Definition
A propeller reduction gear system in which a central sun gear drives or is driven through a set of smaller planet gears that travel around it inside an outer ring gear, allowing the propeller shaft to turn at a slower speed than the engine crankshaft while transmitting high torque in a compact, balanced arrangement.
Plain English
A gear setup that lets the engine spin fast while the propeller spins slower. A center gear, several smaller gears around it, and an outer ring gear share the load and reduce the propeller's RPM compared to the engine's RPM.
Context Anchor
Seen in aircraft engine and powerplant maintenance discussions where engine speed must be matched to a suitable propeller or drive speed.
Derivation
From Greek 'epi' meaning 'upon' or 'around,' and 'kyklos' meaning 'circle.' The smaller planet gears literally travel in circles around the central sun gear, which is what gives this gear arrangement its name.
Why Pilots Care
Propellers are most efficient at lower RPM than the engine produces. Reduction gears let the engine run at its best operating speed while keeping the propeller tips below speeds where efficiency and structural limits become a problem. Knowing the system exists helps a pilot or technician understand why engine RPM and propeller RPM differ on geared engines.
Analogy
Think of a small gear rolling around a larger center gear, like a wheel moving around a hub. That motion lets the system change speed while staying compact.
Intuition Check
Reduction does not mean the engine is making less power here. It means the gear system lowers the turning speed delivered to the propeller or driven part.
Example Sentence 1
The large radial engine used epicyclic reduction gears so the propeller could turn at an efficient speed while the engine ran at its rated RPM.
Example Sentence 2
Proper lubrication of the epicyclic reduction gears prevents overheating during extended high-power climbs.