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Thread: Tailwheel Spring Clamp Failure

  1. #21
    Senior Member HighWing's Avatar
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    Default Re: Tailwheel Spring Clamp Failure

    I have read all the posts and lots of good information. One thing that sort of mystifies me, though, is the side load necessary to shear a bolt on a tailwheel spring mount. The typical tailwheel has a point where side loads would unlock the tail wheel and allow for full swivel. I am wondering if the release mechanism might be defective or in need of adjustment.

    Then one question. Was the landing while flying Hell,s Canyon at Doug Bar by chance. I have been in there once during one of our our flights of 6.
    Lowell Fitt
    Goodyear, AZ


    My You Tube Channel

  2. #22
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    Default Re: Tailwheel Spring Clamp Failure

    I bet if you hit a rock or something like that in a tight turn. Even with the wheel unlocked, that tail spring will act just like a bolt cutter and shear the
    bolt off. That's my guess.

    Jeff

  3. #23
    Senior Member AirFox's Avatar
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    Default Re: Tailwheel Spring Clamp Failure

    Hello Lowell,

    I did a wheel landing at Dug bar, with no unusual bumps. I'm not sure exactly when the bolt snapped, but I'm thinking it snapped when I was taxiing over to park at Dug Bar. The ground at Dug bar was dried out and there were dried ruts in the runway and taxi area. After inspecting the bolts when I disassembled the spring I noticed the grip on the AN3-12a was not long enough to clear the top mounting plate. Right at the shoulder of the grip was shinny due to a small amount of rubbing. Just into the thread of the bolt is where the bolt sheered off. With only 25 in-lb torque on the bolts there is not enough compression to squeeze the 3 springs together completely. That spring play is where you get the movement right at the shoulder of the grip on the AN-3-12A. At a minimum to fix the issue you need a bolt with a longer grip AN3-13A. A better fix in my opinion is to replace the bolt with an AN4 along with a block between the plates to distribute the load between the plates. The new design from Kitfox does exactly that. The bottom and top bracket that holds the spring is completely mated and held together with AN4 bolts. I have it installed now and used it for about 8 landings. The grove spring and mounting setup is much stronger. I am much more confident the new spring setup I have will last although I am going to carry and extra set of bolts in the back country. Since this experience I've foand that it is common for pilots to carry extra tail wheel bolts when landing ln the Back Country. Several people that I have talked to even change the tail wheel bolts out every annual.

  4. #24
    Senior Member HighWing's Avatar
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    Default Re: Tailwheel Spring Clamp Failure

    For those interested - and I hope this is permissible on the forum - the following link is to a video I took on our landing at Doug Bar. As you can see, near the end of the runway, the ground toward the left begins sloping toward the river. I did what the yellow Kitfox did and spun my airplane with tail pointing toward the river and the slope was sufficient that I couldn't exit the airplane without someone holding it to keep it from backing down the slope. The main reason I now have a parking brake. http://highwingllc.com/videoarchivepages/dugbar.html
    Lowell Fitt
    Goodyear, AZ


    My You Tube Channel

  5. #25

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    Default Re: Tailwheel Spring Clamp Failure

    Quote Originally Posted by n85ae View Post
    My support plate is .125 4130, and my bolts AN-4 it fit together just as well
    with that hardware as with the AN-3's so to my mind, the AN-3 debate is
    silly, and pointless. Use AN-4's.

    Jeff
    When you are replace the AN-3's with AN-4's are you forgoing the steel sheath? I think this is a worthwhile mod. I'd like to have a machined block of aluminum to fit the spring in place of the plate but I don't have those kind of resources. Anyone want to make one for me?
    Mark Sprague
    914 Series 6 - flying

  6. #26
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    Default Re: Tailwheel Spring Clamp Failure

    No I don't use the bushings. The thick 4130 plate doesn't bend, and I tighten the bolts evenly so the clamping force is uniform. I would wager I would do
    airframe damage before I would shear the bolts in a bad landing.

    Have had it this way for a long time now, and no problems, and better yet
    no worries .... AN-3's make no sense back there.

    I look at a lot of REALLY nicely engineered solutions to the problem, and
    I have to admit they look really good. BUT I don't think they provide any
    better solution, than simply using AN-4's and a thicker plate ...

    Jeff

  7. #27

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    Default Re: Tailwheel Spring Clamp Failure

    My bushings are 5/16". Is there any reason no to step up to AN-5 hardware? I'm afraid if I go with AN-4 bolts that there will be room for the spring to slide around, which I think would be bad.
    Mark Sprague
    914 Series 6 - flying

  8. #28
    Senior Member jiott's Avatar
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    Default Re: Tailwheel Spring Clamp Failure

    When I went to the AN-4 bolts there was almost no room for a bushing wall thickness, but I machined a couple of blocks instead of bushings, with the bolt hole very close to the edge.

    I think if you went to AN-5 bolts there would not be enough room for the spring width.

    The best solution I believe is to machine a 1-piece block that replaces the lower plate and also acts as bushing and spring locator. Then any extreme side load will have to shear off both AN-4 bolts at the same time, making this arrangement twice as strong for side loads as just replacing AN-3 with AN-4 bolts. This is what Grove does with their solid aluminum spring mount kit, which I eventually installed.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  9. #29

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    Default Re: Tailwheel Spring Clamp Failure

    Quote Originally Posted by jiott View Post
    When I went to the AN-4 bolts there was almost no room for a bushing wall thickness, but I machined a couple of blocks instead of bushings, with the bolt hole very close to the edge.

    I think if you went to AN-5 bolts there would not be enough room for the spring width.

    The best solution I believe is to machine a 1-piece block that replaces the lower plate and also acts as bushing and spring locator. Then any extreme side load will have to shear off both AN-4 bolts at the same time, making this arrangement twice as strong for side loads as just replacing AN-3 with AN-4 bolts. This is what Grove does with their solid aluminum spring mount kit, which I eventually installed.
    When you upgraded to the Grove aluminum spring did you have any issues with the drilled out holes on the airframe? That is one thing that is keeping me from doing this mod. Also, I have the older original one piece aluminum spring. That may be why I would have more room for AN-5 bolts but then I am going even bigger on the drilled out holes on the airframe. My concern is that the bushings are 5/16" and if I replace the AN-3 bolts with AN-4 there will be slop in the mount allowing the tailwheel to shift slightly. I don't currently have a way to machine a block which would be ideal.
    Mark Sprague
    914 Series 6 - flying

  10. #30
    Senior Member jiott's Avatar
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    Default Re: Tailwheel Spring Clamp Failure

    Mark, no I didn't have any issues with the airframe holes. The Grove mounting kit fits the original (SS7) airframe holes perfectly. In fact their kit continues to use AN-3 bolts, and I haven't heard of any problems with this, probably because their setup uses the shear strength of two bolts rather than one in a side load impact. Since I had earlier changed to AN-4 bolts my airframe holes were enlarged to fit the bigger bolt. When I got the Grove kit it was a simple matter to drill out the holes in the new Grove bracket to fit AN-4. So my tailwheel spring setup now is Really strong.

    If you stay with the original spring and just go with AN-4 bolts, I would still put a bushing around the bolts and just file/grind a flat spot on one side of the bushing to fit up snug against the side of the spring. In fact a large sturdy bushing, round or square, filed on one side to fit, is in my opinion a real good way to go because it give more stability and support for the clamping plate on the bottom. I believe Floog did this with good results.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

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