What type of propeller relies on a manual oil valve to control pitch?

• A. Constant-speed propeller
• B. Controllable-pitch propeller
• C. Fixed-pitch propeller
• D. Hydraulic propeller

Answer: (B)

Pitch control is the key idea here. A propeller that can change blade angle is a controllable-pitch propeller, and its pitch is moved by oil pressure in a hydraulic mechanism within the hub. A manual oil valve is used to meter that oil, allowing direct, hands-on control of the blade angle. That ability to set pitch with an oil valve is what distinguishes a controllable-pitch propeller.

Fixed-pitch propellers have a non-changeable blade angle, so no oil-valve control is involved. Constant-speed propellers do change pitch automatically to maintain engine RPM via a governor and hydraulic pressure, rather than through manual selection by a valve. While both use oil pressure, the manual valve direct-control aspect points to a controllable-pitch design.