From SKYbrary Wiki
Propeller synchronisation is the process of manually or automatically adjusting the propellers of a multi-engine propeller driven aeroplane so that they all rotate at the same speed.
When the propellers of a multi-engine aircraft are not turning at the same speed, an audible vibration or "beat" results. Although this does not affect the operation or efficiency of the aircraft, it can be very annoying to the passengers and crew. Propeller synchronisation serves mainly to improve the comfort of the aircraft occupants.
In a twin engine aircraft, manual synchronisation is accomplished by slowly adjusting the RPM of one engine until it matches the other. The elimination of the audible "beat" is, in many cases, the only tool available to the pilot to judge success. Some aircraft provide a cockpit indicator to assist in propeller synchronisation. Pilots can use this indicator to help them match propeller speeds manually or, when installed, to decide whether or not to engage electronic propeller synchronisation
Many multi-engine airplanes have an electronic propeller synchroniser installed. To use it, the RPM of each of the propellers is first coarsely matched by the pilot and then the system is engaged. One engine is designated as the "master" and the other engine(s) as the "slave(s)". The synchroniser adjusts the RPM of the “slave” engine to precisely match that of the “master” engine, and then maintains that relationship.
In most installations, the propeller synchronisation must be turned off for takeoff, landing, during single engine operations and when intentionally changing the propeller RPM in flight (for example, from climb RPM to cruise RPM).