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March 2019 Elevator Trim Cable Tension

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  • March 2019 Elevator Trim Cable Tension

    March 2019 Elevator Trim Cable Tension

    ENGINEERING CHANGE NOTICE: Design change that requires immediate attention to affected serial numbers. This NOTICE contains the technical information to correct the unsafe condition.
    Applies to all three types, Bearhawk 4-Place, Bearhawk Patrol, and Bearhawk LSA.

    Check the tension in the 1/16 cable used for the trim system. Cable tension should be approximately 60 pounds. If too loose the airplane will be unstable in pitch trim.

  • #2
    I spoke with Bob on the phone about this today, and he provided some clarity. If the tension is too light, the trim tab will be able to move uncommanded in flight. This can create adverse handling characteristics with regard to the servo action of the tab. The minimum tension specification is intended to create sufficient friction to keep the trim wheel fixed in position when a change is not commanded by the pilot.

    Comment


    • #3
      Also, Bob says a good range is 50-70 pounds .

      Comment


      • davzLSA
        davzLSA commented
        Editing a comment
        would a friction lock on the trim wheel or lever work?

      • jaredyates
        jaredyates commented
        Editing a comment
        Tightening the cable works, do you have reason to not tighten the cable?

      • robcaldwell
        robcaldwell commented
        Editing a comment
        I think a friction lock would be in addition to tightening the cable. It seems to me that if you apply a friction lock without tightening the cable, you would have a locked wheel with a continued wobbly trim tab.

    • #4
      I have a lot of "wobble" in the trim tab because of the fit of the parts in the tail, specifically the trim pushrods where they meet the trim horns in the horizontal stabilisers.
      At the tab trailing edge, they can wobble about 3/16" at most.
      I have not yet experienced any negative consequences from this situation. That said, I can't physically do high speed dives with the dragwheels (bushwheels) attached.

      Comment


      • Mark Goldberg
        Mark Goldberg commented
        Editing a comment
        The bolt that attaches the trim pushrod to the little tab( coming out the bottom of the hor stab) has to be pretty tight but not so tight that it does not allow movement. If it is too loose, then the vibration starts to wear the hole. I had this happen and fixed the hole. After I learned the bolt needed to be tighter I have had better luck. Mark

    • #5
      Responding to Marks comment......
      Below is a screen shot of the Patrol Trim linkage. I think my Patrol Kit came with a 10-32 threaded end on each end of the trim tab push rod. So, I think my assembly has a Rod End Bearing on each and and allows me to fully tighten the nut at the tab under the H-Stab. Does that sound right?

      Second, it would seem to me that any uncommanded movement of the elevator trim tab would be indicated (seen?) by the movement of the trim wheel/lever. So, Can I test the system for proper function be moving the elevator to full travel limits, and watching the trim lever/wheel for any movement? Maybe this could be included when doing the pre-takeoff Control Check by keeping an eye on the trim handle for movement....but I agree its not a substitute for setting the trim cable tension IAW the engineering notice.

      Third, is there a way to measuring cable tension without a $200 to $800 tool?


      Screen Shot 2019-04-01 at 10.43.50 AM.png
      Attached Files
      Brooks Cone
      Southeast Michigan
      Patrol #303, Kit build

      Comment


      • Mark Goldberg
        Mark Goldberg commented
        Editing a comment
        If you have a rod end bearing at each end - you can tighten down the bolt/nut well and not worry. It is the forked end (as called out on the plans) that can vibrate and enlarge the hole. Mark

      • Archer39J
        Archer39J commented
        Editing a comment
        I recall Bob outlining a method to measure tension without the expensive tool. I think it was in a beartracks, I'll post it here if I find it.

      • jaredyates
        jaredyates commented
        Editing a comment
        Bob does not own a tensionometer. He calibrates his feel by hanging weights from a piece of cable. That and from having built a dozen airplanes over the last 50 years or so. Personally, I'm not there yet, so I'll stick to the measuring device!

    • #6
      Originally posted by Mark Goldberg View Post
      The bolt that attaches the trim pushrod to the little tab( coming out the bottom of the hor stab) has to be pretty tight but not so tight that it does not allow movement. If it is too loose, then the vibration starts to wear the hole. I had this happen and fixed the hole. After I learned the bolt needed to be tighter I have had better luck. Mark
      How did you fix the hole, Mark?

      Comment


      • Mark Goldberg
        Mark Goldberg commented
        Editing a comment
        My friend put some weld bead in the hole where it was elongated. Then re drilled. With fabric on the hor stab. Has to be done carefully and using the proper techniques needed to protect the fabric. Mark

    • #7
      Just wondering if everyone has installed the bronze flat washers on the trim wheel install as shown on dwg. 22. On my trim wheel with the washers and the tightened AN4 bolt/castellated nut combined with the cable tension there seems to be more than enough resistance to keep the trim tab as set.

      Dave Plans 865 QB 77 Awaiting final inspection..

      Comment


      • Archer39J
        Archer39J commented
        Editing a comment
        I did, but I couldn’t find a reduced OD bronze washer so I ordered 100 standard ones from McMaster. The standard OD interfered with the trim wheel chain. Not having a lathe I chucked a couple up in my drill press and had at them with a file... I wouldn’t recommend that. I did manage to get two usable ones out of it fortunately. They serve as a self lubricating wear surface, I don’t think running without them is recommended.
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