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Thread: Kitfox 3 Aileron flutter revisited my 2 cents.

  1. #1

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    paul
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    Default Kitfox 3 Aileron flutter revisited my 2 cents.

    Hi All, this is my take observations and comments on earlier Kitfox (1-3) with one mass bass balance and flat bottomed flap/aileron!
    Here goes in point form.

    1. I have a about 100 hours on a slick Kitfox Mk3 that I that painstakingly built over a few years. I'm not shy is saying its well put together and a lot of care and thought went into its construction. The plane is AWESOME to fly I love it!
    2. I'm well aware it has a larger than normal engine in my R2800 radial. I'm ok with that and respectful of useful load and VNE
    3. On my very first test flight during my first turns the ailerons felt very hard, I knew this was not right. I looked down at the flap trim handle and noticed it had pulled itself all the way forward causing the flap/aileron to go reflexed (negative AOA).
    4. As soon as I put the flaps back neutral the plane again rolled properly and with the correct aileron feel, problem solved. But when letting the handle go the flaps would, due to aerodynamic loads, slowly pull back to reflexed again. I completed the test flight holding the flap handle in place manually.
    5. On landing I added more tension to the flap handle friction washer. Next flight, same thing.
    6. I decided to make a positive locking and staged flap mechanism, next flight problem solved. No more flap handle wandering.
    7. Fast forward 100hrs to this week and I had a good fright! During perfectly calm conditions, dead calm, after a touch and go I was on down wind, I was doing only 70 kts, then a sudden THUMP and the plane started shaking (***!!!!) I thought engine / prop issue so pulled the power, still the plane was shaking. Not the engine or prop.
    8. I looked out the side window and I could see that the aileron was fluttering. Ok why, its never done this in the past? Scanning the controls I saw that the flap handle had jumped out of its notch and had gone fully reflexed (negative AOA).
    9. I pulled the flap handle back to the correct notch and the flutter instantly stopped. This all only lasted seconds say 10 max.
    10. I landed without issue and after inspecting the plane, I found it was fine.
    11. I have checked all the lash in the controls and it was as I built it, solid.
    12. I made a new flap locking notch plate that is near fool proof and cant jump out.
    13. NOTE: I have a separate elevator trim, I do not use the flaps for pitch trim.

    Conclusions / Opinions:
    14. I do not think that sloppy linkages should allow flutter to occur in a well designed air frame. This is an aerodynamic issue.
    15. Later Kitfox have more that one mass balance along the length of each aileron. I believe this is related.
    16. Later Kitfox have a much thicker flap/aileron airfoil and resultant heavier torque tube down the length of the aileron. I believe this is related.
    17. Flying an early kitfox (1-3) with the flaps reflexed and inducing a "stick slap" (sudden movement) can induce an aileron flutter.
    18. This is a design issue that can be managed but not eliminated.
    19. Earlier Kitfox have flap/aileron that are thinner in section, compared to the later models. With one large mass balance at the outer length. I speculate in conditions already mentioned above that the aileron can twist and get out of phase with the one larger mass balance and the flutter begins.
    20. I speculate that later model Kitfox (4-7) rectified this, likely, design issue by making the flap/ailerons stiffer in construction, with a larger torque tube, and thicker airfoil, having more than that one mass balance and a revised (better) control system.

    See my plane here: https://youtu.be/2lrEZkFqktc

  2. #2
    Senior Member Maverick's Avatar
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    Default Re: Kitfox 3 Aileron flutter revisited my 2 cents.

    From an EAA article: Static balance - A condition that exists when an object remains stationary while supported on, or suspended from its own center of gravity. It also means balancing a control surface while it is at rest (not in flight). The dynamic balance of aircraft surfaces is similarly effective but homebuilders really have no practical way of working out the dynamic balance of a control surface subjected to the stresses of flight. For this reason, they must fall back on what might be called a `good practices concept' and assume that, for all practical purposes, when a control surface is properly mass balanced in its static condition, it should also be in dynamic balance. If, in principle, the main objective of dynamic balance is to prevent or minimize torsional stress in flight, we can accomplish this adequately by evenly distributing the mass balance weight along the span of the control surface.

    If you only have one weight I doubt that it evenly distributes weight along the span of the control surface. I speculate that the later model Kitfoxes have two weights per flapperon for that reason, to more evenly distribute the weight. I wonder if statically balancing your flapperons with more than one weight would solve the problem?
    Maverick
    EAA, AOPA
    KF5 (N49FK & N36KJ)
    Phoenix, AZ

  3. #3
    Super Moderator Av8r3400's Avatar
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    Default Re: Kitfox 3 Aileron flutter revisited my 2 cents.

    You probably looked at this already, but how snug are the bearings where the torque tube passes through the turtle deck? On the 1-3 models (like an Avid) they must not allow any play.
    Av8r3400
    Kitfox Model IV
    The Mangy Fox
    912UL 105hp Zipper
    YouTube Videos

  4. #4

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    Default Re: Kitfox 3 Aileron flutter revisited my 2 cents.

    I think this happened to me today, I will check everything you mentioned.

  5. #5
    Super Moderator Av8r3400's Avatar
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    Default Re: Kitfox 3 Aileron flutter revisited my 2 cents.

    Quote Originally Posted by Bigshooter View Post
    I think this happened to me today, I will check everything you mentioned.
    This is the Achilles Heel of the 1-3 Kitfoxes and all Avids: The extended rib tails used as flapperon hangers.

    IMO, all of your rib tails need to be inspected and repaired. (Clad with metal on both sides). This looks like a very poor repair that was done previously. (Your photos on Facebook)


    The flutter may have broken the rib tails or the poor repair to the rib tails may be the source. Either way both wings need to be inspected, the bushings at the turtle deck and repaired and all of the mixer system should be closely inspected.
    Av8r3400
    Kitfox Model IV
    The Mangy Fox
    912UL 105hp Zipper
    YouTube Videos

  6. #6

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    Default Re: Kitfox 3 Aileron flutter revisited my 2 cents.

    Quote Originally Posted by Av8r3400 View Post
    You probably looked at this already, but how snug are the bearings where the torque tube passes through the turtle deck? On the 1-3 models (like an Avid) they must not allow any play.
    Do you have a picture of this bearing? I only have bushings on each of the 5 wing tips.

  7. #7

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    Default Re: Kitfox 3 Aileron flutter revisited my 2 cents.

    Here is my flaperon near the fusalage.
    20230730_202532079_iOS.jpg

  8. #8
    Senior Member
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    Default Re: Kitfox 3 Aileron flutter revisited my 2 cents.

    What is happening to the fabric on your wing? JImChuk

  9. #9

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    Default Re: Kitfox 3 Aileron flutter revisited my 2 cents.

    I cut it open to inspect what was done underneath.

  10. #10
    Senior Member Av8r_Sed's Avatar
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    Default Re: Kitfox 3 Aileron flutter revisited my 2 cents.

    Where is the lower bearing supporting the flaperon control horn? In a Model III there should be a lower support attached to the fuselage and an upper half attached to the turtledeck.
    -- Paul S
    Model III SN910
    582 IVO Med

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