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Thought Experiment

Discussion in 'Issues Around the World' started by joseftu, Dec 11, 2006.

  1. joseftu

    joseftu ORIGINAL Pomp-Dumpster

    OK, massive brains...what do you think abou this? I got it from David Pogue--and folks at the Straight Dope discussion board are also a bit stumped.
    I'm thinking that the plane won't be able to actually move relative to the ground, because as the jet engines try to make the plane move forward, the conveyor belt moves under the wheels, and it doesn't go anywhere. So no airflow over the wings, so no takeoff.

    But some people seem to be saying that because the jets push on the air, not on the belt (or the ground), it would accelerate forward anyway, and take off, no matter what the belt does.

    That doesn't make sense to me. What do you think?
  2. Andy



    Force is making the jet go to the right like a clockwise gear, the belt is counteracting it like a counterclockwise gear.

    The whole thing is being held stationary above the ground.

    No airflow over wing, no takeoff.
  3. Steve

    Steve Is that it, then?

    The plane will take off normally.

    The thrust from a plane's engines cause it to move forward. The wheels only rotate because they're in contact with the ground. The wheels have no motive power of their own.
  4. tke711

    tke711 Oink Oink Staff Member

    If the jet engines push the plane forward, but it's not able to move forward because the conveyor is also moving (keeping the actual plane stationary), then I wouldn't think that lift could be created.
  5. joseftu

    joseftu ORIGINAL Pomp-Dumpster

    That's how I read it. No moving, no lift.

    Steve, how would the plane move forward?
  6. Andy


    Null force.
  7. joseftu

    joseftu ORIGINAL Pomp-Dumpster

    If Andy agrees with me, you know it's gotta be true!

    Perfectly cromulent, at least.
  8. Steve

    Steve Is that it, then?

    The plane's engines generate thrust against the atmosphere.

    Again, the wheels only move because they're in contact with the ground. In the scenario you describe, the wheels would move twice as fast because the two motions are complementary.

    Planes aren't like cars, motion is not created by frictional contact with the ground - it's not the plane's spinning wheels that make it go forward. It is the car's spinning wheels that make it go forward.
  9. joseftu

    joseftu ORIGINAL Pomp-Dumpster

    This is making my head hurt!

    Steve, wouldn't the plane's motion (caused by the jets) cause the wheels to turn? That's how it works on the ground, right? But if instead of the ground, it's a moving belt, then the "ground" underbeneath the wheels is moving exactly opposite to the wheels, so, relative to the real ground (not the belt) the plane would be still.


    (Oh, and by the way--isn't it the equal and opposite reaction of spewing out the gas that generates the thrust? Like, it would work even if no atmosphere? But that's a separate issue).
  10. Steve

    Steve Is that it, then?

    Imagine it this way: the plane rests on a perfectly frictionless surface. That's what this thought experiment renders the wheels to be.

    Now, crank up the jets. Will the jet move forward on this perfectly frictionless surface? Of course it will. That's because the wheels serve only one purpose, and that's to keep the fuselage from scraping the ground.
  11. Steve

    Steve Is that it, then?


    The wheels only move because of their frictional contact with the ground.

    If that ground is turned into a conveyor belt such that the belt moves from right to left while the wheels rotate clockwise, the linear speed of the belt is added to the tangential linear speed of the wheels, effectively doubling their rate of rotation.

    What you're forgetting is that the wheels cannot move in the first place unless the plane is moving forward.
  12. cmhbob

    cmhbob Did...did I do that? Staff Member

  13. Steve

    Steve Is that it, then?

    That's a rocket you've described, not a jet engine.
  14. Steve

    Steve Is that it, then?

  15. joseftu

    joseftu ORIGINAL Pomp-Dumpster

    Ohh....That's making more sense to me.

    I think I should stick to word puzzles!

    (And I guess I'd better read up on the difference between a rocket and a jet, too!)
  16. joseftu

    joseftu ORIGINAL Pomp-Dumpster

    Here's what Bob said over there
  17. tke711

    tke711 Oink Oink Staff Member

    Sure it can, if they are on a conveyor. So, the plane isn't actually moving forward, it's sitting still with it's wheels spinning. Much like leaving one foot on the break while giving your real wheeled muscle car full gas to smoke the tires.

    If the plane is not moving forward, causing air movement under the wings to create lift, it cannot take off.
  18. cmhbob

    cmhbob Did...did I do that? Staff Member

    Forgot those weren't public. Oops. Now Jason will be mad at me. Oh. Dear.
  19. Steve

    Steve Is that it, then?

    Now, for the opposite case, where the wheels are moving clockwise and the conveyor belt is moving left to right, so that the frictional action of the conveyor belt cancels the movement of the wheels:

    The easy answer is that the rubber will burn off and then the jet will jerk forward on crippled landing gear.

    The somewhat more complicated answer is that the problem I've just outlined describes little more than a static test frame, commonly used to hold jet engines in place for testing purposes.
  20. Steve

    Steve Is that it, then?

    Nope, sorry, you and Bob both are describing situations where the motive force is engendered by friction of wheels (or rollers) acting upon a surface.

    Jet engines move air. Wheels are secondary to aircraft design.

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