How do airplanes fly? The flight of an airplane depends on four main forces. Lift is the upward force generated by the wings that counteracts the airplane's weight, which is the downward pull of gravity. Thrust is the forward force produced by the engines that overcomes drag, which is the air resistance that slows the plane down. When lift exceeds weight, the airplane climbs. When thrust exceeds drag, the airplane accelerates forward.
The key to understanding how airplanes generate lift is in the special shape of the wing, called an airfoil. The airfoil is curved on top and flatter on the bottom. As air flows around the wing, it must travel a longer distance over the top surface than the bottom. According to Bernoulli's principle, this faster-moving air creates lower pressure above the wing, while slower air below creates higher pressure. This pressure difference generates an upward force called lift. The greater this pressure difference, the more lift is produced.