I tried, and yes, an inverted balsa-wood glider will still fly. Actually just tried right now with a styrofoam glider, upside-down, and it's fine. Marginally

I tried, and yes, an inverted balsa-wood glider will still fly. Actually just tried right now with a styrofoam glider, upside-down, and it's fine. Marginally less range than when flying right-side-up, but it's a very small difference. I suspect that what's going on is that the center-of-gravity interacts with the center-of-lift to create a slight angle of attack regardless of what orientation the plane itself has. Then there is some unknown feedback loop that keeps that angle of attack from getting too large and stalling. It's not unlimited - if you make a paper airplane whose wings are too far forward or center of gravity is too far back, it will still stall - but it keeps most reasonable planes moving forward rather than down.