How can stunt planes fly upside down?
I sort of understand how an airplane wing works in terms of Bernoulli's principle (although I have only a vague idea why Bernoulli's principle is true). I know the wing is shaped to produce more air pressure on its lower surface than its upper surface and thus provide lift.
If that's so, how can stunt planes fly upside down?
Airplane lift is the result of two things: (1) the shape of the wing, which has to do with the Bernoulli principle, and (2) the angle at which the wing meets the wind, known as the "angle of attack." The most efficient wings make use of both factors. But in a pinch (or an air show) you can get by with just the latter.
The Bernoulli principle for now you're going to have to take on faith. Angle of attack is easier. You see angle of attack in action whenever you stick your palm out the window while driving down the highway. Angle your palm upward and the wind forces your hand higher. Angle your palm down and the wind forces your hand lower.
Same with planes. When you're zipping down the runway during takeoff, the Bernoulli principle generates a certain amount of lift. But to get that last crucial boost you pull up the plane's nose. This increases the angle of attack on the wings, popping the welds on the engine and sending you into a flaming cartwheel of … Whoops, wrong index card. Actually, increased angle of attack provides the lift needed to get you flying.
Generating lift is easier at cruising speed, so stunt pilots can flip the thing over and rely on high angle of attack alone to keep them in the air (i.e., they keep the nose up and the tail down). This isn't a very efficient way to fly, since increasing your angle of attack also increases your aerodynamic drag. Too much drag = stall = plane drops like rock. For that reason stunt planes need low-drag wings, heavy-duty construction (so they won't disintegrate in midair), and powerful engines designed so they won't conk out when they're upside down. Also you want a pretty good seat belt. It is SO embarrassing when you're in a stunt plane, it flips over, and the damn pilot falls out.