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A propeller is a spinning wing. It creates lift but in a forward direction, we call thrust. As a propeller spins there are 5 natural forces acting on it. Centrifugal force is a physical force that tends to throw the rotating propeller blades away from the hub. This is the most dominant force on the propeller. Torque bending force, in the form of air resistance, tends to bend the propeller blades in the direction opposite that of rotation. Thrust bending force is the thrust load that tends to bend propeller blades forward as the aircraft is pulled through the air. Aerodynamic twisting force tends to turn the blades to a high blade angle. Centrifugal twisting force, is greater than the aerodynamic twisting force, tends to force the blades toward a low blade angle.

Reference: http://okigihan.blogspot.com/p/propeller-aerodynamic-process-airplane.html