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enter image description here

What is the small fan I've seen on different turboprops?

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    $\begingroup$ Also note that it is located between the "prop rpm" indicators on the Garmin display. $\endgroup$
    – RTO
    May 22 at 15:34

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This is the propeller synchronization (usually just called "prop sync") indicator, or "synchroscope." It rotates one way or the other to show the change in phase alignment of each propeller. During cruise, it's beneficial to manually or automatically (depending on aircraft) synchronize the frequency and often (again depending on aircraft) the phase of the blades to reduce noise and vibrarion (and consequently wear and tear).

In ancient times, before glass cockpits :), these were mechanical indicators with actual moving parts that rotated in response to the electrical signals from magnetic pickups mounted along the prop shaft.

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  • $\begingroup$ Technically a prop synchrophaser. $\endgroup$
    – Carlo Felicione
    May 23 at 22:58
  • $\begingroup$ @CarloFelicione The presence of a synchroscope doesn't mean a synchrophaser is installed, and synchroscopes don't show the phase alignment, only phase rate of change (equivalent to frequency difference). But yes, a device which aligns propeller phase in addition to frequency is properly called a synchrophaser. $\endgroup$
    – TypeIA
    May 23 at 23:34

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