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From an answer on this site:

The governor just knows: Speed too fast, flyweights move out, routes pressure to move the blades more coarse; speed too slow, flyweights move in, routes pressure to move the blades more fine. How coarse or how fine, the governor does not know or care.

So a constant speed propeller will not maintain the best AoA through the flight, at the end of the day it will allow to fly at maximum prop. efficiency only selecting the appropriate MAP/RPM for the flight condition, otherwise it will just keep the rpm selected.

Am I correct? Or am I missing something?

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You may be overthinking it. The governor will set the blade angle that provides the load on the engine that keeps it from speeding up or slowing down from the setting determined by the spring preload that resists the flyweights. You could connect the governor to a big disc brake in place of the prop and it would work the same.

So it sets the blade AOA that gives the required load without knowing what that AOA is. It's just a spindle with weights, a spring to hold them in, a mechanism to preload the spring and an oil metering valve - there's no AOA feedback to the governor, only a speed feedback, and the input command from the pilot is just pushing on a spring.

Whatever blade AOA that is achieved is the blade AOA required to achieve the load on the engine. Whether that blade AOA is 100% optimized to get the maximum thrust from that particular condition is another story, and blade twist, airfoil selection, blade length, number of blades, total blade area, and all that all have to be balanced against various objectives.

As to what particular aircraft speed, prop rpm and engine load gives the highest possible thrust efficiency from the propeller, you'd have to get from the prop designer and there may still be some optimization of all the parameters I mentioned toward a cruise case, or a static thrust case (like seaplane props; desirable to get on the step as quickly as possible), or a maximum speed case.

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  • $\begingroup$ For a good example of a system like the one described above, you should check out the Rolls-Royce (formerly Allison) T-56 turboprop used on older model Lockheed C-130's, P-3's (used by the US Navy), and a few other smaller US Navy aircraft. Rolls-Royce is now manufacturing the AE-2100 as a significantly updated successor to the T-56. $\endgroup$
    – Big_Al_Tx
    Jan 15 at 5:24
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Yes, it will just maintain RPM, and you have to know what RPM is appropriate for your speed and power setting—and what you want to do.

Max RPM gives you most power, minimal RPM gives you least drag on idle. The maximum efficiency RPM is somewhere between, increases with power required—more power means the AoA increases and too high is not efficient—and decreases with increasing speed—forward speed increases the local flow speed around the blades, which increases parasite drag. The exact relation is estimated and verified with flight test.

The pilot will have to know what the appropriate setting is. Since the cruise speed and power setting are generally always the same, it's not a big problem. And keeping the system simple means the pilot will know what to expect from it and it is more reliable.

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