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Tailwheel king pin angle

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  • Tailwheel king pin angle

    I feel I have too much positive king pin angle and taxiing would be improved if set closer to vertical..I have a bob tailwheel, and the flat spring set from Mark. Scratch built fuselage. At this point I already have 4 or 5 large diameter washers between leaf springs and fuselage bottom at front of leaf springs to get closer to vertical. I understand that kingpin should be slightly positive angle at max gross to solve shimmy issues, but as it is now, when tailwheel breaks loose it wants to spin around causing a very difficult taxi at times. I am looking for suggestions to straighten angle. As it is I have it spaced approximately 3/8 of an inch down at the front. Can anyone see a problem to space it down another 1/2 inch with washers to see if handling improves. I would expect a great improvement...or any other suggestions? Thank you all. Gary
    Last edited by Gary Wallace; 07-25-2022, 08:36 PM.

  • #2

    More questions than answers.

    1. What is the current king pin angle?
    2. What is the weight on the tail?
    3. What type of surface do you taxi on?
    4. How much is the arch of the spring deflected?
    5. Is the tire a Tundra?
    6. What is the speed estimate when the shimmy develops?
    7. Do you know if the existing 3/8 of an inch made a noticeable change?
    8. What is breaking the tail wheel loose? Turns or straight ahead caused by shimmy?
    9. Is the centering detent fully engaged with plunger all the way in? The plunger detent into the locking ramp has to be clean and not packed with grease or burr impeded.
    10. What is the Tailwheel Tire pressure?
    11. Patrol or Bearhawk?
    12. Is your tailwheel in balance or out of round?
    13. Message implies that aircraft might not be flying just taxi status?
    14. Is the Tailwheel Spring totally locked into the airframe when unweighted with no play at the saddle or the forward bolt?
    15. Are your tail wheel springs too tight and causing the tailwheel to unlock with very little rudder deflection?

    Sorry to sound like a proctologist butt....... there is a lot going on at the tail end........

    Kevin D

    Not at Oshkosh, time to kill......


    • #3
      Thanks were correct in assuming I hadn't flown it yet. After a very close inspection I did find that when the tailwheel swung around in a tight castering turn as it came up to 90 degrees to leaf spring, there was a little play in the 7/16 bolt that holds tailwheel to leaf springs for some reason..not sure what happened there. That would cause tailwheel to want to swing right around due to increased angle. I tightened that bolt up and made all the difference..I had my first flight yesterday after 7 years. What a great last project was a 1947 cessna 140 rebuild. Been flying that 9 years and 800 hours so bearhawk was a totally different feel. Took me a bit of taxiing to get comfortable with it. Just an O360 4 place A but I think I'm going to love. Can't wait for next flight! On holidays for a week and a half now...will definitely get back if weather is good for some air time. Thanks again..this forum has been so educational! Gary


      • Nev
        Nev commented
        Editing a comment
        Congratulations Gary !

    • #4
      Congratulations Gary! A O360 BH is a lot of fun; there is no “just” about them.

      My opinion on the tw angle: Your going to want as much as you can get, especially with a light engine up front. I think I understand your concern; the angle causes TW to lift the tail as it rotates to straight. When moving it around by hand it can be a little challenging but my experience is that it is a none issue when actually taxiing.

      With a light engine up front the tail is pretty heavy and only get heavier as you add loads. With a full load your kingpin angle will be negative so you will get shimmy but that’s something you’ll have to deal with. Adding washers to decrease the empty kingpin angle won’t bring a satisfactory solution, IMO. But, now that your flying you’ll have plenty of time to determine you own preferences.

      Super excited for you! Hope you post some pics.
      Scratch Built 4-place Bearhawk. Continental IO-360, 88" C203 McCauley prop.


      • #5
        Congratulations on your first flight Gary. I thought I would share my experiences on my personal Bearhawk, as I went through the same thinking process.

        I thought I had too much angle, straightened the leaf springs a bit and later regretted it. See the link below for my experience

        bottom line, keep the angle you have.

        Eric Newton
        Bearhawk Tailwheels LLC
        Eric Newton - Long Beach, MS
        Bearhawk Tailwheels and Builder's Manuals


        • #6
          Thanks guys..I will leave for now...check with loads...I still do have about 7/16 shim I can remove to get more positive angle down the road..or runway I guess. I'll post pics when I figure it out. Imagine Rob Caldwells bearhawk! Mine is it's little brother!


          • #7
            And yes whee that is exactly it...spinning tailwheel to rear lifts the tail.


            • #8
              2nd flight today with extra weight in co pilot seat. Did notice a wee bit of shimmy upon landing. Again, impressed with my new bearhawk!. I expect I will remove the 7/16 of washers between front of leaf springs and fuselage for increased kingpin angle as whee mentioned. I am getting the feel of things,, don't like any shimmy. LOVE IT!!. I will be trying to keep tailwheel up a little longer and slow up a bit more if I can


              • jaredyates
                jaredyates commented
                Editing a comment
                Also, I would recommend no stick deflection aft of neutral unless you are afraid you are going to nose over, which will probably only happen on very soft surfaces.

            • #9
              Congrats Gary! For a fellow 140 driver this is inspiring.
              Scratch building Patrol #275
              Hood River, OR


              • #10
                Are you flying off pavement?


                • #11
                  I've flown 99% of my flying off of grass. 1650 foot grass strip 75 foot trees at one end. Clear the other way. Currently flying off pavement...CYKF. to get used to this transition till I fly into my home strip. Very different but getting the hang of it. Very Very Very early days


                  • #12
                    And thanks Jared...the shimmy was when I pulled stick back a "Whee" bit for the first time. I won't do that again, unless necessary. And Jared...your wheel pants look great on my bird! I expect some day when I graduate, they may be removed the meantime.....I love them!.. soon to post pics.


                    • jaredyates
                      jaredyates commented
                      Editing a comment
                      Can't wait to see the pics!

                  • #13
                    And...maybe I should start posting in a different section...just so used to this!


                    • #14
                      So.....10 hours at this point, still not sure about this tailwheel....ground handling. Again I have the smaller...I think 8 inch bob tailwheel...bearhawk leaf spring. First taxis by myself I was concerned about tailwheel wanting to castor forward at slow speeds...well over sideways anyway, I'd have to throttle up and opposite brake to get tailwheel to swing around and lock again. This was with approximately 5/8 inch of washers between front of leaf springs and fuselage. As I started to load up plane..and finally to max gross, I did remove the 5/8 spacer to get more positive king pin angle as leaf straightened out. At max gross kingpin was pretty much vertical...with a wee bitt of shimmy on landing. After the climb test, I still have spacers out at front of leaf springs for maximum positive king pin angle, but taxiing solo now is horrible with tailwheel always wanting to swing around and very difficult to get rearward and lock for steering. I guess I need to pick a happy medium and deal with the shimmy? Maybe another leafspring? I'm confused which direction to go here. Is this typical? Thanks...Gary


                      • #15
                        I wonder if it isn't locking well. Is the locking pin moving freely into its hole, free of burs and perhaps with the slightest chamfer on the front? Lift up the tail and try to se how easily it locks and unlocks with your hands on the wheel.

                        Also, 5/8 sounds like a big chunk, were you removing the washers incrementally or all at once? Perhaps at some intermediate setting it is more acceptable across your range of operating weights? Maybe use a little aft stick during forward cg operations?
                        Last edited by jaredyates; 08-11-2022, 09:15 PM.