Which forces cause blade angle changes in constant-speed counterweight propellers and two-position counterweight propellers?

• A. Thermal and mechanical stresses
• B. Electrical and magnetic forces
• C. Hydraulic and centrifugal forces
• D. Aerodynamic forces

Answer: (C)

Blade angle changes in these propeller systems are driven by two interacting forces: hydraulic pressure from the governor and centrifugal force on the counterweights. The governor uses engine oil pressure to push or pull a piston in the propeller hub, which adjusts the blade pitch. At the same time, as RPM changes, the counterweights experience centrifugal force that tends to move the linkage and set the blade into a different angle. When RPM tends to rise, the centrifugal force drives the weights outward and, through the linkage, moves blades to a coarser pitch to load the engine. When RPM tends to fall, the oil pressure drops and the restoring springs, aided by the reduced centrifugal effect, allow the blades to move toward a finer pitch to reduce load and bring RPM back up. In a two-position counterweight setup, the same centrifugal action on the weights selects one of two preset angles, with the hydraulic system providing the means to hold or transition between those positions as commanded.