Answer: its shape. The shape changes over the length of the propeller blade, resulting in a twist. The twist is what makes the propeller produce thrust. To get the twist right is no easy task. The following illustrations will give you the major ideas behind a propeller's shape, which must all be considered together when designing a propeller. They were all developed by the Wrights in 1902-1903, and are still in use today.

Since a horizontal wing generates lift in a vertical direction, a vertical wing (the propeller), if spinning, would generate thrust in a horizontal direction.

When spinning, the sections of the propeller blade closer to the center move slower than those at the tips.

A wing meets the air at an angle called the angle of attack. The slower the speed, the steeper the angle of attack must be to generate lift. Therefore, the shape of the propeller's airfoil (cross section) must change from the center to the tips.

The changing shape of the airfoil (cross section) across the blade results in the twisting shape of the propeller.

There are many other factors to consider when designing a propeller, including the weight of the aircraft, the speed of the propeller(rpm), the power of the engine, and the final thrust required to maintain flight.