chumdawg

Would your plane take off if instead of wheels it had pegs supporting it off the ground? C'mon, you're a pilot. Would you approve of that setup?

You are being intellectually dishonest to ignore the role that traction against the runway plays in building airspeed for the plane. You cite the 20mph headwind as aiding the plane in taking off, yet you seem to believe that the plane going 20mph forward does not offer an equal advantage. It seems you would like to have your cake and eat it too. Question for you... If the conveyor belt were moving backward at 700mph, and the plane's max airspeed were 700mph, could the plane take off?

If we suspended you in mid-air, you could run your legs back and forth all day long but you wouldn't go anywhere. The wheels--and your feet, in this example--play a very important role. You ever see a car spin its wheels on ice? That engine can rev as high as you like, and as long as those wheels don't get traction the car isn't going anywhere.

Of course it has to do with the wheels, because the wheels give the plane something to push off against. If not for the wheels pushing off against the runway, then all the plane has to push off against is air--which is exactly how rockets work. Commercial aircraft are not designed to push off against just air for takeoff.

If the plane were tethered to a building, would it take off? Of course not. It needs relative ground speed. To achieve relative ground speed it needs friction. If there is no friction at the wheels, it never gains relative ground speed. This is really not that complicated. If the hypothetical is taken to mean that the wheels never gain any traction on the fast-as-it-needs-to-be treadmill, then you don't get flight.

The way the question is posed, the runway is engineered to move exactly as fast as the wheels turn. So this talk of the wheels turning twice as fast as the runway moves runs afoul of the hypothetical. The hypothetical asks if an airplane could thrust forward and ultimately take flight in a frictionless environment at the wheels. Obviously a large enough rocket could do it, since we have managed to send space shuttles into flight at ninety degrees to the horizontal, overcoming 100% of the force of gravity. Whether a jet airplane could do it is far from obvious. If the hypothetical is taken at its word, the feat is exactly the same as an aircraft hovering in air and then achieving flight. It's possible, yes, but there is no commerical aircraft that can do it.