Originally Posted by
AndyAndrews
no I didn't. Gliders can be launched in the air being towed by an airplane.
That's true. And when they are launched that way, nobody cares a bit whether the glider pilot releases the towrope while the glider and the towplane are flying upwind, downwind, or in a crosswind. So unless you were just being silly, your reference to "taking off downwind" must have assumed a takeoff from the ground. Are you just quibbling about words, or do you really think the original post is wrong?
Your paper airplane example shows that you need air moving over the wings (from front to back) to fly. Nobody disputes that. It has nothing to do with the forces acting on a plane that's flying.