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On an airplane with a constant speed prop, the prop lever typically controls the RPM (pitch) of prop, and the throttle controls the power, measured as Manifold Pressure ("MP"). However, at low power settings with the prop lever full forward, there is a point where the throttle control begins to act like a throttle on a constant pitch airplane where the throttle controls the RPM. This is the point that I'm asking about.

Does this point occur at the same MP on all aircraft? Conceptually I understand that when the prop is set to max RPM the prop will adjust pitch finer and finer as I reduce power until it gets to the point where it can't flatten out the pitch anymore as I continue to reduce power, then the RPM will begin to drop. Is this a point that is specific to engine/prop combination or is it adjusted by mechanic?

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  • $\begingroup$ When the prop can no longer pivot in the hub. $\endgroup$
    – acpilot
    Sep 18, 2019 at 20:43
  • $\begingroup$ This can only be adjusted to a point. At some point your blades are completely flat, any more and you'll basically go into "beta", which is something you don't want to do on an aircraft that doesn't explicitly support it. $\endgroup$
    – Ron Beyer
    Sep 18, 2019 at 20:46
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    $\begingroup$ Nah. There are stops built into props that only allow travel to a certain limit. Since the op includes MP in the question, we know he is not asking about turboprops, so beta is not relevant. The prop will hit the stops and the will be effected by the throttle at low and high rpm limits. $\endgroup$
    – acpilot
    Sep 20, 2019 at 20:28

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What happens is when you're on final and you move the prop to MAX RPM, the prop governor moves the blades finer until the RPM is at red line, then governs RPM there by adjusting blade angle as required. As you reduce throttle, the governor moves the blades finer and finer trying to keep the RPM that has been set (maximum or redline RPM in this case).

As throttle is further reduced the blades hit the fine pitch limits of the propeller hub and can't go any finer, so the governor can't unload the engine any more to keep the RPM up. As you continue to reduce throttle you basically now have a fixed pitch prop with very finely pitched blades and down comes the RPM. As long as throttle is below the level that will speed the prop up above redline (or whatever the governor setting is), RPM will vary directly with throttle, like a fixed pitch prop engine.

Add power and RPM rises until it hits the governor setting, then the governor starts to coarsen pitch to load up the engine, to keep the RPM at the setting.

The point at which the prop "comes off the governor" is determined by the pitch limits of the blades within the hub. The part the mechanic can adjust is the RPM set by the governor when the prop lever is at MAX, which is normally redline RPM +/-.

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    $\begingroup$ excellent exposition. thanks for posting this. $\endgroup$
    – niels nielsen
    Sep 19, 2019 at 2:20
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    $\begingroup$ "Hitting the (pitch) limits" is formally called actuator saturation. Google it yourself, because the results' comprehensibility depend strongly on your engineering background. $\endgroup$
    – Camille Goudeseune
    Sep 19, 2019 at 3:53
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    $\begingroup$ Well my answers are written for laypeople, not engineers, so I try to minimize engineer-speak. Fine pitch limits, fine pitch stops, it's simply referring to a mechanical travel limit of something that moves. No need to get too deeply into sematics. $\endgroup$
    – John K
    Sep 19, 2019 at 4:02
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    $\begingroup$ Fine pitch limits, fine pitch stops... terms such as these work just fine for this 7,000-hour commercial seaplane pilot. Thanks, John K. $\endgroup$
    – Stu Smith
    Sep 19, 2019 at 4:59
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    $\begingroup$ It's the physical limit on the blade's rotation within the hub. When it gets there depends on the torque being applied and it will vary. When torque is no longer sufficient to sustain the RPM at the fine pitch limit, RPM will start to drop. This will be at some fairly low manifold pressure and will also vary with airspeed, maybe between half and 1/3rd throttle, say between 15-20" MP (I haven't flown an airplane with a CS prop in some years so I can't remember). But if you have set the prop to MAX RPM, and RPM starts to drop below red line, you are there; the blades are as fine as they'll go. $\endgroup$
    – John K
    Sep 19, 2019 at 21:32

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