Propeller driven airplanes use either a fixed pitch or constant speed propeller. A fixed pitch propeller's angle is set by the manufacturer. This means unless some type of gearing is involved, the propeller turns at the same speed as the engine via the crankshaft. Whereas the angle of a constant speed propeller can be adjusted in flight, a fixed pitch propeller cannot. With a fixed pitch propeller, the tachometer measures the speed in terms of RPM, which is how we read the engine's power output.

A Constant Speed Propeller has movable blades the pilot can adjust in flight. Changing the blade angle allows the pilot to set a specific RPM. As the throttle changes, a governor allows the blades to automatically twist and maintain the established RPM. The “pitch setting” is determined by the blade angle. With a constant speed prop, power output is read from a manifold pressure gauge compared to a fixed pitch prop where power is read directly from the tachometer. When increasing power on a cons

What are constant-speed propellers? | Pilot Institute Airplanes

The 54H60 propeller assembly is a constant speed, fully feathering, hydro-mechanical propeller. During aircraft operation, the 54H60 prop system has two distinct ranges, beta (ground) and alpha (flight) ranges. These ranges are differentiated by what component is controlling blade angle. | C&S Propeller