Ok, I'm working with Iraqis and have now become a master of trying to explain things in tiny detail - also works for U.S. Army staff officers who have never left the wire. Here are the conditions we are operating under: An airplane traveling at say 300mph, thrust being provided by its engines. A tread mill under the bird is traveling at 300mph in the opposite direction (this is drag). Since thrust must be more than drag, by an amount that differs from bird to bird, this particular aircraft won't fly until more thrust is provided, assuming there is no head, tail, or side wind. The treadmill is essentially pushing the aircraft backward while the engines push forward. Everyone knows there are 4 forces working on an airplane, right? Thrust must exceed drag by a certain amount to generate lift, which is caused by air flowing over the wings. There are a number of physical laws, principles, and effects involved and you can read here if you are interested. Suffice to say that the wing has to be moving relative to a fixed point on the ground, in the absence of any air movement, in order to fly. So, if the thrust does not overcome the rearward speed of the treadmill, it matters not the length of the treadmill. The aircraft won't fly until it generates enough thrust to move the wings through the air. Bottom line, on a calm day, you can not stand next to the tread mill, facing the side of the fuselage and watch the plane take off without moving any part of your body relative to the earth.