I've thought about this many many times, and your only hope is to make an animated texture for the spinning rotors, and have everything else a func_train, that way it can follow a path in teh sky and land when it needs to, and then take off again.
It is possible though, would be a bit heavy on the r_speeds, think about how high it might have to fly, and how much extra shizzle is gonna be rendered by looking down onto teh map.