Unless the laws of physics have changed since I was at school there is no way that a force can cause an object to move in a direction it doesn't have a component in. So far as I am aware there is no downward component to drag so by the Newtonian physics I was taught there is no way that drag is responsible for the downward movement of the glider.
Think of my no-gravity and no-drag scenarious. Do you find a flaw in them? You also haven't disputed my statement that there is no downward component to drag, unless you are trying to do so by the 'guess what's in my mind' method sadly all too beloved of some instructors.
If you can produce a nice diagram showing how drag has a downward component that would be great.
PS try reading the following, especially page 4:
http://williams.best.vwh.net/smxgigpdf/SMX99tot.pdf