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6 Answers

RPM and Angle of attack in fixed pitched propeller

Asked by: 2406 views Aerodynamics, Flight Instructor

Having problem explaining how Prop Angle of Attack is increased with ONLY an increase in RPM. I can diagram it but just the basic understanding of how the blade AOA increases with an increase of prop RPM only.

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6 Answers



  1. Craig on Oct 07, 2019

    Take a look at this video. It explains it using vectors…..

    https://www.youtube.com/watch?v=R7hyrD9-R4A

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  2. Astrodog on Oct 07, 2019

    Thank you. Have seen this vector. My problem is understanding why an increase or decrease in AOA as a result of Increase or Decrease in RPM, only. The video shows it and I do understand it. But cant understand the following:
    Given TAS say 0 on the ground
    Increase in RPM= Increase in AOA
    Decrease in RPM = decrease in AOA
    Its probably right in front of me but other than the vector, cant explain it
    Don’t see how AOA changes just with change in ONLY RPM.
    Great video thanks!!

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  3. Mark Kolber on Oct 07, 2019

    The principle is related to constant speed propellers. A constant speed propeller automatically changes the angle of attack of the prop to maintain rom. With a fixed pitch prop *you* are changing the prop’s AoA manually without the prop conpensatung. Climb and the AiA increases, reducing rpm. Reverse for a descent.

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  4. Astrodog on Oct 07, 2019

    Thanks and I totally get your point. My question comes from looking and understanding the vector diagrams in the above mentioned video.
    With a FIXED pitch and NOT changing the AOA by varying the attitude “as in a climb or descent”, like sitting on the ground doing a run up. According to the vector diagrams your AOA or relative wind the blade “sees” can change based on TAS or RPM. Either or both separately can change the relative wind an in effect change AOA, proportionally.
    Thats the point of the video, and its obvious on a diagram.
    Simply put how does an increase in RPM increase the AOA sitting on the ground.
    Thanks for the input. Maybe my question is clearer now.

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  5. Best Answer


    Craig on Oct 07, 2019

    This vid might help too… https://www.youtube.com/watch?v=uzhPkrx_q2U

    In absolutely 0 relative wind, I’m not sure it would actually increase the prop AOA. Otherwise, I can kind of think of it like this:

    I’ll assume you understand how increasing the relative wind component decreases the AOA.
    So the relative wind and the prop AOA are inversely related. An imaginary formula might be (prop AOA = 1/relative wind). There’s probably a real formula, but I don’t know it.
    Then think of the increase in RPM as basically the reverse in that you basically decrease the amount of relative wind per revolution of the prop. For a thought experiment, lets imagine the plane doesn’t move on the ground, but wind is blowing directly down the plane from nose to tail. Similar to the relative wind vector, let’s say I get a “length” of air (relative wind) of 5 kts per 1 revolution of the prop blade at 100 rpm. At 200 rpm, my blade is moving twice as fast, but the “length” of the relative wind remains the same (5 kts), so I might say I chop that “length” of air (relative wind) in half so I get 2.5 kts of air.

    Going back to our imaginary formula of prop AOA = 1/relative wind:
    At 100 rpm the prop AOA = 1 / 5 = 0.2 for my AOA.
    At 200 rpm, the prop AOA = 1 / 2.5 = 0.4 for my AOA.

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  6. Astrodog on Oct 07, 2019

    Going to work through this answer, many thanks!!

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