Definition
A gear assembly between the engine and the propeller shaft that lowers the propeller's rotational speed to a value less than the engine's crankshaft speed. It allows the engine to run at its efficient high rpm while the propeller turns slower, keeping the propeller tips below speeds where they lose efficiency.
Plain English
A set of gears that lets the engine spin fast while the propeller spins more slowly. The engine works best at high rpm, but a propeller works best at lower rpm, so gears in between handle the difference.
Context Anchor
Seen in fixed-shaft turboprop engine discussions, where the engine and propeller are mechanically connected but need to turn at different speeds.
Derivation
From Latin reducere, meaning to bring back or lead down. Here, reduction simply means making the rpm number smaller as power passes through the gears from engine to propeller.
Why Pilots Care
Allows the engine to produce full power at efficient high RPM while keeping propeller tip speeds safe and efficient.
Intuition Check
Do not read “gear” here as landing gear. In “reduction gear,” it means a mechanical gear system that reduces engine speed before it reaches the propeller.
Example Sentence 1
On the fixed-shaft turboprop, the engine spins at very high rpm, and the reduction gear brings the propeller down to a usable speed.
Example Sentence 2
During preflight the pilot checks the reduction gear oil level to confirm proper lubrication.