Question regarding the AoA (Angle of Attack) on a Constant speed.

The CSU (governor) sets the blade angle based on the desired RPM, and the flyweights keep it from speeding up or slowing down using the the spring preload that resists the flyweights.

So it sets the blade pitch.

What about the AoA?

E.g. on a run-up (no forward motion), the constant speed prop would speed up to a certain RPM, what happens to the AoA and why?

What about when accelerating down the runway? In this case the relative airflow would change angle, what about the AoA?

Many thanks!


1 Answer 1


FYI, some constant speed units are hydraulically driven. Others are even electric. I don't think you actually care about the mechanism that does the changing.

As airspeed changes -- whether in flight or running down the runway, the constant speed prop is adjusted to maintain constant RPM. In fact, the main thing it responds to is changing airspeed.

You are perhaps forgetting the other primary engine control -- the throttle. The throttle sets (roughly speaking) the power output of the engine.

The constant speed propeller adjust the prop such that it absorbs the engine's power at the desired speed.

Consider for a moment a fixed pitch prop. For a fixed geometry prop (at a fixed airspeed and altitude), the power absorbed by the prop goes with the cube of RPM.

If you put an airplane with a fixed pitch prop into a dive, the airspeed will increase. If you don't reduce the throttle, the RPM will increase -- perhaps to the point where you could over-speed the engine. On an aircraft with a fixed pitch prop, the pilot manually adjusts the throttle to keep the engine/propeller within the safe operating range of the engine.

Back to a constant speed prop... If you put an aircraft with a constant speed prop into a dive, the airspeed will increase. The propeller un-loads from increased airspeed and would want to increase RPM. However, the constant speed governor increases pitch to prevent RPM from increasing. So, the constant speed unit helps the pilot safely operate the engine by preventing over-speeding.

Throughout all of these processes, the realized blade angle of attack can be whatever it needs to be. It is certainly not constant. It is also not at the point for maximum efficiency. A constant speed propeller does not achieve maximum efficiency. A CS prop has nothing to do with efficiency -- it has everything to do with power.


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