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Patrol clip lock turnbuckles

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  • Patrol clip lock turnbuckles

    I noticed a general preference for the clip lock turnbuckles. There is plenty of info how to do the substitution for the 4P but not for the Patrol.

    This is the list I have for that standard safety wire turnbuckles:


    I then followed the ACS guide for substitution and came up with:


    The issue I ran into is with the min braking strength. The Patrol requires 2200 lb apart from the trim cable. The specs for the clip-lock components are either 1600 lb or 3200 lb. Is there any reason the 3200 lb components would not work? Is there enough space for slightly bigger turnbuckles in the Patrol? How about the forks? Do the size 5 forks (as used in the 4P) work?
    Last edited by noema; 01-11-2021, 01:29 AM. Reason: typo
    Bearhawk Patrol, O-360, Trailblazer 80", 26" Goodyear, Stewart Systems

  • #2
    Its not clear what your concern is, but i thought I might share my view of trim cable strength in the Patrol.

    I never considered the breaking strength of the trim cable when I installed it. I merely used the 1/16" 7x7 cable that is called out in the plans. I then chose to use the MS21251 barrel and there associated accessories to make up a turnbuckle assembly so that I could use the clips to avoid safety wiring them. I didn't know that the trunbuckles with the clips sacrifice strength, but I think the most force I could ever apply to the trim handle might be 10% of its weakest part. A design goal seems to factor in a degradation in strength of the trim cable after some of the 7x7 cable strands break due to wear and tear over time.

    So, the weakest link in the trim cable is me....until the cable wear through. No matter what, its never going to be the turnbuckle. It seems to me you should feel free to choose what ever turnbuckle you want.
    Brooks Cone
    Southeast Michigan
    Patrol #303, Kit build


    • #3
      Hi Brooks, thanks for the input. Great to have you here. I am working off your Patrol hardware list and post.

      Looking at your turnbuckle parts list, all parts are rated 3200 lb but the fork ends. Is there any obvious reason that going to the next bigger fork would not work on the Patrol? Basically replacing MS21252-4RS with a size 5 fork end (MS21252-5RS or the left-threaded one in my case)? If it still fits, that would be my personal preference even though you are probably right that's not the weak link in the system.

      MS21251-B5S (5) Barrel
      MS21252-4RS (4) Fork End
      MS21256-1 (10) Clip
      MS21255-5LS (2) cable eye left thread
      MS21255-5RS (2) cable eye right thread
      Bearhawk Patrol, O-360, Trailblazer 80", 26" Goodyear, Stewart Systems


      • #4
        Once, when finishing my FlyBaby, I tested 1/8" cables to failure. They all broke around 2,000 lbs while trying different variations of cable manufacturers and Nico press technique. In the FlyBaby - these cables hold the wings on. The 1/16" trim cable is never going to make even the 2,000 lbs. Mark


        • #5
          1/8" cable is only around 1900 or 2k, depending on type. 1/16" 7x7 is only 480 lbs. Any strength above that for terminal parts is just extra weight. The best cable end terminals are simply rated for the max strength of cable. Swaged or compression type.

          To save some weight is seems Bob used 3/16" holes to connect some control cables. I think that is why the assortment of bushings you need to bush stuff up or down from 3/16" to 1/4". The 3/16" saved some weight, but most of the terminals for 1/8 cable are 1/4".


          • #6
            I understand, if I go with the bigger fork ends I also need to match the 3/16 holes on the bellcrank with the 1/4 holes of the fork ends.

            Ok, here is a wild suspicion ... ACS just spec'ed the size 4 parts incorrectly in the catalog.

            The logic for substituting the turnbuckles seems to be:
            size 16 -> size 3 (1600 lb)
            size 22 -> size 4 (2200 lb)
            size 32 -> size 5 (3200 lb)

            For example:
            AN161-16RS -> MS21252-3RS
            AN161-22RS -> MS21252-4RS
            AN161-32RS -> MS21252-5RS

            ACS page 136:


            For reference, size 4 fork ends are listed with 2200 lb in other places (which is what you would expect):



            I sent an email to ACS to clarify.

            Bearhawk Patrol, O-360, Trailblazer 80", 26" Goodyear, Stewart Systems


            • #7
              Originally posted by noema View Post
              Looking at your turnbuckle parts list, all parts are rated 3200 lb but the fork ends. Is there any obvious reason that going to the next bigger fork would not work on the Patrol?
              Building an airplane takes a lot of energy. The Energy we have is not unlimited, varies from one person to another, and sometimes it runs out without any warning, like an injury or death.

              Some builders are efficient, others, like me, are not. If you run out of energy you will fail. I want you to succeed. This hardware issue is insignificant but consumes energy. My advice is don’t sweat the small stuff, build it IAW the plan and move on. Known consequences exist if you don’t includes weight gain. Unknown consequences may exist that I don’t know of.

              I think it’s likely that if you go the next larger fork, things may not fit and require changes that may compromise the design or take time and energy to fix. Will the next larger fork fit through the fitting that holds the fairlead in place?

              I built my trim cables IAW the plan and used the turnbuckles that are designed for that small cable and they fit and meet the design criteria. Bigger is not better, bigger is a compromise. 100 compromises (One that is justified as an improvements) might add up to 100 pounds and will effect real world capabilities, performance, Efficiency and safety.
              Last edited by Bcone1381; 01-11-2021, 12:30 PM.
              Brooks Cone
              Southeast Michigan
              Patrol #303, Kit build


              • noema
                noema commented
                Editing a comment
                Right, I know. I was happily ordering some things. The next moment I am shaving a yak knee deep in specs of obscure parts. Why is this always happening. For decades 😊
                Last edited by noema; 01-11-2021, 03:08 PM.

            • #8
              If you are concerned about some of the attach holes being 1/4" and others 3/16" - you get with the kit and hardware package some 1/4" x .028 tubing. This is for bushings for exactly this type of situation. Also 5/16 x .028, and 3/8 x .028 tubing is shipped with the hardware package. That solves this type pf problem in a few places on the planes. Mark


              • noema
                noema commented
                Editing a comment
                Sounds good.

            • #9
              I was thinking about a test-to-failure video that related to this when I was reading the posts, and in finding it I realised it’s not about turnbuckles but testing the rod ends and composite ends of flying wires.
              Still, it’s good info if you’re into the analytical end and you are done building for the day.
              Through engineering, designs are created to allow you to not have to design/engineer/test it. Not to say there isn’t alternative methods, but as mentioned above, staying on course can mean accepting the plans as printed.

              Now, watch this video and write down your guesses as what order the components will fail before the actual testing. It’s a good education about how strong some components are.


              • noema
                noema commented
                Editing a comment
                Mike Patey's videos always make me ... hmm maybe I should build this out of carbon fiber