Heavier-than-air flight began with gliders -- light aircraft that could fly for long periods without using an engine. Gliders were the flying squirrels of aviation, but pioneers like Wilbur and Orville Wright desired a machine that could emulate falcons, with strong, powered flight. That required a propulsion system to provide thrust. The brothers designed and built the first airplane propellers, as well as dedicated four-cylinder, water-cooled engines to spin them.
Today, propeller design and theory has come a long way. In essence, a propeller functions like a spinning wing, providing lift but in a forward direction. They come in a variety of configurations, from two-blade, fixed-pitch propellers to four- and eight-blade models with variable pitch, but they all do the same thing. As the blades rotate, they deflect air backward, and this air, thanks to Newton's action-reaction law, pushes forward on the blades. That force is known as thrust and works to oppose drag, the force that retards the forward motion of an aircraft.