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An evil brain teaser

Geof posted this over on the Rumor Forum and it’s been so much fun I thought I’d share it with everybody. (Geof promises his solution tomorrow.)

Question:

An aircraft is on a runway. This runway is outfitted with a conveyor belt that has the capability to exactly match the speed of the aircraft landing gear’s wheels. Can the plane take off? Why or why not?

7 Comments

  1. Rod Rod

    Heard this on the Neal Boortz show a coupla months ago. Neal thought it would fly, which I thought strange since he is a pilot. I still don’t. Apparent groundspeed is irrelevent, what creates lift is the wing moving through the air. If the plane is not moving through the air, the windspeed is zero & therefore the lift is zero.

  2. Aaron Aaron

    Of course it would fly.. The thrust created by the jet engines would be the same, and has nothing to do with the speed that the wheels are turning. The wheels would have to turn twice as fast, but since the forward motion of the plane has nothing to do with power from the wheels, the plane would take off as normal.

  3. Rod Rod

    If the conveyor belt can exactly match the speed of the planes wheels, does the plane actually move? That’s the key part of the question.

    I think Aaron’s right if the thrust can overcome the belt speed such that the plane can physically move through the air. Wheel speed doesn’t count, it’s airspeed that’s the issue.

    My initial reaction had to do with how Boortz was spinning the question. He was saying the conveyor belt system would replace the need for a runway…all you would need is a short belt to take off because the plane is not physically moving. I’m pretty sure that can’t work for the reason I stated.

    In my mind, Boortz had it exactly backward. To shorten takeoff runs what you really want is the belt going the same direction as the plane. That increases the airflow and shortens takeoff runs accordingly. The same principle has been used with aircraft carriers forever…they come up to full speed and turn into the wind in order to increase the effective airspeed over their deck, thus helping the planes get airborne. Well…it was used pre-steam catapult anyway. I’m assuming they do the same thing event though they shoot the planes into the air these days.

  4. Aaron Aaron

    Yeah, so after I wrote the above comment I kept thinking about the quesiton and decided it wasn’t quite as simple as I made it out to be. I’m not even sure I totally understood the question. If we’re talking about having a shorter take off distance because of the conveyor belt, then I agree with Rod that you would want the belt going in the take off direction to “launch” the plane. A plane taking off on a belt going the opposite direction would probably just somewhat increse the take off distance.

    I was taking the question from the standpoint of if it’s “possible” for a plane to take off under these circumstances. I still think the plane would lift off in close to the same distance as on a normal runway, because the forward thrust of the plane from the engines wouldn’t be affected by the movement beneeth the plane.

    It’s pretty fun to think about anyway….

  5. Aaron Aaron

    And one more thing… If the conveyor belt having the ability to exactly match the speed of the wheels, implies that that’s what they are actually doing, then I think Rod could rightly say that the plane is not moving.

  6. Thanks for the comment again πŸ™‚ sort of wondering “to what do i owe this pleasure?” but maybe i should just be content as i am. : p anyways.. thats an interesting question..those who commented before me really gave it some thought..! Not sure that i know enough background info on planes to have a say.. : ) well, ttys

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