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I am trying to understand why electric propeller governors are not (yet) prevalent given that they offer the ability to control the RPM with high accuracy. What is the typical architecture for an electric governor or electronically controlled governor? What are their pros and con?

As far as I know, there are two types of electric governors:

  1. The governor has an electric actuator in the propeller hub like the Airmaster Governor. This removes a lever, but adds a number of switches in the cabin.

  2. The governor is still hydraulic, but an electric actuator controls its position. (I think the PC-12 and TBM700 work like this?)

I am assuming that option 2 is not ideal as you need a separate actuator to control the governor cable.

I guess I do not understand how airplanes like a PC-12 or TBM700 (that only have a single lever in the cabin) regulate propeller RPM.

Thanks!

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    $\begingroup$ The presence of manual control is rather orthogonal to the mechanism used for governor and actuator, just like you can have autopilot with mechanical or hydraulic controls. Are you asking about how the automatic RPM control (of e.g. PC-12) works (which still allows different kinds of actuation) or how the electric governor + actuator work (which still may be controlled manually or automatically)? $\endgroup$
    – Jan Hudec
    Commented Apr 7, 2021 at 21:28
  • $\begingroup$ In short, I wanted to know both! It basically seems to me like the number of functions the governor/actuator mechanism needs to perform are too high for a single lever to control everything. Edit: I have seen diagrams for the PC-12's hydraulic governor that keeps RPM constant. It does not allow for varying RPMs however. $\endgroup$
    – Gerry
    Commented Apr 8, 2021 at 19:32

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They are much more prevalent than you think, and have been in use for more than 70 years. Rotax 912 and 914 engines can be installed with electric governors from MT Propeller. Airmaster sells them as well. You may also be surprised to learn that nearly 90% of German operational Luftwaffe aircraft during WW2 used an electrically-actuated propeller system designed by Dr. Hans Ebert of the VDM (Vereinigte Deutsche Metallwerke, or United German Metalworks). One example of these excellent propeller systems is in the Smithsonian's National Air and Space Museum.

This article does a pretty good job of explaining how they work.

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    $\begingroup$ The WW2 VDM units were not even constant-speed though, were they? My understanding is that they were just electrically actuated, but the pilot set pitch, not RPM, so they had no governor (governor is the part that senses RPM for the constant-speed feedback loop). Or did they? $\endgroup$
    – Jan Hudec
    Commented Apr 7, 2021 at 21:24
  • $\begingroup$ I didn't say the German props had a governor. Also, many of the electrically actuated props on modern aircraft are not governors either. You just select the pitch you want, often from a fixed number of options. $\endgroup$
    – Juan Jimenez
    Commented Apr 8, 2021 at 13:11
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    $\begingroup$ Indeed, but the question asks about governors, so it would deserve to be stated explicitly. $\endgroup$
    – Jan Hudec
    Commented Apr 8, 2021 at 14:43
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    $\begingroup$ Thanks for sharing the MT Propeller link and the WW2 era technology. I also found pictures of the MT Propeller's control box on the instrument panel, and it looks similar to Airmaster's: backcountrypilot.org/forum/mt-electric-adjustable-prop-12019 $\endgroup$
    – Gerry
    Commented Apr 8, 2021 at 19:37
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Electric stephead motors (governor motors) have been around since prior to the second world war. I used to run them on various old radials such as R-2600's. These accomplished the same thing a manual control would do, in changing tension on the speeder spring in the prop governor. Instead of moving a lever in the cockpit, one moved an electric switch in the cockpit.

In the case of the old radials, setting RPM using a stephead motor assembly was more about finesse; the change dind't occur immediately or in a liner relation to holding the switch fo ra given period of time. One could hold the switch and see no immediate change, and assume it required being held longer. Then one ended up holding it too long, and the RPM rolled back too far. Setting RPM on takeoff, or fine tuning it, was purely a matter of experience (trial and error).

Most constant speed propellers, from the hamilton standard hydromatic three quarters of a century ago, to most propellers today, use the same basic function of using a pilot valve to admit oil to the propeller mechanism, and that oil, under pressure, drives prop blades to the angle necessary to maintain a specific RPM. The pilot valve is controlled by a spinning flyweight, and the flyweight travel and action is controlled by spring tension against the pilot valve. The electric motor controls the spring tension.

Various engine installations also use feedback from other engine components, such as power lever, and governing comes from several sources; RPM may be held constant, and engine power is regulated using fuel governors, overspeed governors, underspeed governors, and fuel topping governors.

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  • $\begingroup$ Interesting! I did not know that the old radials had electric stephead motors to adjust propeller pitch! I will look into the R-2600s. I only have some exposure to the R985 with the hydromatic. It is amazing to see the many ways to optimize the propeller to the engine/motors. $\endgroup$
    – Gerry
    Commented Apr 18, 2021 at 5:30

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