Definition
A system of gears placed between the engine crankshaft and the propeller shaft that causes the propeller to turn at a slower rotational speed than the engine. This allows the engine to operate efficiently at high rpm while keeping the propeller tips below the speeds at which their efficiency drops sharply.
Plain English
A set of gears that lets the engine spin fast while the propeller spins slower. The engine runs where it makes good power; the propeller turns at a speed where it works well.
Context Anchor
Seen in engine descriptions, aircraft systems discussions, and operating information for airplanes whose engines do not turn the propeller directly at engine speed.
Derivation
From 'gear' (a toothed wheel that transmits motion) and 'reduction' (from Latin 'reducere,' to bring back or lower). The drive 'reduces' the engine's rotational speed before it reaches the propeller.
Why Pilots Care
It keeps propeller tip speeds below supersonic levels, reduces noise and vibration, and lets the engine produce maximum power without exceeding propeller design limits.
Analogy
It is like using a lower gear on a bicycle: your legs can move at one speed while the wheel turns at a different speed. The gear system changes the speed between the source of power and the thing being turned.
Intuition Check
Do not read “gear” here as landing gear. In gear-reduction drive, “gear” means toothed wheels inside the engine drive system, and “reduction” means the propeller RPM is reduced compared with engine RPM.
Example Sentence 1
The turboprop's gear-reduction drive lets the engine turn at high rpm while the propeller spins at a much slower, more efficient speed.
Example Sentence 2
During preflight the pilot confirms the gear-reduction drive ratio matches the engine and propeller specifications.