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Thread: P-factor and vibration at high angle of atack

  1. #11
    Administrator DesertFox4's Avatar
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    Default Re: P-factor and vibration at high angle of atack

    I on occasion notice a drumming on the turtle deck but haven’t paid close attention to what conditions trigger this. Noticed it on more than one Kitfox over the years.


    DesertFox4
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    7 Super Sport
    912 ULS Tri-gear


  2. #12
    Senior Member Delta Whisky's Avatar
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    Default Re: P-factor and vibration at high angle of atack

    Dorsal - FWIW, what you are describing DID occur on my RV-6. Don't know if it translates but certainly could be your issue.

  3. #13
    Senior Member Slyfox's Avatar
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    Default Re: P-factor and vibration at high angle of atack

    ok, got some new thoughts for you. a few months ago I ran out of fuel in my header tank, aka a vent plugged. I got 4 miles out from take off and I got this very fine strong vibration that lasted a few seconds and than quit. I quickly turned around and headed back to the airfield. it did this about 3 times than it started running rough at full throttle, pull it back and it smoothed out. than the engine quit. I tried one restart and said, there's a good place to land, so I did that.
    What I'm trying to say is it could be you are running out of fuel. fuel pump or a restriction between the header tank and engine. just a thought at this time.
    steve
    slyfox
    model IV 1200-flying
    912uls
    IVO medium in-flight
    RV7A-flying
    IO-360
    constant speed prop

  4. #14
    Senior Member Dorsal's Avatar
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    Default Re: P-factor and vibration at high angle of atack

    Thanks Slyfox, good things to think about. In my case this vibration has been there from the start (510 hrs now). I had always attributed it to the floppy lower cowl or loose belly fabric but since I fixed that i'm looking for another explanation. If anyone want to run an experiment for me (preferably in a Rotax 912ULS equipped fixed pitch 3-blade ship) fly straight and level at about 5200 RPM, then WOT steepest sustainable climb (should still be around 5200 RPM). Let me know if you feel a difference in the vibration levels.
    Dorsal ~~^~~
    Series 7 - Tri-Gear
    912 ULS Warp Drive

  5. #15
    Senior Member Slyfox's Avatar
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    Default Re: P-factor and vibration at high angle of atack

    Quote Originally Posted by Dorsal View Post
    Thanks Slyfox, good things to think about. In my case this vibration has been there from the start (510 hrs now). I had always attributed it to the floppy lower cowl or loose belly fabric but since I fixed that i'm looking for another explanation. If anyone want to run an experiment for me (preferably in a Rotax 912ULS equipped fixed pitch 3-blade ship) fly straight and level at about 5200 RPM, then WOT steepest sustainable climb (should still be around 5200 RPM). Let me know if you feel a difference in the vibration levels.
    do check out for fuel starvation. the vibration I got was really awakening. another thing to check out would be the elevator. have someone hold the stick in different positions and take and grab the elevator and check for looseness through the system. than grab your elevator on one side and do a left to right or up on one side and down on the other checking for slop in the hinges. than take your horizontal and check for slop in all it's connecting spots. be wary of control flutter.
    steve
    slyfox
    model IV 1200-flying
    912uls
    IVO medium in-flight
    RV7A-flying
    IO-360
    constant speed prop

  6. #16
    Senior Member Dorsal's Avatar
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    Default Re: P-factor and vibration at high angle of atack

    Fuel flow and pressure remain stable throughout and given TAS is significantly slower in a steep climb I think it is unlikely to be related to control surfaces. This is not the kind of thing that grabs your attention, in fact you might not notice it if you weren't looking (feeling) for it.
    Dorsal ~~^~~
    Series 7 - Tri-Gear
    912 ULS Warp Drive

  7. #17
    Senior Member Slyfox's Avatar
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    Default Re: P-factor and vibration at high angle of atack

    if you got a camera, set it up at different spot on the aircraft and duplicate and watch your video. only thing left for you to do.
    steve
    slyfox
    model IV 1200-flying
    912uls
    IVO medium in-flight
    RV7A-flying
    IO-360
    constant speed prop

  8. #18

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    Default Re: P-factor and vibration at high angle of atack

    There is definitely a vibration with p-factor. As the airplane pitches up each blade goes through an AOA variation. The lift on the blade varies as it rotates. The vibration is at the N-per of the blade. So a 2-bladed prop will have a 2-per (twice per rotation vibration) and a 3-bladed prop will have a 3-per vibration (three times per vibration).

    If the prop is spinning at 2400 RPM, that would be 40 Hz, then for a 3-bladed prop it should feel like a 120 Hz buzz. If it is a lower frequency then it may be something interacting with the airframe.

    If someone near you has a DynaVibe GX (www.rpxtech.com) , it can tell you the frequency of the vibration which isolates the cause of the vibration. I fly with mine on my Kitfox III.

    Matt

  9. #19
    Senior Member Dorsal's Avatar
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    Default Re: P-factor and vibration at high angle of atack

    mdock, This is what I expect I am experiencing. I do have the original Dynavibe but I don't think that will give me frequency. I would have pegged the "buzz" between 100-200 hz.
    Dorsal ~~^~~
    Series 7 - Tri-Gear
    912 ULS Warp Drive

  10. #20

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    Default Re: P-factor and vibration at high angle of atack

    editedosted in wrong thread
    Last edited by KitKarson; 08-01-2018 at 03:55 PM.

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