What mechanism is primarily used to change the RPM of a propeller?

The primary mechanism used to change the RPM of a propeller is oil pressure. In multi-engine aircraft, the propeller pitch is typically controlled by a constant speed propeller system that utilizes oil pressure to adjust the angle of the blades. This adjustment allows for changes in the propeller's RPM while maximizing efficiency and performance according to flight conditions.

When the RPM needs to be increased, oil is directed into the propeller hub, causing the blades to change to a finer pitch, thereby reducing drag and allowing the engine to increase RPM. Conversely, if a decrease in RPM is needed, the oil supply can be altered to change the blade pitch to a coarser setting, allowing for better control during engine management, particularly in situations such as go-arounds or when power is reduced.

The other mechanisms listed—an electric motor, mechanical linkage, and magnetic field—are not typically used in this context for changing the RPM of propellers in multi-engine aircraft. Electric motors may be utilized for other components in some systems, and while mechanical linkages could theoretically adjust pitch, they do not provide the precision and responsiveness that an oil pressure system does in a continuously variable pitch scenario. Magnetic fields are not a principle mechanism in propeller control systems.