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  • #16

    I can see I need to revisit this too. I messed with McMaster Carr today. They have no spring that is 2.5” relaxed, 1 inch in diameter that will stretch to 6”.

    A smaller diameter spring is the only option.

    McMaster springs data shows that a springs wire diameter is a key design component. A smaller wire with more windings will give the spring more reach. Smaller wire will also decrease the springs Max load. So a spring wound with .075” wire will stretch farther than my spring with .105 “ wire but it’s Max load is less.

    I will show some Spring data from MacMaster for a free length of 2.5” And 3” springs that will reach over 6 inches.

    Spring Data is relaxed length followed with spring outside diameter, wire diameter, Max length, Max load, type of steel.

    2.5” long, .500” OD, .045” wire, 7.44” max, 9.50 lb load, zinc plated music wire. https://www.mcmaster.com/9432k69
    3.0” long, .625” OD, .062” wire, 7.27” Max, 20.0 lb load, zinc plated music wire. https://www.mcmaster.com/9432K119/
    3.0” long, .750” OD, .075” wire, 6.00” max, 22.9 lb load, black oxide music wire. https://www.mcmaster.com/3114T114/
    3.0” long, .750” OD, .075” wire, 6.78” max, 27.3 lb load, zinc plated music wire. https://www.mcmaster.com/9432k124

    The most expensive option above was about $12 for 3 springs. Watch out for shipping charges at McMaster.

    If a 3 inch spring is a bit long, then a bolt and fender washer works pretty good so it won’t sneak off the rudder pedal.


    Brooks Cone
    Southeast Michigan
    Patrol #303, Kit build

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    • #17
      I tried out this spring on my Patrol. https://www.mcmaster.com/9432K119/
      My rudder moves the spring attach point from 3 inches to 5 inches. from stop to stop. It fits perfect! Its way better than the home fabricated .105 wire spring I had. There is no way this spring comes close to reaching its max limit.

      Engineering theory of springs IAW McMaster/Carr says my spring has a "rate" of the force is 4.3 lbs/inch. Rate is kind of like stiffness, or tension...its how much force it takes to move the spring an inch.. If we had a slinky, its rate might be .08 lb./inch.

      On post #15.3 Jared refers to a very good article that compares spring rate with yaw stability. The author said a rudder spring with a 5 lb rate improved the stiffness and autopilot compared with a 2 lb rate spring.

      My spring has a 4.3 lb./inch rate. This spring has a 6.7 lb./inch rate. https://www.mcmaster.com/9432K121/
      Because its higher, the rudder will stiffen up a bit, and this may give some passive directional stability IAW the Bearhawktracks article.

      I am sending my spring to Bob along with the engineering data of both springs I mentioned above.

      So, I love the new Spring and am curious what Bob will say.
      Brooks Cone
      Southeast Michigan
      Patrol #303, Kit build

      Comment


      • #18
        Hey Brooks, did you ever get any feedback from Bob regarding your spring selection? Just starting to work on this... Cheers!

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        • #19
          Bob and Mark settled on a spring that Mark now provides with the kit. I don't recall how this ended, but I think I have the 6.7 lb. MMC spring installed on my Partol. I recall Mark had a better spring supplier than MMC. More choices, etc. Maybe Mark will chime in.
          Brooks Cone
          Southeast Michigan
          Patrol #303, Kit build

          Comment


          • DBeaulieu
            DBeaulieu commented
            Editing a comment
            Mark reached out to check I had them in my kit - which I did.
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