What is the principal advantage of using propeller reduction gears?

The principal advantage of using propeller reduction gears lies in their ability to decouple the rotational speeds of the engine and the propeller. This means that while the engine can operate at a higher RPM, which is often necessary for optimal power output, the propeller can maintain a lower, more efficient RPM. A propeller's performance often diminishes at higher speeds due to increased drag and potential issues with efficiency.

By using reduction gears, the aircraft's design benefits from the high output of the engine while ensuring that the propeller operates within its optimal performance range. This balance leads to improved efficiency and performance during flight, particularly during critical phases like takeoff and climb, where engine power is paramount.

The other choices present alternative considerations regarding engine RPM and operational smoothness, but they do not specifically address the core benefit of allowing the engine to produce maximum power at higher RPMs while keeping the propeller operating efficiently at lower RPMs.