Backwash Causes Lift?

On Oct 3, 6:33 pm, "Morgans" wrote:
It goes something like this.

An airplane is about to takeoff on a runway, that is really a treadmill; a
very expensive treadmill.

The treadmill senses the airplane's speed, and matches the aircraft's
speed, with speed increases of its own.

Can the airplane takeoff? Why or why not?

I forgot one important qualifier of the treadmill's operation.

It goes in the opposite direction of the intended direction of travel for
the airplane.

Tricky question. I will take a stab at it.

You said "sense the airplanes speed". Relative to what?

Assuming no friction between wheels and treadmill, the prop of the
plane will cause the airplane to move in the wind. The speed
"sensing" of the airplane by the treadmill does not really make sense
and less you mean that the treadmill senses relative to the wind.

If the treadmill senses that the plane has a relative wind speed of
Vpw, and starts moving in the opposite direction, and the relative
speed of wind against treadwill were nil before all this started, then
the treadmill will only cause the wheels to turn twice as fast as they
would have if the treadmill had not been moving. In this case, the
speed of the plane relative to the treadmill will simply be twice the
speed of the plane relative to the win, in opposite direction, of
course. The plane will take off.

If you mean to imply that there is some way for the treadmill to sense
the speed of the plane relative to the treadmill, then adjust the
speed of treadmill accordingly, of course, that won't work, as it is a
circuitous proposition.

-Le Chaud Lapin-