Which engine uses a reduction gearing arrangement to drive a propeller?

• A. Turbojet engine
• B. Turboprop engine
• C. Turbofan engine
• D. Turboshaft engine

Answer: (A)

Propellers need to turn much more slowly than a gas turbine runs, otherwise they overspeed and become inefficient or damaged. A turboprop engine is built to take the high-speed rotation of the turbine and use a reduction gearbox to drop the RPM to a suitable propeller speed while increasing torque. This gearing lets the engine produce strong, efficient thrust through the propeller at typical cruise and climb speeds.

Turbojet, on the other hand, makes thrust by accelerating air through the engine with no propeller drive. Turbofan uses a large front fan and the core’s turbine drives that fan, not a separate propeller that requires reduction gearing. A turboshaft is designed to power a rotor (as in helicopters) rather than a fixed-wing propeller, though it also uses gearing to reach the correct rotor speed. The feature of reducing speed with a gearbox specifically to drive a propeller points to a turboprop.