Some aircraft cockpits feature a large blue lever located next to the throttle. This the propeller controller, and is used to set the propeller and engine speed for an aircraft with a constant speed propeller. Constant speed propellers work by varying the pitch of the propeller blades, which alters the in-flight properties of the propeller. As the propeller blade angle is increased, it produces more thrust, but also requires more torque to spin the propeller, which slows down the engine. Inversely, when the blade angle is decreased, the torque required is decreased, and the engine speeds up.
Constant speed propellers are named such not because they always operate at the same speed all the time, but because the operator can set their RPM, which the propeller then maintains until the operator changes. During takeoff, high RPM is best to achieve maximum power, but during cruising operations pulling RPM back is better for fuel economy.
The propeller’s blade pitch is altered hydraulically by using engine oil. This is the same oil that goes through cylinders to keep them cool and lubricated, and is paired with a spring at the back of the propeller hub assembly that helps the propeller return to a low pitch/high RPM setting. For most single-engine airplanes, there are stops installed so that the blades cannot be fully feathered or flattened.
The job of governing the movement of the propeller falls to an assembly called a governor. The governor moves oil back and forth through the propeller hub to make sure the propeller is at the pitch and speed you want. The governor is comprised of: