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.