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Variable-pitch props allow pilots to adjust the angle of attack of the propeller to change the load on the engine.

The most common implementation of this is a constant speed prop where the prop governor adjusts pitch to maintain the chosen RPM.

From a pilot's perspective, we only really care about two things:

  • Power output
  • Fuel economy

Using a constant speed prop, you can't directly set the output power - you set the throttle and prop speed and observe the power output.

Using a constant speed prop, you also can't directly set the fuel flow rate - you set the throttle and prop speed and observe the fuel flow (or look up the POH).

Using a constant power control law instead the pilot could set the desired power and have the prop pitch adjusted automatically to maintain that (or if impossible, fail in a similar way to a constant speed prop going off the governor).

It is also possible to pick the most fuel efficient RPM for a given engine load which means that the two inputs (throttle + prop speed) get reduced to one: desired power.

The benefits I see are:

  • Reduced workload
  • Increased efficiency

The drawbacks I see are:

  • Inability to operate the engine at anything other than best efficiency for a given power - but I don't actually know what use case that would be useful in.
  • System complexity
  • Certification cost

Has a constant power control system ever been used in aviation? Are there other drawbacks that I'm missing?

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  • $\begingroup$ Are you looking for an ECU whose input is directly translated in percentage of available power? or directly in a given power (let's say throttle full forward correspond to 200hp, midcourse to 125hp,...)? $\endgroup$
    – Manu H
    Commented Jul 29, 2020 at 6:35
  • $\begingroup$ In terms of (combustion) engine performance, rpm matters a lot. This surely partly explains why there are constant speed props. In fact, you generally design your engine to have a maximum power for take-off speed, then maximum efficiency for cruise speed. Also, some unstable phenomena forbid to stay in some areas of the operating range (especially for turboengines). $\endgroup$
    – BambOo
    Commented Jul 29, 2020 at 13:31
  • $\begingroup$ It isn't clear to me what you are actually hoping to achieve. For example, your comment that you cannot set fuel flow but can only observe it is incorrect. You absolutely can set the fuel flow. Every time you move the throttle the fuel flow changes. If you want to set a specific fuel flow simply adjust power and prop to a setting that will achieve it. $\endgroup$
    – Michael Hall
    Commented Jul 29, 2020 at 17:59
  • $\begingroup$ @ManuH yes, but also factoring in available prop pitches to pick the most efficient way to produce 125hp etc $\endgroup$
    – digitalPhonix
    Commented Jul 30, 2020 at 5:08
  • $\begingroup$ @MichaelHall Throttle does not set fuel flow (but does affect it) - this is obvious by the fact that changing the prop setting while keeping the throttle constant changes your fuel flow (ie. reducing RPM with a constant throttle will reduce the fuel flow) $\endgroup$
    – digitalPhonix
    Commented Jul 30, 2020 at 5:11

2 Answers 2

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There are some aircraft that indeed have just power lever and set the RPM on schedule, e.g. the Pilatus PC-7, PC-9, PC-21 line of trainers. Since they are primarily designed for training future jet pilots, and jet aircraft only have power levers, it makes sense to skip the complexity of separate propeller and condition controls.

However normally there are some cases where you do want to set other than most efficient RPM for current power and true airspeed:

  • The most common is descent and approach. These are flown with high RPM because

    • the engine is already spinning fast, so it does not have to spool up if you need to add power to go around and
    • with retarded power it produces more drag, allowing to expedite the descent when needed (the Pilatus PC-7/9/21 have a ventral speed brake; most turboprops don't).

  • Failure modes, though for most a feather switch would probably be sufficient.

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  • $\begingroup$ Do you know how those aircraft measure the shaft power and adjust throttle/pitch to maintain it? $\endgroup$
    – digitalPhonix
    Commented Jul 31, 2020 at 21:26
  • $\begingroup$ And thanks for those two cases! $\endgroup$
    – digitalPhonix
    Commented Jul 31, 2020 at 21:26
  • $\begingroup$ @digitalPhonix, I don't know, but I don't think they'd actually measure power, just have the power settings mapped to fuel flow and RPM (it is a turbine engine, so no mixture to fiddle with). $\endgroup$
    – Jan Hudec
    Commented Jul 31, 2020 at 21:34
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A constant-speed prop lets the pilot select the speed setpoint for the prop/engine system with the prop control. With that control in the full-forward (fine pitch) position, full throttle applied to the engine will result in the engine revving up to its maximum rated RPM.

Then adding pitch to the prop so as to load down the engine (at full throttle) will cause it to slow down to any desired RPM setpoint. At that RPM setpoint, pulling back the throttle will cause the pitch to decrease as the governor system strives to maintain the RPM setpoint.

Working the throttle and the prop controls in this way lets the pilot select any possible combination of prop pitch and engine RPM desired. The operation manual contains charts that specify the power output of the engine for every possible combination of those parameters, and on those charts the "best power" and "best economy" combinations will be marked.

So, if the pilot has to execute a climb during cruise, they will select the "best power" combination of manifold pressure and RPM for that altitude by adjusting the throttle and prop controls necessary to produce it- and then monitor the resulting cylinder head and oil temps to make sure they don't go into the red. Once they arrive at the assigned altitude, then they will go back to the "best economy" setting for that (new) altitude.

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  • $\begingroup$ Why would a pilot ever want any setting that isn't best economy for a given power setting? Eg. In a C177 I can produce ~125bhp @ 2000ft using: 2100rpm/24" (burning 9.0 gph), 2200rpm/23" (burning 8.9 gph), 2300rpm/22.5" (burning 8.7 gph), 2400rpm/22" (burning 9.0 gph) and 2500rpm/21.5" (burning 9.2 gph). Why would I ever pick anything other than 2300rpm/22.5" if I wanted to produce 125bhp? $\endgroup$
    – digitalPhonix
    Commented Jul 30, 2020 at 5:07
  • $\begingroup$ And yes, I understand it allows you to select any possible operating point - I'm asking why are the inputs speed and throttle? Power and throttle would also allow you to pick any operating point; as would power and rpm. (The second part of the question was asking about dropping the second input, in which case you can't select any operating point) $\endgroup$
    – digitalPhonix
    Commented Jul 30, 2020 at 5:10
  • $\begingroup$ there's no way to measure power directly; you need to put RPM and manifold pressure onto a chart from which you then read off the resulting horsepower. $\endgroup$
    – niels nielsen
    Commented Jul 30, 2020 at 5:20
  • $\begingroup$ @nielsnielsen, that most GA aircraft don't have a power indicator installed does not mean there is no way to measure it. Larger piston airliners usually had one (often labelled BMEP) and it was used as a reference to set mixture for best power. These days turboprops usually only have torque and RPM though. $\endgroup$
    – Jan Hudec
    Commented Jul 31, 2020 at 5:43
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    $\begingroup$ I know in maritime applications too the shaft power is directly measured so the measurement part is a solved problem (maybe not within the weight/cost constraints of GA - in which case that would be the answer to my question). $\endgroup$
    – digitalPhonix
    Commented Jul 31, 2020 at 6:46

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