Definition
A set of gears positioned between a turbine engine's high-speed output shaft and the propeller, designed to reduce the rotational speed delivered to the propeller to a value at which the propeller can operate efficiently.
Plain English
A gearbox that slows down the very fast spinning of a turbine engine so the propeller turns at a much slower, usable speed.
Context Anchor
Seen in turboprop engine descriptions, especially where a free turbine or power section drives the propeller through gearing instead of turning it directly at turbine speed.
Derivation
Reduction' comes from the Latin reducere, meaning 'to bring back' or 'lessen.' In this context, it lessens the rotational speed. The gearbox 'reduces' thousands of turbine RPM down to a propeller-friendly speed.
Why Pilots Care
Allows the propeller to rotate at efficient speeds while the turbine operates at its optimal high RPM, improving performance and preventing propeller tip damage.
Analogy
It is like using a lower gear on a bicycle: your legs and the wheel do not have to turn at exactly the same speed. The gears change the speed relationship so the power is usable.
Intuition Check
Do not read reduction as reducing engine power. Here, reduction means reducing rotational speed between the engine shaft and the propeller.
Example Sentence 1
The turbine spins at over 30,000 RPM, but the reduction gearbox brings the propeller speed down to around 2,000 RPM.
Example Sentence 2
Pilots must monitor the reduction gearbox oil pressure during preflight to ensure proper lubrication of the gear system.