Definition
A gear arrangement consisting of a central sun gear, several planet gears that mesh with the sun gear and rotate around it, a ring gear (with internal teeth) that surrounds and meshes with the planet gears, and a planet carrier that holds the planet gears in position. By holding one element fixed and driving another, the system produces a change in speed and torque between the input and output.
Plain English
A set of gears arranged with one in the middle, several smaller gears circling around it, and an outer ring with teeth on the inside. This layout lets one shaft turn another at a different speed, with the smaller gears moving like planets around a sun.
Context Anchor
Seen in aircraft engine reduction gearboxes, starter drives, and other powerplant assemblies where engine power must be geared before it turns another part.
Derivation
Named after the solar system. The center gear is called the 'sun,' the gears that orbit around it are 'planets,' and they all sit inside an outer 'ring.' The picture of planets circling a sun makes the layout easy to remember.
Why Pilots Care
Planetary gear trains allow engines to produce full power at high RPM while driving propellers at lower, more efficient speeds, improving performance and reducing blade stress.
Analogy
Think of several small wheels rolling around a larger center wheel while all staying held together in one frame. That is the basic motion idea behind a planetary gear train.
Intuition Check
Planetary does not mean the gear has anything to do with space flight. It describes the motion pattern: smaller gears move around a central gear, like planets around the sun.
Example Sentence 1
The turboprop's reduction gearbox uses a planetary gear train to bring the engine's output speed down to a range suitable for the propeller.
Example Sentence 2
A failing planetary gear train in the reduction gearbox can cause vibration and loss of propeller control.