Posted December 13th, 2007 @ 11:47am by Erik J. Barzeski
Because MythBusters was supposed to show the "plane on a conveyor belt" show last night, I stumbled onto the original question at Jason Kottke's blog. The question is as follows:
A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. 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). Can the plane take off?
It was interesting reading the comments to see people going back and forth, but the answer is really quite easy and simple: the plane takes off.
The solution and a very quick explanation below, if you want to read it…
The problem some people have is that they seem to believe the conveyor belt going backwards can counter the effect of the airplane pulling itself forward through the air. In other words, they believe the plane remains stationary.
The best way I can think to describe the result is simply to imagine the plane resting on a frictionless conveyor belt. The belt can move a billion miles an hour in any direction and the plane won't move (frictionless). Wheels aren't frictionless, but they're close. That's the whole purpose of putting wheels on a plane. The small amount of friction that wheels add (particularly when they've got fresh ball bearings and grease) are nowhere near enough to slow the plane's forward movement.
Some people are misled by earlier comments that imply that the engines or propellers would have to push air over the entire wing structure. They miss out on the fact that in this case, the propellers and engines act on the air, and pull the entire plane forward. That's what generates the airspeed - the plane moving against a motionless block of air, not air moving over a stationary wing.
Or try this: imagine sitting in a wagon on a conveyor belt moving backwards. You're holding onto a rope that's anchored to a wall in front of you. The conveyor belt starts spinning. You begin pulling on the rope. You move closer to the wall.1
- Engines or props on a plane push against the air behind the plane rather than pulling forward, but since they're equal and opposite, the resulting force is in the same direction. Pulling forward is the same as pushing backwards.↩