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Thread: Trim Actuator

  1. #1
    Senior Member Flybyjim's Avatar
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    Default Trim Actuator

    I am very frustrated with adjusting the trim actuator and trying to get this to work. I have searched past posts and found John P. post from 2014 but my situation and his resolve do not seem to match.
    My actuator stroke is 4 inches, with the rod end all the way in at full stroke I have 3/4 inch between tubes. If I turn the rod end out (you can not turn the actuator rod to try to lengthen the rod end even if you remove the jam nut) to get the 7/16" called for in the instructions my rod end is only turned in 6 turns leaving 12 turns out of the actuator, this can not be correct. What am I doing wrong? Is 4 inches correct for the stroke of the piston.

    Jim

  2. #2
    Senior Member Flybyjim's Avatar
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    Default Re: Trim Actuator

    Anyone out there with advice?

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    Senior Member jiott's Avatar
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    Default Re: Trim Actuator

    With the 7/16" called for in the book, when I started to fly I found that I didn't have near enough nose-up trim to operate with full flaps. I also found that I had more than enough nose-down trim. So I ended up screwing the rod end farther into the actuator rod (I can't remember how many turns) to get more nose-up and of course less nose-down. The sweet spot for me ended up with the stabilizer front tube almost touching the bottom of the slots in the covers with full nose-up trim. This still requires a little back-pressure on the stick when approaching to land with full flaps, but that is OK. Still have never found a situation where I needed full nose-down trim. This was my experience on my SS7. Don't be afraid to experiment with this, you can be quite a ways off in proper trim and still compensate for it easily with stick pressure (I have been known to forget to set the trim for T/O and leave it in the nose-up landing position-quite a surprise but easily overcome with forward stick).
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  4. #4
    Senior Member PapuaPilot's Avatar
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    Default Re: Trim Actuator

    I have found the same thing to be true as Jim did. You want the stabilizer leading edge to go down as far as possible.

    Rigging this is not hard. The jack screw goes up or down until it gets to its internal stops. The stroke is ~4" and it is not adjustable. To rig the trim run the jackscrew either to the top or bottom of its travel. DO NOT turn the jackscrew by hand. Now all you need to do is adjust the length of the rod end (without spinning the jackscrew). I would do it like Jim said, get leading edge tube of the stabilizer as close as you can to the bottom of the slots in the panels.

    FYI for your first flight I would put the trim in the middle of its travel and then figure out where it needs to be for subsequent takeoffs.
    Phil Nelson
    A&P-IA, Maintenance Instructor
    KF 5 Outback, Cont. IO-240
    Flying since 2016

  5. #5
    Senior Member Flybyjim's Avatar
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    Default Re: Trim Actuator

    Okay, I can go with this but what about the number of threads that should be the minimum into the actuator?

    Thanks

  6. #6
    Senior Member jiott's Avatar
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    Default Re: Trim Actuator

    As Phil says, trim in the middle for first flights and you will be fine. My sweet spot for T/O (after rod end adjustments as described earlier) ended up being about 3/4 of the way to full nose-down. This feels good on T/O with half flaps and then after pulling off the flaps I hardly need to adjust trim for straight cruise flight, sometimes just a quick stab on the nose-down switch.

    I wish I had, but didn't install a trim position indicator. Now I don't need it because I can easily adjust trim for T/O without having to look at the stabilizer. I just hold the trim switch until it reaches full nose-down (you can see when the stick quits moving) then I press nose-up for 2 seconds (count one thousand one, one thousand two) and its in perfect T/O position.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  7. #7
    Senior Member jiott's Avatar
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    Default Re: Trim Actuator

    If you adjust it somewhere near like Phil and I suggest, I believe you will have more than enough threads engaged. You are correct though, there is a proper minimum number of thread engagement, maybe someone knows this number. I know on a female rod end there is a little hole that you can use to see if the male end is in far enough.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  8. #8
    Senior Member jiott's Avatar
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    Default Re: Trim Actuator

    I just found it on the Aurora (they make most rod ends) website: Minimum thread engagement, unless otherwise stated, is 1.5 times the major thread diameter will support the advertised max load.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  9. #9
    Senior Member PapuaPilot's Avatar
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    Default Re: Trim Actuator

    Good job Jim, I didn't have any idea how to answer that question.
    Phil Nelson
    A&P-IA, Maintenance Instructor
    KF 5 Outback, Cont. IO-240
    Flying since 2016

  10. #10
    Senior Member Flybyjim's Avatar
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    Default Re: Trim Actuator

    Good info to know. So if I understand this: If the dia. of the threaded rod end is 1/2 inch then the minimum length you need to thread the shaft into the socket is
    1/2 plus 1/4 equals 3/4 inch of thread.
    You fellows are of great help, much experience in this group.

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