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  • Electric Rudder Trim

    What you see here is conceptual but I have the motor in hand and have been tuning that to see if this is even feasible, I recently got it to work well so I thought I'd post this design. Not wanting to weld or otherwise modify the structure on my kit this is what I'm thinking for a bolt-on solution. Travel is limited to +-25 degrees per the RAC design guide. With the horn 1" from the hinge pivot I get 0.85" of travel stop to stop. I haven't worked out whether I'll attach a larger tab and right now you can see it's just the hinge leg, but this is an easy enough part to swap out and find what works. I'd rather have too small a tab to start and sneak up on the ideal size. I'm using an extension rod to minimize the cutout I'll need in the rudder skin as connecting it directly to the actuator made for too large a cross section intersecting the skin. The aluminum extension rod will thread onto the end of the actuator then pass through the bracket and nylon bushing. I'm thinking about the RAC threaded rod and clevis kit for the jump from the extension to the horn.

    You can see I've put a housing around the actuator to prevent drumming of the fabric against the actuator body and to provide a mount for an access plate. There will need to be a pass-through for the rod going to the rudder trim horn, the standard one I've seen around is the plan there.

    The actuator is an Actuonix P16-P, 50mm stroke, 256:1 gearing, coming in at a hefty 0.21lbs (95g). I'm using their control board to limit the stroke and speed, ~.85" and 20% respectively. I'm also using a 10-turn pot as the control which I think will work well, it seems to give very fine adjustment and will emulate the manual rudder trim knob I'm use to in the PA-28s I fly. Currently it takes 4 full turns of the pot to move the actuator .85", or stop to stop in this system, so it's a little more sensitive but still feels like enough throw on the adjustment. I'll have a position indicator on the panel above the control knob. Unfortunately the indicator isn't adjustable in relation to the stroke so at full deflection the indicator will only show 1/2 or so, but I think that will be ok so long as it's centered about the neutral point.

    Weight wise the components shown add up to 0.52 lbs, though there are a few other brackets to account for. Using a conservative 0.75 lbs and a moment arm of 192" (estimated, I've yet to measure) I get an aft CG penalty of 0.09", or 0.75%. This doesn't seem too bad given the functionality.

    I will need to wire into the rudder now, so another hole in the skin and possible weather ingress. The rudder also swings around so much these components are going to get knocked about quite a bit, I'm striving to make everything as tight as I can to reduce vibration and wear. The actuator itself is close to the rudder pivot so it won't see too much of this. With just the hinge leg as a tab this sticks out 1.25" from the rudder end and will be prone to damage easily, but it will also be easy (and cheap) to replace so that's not too much of a worry.

    Rudder 1.png
    Rudder 2.png
    Rudder 3.png
    Last edited by Archer39J; 03-09-2018, 03:59 PM.
    Dave B.
    Plane Grips Co.
    www.planegrips.com

  • #2
    This is what I'm using... Will modify the trailing edge of the rudder....will be similar to the elevator trim tabs. http://www.aerotriminc.com/id55.html.

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    • #3
      Mark, Do you not like the Steve's Aircraft system in your Pacer?
      Scratch Built 4-place Bearhawk. Continental IO-360, 88" C203 McCauley prop.

      Comment


      • #4
        Originally posted by Mark Moyle View Post
        This is what I'm using... Will modify the trailing edge of the rudder....will be similar to the elevator trim tabs. http://www.aerotriminc.com/id55.html.
        Nice. I'm trying to avoid any major modification. If I don't like how this works I can just pull everything and be back to stock (mostly).
        Dave B.
        Plane Grips Co.
        www.planegrips.com

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        • #5
          I think I've seen some examples of this, but I don't recall exactly where. Are folks profiling the rudder ribs?
          Dave B.
          Plane Grips Co.
          www.planegrips.com

          Comment


          • Mark Goldberg
            Mark Goldberg commented
            Editing a comment
            Not needed Dave. Only the hor stabs and vert stab. Mark

          • Archer39J
            Archer39J commented
            Editing a comment
            Yeah I just recall seeing that and was wondering if there was a justifiable benefit.

        • #6
          I've seen adjustable rudder trim mentioned a few times. Initial flights, in my Patrol, revealled the need for left rudder pressure at cruise. I added a small fixed trim tab, which solved the issue. Now, a little right rudder is needed during climbout (which I consider normal) but all other phases of flight are pretty well balanced. Personally, I wouldn't suggest anyone bother to go to the trouble of a controllable rudder trim system on a Patrol. The fixed tab does the job.

          It's been quite a while since I logged time in a 4 place, but I don't remember wishing for a rudder trim system. I'm kind of wondering if the electric rudder trim systems are intended to fix an issue that might not be a big deal. So, to those of you flying your 4 place Bearhawks, do y'all feel a real need for a rudder trim system? Not trying to rain on anyone's parade here. Just curious if it's a bigger deal than I remember.

          Bill

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          • #7
            People generally seem to be happy with a fixed rudder trim tab. It has been my experience having an adjustable rudder trim is a nice convenience, so for relatively few negatives I think I'll give this a try.
            Dave B.
            Plane Grips Co.
            www.planegrips.com

            Comment


            • #8
              You know Dave, that's the best explanation I could imagine. Makes perfect sense to me!

              Bill

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              • #9
                I wonder if you could use that with the autopilot since they really can't fly the airplane due to adverse yaw.

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                • #10
                  Originally posted by schu View Post
                  I wonder if you could use that with the autopilot since they really can't fly the airplane due to adverse yaw.
                  This motor probably isn’t fast enough, but it feels like you could go for a faster one with less force.

                  There are a number of two axis AP installs and to my knowledge they work quite well. There’s been some discussion if a 3rd axis would be useful, but I don’t think they’re incapable of flying the airplane. If they are then I’ll have to rethink some things
                  Dave B.
                  Plane Grips Co.
                  www.planegrips.com

                  Comment


                  • #11
                    I haven't heard of a 2-axis autopilot on a bearhawk able to fly the airplane. From what I've heard it's basically just a wing leveler due to the adverse yaw.

                    Comment


                    • #12
                      Originally posted by schu View Post
                      I haven't heard of a 2-axis autopilot on a bearhawk able to fly the airplane. From what I've heard it's basically just a wing leveler due to the adverse yaw.
                      Check out this thread: https://bearhawkforums.com/forum/bea...topilot-advice

                      Mix of 4-place and Patrol autopilot feedback. Seems to work well for folks.
                      Dave B.
                      Plane Grips Co.
                      www.planegrips.com

                      Comment


                      • #13
                        Originally posted by Archer39J View Post

                        Check out this thread: https://bearhawkforums.com/forum/bea...topilot-advice

                        Mix of 4-place and Patrol autopilot feedback. Seems to work well for folks.
                        It all depends what you mean by "fly the aeroplane".

                        The AP works well to hold a course in a BH, but you have to keep your feet on the pedals if it needs to turn, climb, descend. That is my experience, I have only done a few hours with a BH autopilot. The autopilot is not going to "fly the plane" without you managing the rudder, but it will hold course and altitude provided you keep the rudder stationary one way or another.

                        Rudder trim.
                        I agree that rudder trim is unnecessary, Bill nailed in with post #6. It would be nice to have provided it worked well, but not necessary. I imagine electric trim would be twitchy, if the electric elevator trim is any indication. You would need very fine adjustment.

                        Comment


                        • #14
                          Originally posted by Battson View Post

                          It all depends what you mean by "fly the aeroplane".

                          The AP works well to hold a course in a BH, but you have to keep your feet on the pedals if it needs to turn, climb, descend. That is my experience, I have only done a few hours with a BH autopilot. The autopilot is not going to "fly the plane" without you managing the rudder, but it will hold course and altitude provided you keep the rudder stationary one way or another.

                          Rudder trim.
                          I agree that rudder trim is unnecessary, Bill nailed in with post #6. It would be nice to have provided it worked well, but not necessary. I imagine electric trim would be twitchy, if the electric elevator trim is any indication. You would need very fine adjustment.
                          I was taking Schu's meaning to be that the AP was incapable of working, which his followup indicates may not have been correctly interpenetrated. Feedback is contradictory, sometimes it will fly a GPS course, sometimes it's only good as a wing leveler. Sometimes there's little to no adverse yaw, sometimes there's a lot. If I have to give a little rudder input I'll still consider it "working".

                          This actuator, especially coupled with a rotary potentiometer, gives quite fine adjustment. It's a fundamentally different way of controlling the actuator in that your setting a position and the actuator is matching it within a percentage. I could see a "dumb" actuator on a DPDT switch being hard to fine-tune.
                          Dave B.
                          Plane Grips Co.
                          www.planegrips.com

                          Comment


                          • #15
                            There was a thread about this in the past 6 months. I did a little googling on VAF on 3rd axis autopilot servos/yaw dampers. They are out there. I did find a post that said numerous RV-10 guys with yaw servos will turn the yaw axis "on" right after takeoff and leave it on until right before landing. Than means even with the autopilot off, the autopilot was working in yaw.

                            So some, or maybe all EFIS companies have this function built in, and there are 3rd party solutions as well. How much work is it to get it to keep the ball centered while flying/turning with the autopilot on? I don't know. Maybe plug and play. Maybe 6 months of trial and error adjusting gains/rates.

                            But it is out there.

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