Final answer:
The propeller governor maintains a constant RPM by regulating oil pressure to change the propeller blade pitch, adjusting to load changes and speed variations.
Step-by-step explanation:
The unit that adjusts the propeller to maintain a constant RPM (Revolutions Per Minute) is called a propeller governor. The governor works by regulating the oil pressure that's applied to a mechanism attached to the propeller hub. This mechanism can change the pitch of the propeller blades, that is, the angle between the blades and the plane of rotation. When an aircraft increases in speed or the engine load changes, the governor senses the change in RPM and adjusts the blade pitch to maintain the desired RPM setting. An increase in blade pitch will cause a decrease in RPM, while a decrease in pitch will result in an increase in RPM, thereby maintaining a balanced power and efficiency of the propeller.