Kian Pham, Australia asked:
A plane is standing on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction). Can the plane take off?
Tim Sabey, Pilot & Ex-physicist:
Won't the plane still move forward as the wheels aren't driven? Even if the conveyor moves, the wheels will be free to move and the plane will still go forward.
Imagine, for a moment, that there's no air resistance (It's OK, I will come back to that one).
If there is no friction between the plane an the runway then the runway can have no effect because it cannot impress a force on the plane, forwards or backwards. Wheel bearings are pretty good and, let's face it, would only have to work at twice their normal rate if the runway is running 'backwards'. Simplest, ideal case - no effect at all. The plane uses air being pushed backwards(reaction) to produce its thrust. If it tried to use driven wheels to aid its takeoff then the runway might have an effect. lyner, Wed, 23rd Jan 2008
The plane will not take off as all will be equal. (providing "take off" also means flying)
"But, it don't say how the plane is powered, with jets or a prop?"
If there were significant drag between the runway and the air I think you could see the runway pulling some air at low level back with it but the air higher up, unaffected by the runway, would be still wrt the ground and so the plane migt be able to take off by remaining stationary relative to a (very fast moving) runway because of the air flow it induced, but then rise above the level of the disturbed air, into the air that was still, and discover that it had suddenly "stopped" at which point it would fall back to earth.
"the plane migt be able to take off by remaining stationary relative to a (very fast moving) runway because of the air flow it induced, but then rise above the level of the disturbed air, into the air that was still, and discover that it had suddenly "stopped" at which point it would fall back to earth."
" couldn't imagine a runway providing a huge effect here, but it'd be there all the same. "
Agreed, I was being nit-picky. Aircraft fly from airspeed, ground speed is irrelevant unless you fly into it (tree, mountain etc). Cameron Lapworth, Mon, 28th Jan 2008