Re: What is "synchronizing" propellers and why was it done?

Date:         21 Dec 97 02:32:40 
From:         Seth Dillon <bdillon1@mindspring.com>
Organization: MindSpring Enterprises
References:   1 2
Followups:    1
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Bob Mann wrote:
> Multi-engine equipment used electrical or mechanical propeller synchronizers,
> phasers and in some cases, combined synchro-phasers, to reduce the frequency
> and magnitude of vibrations associated with "beating" of propellers that were
> out of speed and phase (angular/rotational) synchronization.  Reduced
> vibration contributed to lower cabin noise and lower vibration/cycle fatigue.
> Phasing was the later innovation, and common on larger-bladed equipment like
> P-3/Electra.

This feature is also found on MD80's. There is a N1 mode, and an N2 sync
mode.  It stops that droning noise which is really bad if seated in the
back between the engines.

The pilot match the N1's within 2% and the sync will pull the left
engine up or down to match the right.  N2 must be matched within 1%.  It
seems from talking to crews that it is seldom used.  The pilots prefer
to fly with the EPR matched using the AFCS, letting it control the
throttles to produce the set EPR, with the engines turning at whatever
rpm is needed to produce that EPR.
-Seth