The good old theory of flight !!!
Imagine a flat wing section, pulled into the air by a good old MDS engine, it flies, until the engine stops, then it basically falls out of the sky, all be with a shallow glide, then lets imagine a fully aerofoil section ( english) , pulled again by the good old MDS , it gains height, the engine stops, it falls to earth.
It is NOT the wing section at all making the aeroplne fly, it is the engine pulling the aeroplane through the sky that keeps it up there. so the theory of flight, as good as it sounds, relies upon something pulling the aircraft through the air, most of us have flown flat plate wing models, and they fly, some cases very well, What happens when the engine stops? Yep, it falls to earth, what about the latest NACA aerofoil section ? works while being pulled through the air, engine stops, it falls to earth, just a bit quicker than the flat plate. ? don't believe me? try it.

Footnote, we are talking powered models here, gliders fall to earth no matter what. !!!