the intuition fails…

An airplane taxies in one direction on a moving conveyor belt going the opposite direction. Can the plane take off?  http://www.straightdope.com/columns/060203.html

[via this Straight Dope hacked RSS feed I found]

Is the situation clear? A plane, 747 say, is put on a giant treadmill in such a way that the plane is stationary to an outside observer. The plane gets going faster and faster and the belt and wheels spin furiously, but the theoretically the plane is supposed to be able to take off. Supposedly the engines will pull the air backwards and allow the jet to takeoff normally.

Here’s a good case where some visualization would really help me believe this (I mean I guess I believe this to the extent that Cecil is a smart person(a) but I have doubts at the core my inner engineer). Sounds like a good job for the Mythbusters.

Published

5 comments

  1. I am going to agree with you Corey. I sent the link to an aeronautical engineer friend for a confrimation.

    The issue is: is there sufficient lift on the wings to support flight? Lift is caused by the air pressure discrepancy between the bottom and top of the wing. In your hypo, the plane is a 747 which pushes the plane by jet and doesn’t actually send air over the wings (its the forward movement realtive to the air caused by the thrust which creates lift). Assuming arguendo, the plane is in fact a nose mounted pull type single engine prop plane with traditional wing placement and all of the thrust pushes air more or less over the wing, you would still stall as the design is such that the propller, to move the plane forward, is only pushing a small quantity of the air over the wing compared to the wind passing over the wing if it were moving forward relative to the air. Its the air on (actually under) the wing, not the thust of the motor that causes lift in a fixed wing aircraft.

    Peace.

  2. here is the response from a friend who is an aeronautical engineer:

    (Whether by push or pull, in the real world it’s not the engines that generate the Bernoulli/Navier-Stokes airflow over and under the wing to create lift. Maybe there is some lift, but my intuition says it’s too turbulent and wouldn’t amount to much. It’s the whole wing moving through the air with substantially laminar flow that creates the lift.)

    << THE HYPOTHETICAL (cut/paste from web site but footnoted):

    "A plane is standing on a runway that can move (1) (some sort of band

    conveyer). The plane moves in one direction (2), while the conveyer

    moves in the opposite direction(3). 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)(4). Can the plane take

    off?"

    Footnotes-

    Assumption: Absolute point of reference is solid ground.

    (1) The plane is "standing" so it’s not moving with respect to ground.

    (2) Okay, so now the plane "moves" with respect to ground.

    (3) The conveyer is moving in the opposite direction. So what. Doesn’t

    matter. The plane "moves" with respect to ground.

    (4) The plane is still moving according to the hypothetical. So if the

    plane is moving fast enough, it will take off. At the point of take

    off, the conveyer would be moving at the exact same speed but in the

    opposite direction. The wings don’t care.

    Don’t even think about messing with the point of reference, or the

    hypothetical falls apart.

    The only confusion is the poor writing.

    STRAIGHT DOPE’S DISCUSSION

    The Straight Dope’s discussion is much ado about nothing. The Straight

    Dope equates wheel rotation with "moves." In engineering terms, one is

    angular velocity and one is linear velocity. They are arguing apples

    and oranges. The hypothetical says the "plane" is moving. If the plane

    is on a conveyer, the wheels can be moving with the wheels stationary.

    The hypothetical says nothing about the wheels. Straight Dope is high.

    Come to think of it, even the hypothetical is messed up because it

    states the "conveyer" is moving (who knows what the conveyer’s BELT is

    doing). . ALTHOUGH… if the CONVEYER itself is moving with the plane on top, then we’re back to Mark’s solution. The plane is thrown from the conveyer as they each go their separate directions, the airplane crashes onto its belly, the fuel tank explodes == big boom.

    Cool.

  3. The plane will move foward and take off normally. The forces acting on the plane’s body are not changed at all by the moving runway. The wheels spin faster, but that does not produce any additional thrust-opposing force.

    A conveyor belt cannot keep the plane stationary, no matter how fast it moves.

    The answer has nothing to do with the engines pulling the air backward over the wings. The plane moves foward normally and takes off normally.

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