Section 17.4

Propeller Synchronising

Synchronising RPM to remove the beat, and synchrophasing to reduce cabin noise.

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Main ideas

On a multi-engine aircraft, small differences in propeller RPM produce a slow, throbbing 'beat' as the sound waves drift in and out of phase — annoying and fatiguing for crew and passengers.
A SYNCHRONISING system matches the RPM of all propellers to a chosen master engine, removing the beat; the synchroniser trims the slave engine governors to match the master.
SYNCHROPHASING goes a step further: it not only matches RPM but also holds a selected angular PHASE relationship between the propellers (e.g. one blade set a few degrees behind another), which further reduces cabin noise and vibration.
One engine is the master and the others are slaves; the system has a limited authority so it trims, rather than grossly changes, the slave RPM, and is switched off for take-off/landing.
The pilot still sets the approximate RPM; the synchroniser makes the fine match.
⚠ Exam trap: SYNCHRONISING matches RPM to remove the beat; SYNCHROPHASING additionally fixes the blade phase relationship between propellers to further cut noise — synchrophasing controls phase, not just speed.

Solved examples

A twin's cabin has a slow rhythmic throbbing in the engine noise. What causes it and what system removes it?
The two propellers are turning at very slightly different RPM, so their sound waves alternately reinforce and cancel, producing a slow 'beat'. A propeller synchronising system matches the slave engine's RPM to the master, eliminating the frequency difference and the throb; synchrophasing can then hold a fixed blade phase to reduce the residual noise further.

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