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When a constant speed propeller changes its pitch from low to high, RPM would decrease.

Why is that the case?

Wouldn't the RPM increase when we increase the blade angle since there's less drag and the blades are more aligned to the relative wind, compared to when it's in a low pitch angle? Also, does higher blade pitch allow us to fly any faster?

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  • $\begingroup$ It seems as though you are agreeing with yourself with your first question. Perhaps you could reword the sentence. $\endgroup$
    – user22445
    Commented Jul 30, 2017 at 0:17
  • $\begingroup$ You're right, it's correct now. $\endgroup$
    – nyorkr23
    Commented Jul 30, 2017 at 0:19
  • $\begingroup$ @nyokr23 if the power setting, aircraft pitch attitude, etc., remained unchanged, increasing the blade angle would decrease the RPM. $\endgroup$
    – user22445
    Commented Jul 30, 2017 at 0:21
  • $\begingroup$ Yes, but why is that the case is what I'm trying to figure out. $\endgroup$
    – nyorkr23
    Commented Jul 30, 2017 at 0:22
  • $\begingroup$ increased pitch in a propeller increases the blade's angle of attack creating more induced drag - which would decrease the RPM. I'm sure some of the engineers and aerodynamic experts could explain it a bit more eloquently. $\endgroup$
    – user22445
    Commented Jul 30, 2017 at 0:25

3 Answers 3

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The propeller blades indeed have least resistance when aligned with the inflowing airstream, however the inflowing airstream is not the free stream at infinity, but the vector sum of free stream velocity $V$ and rotational velocity of the propeller blade $\omega \cdot r$.

enter image description here

In the picture the local Angle of Attack is indicated as $\alpha$. Increasing blade pitch results in increasing $\alpha$, which means more air accelerated backwards: more thrust delivered, and more torque required.

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Koyovis explained it with maths and cool diagrams, here is my layman's version, if it helps.

You're confusing aircraft drag with the drag on the prop. Since the prop is powered, it typically produces thrust not drag with respect to the aircraft. It only produces a net drag if it's windmilling (that is being turned more by the airflow than by the engine) - which is why you would feather the prop on a dead engine in a multi-engine aircraft.

Each prop-blade however produces drag in the direction they're travelling (i.e. sideways) in order to produce lift in the direction the aircraft is moving (i.e. thrust) - and it's this drag the engine is fighting. Increasing the pitch makes it harder to swat that thing through the air (but also produces more thrust) thus the engine RPM goes down.

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    $\begingroup$ Best answer. For basic questions such as this the layman's explanation is generally always preferred... $\endgroup$
    – Michael Hall
    Commented Mar 26, 2019 at 15:42
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This is based on another answer I wrote:

enter image description here
(Own work)

Plane moving forward plus blade moving down gives the prop a new velocity vector, shown above in red. Relative air(speed) is still coming from ahead—but this now no longer concerns the propeller as it has its own velocity vector.

As you can see, if the RPM remained constant (down arrow), and the plane flew faster, this will also change the prop angle of attack (lower it). (Given that the prop pitch governor does not change its input, or in the case of a fixed-pitch prop.)

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