what does Pro Sync do?

PROPELLER SYNCHRONIZATION

Most four engine, and many twin engine, aircraft are equipped with propeller synchronization systems. Synchronization systems provide a means of controlling and synchronizing engine rpm. Synchronization reduces vibration and eliminates the unpleasant beat produced by unsynchronized propeller operation. There are several types of synchronizer systems in use (AC65-12A).

Master Motor Synchronizer

An early type, still in use on some operating aircraft, consists of a synchronizer master unit, four alternators, a tachometer, engine rpm control levers, switches, and wiring. These components automatically control the speed of each engine and synchronize all engines at any desired rpm.

A synchronizer master unit incorporates a master motor which mechanically drives four contactor units; each contactor unit is electrically connected to an alternator. The alternator is a small, three phase, alternating current generator driven by an accessory drive of the engine. The frequency of the voltage produced by the generator is directly proportional to the engine accessory speed. In automatic operation, the desired engine rpm may be set by manually adjusting the rpm control lever until a master tachometer indicator on the instrument panel indicates the desired rpm. Any difference in rpm between an engine and the master motor will cause the corresponding contactor unit to operate the pitch changing mechanism of the propeller until the engine is on speed (at correctly desired rpm).

One Engine Master System

Synchronizer systems are also installed in light twin engine aircraft. Typically, such systems consist of a special propeller governor on the left-hand engine, a slave governor on the right-hand engine, a synchronizer control unit and an actuator in the right-hand engine nacelle.
The propeller governors are equipped with magnetic pickups that count the propeller revolutions and send a signal to the synchronizer unit. The synchronizer, which is usually a transistorized unit, compares the signal from the two propeller governor pickups. If the two signals are different, the propellers are out of synchronization, and the synchronizer control generates a dc pulse which is sent to the slave propeller unit.
The control signal is sent to an actuator, which consists of two rotary solenoids mounted to operate on a common shaft. A signal to increase the rpm of the slave propeller is sent to one of the solenoids, which rotates the shaft clockwise. A signal to decrease rpm is sent to the other solenoid, which moves the shaft in the opposite direction.
Each pulse signal rotates the shaft a fixed amount. This distance is called a "step." Attached to the shaft is a flexible cable, which is connected on its other end to a trimming unit. The vernier action of the trimming unit regulates the governor arm (AC65-12A).

so not synchronizing doesn't really affect the flight, except that it's more pleasant when the propellers are synchronized?

SeanGa wrote:

so not synchronizing doesn't really affect the flight, except that it's more pleasant when the propellers are synchronized?

But you always need satisfied passengers 😉

jolegg, thanks for this detailed explanation of sync prop feature.
🙂

then if I move the throttle levers separately in order to offset the crosswind, would the sync prop still work?

I don't think so no. The prop sinc synconises the RPM's, so if there is less in one engine then I don't think it is syncronised.

Is reducing the throttle on one engine common practice for a crosswind landing? I know about slipping / crabbing but not the throttle idea.

Thanks

😉