Flight has fascinated humans for centuries. How do massive airplanes weighing hundreds of tons stay in the air? The answer lies in understanding four fundamental forces that act on every aircraft. Lift is the upward force that opposes the airplane's weight. Weight is the downward gravitational force pulling the aircraft toward Earth. Thrust is the forward force generated by the engines that propels the airplane through the air. And drag is the backward resistance force that opposes the airplane's motion through the air. The balance and interaction of these four forces determine whether an airplane can fly, climb, descend, or maneuver through the sky.
Lift generation is the key to flight, and it works through two complementary principles. First, Bernoulli's principle explains that the curved shape of an airplane wing, called an airfoil, causes air to move faster over the top surface than the bottom surface. According to Bernoulli's principle, faster-moving air creates lower pressure, while slower-moving air creates higher pressure. This pressure difference between the top and bottom of the wing generates an upward force called lift. Second, Newton's third law of motion also contributes to lift generation. As the wing moves through the air, it deflects air downward. According to Newton's third law, for every action there is an equal and opposite reaction, so when the wing pushes air down, the air pushes the wing up, creating additional lift force.