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  • Access to trim horns

    A question for the group:

    Over the years my trim horns have become flogged out (the hole at the end of the horn), and there is now about 3/64" clearance around the bolts. I don't have access into the horizontal stabiliser for those trim horns which bolt on, I never foresaw a need to maintain them.

    3/64" is a lot of play when you see it firsthand (!) and sloppy trim is not good. It's worsening over time. I need a resolution.

    I have spent about 12 months racking my brains for ways to resolve this issue, while avoiding a major re-design of the trim drive and avoiding recovering the h-stab with new fabric. Suggestions welcome!

    At this stage, all I can think of is installing some new access holes. I can't see any way of repairing the horns without removing them.

    Next question, what provision have other builders included for access to those trim horns?

    - I thought about cutting a hole and trying to glue a ring into the inside of the fabric. From experience, cutting new holes in old fabric can be fraught.
    - I thought about doing the same, but gluing the ring to the outside prior to cutting to reduce the risk of unintended consequences. This would be hidden under the cover plate once complete.

    Both these options leave me wondering about stones hitting the cover plate / access hole and causing damage. Particularly given it wouldn't be as strong as possible, given I am installing it after the fact.

    All suggestions appreciated.
    Last edited by Battson; 10-19-2020, 04:48 PM. Reason: Clarify which hole is flogged out.

  • #2
    I don't suppose you can up-size the bolts to 1/4?

    Comment


    • Battson
      Battson commented
      Editing a comment
      I think the edge distance would be marginal. The part would look very weak, at least.

  • #3
    1: Drill out and press fit a bushing to resize the hole. Then use rod end bearings on both ends of the pushrods.

    2: Weld the holes closed then re-drill. Drilling is much harder because if the HAZ.

    I’m not sure how other paint systems would hold up to the heat but Stewart’s is tough and has held up to the heat the couple times I’ve had to weld next to it.

    Rod end bearing seem like a good idea.
    Scratch Built 4-place Bearhawk. Continental IO-360, 88” C203 McCauley prop.

    Comment


    • Battson
      Battson commented
      Editing a comment
      Press-fitting a bushing could be a possibility, I wonder what the closest stock rod size would be.

      Polyfiber melts like plastic when heated :/

  • #4
    Good point Battson - I've just installed these outboard trim arms, and took your suggestion of making sure the smooth part of the shank clears the holes, to prevent flogging out. I also used a MS27039-1-17 machine screw, so the head would clear the torque arm at this shallow socket. I don't think you could drill the hole for a 1/4" bolt in situ without breaking out either the socket or the torque tube.

    Now, it's not a solution, but in the way of prevention, do you think I should also bond my torque arms on with some hysol structural adhesive? To my mind, the torque forces would be carried by the adhesive, and the bolt would only come in to play once the adhesive has failed.

    You could ream and put in an oversize hi-lok, with a shear collar so you don't collapse the tube? Maybe hilok+hysol (a winning combination, surely!).

    Don't know what to do about access, sorry mate. If you had to cut it open, I'd be going for an all-new torque tube assy.

    James

    PS - just saw Whee's suggestion of welding - we use one of those fibreglass blankets at work, it's surprising how well they protect stuff around the area.
    Last edited by James; 10-18-2020, 10:22 PM.
    The Barrows Bearhawk: Who knew my wife could get jealous of a plane?

    Comment


    • #5
      Originally posted by James View Post
      Now, it's not a solution, but in the way of prevention, do you think I should also bond my torque arms on with some hysol structural adhesive? To my mind, the torque forces would be carried by the adhesive, and the bolt would only come in to play once the adhesive has failed.
      Are we talking about the same end of the horn?
      Because the trim pushrod moves relative to the drive horn as the elevator travels up and down, you can't use adhesive.

      I think the best prevention might be either using a rod end bearing as Jon suggested, or welding a doubler to the end of the drive arm, so its meatier.

      Comment


      • #6
        Ah, sorry Chief - I thought you were talking about the pivot end, not the swing end!

        There's a fair amount of meat there, you might double and rebush if you needed to. Was there much flog in the pushrod clevis holes, or just in the torque arm hole?

        With the rod ends at the aft end of the pushrod, I couldn't bring myself to just attach it with an AN3 bolt - I sandwiched mine on with penny washes, in case the bearing fails.

        James
        The Barrows Bearhawk: Who knew my wife could get jealous of a plane?

        Comment


        • #7
          Originally posted by James View Post
          Was there much flog in the pushrod clevis holes, or just in the torque arm hole?
          It was practically only the torque arm end.

          Comment


          • #8
            I don't see a solution that saves the fabric. If you only went in from the bottom, cosmetics might not be a problem other than you know it isn't pretty. I have held off covering the H stabs because of your previous post about this. Excess play isn't good, and can easily cause flutter.

            I think the tubing doubler, rosette welded to the main tube, or welding in a compression tube into the torque tube.

            Is the hole in the actuating arm also elongated, or just the torque tube?

            Comment


            • #9
              About 10 years ago I had the same wallowing out of hole in the "arm" of the trim system that extends down below the hor stab. A friend TIG welded up the hole and we re drilled. Bob advised me then to TIGHTEN the nut that attaches the trim pushrod to the arm. Not to full torque but pretty snug. It has lasted fine since then with the hole staying in good shape.

              It is also important that the AN3 bolt that attaches the top of this part to the 1/2" tube that runs inboard inside the hor stab also be tight.

              In your case Jonathan - I would glue an inspection ring to the outside. Then cut out the inside once the glue had dried. Then you can remove that part and re weld the hole. Then glue another inspection "ring" on the inside of the fabric and use a "high speed inspection plate" there.

              Bob has drawn that trim pushrod two different ways. One with MM3 rod end bearings on BOTH ends. And another design with the forked .050 forward end. I kind of like the rod end bearings on both ends but since Bob has drawn that part the other way in most places, we have tried to have our parts match the plans. Certainly a rod end bearing on the front can be tightly bolted to the sheet metal arm. That way the vibration back there (which is what causes this problem) is minimized as the parts are tightly held together.

              Those of you flying as well as Jonathan - please let us know your thoughts on what you have seen on your planes. Mark

              Comment


              • James
                James commented
                Editing a comment
                Mark - when you say you do this bolt up "snug", I assume that means you have a rod end at the front of the pushrod as well?
                Or else you collapse the pushrod clevis fork?

                James

              • Mark Goldberg
                Mark Goldberg commented
                Editing a comment
                Some kits have had trim pushrods with rod end bearings at both ends. But most have been like Bob shows on the plans with a fork on the forward end of the trim pushrod. Mark

              • James
                James commented
                Editing a comment
                Yeah, righto Mark - so "snug" for both cases. Thanks

            • #10
              I am a little confused. Is the worn out hole Jon is talking about, the hole in the actuating horn, or the hole that goes through the torque tube and the tube portion of the actuating arm?

              Comment


              • #11
                Originally posted by svyolo View Post
                I am a little confused. Is the worn out hole Jon is talking about, the hole in the actuating horn, or the hole that goes through the torque tube and the tube portion of the actuating arm?
                Sorry - it's the hole at the end of the arm, not the one through the torque tube.

                Comment


                • #12
                  Not an ideal solution and it wouldn’t be fun: Open the hole just enough to remove the arm from the torque tube. Repair or replace the arm. Use a glue on cable fairing to cover the hole.
                  Scratch Built 4-place Bearhawk. Continental IO-360, 88” C203 McCauley prop.

                  Comment


                  • #13
                    Maybe go up to the next metric size. 3/16" is 4.75 mm. Would 5mm make the hole round enough, or at least better? 6mm is still smaller than 1/4". Or a very thin walled steel bushing on the 3/16" bolt?

                    Comment


                    • svyolo
                      svyolo commented
                      Editing a comment
                      Going forward for the rest of us still building or without a lot of hours, I wonder if a bronze fastener or shear pin would wear better than the steel on steel. It might not be a whole lot of fun to assemble and disassemble with the roll pin. I can see punching a hole in the fabric, but that is just me!
                      Last edited by svyolo; 10-20-2020, 04:48 PM.

                    • James
                      James commented
                      Editing a comment
                      What about a 3/16 roll pin, with a loop of lockwire through it so it can't fall out? With the rod end version of the pushrod, just torque the bolt down like Mark says, but with the fork version, I'm thinking we want something as tight as possible jammed into the torque arm, so that it's the pushrod holes that wear.

                    • svyolo
                      svyolo commented
                      Editing a comment
                      The rod end might be the way to go I think.

                  • #14
                    I’m right at this point and apologies for some thread drift in advance - but I’d like to ask a question for my own benefit.
                    Battson, thanks for the heads up, I’ve installed some access ports as a result, albeit from the outside rather than inside. It seemed to have worked ok.
                    My plans clearly show a rod end bearing at both ends of the trim linkage. How ever the linkage I’ve got only has the rod end at the rear, the front needs a bolt or clevis pin.
                    My question is: is a clevis pin acceptable? It will give a little more clearance and as it’s in the airstream, would be slightly less draggy. Would you be doing this differently knowing the issues above ?

                    C3FDB7FC-0E09-409B-ACDF-A0B01750A8ED.jpeg
                    Nev Bailey
                    Christchurch, NZ
                    Builders-log

                    Comment


                    • Mark Goldberg
                      Mark Goldberg commented
                      Editing a comment
                      Nev - you need a bolt and nut so you can tighten the nut up. If not it will vibrate and enlarge the holes in the sheet metal piece sticking down from the hor stab. Mark

                    • Battson
                      Battson commented
                      Editing a comment
                      Looks to me like that plate is going to rub against the bolt head or nut, and make a smokey mess.
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