Physicist: The narrative that usually leads to this question is something like: “It was the Wright brother’s brilliant wing shape, among other design innovations, that first made manned flight possible”. So if the wing shape is so important, why does it still work if it’s flipped upside-down? A wing (or airfoil or whatever) creates lift by taking advantage of a combination of the Bernoulli force and “angle of attack”. Using the engines we have today (jets and whatnot) you could fly a brick, so long as the nose is pointed up. So to actually answer the question; back in the day planes couldn’t fly upside-down.