Great question, but the physics is still the same. The aircraft still generates lift by direction air in a downwards direction. For some aircrafts (based on the

Great question, but the physics is still the same. The aircraft still generates lift by direction air in a downwards direction. For some aircrafts (based on the wing geometry), you can achieve this upside down, within a certain range of angle-of-attack of the wing. Obviously, the aerodynamic efficiency is not the same. Airfoil sections are anti-symmetric across this direction because these designs achieve higher L/D (lift to drag) ratios.