Welcome Guest. Sign in or Signup

6 Answers

RPM and Angle of attack in fixed pitched propeller

Asked by: 2369 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.

Ace Any FAA Written Test!
Actual FAA Questions / Free Lifetime Updates
The best explanations in the business
Fast, efficient study.
Pass Your Checkride With Confidence!
FAA Practical Test prep that reflects actual checkrides.
Any checkride: Airplane, Helicopter, Glider, etc.
Written and maintained by actual pilot examiners and master CFIs.
The World's Most Trusted eLogbook
Be Organized, Current, Professional, and Safe.
Highly customizable - for student pilots through pros.
Free Transition Service for users of other eLogs.
Our sincere thanks to pilots such as yourself who support AskACFI while helping themselves by using the awesome PC, Mac, iPhone/iPad, and Android aviation apps of our sponsors.

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

    0 Votes Thumb up 0 Votes Thumb down 0 Votes



  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!!

    0 Votes Thumb up 0 Votes Thumb down 0 Votes



  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.

    0 Votes Thumb up 0 Votes Thumb down 0 Votes



  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.

    0 Votes Thumb up 0 Votes Thumb down 0 Votes

  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.

    +1 Votes Thumb up 1 Votes Thumb down 0 Votes



  6. Astrodog on Oct 07, 2019

    Going to work through this answer, many thanks!!

    0 Votes Thumb up 0 Votes Thumb down 0 Votes


The following terms have been auto-detected the question above and any answers or discussion provided. Click on a term to see its definition from the Dauntless Aviation JargonBuster Glossary.

Answer Question

Our sincere thanks to all who contribute constructively to this forum in answering flight training questions. If you are a flight instructor or represent a flight school / FBO offering flight instruction, you are welcome to include links to your site and related contact information as it pertains to offering local flight instruction in a specific geographic area. Additionally, direct links to FAA and related official government sources of information are welcome. However we thank you for your understanding that links to other sites or text that may be construed as explicit or implicit advertising of other business, sites, or goods/services are not permitted even if such links nominally are relevant to the question asked.