cjcampbell wrote:
Saw this question on "The Straight Dope" and I thought it was amusing.

http://www.straightdope.com/columns/060203.html

The question goes like this:

"An airplane on a runway sits on a conveyer belt that moves in the
opposite direction at exactly the speed that the airplane is moving
forward. Does the airplane take off?" (Assuming the tires hold out, of
course.)

Cecil Adams (world's smartest human being) says that it will take off
normally.

I confess to having difficulty understanding the difference between a
conveyer belt and a treadmill, although it seems important to some
folks.

Cecil does not seem to be arguing that prop wash or jet thrust are
generating enough lift by providing downwash over the wings. He claims
the airplane will take off normally -- that is, the airplane will move
forward just as in a normal takeoff, developing airspeed as it goes. He
also claims that this will not take any extra thrust.

He likens the situation to someone on rollerblades on a treadmill who
is holding onto a rope fixed to an object at the front of the
treadmill. No matter how fast the treadmill is moving, it takes the
same energy to pull yourself to the front of the treadmill as it would
if the treadmill was stopped. The only difference is that the wheels
would turn twice as fast.

Cecil appears to be saying that the airplane cannot be prevented by the
conveyer belt from moving forward, no matter how fast the conveyer belt
moves, because the thrust is generated independently from the wheels.
The airplane will accelerate and take off, using the same length of
conveyer belt as it would an ordinary runway. The only difference is
that the wheels will be spinning twice as fast at rotation as they
would otherwise, thus the caveat "assuming the wheels hold out."

I am not sure I understand why Cecil thinks this. If I understand him
correctly, the drag of the treadmill against the wheels does not
increase just because the treadmill is moving. Okay, I guess I can
believe that. Still, it seems counterintuitive to me that if a plane is
sitting on a conveyer that is moving backwards at exactly the same
speed (I assume they mean groundspeed here) as the airplane is moving
forward that the airplane will move forward at the same speed as if it
was not on a conveyer at all.