When does a constant-speed propeller maintain optimal engine efficiency?

A constant-speed propeller maintains optimal engine efficiency by providing a uniform speed at varying conditions. This design allows the propeller to automatically adjust its blade angle (or pitch) based on the engine's power output and the aerodynamic conditions it encounters. By maintaining a consistent RPM, the propeller ensures that the engine operates within its ideal power range, thus maximizing fuel efficiency and performance.

The inherent design enables the pilot to set a desired RPM and the propeller system will adjust the pitch accordingly, responding dynamically to changes in speed, climb rate, and other flight conditions. This response is crucial for optimizing the engine's power output and maintaining efficient flight, especially during varying phases of flight such as takeoff, climb, cruise, and descent.

The other options describe different concepts related to propeller operation. Adjusting pitch based on altitude changes is part of achieving optimal performance but does not directly explain the function of maintaining a uniform speed. Altering RPM with throttle position is connected but does not encapsulate the broader efficiency aspect. Lastly, pairing with a fixed gear ratio is not relevant to the operation of a constant-speed propeller, which is designed to independently vary pitch as needed for optimal performance.