A Propeller Is a Wing with a Twist
In cross section, a propeller is shaped like a wing to produce higher air pressure on one surface and lower air pressure on the other.
Propellers and Pitch
Angle of attack is the angle a wing makes with the oncoming airflow. Pitch angle is the angle a propeller blade makes with its plane of rotation. A wing has nearly the same angle of attack across its entire length. But a propeller blade has a twist, so its pitch angle varies along its length.
On a controllable-pitch propeller, the pitch of the entire blade can be altered during flight to give the best performance at different air speeds. This is similar to changing gears with a car or bicycle.
A ship’s propellers create thrust in water in much the same way an airplane’s propellers create thrust in air.
In order for a propeller blade to spin, it usually needs the help of an engine.