Here is another one of those things I think about as I take my morning walk. How can an airplane fly upside down? Having studied Physics sixty years ago, I am a little rusty, but we all remember Bernoulli's Principle [same guy -- Daniel Bernoulli -- who solved the St. Petersburg Paradox and gave rise to modern utility theory. There were giants in the earth in those days. Genesis 6:4] You know, flow of air over a wing whose upper side is curved, producing differential pressure below and above the wing, known as the "lift." The faster the plane goes, the more the lift. Hence planes always take off and land into the wind, increasing the airspeed [which is all that matters] over the ground speed. So, when you roll a plane over, as we have all seen done in the movies, if not in real life, how does the plane fly? Why don't the same forces drive it into the ground?
Well, I googled the question, and found what looks like a very plausible answer. I also checked Wikipedia, and found a monstrously complicated discussion of the general subject that makes it clear that it is not at all obvious exactly why airplanes fly. The simple answer is that although flying upside down does indeed put downward pressure on the airplane, one can, by tilting the plane up [which is accomplished, upside down, by pushing the joystick down, of course], create an angle between the wing and the air that results in the onrushing air pushing the airplane up. If you balance things just right -- which the author of this answer, an Air Force pilot, says one can do -- the result is that you can fly upside down horizontally.
I am glad I thought to look this up after getting home from Europe! I do not want to have to count on US Airways to understand these things.
