I've watched a lot of youtube videos on how planes fly and they all gloss over one detail that I can't understand. They say that the foil is shaped such that the air on top must travel faster than the air on the bottom. By Bernouille's equation, this creates higher pressure on the bottom than on the top since if P1 + v12 = P2 + v22 then if P decreases on one side, then it must be compensated by higher velocity on the other side of the equation. I get that part. But what I don't understand is why the foil causes the air on top to go faster. I have an idea, but I want to be sure if it's right. It's the same reason why water flows faster in a narrow pipe than in a wide pipe. It relates to the equation of continuity: A1v1 = A2v2 The area above the wing becomes less so this must be compensated for by a higher velocity. I realize it's a bit hard to imagine how a foil could actually decrease the amount of space, since the amount of space above the wing is so large, effectively all of the Earth's atmosphere, but that is the only explanation I can think of.