Refer to figure.
A propeller is a fan, fitted with between two and six aerodynamic blades that is used to convert rotational motion into forward thrust for the propulsion of an aeroplane through the air.

The blades of the propeller are small aerodynamic aerofoils mounted vertically on a horizontal shaft that is rotated by either a piston engine or a turbine-driven engine.

BLADE ANGLE (PITCH)

The angle between the blade chord and the plane of rotation.

ANGLE OF ATTACK

The path of the propeller through the air determines the direction of the relative airflow. The angle between the blade chord and the relative airflow is the angle of attack (α). The angle of attack (α) is the result of propeller rotational velocity (RPM) and aircraft forward velocity (TAS).

Because the rotational velocity of the blade increases with distance from the propeller shaft and the fact that the aeroplane is moving forward, then the angle of attack of the blade varies at different distances from the shaft.

FIXED PITCH PROPELLER

The blade angle of a fixed-pitch propeller is constant but the helix angle and the angle of attack will vary according to its forward speed. To ensure its efficient operation the blade angle of attack of a fixed-pitch propeller must be close to the optimum and is that which produces the greatest excess of horizontal lift (thrust) over drag.

To operate the propeller in this manner will restrict the range of speeds within which the aeroplane can function. Thus, the blade angle selected during the design of the aeroplane is dependent on the operational requirements for that aeroplane.

Figure shows a “fixed pitch” propeller at constant RPM. Increasing TAS decreases the angle of attack of the propeller.