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Let's try this: The Mystery of the Stiff Rudder and Red Herrings

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  • Let's try this: The Mystery of the Stiff Rudder and Red Herrings

    Something about going to Oshkosh brings out the mechanical problems. I think it's that we tend to not fly our airplanes as much in a short time, or park them outside as much, as we do for the big show. So far our 2016 trip has been pretty uneventful mechanically, but it's only half complete. I've tied the airplane down at an airport with airline service so that I can go back to work for a few days, being that the weather was not going to allow me to fly it home today. But there was one pretty serious mechanical problem that I encountered and fixed today.

    Rather than make it easy and just tell you what it was and how I fixed it, I thought it might be fun to work through a little public aeronautical decision making and troubleshooting, so that we can increase engagement and learning. Maybe it won't be fun, but there's only one way to find out.

    So here's the background. I've been running a big tailwheel that tended to shimmy a lot. One of the fixes to minimize shimmy has been to add friction to the king pin, by way of a soft bolt that threads into the tailwheel body. To prevent shimmy on the big wheel, this bolt has to be pretty tight- so tight that I was starting to notice heavier rudder pedal action, especially on the taxi.

    Fast forward to our departure from Oshkosh early on Wednesday morning. We had a big load and a long taxi. There was a steadily-moving and solid east-west oriented line of convective weather approaching from the north. It was my preference to depart before this line of weather reached the airport. There were lots of good reasons to depart before the weather. As we made our way down the long taxiway to take off, I noticed that the rudder pedals were much more stiff than usual. This is decision fork location 1.

    If I were at my home airport getting ready to go, I probably would have stopped the plane and investigated the cause of the rudder pedal resistance. The most plausible explanation to me at that point was that it was cargo related, being that we had just loaded up ourselves and our 200 pounds of gear. Perhaps the rudder cable was impinged by the cargo somewhere. So here's discussion question number 1: Would you have chosen to go, or not to go in this scenario? Keep in mind that lots of folks are watching, and it's going to take you a few minutes to get it tied down to weather the approaching storm. On the other hand, mechanical problems seldom get better in the air, unless they are related to your tailwheel being on the ground.

    I decided to continue the takeoff, and did complete the flight safely. I will say though that by the time I landed an hour later, I relied mostly on differential braking to steer the airplane on the ground.

    Discussion question 2: If you were in this situation, knowing what you know about the Bearhawk rudder system, where would you start your post-flight mechanical investigation? Just to expedite things, I'll tell you that the problem did not change after emptying the plane. In other words, the problem was not related to the cargo or people. Disregard that point in your consideration of question number 1.

    Last edited by jaredyates; 07-28-2016, 09:45 PM.

  • #2
    One cause of tailwheel shimmy is the king pin caster angle. If the tailwheel was changed for a larger wheel then it may have gone from slightly positive to negative as it would have lifted the tail to change the kingpin angle. Shimmy is a negative camber condition if there is no mechanical looseness. If the caster is not positive then the wheel cannot not caster behind the kingpin. The centerline of the king pin has to be angled to point slightly in front of the tire contact on the ground. This way the wheel will naturally trail behind the kingpin. If the king pin centerline is aimed at the wheel contact or behind it then the wheel will try to get behind the kingpin centerline pivot but cant because of the direction that it is travelling will not allow it. The negative caster causes the tailwheel to shimmy wildly as it cannot caster. In a negative situation the kingpin centerline is behind the wheel contact so the wheel struggles to caster behind the kingpin centerline which is the shimmy as the wheel fights to swing behind the kingpin but the physics won't allow castering. Tightening the tailwheel will only transfer mechanical resistance to the rudder and rudder controls but will not fix the condition. Tightening of the tailwheel to resist shimmy would make the controls very heavy. Weight would make the condition worse by increasing the negative camber but removing the weight would not correct the negative caster if it exists in the no load condition.

    My guess.

    What would have happened if the over tightened wheel seized up due to a shimmy induced mechanical failure while running down the runway to freeze the rudder controls?
    Last edited by Glenn Patterson; 07-29-2016, 12:15 AM.


    • #3
      Unless I felt there was a risk of the rudder locking up, I probably would have continued. Leaving a show like Oshkosh with an audience, approaching weather and no great place to stop and check things adds a lot of pressure to the equation.

      I'd start looking back at the rudder most likely, try to see where the binding might be occuring and work my way back to the pedals.


      • #4
        Hmmm... Thirty years ago, I'd have continued on! Damn the torpedos!!! Now, I'd have to think about it. Is your tailwheel connected to the rudder horn through springs? Can you move the rudder through rull range with the stretch of the springs? If the rudder moved easily, by a quick hand shake, I'd probably disconnect the rudder-tailwheel springs and continue on. If the rudder does not wiggle easily with a quick shake, the plane stays parked until the problem is fixed. My guess is that the 'soft bolt' was galling and shed enough metal to start seizing the tailwheel pivot assy. I've landed and taxied a fairly heavy plane, with a seized tailwheel assy, using brakes only. Not really a big deal.

        You are exactly right about one thing for sure; stuff like this seems to happen more often at big events...



        • #5
          The caster angle is good when the plane is stationary, though I do think the tailwheel spring flexes when the elevator is deflected aft in the landing rollout. Bob is flying with the prototype version of the same wheel, and he says his often shimmies on the pavement. It's crucial to go to a neutral elevator position right after touchdown.

          Getting back to the story, here's what I did. After parking the plane overnight in the rain, I came back out to investigate, also in the rain. I wiggled the rudder from the trailing edge and it was pretty much stuck. There are springs in the connection to the tailwheel, so it didn't make sense that the tailwheel would be responsible for the stiffness. My best guess at that point was that the rudder cable had been cutting into a fairlead to a progressively deeper degree. To rule that out, I disconnected the right rudder cable from the rudder and gave it a pull. It felt about as it should, with free movement and a little resistance from the rudder return spring. Next I disconnected the left cable and did the same, with the same result. So that ruled out any problem with the rudder cables. Next I applied pressure to the trailing edge and watched the hinges. The top hinge movement between the rudder and vertical stab to correspond with the slight deflection of the rudder, but the bottom did not. This was because the forward tube of the rudder was twisting, because the top hinge was free but the bottom was seized. This explains the progressive nature of the problem. I've not been flying much in recent months, and the Oshkosh trip did expose the airframe to several hours of rainfall in our 7 days of camping. Rust was slowly building up in the tiny space inside the hinge, rendering it less and less free.
          To fix it, I went to the auto parts store and got a can of penetrating oil and a can of carburetor cleaner. Carb cleaner is probably not paint safe, but I figured it would dissolve any residue of prior lubricant and I used it sparingly.
          With repeated applications of the penetrating oil combined with wiggles of the rudder from the trailing edge, the hinge started to free up and tiny flecks of brown rust ran out with the oil. I let it sit for a while and repeated the process a few times on both hinges, and before long it was good and free.
          This doesn't mean that seizing hinges are a big crisis to be concerned about... the problem was certainly progressive had a very slow onset. The elevator hinges are much easier to lubricate, being that they are horizontal, and they are still fine. As I looked closer at the bottom rudder hinge, I think one contributing factor was that my covering very slightly overlapped the top of the hinge (see photo). This blocked the lubricant from entering at the top. I assumed that lubricating through the little hole would be sufficient, but after this experience it looks like the more effective method is to apply it to the outer edges of the hinge in addition to the little hole, especially on the vertically-oriented rudder hinges. I used a sharp knife to trim back that little boot of covering, and after that I was able to get much more oil into the important part of the hinge.
          The bigger lesson for me is about not getting distracted by red herrings like the tailwheel. I should have done more investigating of the advancing rudder stiffness, rather than just blaming it on a plausible explanation like the tailwheel or the cabin loading. Also, the problem was so slowly coming on, that I probably didn't realize just how stiff it was getting. And from a maintenance standpoint, it's worth making sure that there is a clear path for lubrication on the extreme ends of the hinge. It's likely that the upper side of that hinge has only been lubricated by capillary action in the past 200 hours and 2.5 years. If I were him, I'd probably have gotten a little rusty too.


          • #6

            1) What are peoples thoughts about having a sintered bronze oil impregnated bearing at the rudder and elevator hinges? I know its too late now for that, but others might think about it.
            They are cheap.....

            2) Is it reasonable during the preflight inspection to move the rudder and elevator controls by hand to check for normal travel, and freedom? I am asking myself "How could we all catch something like this in the future?"

            3) I would like to see a drop of lubricant placed on the top side of that hinge with some kind of tiny oil can applicator. Is there enough clearance that the rudder can be lifted slightly (1/16" maybe) do that? BTW, I believe Jared's experienced may be a more common issue than I ever realized with aircraft that have had periods of rest after this and talking with DemoDollCate at the picnic.

            Brooks Cone
            Chelsea, MI
            Building Patrol #303
            Brooks Cone
            Southeast Michigan
            Patrol #303, Kit build


            • #7
              1- I don't think it's necessary. It hasn't really been a problem for folks who maintain their airplanes properly, and there have been lots of planes flying with this type of hinge system for a long time.
              2- Yes, and a more thorough exterior inspection might have made the problem obvious on departure from OSH. This isn't the first time I've reminded myself to be more methodical on the exterior inspection, regardless of mission pressures or distractions. Departing OSH is a big distraction, and I didn't spend as much time as I might have otherwise. This was a deviation from preferred pilot performance, shall we say.
              3- In our Bearhawk there is not any play. I carry an eye dropper of lubricant in my tool kit, and when I'm at home I have a little drippy bottle that I got from an art supply store. Now that the covering lip is out of the way I should be able to apply the lubricant more effectively.


              • #8
                I built Three Sigma with grease fittings on each of the tail hinges. Each year I squeeze some more bearing grease in there and haven't had anymore problems. I think now I'm glad I did it that way.

                I would think you would have noticed this while doing control sweeps during the preflight inspection. I would hope you would have investigated when you first noticed the increase in resistance. Of course, if you first noticed it at KOSH then I see your dilemma.
                Russ Erb
                Bearhawk #164 "Three Sigma" (flying), Rosamond CA
                Bearhawk Reference CD


                • #9
                  Thru a burst of insight(or just dumb luck), I thought about this when building my LSA rudder.

                  Instead of putting an oil hole in the side of the rudder hinges, I filed a couple small round notches in the top of the hinge barrel(and bottom of the rudder horn where the hinge butts up) so they would be oiled from the top.


                  • #10
                    The hole in the side of the hinge, per your picture there Jared, it's a grease hole yes?
                    I was a little surprised by the talk about oil.

                    At least, grease is what I've been injecting into there every 100hrs. There is always a healthy line of grease at both edges of the hinge as a result.
                    Last edited by Battson; 08-29-2016, 03:27 PM.


                    • #11
                      The lubricant choice may indeed be a factor. How do you force the grease into the hole, and what kind of grease are you using?


                      • #12
                        Originally posted by jaredyates View Post
                        .... How do you force the grease into the hole,....
                        I was planning on using a grease needle, like you would use to grease the gears in the head of your angle drill and other air tools. Harbor Freight has the little guns with the needle built in or you can get the needles to fit a regular grease gun.

                        Another example:


                        Scratch building Patrol #254


                        • #13
                          Thanks for sharing your experience Jared, I was wondering about this very thing as I looked over my tail surfaces. These type of hinges use two pieces of chromoly tubing rubing directly against eachother, one of which acts as a spar for the control surface. There is no softer, sacrificial, and replaceable bushing or bolt. Any substantial corrosion or circular wear marks would weaken the control surface. I would think some type of synthetic grease pushed in the joint using a grease needle, like Doug mentioned, would form a film between the pieces and better resist gravity/water.


                          • #14
                            A grease I have been pleased with for years is Red Line CV-2 grease. I use it to lube my air tools, CV joints and wheel bearings in my cars and anywhere I need to grease something. I think it would be a perfect hinge lube for any of the Bearhawks.

                            The specs can be found at:


                            • #15
                              Originally posted by jaredyates View Post
                              The lubricant choice may indeed be a factor. How do you force the grease into the hole, and what kind of grease are you using?
                              I have been using Grease 22, although that's a little thin. You might like a more-viscous number. All available from Spruce of course.

                              I have a medical-type disposable syringe which is exactly the same size as the hole, to force grease in under pressure. Moving the control surface back and forth as I squeeze helps to distribute grease around the hinge by capillary action.