I learned from an old Navy pilot. His brand of heresy never left me. An aircraft in flight is not lifted by the upper surface, it is pushed, from beneath. The upper surface is not relevant to flight, except to say that it provides a place for the underside to rest against.
It is obvious I am in . . hallowed or indeed sacred company here. e the company of pilots or engineers.
The above is my theory too - problem is that if the upper surface gets rough with hoar frost say, then the thing does not work and stuffs the aircraft into the hedge, during takeoff. Therefore, the airflow must be smooooth for some reason. . . and, say just if the lift is the "push" from below, and that the aerofoil surfs along then, for some reason, only appreciated by engineers, the lift drag thang must be more delicate than we can see. . .meaning, even the slightest . . interruption of the airflow over the upper surface is going to send it all down the Swanny. (or into the hedge)