speed. I don't care how much drag you take off of the wheels. Put it on a high speed dynamo. It's not going to lift off.
     
  12. livesounder

    livesounder New Member

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    I'll :ditto: your ditto.

    As the tractive force is not the wheels against the conveyor belt but the propeller against the air it makes not a whit's difference whether it is wheels, skids, or floats.

    Factor in a headwind that matches the aircraft's forward motion and it's another story, though.[/quote:ncanmxdr]

    Well, yes and no.

    If the velocity of the forward motion were providing sufficient lift for the craft to become airborne, in the theory the craft could remain airborne while staying in a static position relative to the ground and with the power off.
     
  13. jsmn4vu

    jsmn4vu New Member

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    Exactly

    Uh, as long as it's free to move forward (the treadmill will do nothing to stop it), it will take off.
     
  14. Sine Nomen

    Sine Nomen New Member

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    I'll :ditto: your ditto.

    As the tractive force is not the wheels against the conveyor belt but the propeller against the air it makes not a whit's difference whether it is wheels, skids, or floats.

    Factor in a headwind that matches the aircraft's forward motion and it's another story, though.[/quote:1udkiqsq]
    :ditto: your :ditto: to a :ditto: with my own :ditto:

    Note: with a headwind equal to or greater than the airplane's stall speed, it would be able to "hover" over a fixed spot on the ground. How cool would that be to watch?
     
  15. RevolverDan

    RevolverDan Active Member

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    I think what most people have a hard time with is the idea that since there is no air running over the wings, they don't understand where the lift comes from.

    In the Mythbusters test, they used planes that have massive power and very little weight. These planes should be able to hang in the air pointed straight up.
     
  16. CountryGun

    CountryGun New Member

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    Big difference between lift and thrust. You can put an airplane in a wind tunnel. If you can get the wind speed up high enough, it will lift, whether the wheels turn or the engine is even started. Yes, some engines are powerful enough to pull a plane straight up. No doubt, but that's not flight. It's a missile.
     
  17. gunsmoker

    gunsmoker Lawyer and Gun Activist

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    why do you say there won't be air running across the wings of the aircraft?
    Why do you think that the treadmill will impede the forward motion of the aircraft's body and wings?
    All I can see the treadmill doing is make the landing gear spin like crazy. But no matter how fast the wheels spin, even 10x faster than the airspeed, the plane will go forward and there will be ever-increasing airspeed.
     
  18. Sine Nomen

    Sine Nomen New Member

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    You don't need any such plane to do this. A Cessna 152 could take off from a high speed conveyor belt so long as the wheels, tires, and wheel bearings were in good shape...
     
  19. RevolverDan

    RevolverDan Active Member

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    If the speed of the treadmill equals the thrust of the plane, then the plane is not moving, thus no air passing over the wings, thus no lift on the wings. In the tv test, the plane took off b/c the thrust from the propeller was enough to overpower it's weight.