I followed discussion here: In a constant speed propeller, why does the RPM drop with a prop pitch increase?, I got confused about the RPM there. As the opening statement said in that discussion, "When a constant speed propeller changes its pitch from low to high, RPM would decrease."

From that discussion said that the RPM there is not the RPM of the engine that used to power the airplane, but the RPM of the propeller itself, how fast it rotating after the pitch was changed. The more pitch of course will produce more drag and as the product, the propeller will rotate slower than previously.

Please you masters of the aviation, please confirm if it is true, the RPM is the rotation of the propeller itself and not the RPM of the engine/turbine used to power of the airplane. That have confused me for quite long time and made me very difficult to understand many explanation.

  • $\begingroup$ Propeller RPM will always be the same as engine RPM unless there’s a gearbox between the two - or in a shaft failure case of course. Not sure I understand your question correctly...? $\endgroup$ – Cpt Reynolds Oct 20 '18 at 8:32
  • $\begingroup$ I mean, the RPM forcibly reduced by the induced drag rather than the RPM decreased by the pilot? $\endgroup$ – AirCraft Lover Oct 20 '18 at 10:25

Yes it is the propeller RPM. If the engine is direct drive, the engine and propeller RPM are the same. If the engine has a propeller reduction gearbox, then engine RPM and propeller RPM are different, the propeller RPM being the engine RPM minus the gear reduction.

Propeller RPM is controlled by a flyweight governor, like those spinning ball speed regulators you see on old steam engines. The flyweights control the oil flow to the prop to vary the pitch. The prop control lever applies a spring preload to the governor flyweights and the amount of preload determines at what RPM there is a state of hydraulic equilibrium so there is no pitch change happening. The speed of the governor is a function of the speed of the propeller in either a direct drive or geared engine.

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    $\begingroup$ I think you mean "propeller RPM being the engine RPM divided by the gear reduction." $\endgroup$ – Paul Oct 20 '18 at 17:10
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    $\begingroup$ Yeah close enough lol. $\endgroup$ – John K Oct 20 '18 at 21:01
  • $\begingroup$ So @John-K, can I say that the propeller's RPM decrease is due to the load that affecting (decreasing) the engine's RPM? Just like a truck's wheel that slowing (decreasing RPM) due to its load or due to the climbing road that finally will decrease the engine RPM? Is that my analogy acceptable? $\endgroup$ – AirCraft Lover Oct 21 '18 at 7:09
  • $\begingroup$ My concern here is to avoid my misunderstanding that the propeller's RPM decreases is due to the load (because of the higher pitch angle) and not due to the pilot decrease the engine's RPM through throttle. They are very different. $\endgroup$ – AirCraft Lover Oct 21 '18 at 7:13
  • $\begingroup$ With a constant speed propeller, the throttle only affects RPM when the prop is still at its fine pitch stop. When there is enough power to exceed the RPM at which the governor is set, the governor starts regulating RPM by increasing pitch. While the prop is on the governor, changing throttle position doesn't have an effect on RPM unless the throttle is reduced enough to cause the prop to hit its fine pitch limit. This is something important to understand because at normal power settings something that reduces power (like carb ice) doesn't cause an RPM drop until its too late. $\endgroup$ – John K Oct 21 '18 at 17:49

In the context of that discussion, it doesn't matter much whether it's the engine or propeller.

RPM is just a unit of measure, and you can apply it to anything that turns.

For the pilot, the prop speed is one of the most important parameters, so it is the prop RPM that is usually displayed.

Many, perhaps most small aircraft have 'direct drive' prop, where the engine shaft drives the propeller directly. Obviously, their RPM will be exactly the same.

Some have a gearbox (nearly always a reductor). In that case, as I said, usually the prop RPM are displayed, but because the engine and the prop are still hard-linked, the measurement can actually be made on the engine shaft and then recalculated.

Turboprops always have a gearbox. But added to that, they nearly always have two turbines (in each engine). One drives the prop, and it is normally linked to the gearbox just like a piston engine. So their RPM change together with the prop. But the other turbine drives the compressor and is more or less independent. And even though it doesn't affect the power directly, its RPM are also important and get displayed separately. But the pilots specifically mention that when they talk about this turbine.

So again, in the context of that discussion, the important points were how RPMs are changed. And they change together, they are hard-linked (lest there is a horrible failure): if one drops, the other does too. The numbers may be different if there is a gearbox, but who cares.

If you are still interested why things happen as they do, you can ask a separate question.


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