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?