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5 Answers

Propeller Pitch and Aerodynamic Forces

Asked by: 5575 views , , , ,
Aerodynamics, Aircraft Systems, Commercial Pilot

Hello everyone. I'm studying for my comm/instrument multi-engine add-on and I've found what appears to be conflicting information. I'm hoping someone can give me some clarification on a nuance of constant speed propellers.  Specifically, with respect to the aerodynamic forces acting on a rotating propeller and whether or not those forces tend to twist the blade towards the low pitch (high RPM) or high pitch (low RPM) position.

I understand the that the aerodynamic forces are not the only forces acting to twist the blade one way or the other and, I understand how single engine and multi-engine airplanes differ in how the propeller reacts to a loss of engine oil pressure.  However, in my reading one text says the aerodynamic forces act to push the blade to low pitch and another place I looked says the opposite.  

It all boils down to the location of the center of pressure of the blade, is it forward or aft of the propeller's central axis?  If it's aft (what I would expect) the blade would get pushed towards the low pitch setting by the aerodynamics forces, if it's forward, the opposite would occur.  I have always thought the blade gets pushed towards low pitch (high RPM) but this other possibly incorrect source confused me.

So, which is it?  Is it always one way or does it depend on the specific propeller?  Does anyone have a reliable source of information they can point me to?

Thanks in advance for your help.

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5 Answers



  1. Kris Kortokrax on Feb 23, 2015

    It seems to be the convention that on a single engine airplane, the prop is driven to the low pitch, high RPM setting in the event of a loss of oil pressure.

    In a multiengine airplane, the propeller is driven to the feathered position.

    Oftentimes, counterweights are used to drive the prop to the desired position.

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  2. Dmitriy on Feb 23, 2015

    As Kris states, the direction the propeller pitch is naturally forced depends on the need of the propeller. Generally, propellers that can feather will have the propeller move to a higher pitch position, and vice versa for those that don’t feather. The system itself is the same in both cases, with the exception of the pilot valve inside the propeller governor is backwards, meaning it opens the opposite flow line to/from the prop hub (I’m assuming simple reciprocating engine, non-reversible propeller systems).

    Also, just to help clarify, the forces that change propeller pitch are not “aerodynamic” forces. There are only two types of aerodynamic forces, lift (or thrust for a propeller or rotor) and drag. So for example, a propeller will windmill due to aerodynamic forces, but that’s about it.

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  3. Best Answer


    Russ Roslewski on Feb 24, 2015

    The question wasn’t about simple topics like oil pressure changing blade pitch, it was clearly for a more advanced understanding of how aerodynamic forces affect the pitch of a blade. Of course they do, any time an airfoil’s center of pressure is not located right on the “mounting axis” there will be a twisting component due to lift developed. There’s a pretty good picture here:

    http://www.datwiki.net/images2/ATF.jpg

    Since the center of pressure is ahead of the pivot point of the blade, the lift developed is going to want to rotate the blade to coarser pitch. The constant-speed hub fights this of course, so the rpm does not actually drop, but the force is definitely there.

    If that oil pressure is lost, any counterweights/springs/etc are strong enough to overcome this force and twist the prop to the desired position.

    Admittedly, this is a more advanced aerodynamic topic that I’ve never seen covered in pilot training, and really doesn’t affect the operation of the aircraft at all. But it’s a valid question.

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  4. Brian on Mar 01, 2015

    Russ,

    How do we know whether CP is ahead or behind the the rotation. Given that CP’s location is predicated on AOA, propellers have a large twist in them, forward velocity greatly effects the propellers AOA, and so on. In other words, a higher AOA moves CP back significantly. So it would seem that CP may be ahead in some instances and behind in others.

    As you stated I don’t see this being at all relevant to the pilot as it has no effect on the aircraft. However, it has spiked my curiosity so do you have a source where I may read up more on this. Or, even better, a link to some data. I’m sure NACA has something, but I’ve turned up nothing yet.

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  5. vne147 on Mar 07, 2015

    Thanks everyone for your input.

    Russ, you most directly answered my question. I agree with you that the aerodynamic forces in question and their tendency to twist the propeller blade one way or the other have little to no practical purpose in actually flying the airplane. However, while I was studying constant speed propeller systems I realized that this topic was inadequately addressed. That’s unfortunate because I think an understanding of these concepts can go a long way towards understanding the how and why of these system’s design and operation.

    So, I’d like to echo Brian’s question. Do you know of or have any sources of information that more thoroughly discuss this topic?

    Thanks again.

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