Definition
In a multi-engine airplane equipped with a propeller synchronization system, the slave engine is the engine whose propeller RPM is automatically adjusted to match the RPM of the master engine. A control unit senses the difference between the two propeller speeds and signals the slave engine's propeller governor to bring its RPM into precise alignment with the master.
Plain English
On a twin or multi-engine airplane, one engine is chosen as the reference and the other engine's propeller speed is automatically tweaked to match it. The one being adjusted to match is the slave engine.
Context Anchor
Used in multiengine airplane discussions of propeller synchronization and synchrophasing systems.
Derivation
The word 'slave' here is a mechanical engineering term, used long before aviation, to describe a component that follows the lead of another. It comes from the older sense of one part being controlled by another. In aviation, it simply means the engine that follows -- not the one in charge.
Why Pilots Care
Correct synchronization of slave engines with the master reduces vibration, noise, and structural stress on the airframe.
Intuition Check
“Slave engine” does not mean the engine is weaker, less important, or producing less power. It means that, for the synchronization system, that engine follows the master engine’s propeller speed reference.
Example Sentence 1
With prop sync engaged, the right engine acts as the slave engine and its RPM automatically trims to match the left.
Example Sentence 2
The synchronization system prevents the slave engines from operating at a slightly different speed that would cause an annoying beat frequency.