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  • Spherical rod ends in the rotor control system.

    I use quality spherical rod ends in my rotor control system and advise others to do the same.

    The specifications for even the poorest quality spherical rod ends exceed the loads in the control system by a very wide margin. A ¼ 28 commercial grade Heim brand will handle 1,700 pounds of static load. Most people use at least 5/16 24 for a static load of 2,500 pounds.

    If the spherical rod end is not captured on both sides I like to have some sort of retention like a large washer incase the spherical bearing fails.

    As with all things that rotate I feel there should be a secondary form of retention on the bolt that goes through the ball. I prefer mechanical but fresh elastic stop nuts will do the job.

    All of the spherical rod ends I have seen fail broke because their maximum misalignment was exceeded and they were repeatedly put in bending.

    If I find a bent one in the control system I replace it before the next flight and correct whatever caused it to bind.

    In a crash it is possible to create loads that will pull a spherical rod end apart.

    I have found high misalignment spherical rod ends for special applications.

    There have been times I have made special coned washers so that the body of the spherical rod end doesn’t bind.

    I recently inspected a client’s aircraft and found the control rod ends exceeding the maximum misalignment and were in bending.

    It is my observation that even the highest quality spherical rod ends will fail if they are put in bending and may in fact fail sooner in bending because they are usually harder.

    The way I check the spherical rod ends for bending is see if they still have play throughout their travel while someone slowly operates the cyclic through its full travel in all directions.

    If they get tight anywhere the flexing of the mast and other parts of the frame will probably be enough to put them in bending. It doesn’t take much bending to cause a failure.

    Even on a well-designed control system it may be possible to put the spherical rod ends in bending by how the jam nuts are tightened. I watch how it works and pay close attention to this flight critical part.

    I have found high misalignment spherical rod ends for special applications.

    Part of the rotor control system on The Predator. High misalignment spherical rod ends with cones. An Aurora Spherical rod end.
    Regards, Vance Breese Gyroplane CFI http://www.breeseaircraft.com/

  • #2
    Also check to see if color anodizing has been used on the control rods.

    Comment


    • #3
      Vance,

      In the 2'nd photo with the cones, do you have a supplier for those? I am in need of a good source for that item. Thanks.

      Comment


      • #4
        I have always made my own so I googled 'high misalignment spherical rod ends" Brian.

        http://www.midwestcontrol.com/series.php?id=126

        I don’t know anything about these people.

        The pictures I posted were just something off the internet so people could understand what I was describing. I was not down at the hangar and it is not something I would usually take pictures of. I have some in my upper vertical stabilizer mount so I could use a smaller bolt.

        Binding of the control rod spherical rod ends is more common than people realize.

        It is one of the first things I check before flying any gyroplane new to me gyroplane.

        It is easy to check and usually a quick fix unless it is a bad design and the design needs to be fixed.

        Blaming the spherical rod ends when they break just above the jam nut misdirects people trying avoid control system failure.

        I recently redid that upper control bar and that is why I had a picture of it. I found a crack during preflight. I made the doubler and trim spring attachment points longer and thicker and I hope that will prevent further cracks.

        This is a good example of why preflight is so important and my point in posting one of the things I have found during preflight.
        Regards, Vance Breese Gyroplane CFI http://www.breeseaircraft.com/

        Comment


        • #5
          McMaster.com has them, but they only go down to 3/8" in the "Super Swivel" high angled ones.
          https://www.mcmaster.com/#rod-ends/=1bhxs3m
          Bobby Munroe
          Private Pilot (SEL)
          PRA Chapter 62 #42748
          EAA #1160523

          Comment


          • #6
            Thank you gentlemen very much. I also found an additional supplier but I don't have the link right now... I think it was Aurora Bearing. Fairly beefy piece of hardware but necessary for the application. I didn't know if one manufacturer was favored over another for aircraft use.

            Comment


            • #7
              I have used Aurora Bearing spherical rod ends with success.

              Those were purported to be Aroura high misalignment spherical rod ends in the picture I posted.

              Some of their series are better than others and more appropriate for the application.

              I like Heim also with the same caveat.

              Read the charts and you will see that for the rotor control I could not approach the load limits.
              Last edited by Vance; 02-09-2018, 11:48 AM.
              Regards, Vance Breese Gyroplane CFI http://www.breeseaircraft.com/

              Comment


              • #8
                Originally posted by klyde View Post
                Also check to see if color anodizing has been used on the control rods.
                Stuart brings up a good point.

                The control rods themselves may fail if not made of the correct material and handled correctly.

                Many of the control rods I have seen are aluminum and it needs to be the correct alloy and thickness with the ends attached in the correct way.

                Anodizing or powder coat may cause problems with aluminum control rods.

                Because of where I live near the Pacific Ocean corrosion can be a problem and it is not easy to inspect my steel control rods.

                I have stripped and repainted them several times.
                Regards, Vance Breese Gyroplane CFI http://www.breeseaircraft.com/

                Comment


                • #9
                  All of the spherical rod ends I have seen fail broke because their maximum misalignment was exceeded and they were repeatedly put in bending.
                  I have seen an example that experienced no misalignment at all, yet nonetheless failed in flight, killing two.
                  NTSB probable cause of N5002E: "
                  failure of the flight control rod bearing".

                  It was one of the lowest control rod ends on an RAF, on the lower control yoke.
                  It had cracked long before it finally separated, presumably from taxiing on rough ground while the control lock was in place.

                  Bending from excessive misalignment may be the most common cause of rod end cracking, but it is not the only cause.



                  Click image for larger version  Name:	N5002E corroded control rod end failure.jpg Views:	1 Size:	203.4 KB ID:	1130149



                  Here is an earlier thread, related more to the RAF.
                  My posts in it began in March 2015.

                  In my experience with RAFs, the OEM control rod ends (which I had analyzed; they are AISI 1112b Grade 0 low-carbon)
                  have lower quality anti-corrosion plating (zinc, not cadmium), as well as construction.

                  If you have an RAF with OEM control rod ends, I strongly urge their immediate replacement for Heims or Auroras, regardless of apparent condition.
                  Barring that, please at least remove them from their push tubes and inspect for hidden corrosion or cracking.

                  I consider the smaller 1/4 inch rod end on rear of the pitch control rod in the lower control yoke to be especially vulnerable to damage and corrosion.
                  In that 2015 thread, I outlined Jim Mayfield's 5/16" mod with a stronger Aurora XAM-4T part. It's a mod I incorporated myself, and am relieved to have done so.

                  Safe flying, Kolibri


                  PP - ASEL (Piper 180, C172, RV-7A), SP - Gyro (Calidus, RAF), soloed in gliders; checkride soon

                  Wasn't happy with my RAF's pitch instability, so I installed a Boyer H-Stab to my great satisfaction!

                  "My expectations: disclose the truth I need to fly safely, and act like you truly care about quality. Anything less is greed or laziness."

                  Comment


                  • #10
                    Click image for larger version Name:	N5002E corroded control rod end failure.jpg Views:	1 Size:	203.4 KB ID:	1130149



                    In my opinion this is a picture of a spherical rod end that broke from bending.

                    The corrosion most of the way through shows that it was cracked for a long time before it failed.

                    In my opinion it cracked because the threaded portion was in bending from exceeding the maximum misalignment of the spherical bearing.

                    It breaks next to the nut because it is the first place it can bend.

                    In my experience it would look very different if it had failed from poor materials and it most likely would have failed near the ball because that is the weakest part of a spherical rod end and where they commonly fail from overload.




                    Last edited by Vance; 02-10-2018, 04:44 PM.
                    Regards, Vance Breese Gyroplane CFI http://www.breeseaircraft.com/

                    Comment


                    • #11
                      Good stuff, Vance.

                      A few other control-rod tidbits:

                      Rod ends come in different grades. Commercial-grade rod ends may be assembled by welding through the rim of the ring. I have heard tales of these peripheral welds failing. Yet, my 1986 model Air Command gyro came with them. When using commercial-grade rod ends, I've had them magnafluxed to check the weld quality.

                      Aircraft-grade rod ends have a soft-metal liner inside the steel outer ring. Commercial rod ends are 2-piece, steel-on-steel.

                      In my opinion, 1/4" shank rod ends simply lack the strength for real-world gyro operations, despite their rated strength (which can only be achieved in an abuse-free environment, not when slamming about on grass or occasionally flapping blades).

                      An often-neglected issue is the rigidity of the pushrods in compression. As gyros have gotten taller (to accommodate longer props and rotorblades), in some cases designers have stuck with a single pushrod from the base of the mast to the rotorhead. Others have put a bellcrank ("scissors") assembly around seatback height, with upper and lower rods and additional bearings. When using a single rod for the whole distance, the rod will carry a much lower load in compression than in tension.

                      The rod functions as a slender column. The new, longer rods, if built of the same diameter tubing as the old Bensens, are apt to have a buckling strength of only about 150 pounds. To add stability to a column, you must either use stiffer material or increase the diameter. 7/8" or 1" diameters are more suitable for today's six-foot rods than the 3/4" tubing on a Bensen. Even then, the buckling strength is only in the 250-pound-ish range, while the tensile strength is a couple tons.

                      Long push tubes can be further stiffened with internal or external sleeves a couple feet long and centered on the tube's length. Don't drill holes near the mid-span of the tube, however.

                      Intentional bends in push tubes are not OK at all, despite what you may see on old Quicksilver ultralights. If you can't use a straight run, go to scissors.

                      Comment


                      • #12
                        And this is why I read all your posts, Doug. Always something of value to learn.

                        Comment


                        • #13
                          Good point about the buckling pushrods, thanks Doug.

                          _________
                          In my opinion it cracked because the threaded portion was in bending from exceeding the maximum misalignment of the spherical bearing.
                          I disagree, Vance, since I've had a pretty good look at N5002E in person.
                          The lower control arm rod ends properly had coned outside washers, and I saw nothing to indicate that the rod ends were binding up at max angles.
                          Owning an RAF I'm very familiar with its lower control yoke range of motion, and doubt that this failed rod end was bent from excessive misalignment.

                          The OEM RAF rod ends were so poorly made that impurities could be seen by eye in broken shanks.
                          I read of one snapping from merely its jam nut being tightened.


                          Furthermore, the steel used had a high concentration of manganese sulphide 'stringers' which were able to significantly further reduce the expected bending strength and stiffness.

                          http://www.aaib.gov.uk/cms_resources...pdf_027262.pdf
                          Thus, such an inferior rod end could have begun to crack within its normal range of use, and/or from ground handling forces.

                          Also, since some of N5002E's rod ends had been installed with more exposed threaded shank than prudent,
                          such could have been the case with the failed one, which provided more leverage to bend/crack at the jam nut even within normal operation range.



                          It is my observation that even the highest quality spherical rod ends will fail if they are put in bending and may in fact fail sooner in bending because they are usually harder.
                          Perhaps without meaning to, you're supplying justification for some people not to replace their OEM parts with better ones.

                          I think the best advice is this:
                          Install high-quality control rod ends (such as Heim or Aurora) without any bending in their normal range of cyclic motion.
                          Confirm during pre-flight that their range of misalignment is not exceeded.


                          But, to not unequivocally state such is to mollify some RAF owners to retain their old <Grade 0 discredited rod ends so long is they're not being bent.

                          On this point, Vance, if somebody has 650+ hour original RAF control rod ends (which are not binding up at max cyclic), should he replace them or keep them?
                          Simple question; simple answer.

                          Regards, Kolibri
                          Last edited by Kolibri; 02-13-2018, 02:18 PM.
                          PP - ASEL (Piper 180, C172, RV-7A), SP - Gyro (Calidus, RAF), soloed in gliders; checkride soon

                          Wasn't happy with my RAF's pitch instability, so I installed a Boyer H-Stab to my great satisfaction!

                          "My expectations: disclose the truth I need to fly safely, and act like you truly care about quality. Anything less is greed or laziness."

                          Comment


                          • #14
                            It appears to me that some people may not understand what is causing the misalignment and bending of the control rod ends so I will attempt to explain how I perceive the problem and how I check them.

                            There are jam nuts on each end of the control rod to secure the control rod to the spherical rod ends. If the control rod is rotated incorrectly and tightened it may work just fine in the middle of the travel but somewhere in the travel it will bind up putting a bending load on the spherical rod end.

                            I have a friend move the cyclic slowly through its full travel in all directions to see that the rod ends with the jam nuts locked down have play in them throughout their travel.

                            If the control rods won’t rotate slightly throughout their travel they will likely be in bending in flight.

                            If I were examining a wrecked aircraft it would be very difficult to find it the spherical rod ends were in bending when it happened and impossible to tell if they had ever been in bending at some time in their service life.

                            It is my observation that spherical rod ends break around the ball if they fail from overstress or low quality.

                            In my opinion a poor quality spherical rod end may break anywhere and because it is a flight critical part I prefer a quality rod end.

                            Putting a life limit on something that should outlast the aircraft with proper maintenance suggests to me a lack of understanding.

                            Spherical rod ends should be regularly inspected for corrosion and dirt.

                            Spherical rod ends should be lubricated if it is a rod end that needs lubrication. Many rod ends don’t require lubrication.

                            I have seen the referenced report and pulled this excerpt from it:

                            “The rotor control system was otherwise complete and, although the mast was damaged, could still be functioned. A visual examination of the fractured eye end showed it to have plastically deformed in bending before final fracture, indicating overload conditions.”

                            The way I read the report is; the spherical rod end had been damaged from overstress and there was evidence of previous bending before the final fracture.

                            Further along in the same report:
                            “Control rod:
                            The metallurgical report found that the eye end of the control rod had fractured by overload in bending, as previously thought. The surface appearance and distortion of the fitting suggested, at the most, a few load cycles to final fracture. The eye end was a commercial part closely resembling an aerospace quality component in appearance. The main difference however was that the body of the fitting was manufactured from a free-machining low carbon steel with a tensile strength of around 34 tons/in2 . In an aerospace application, a low alloy steel of around 50-60 tons/in2 would be expected, however there was no requirement to use an approved part in this application. Furthermore, the steel used had a high concentration of manganese sulphide 'stringers' which were able to significantly further reduce the expected bending strength and stiffness.”

                            The way I read this is the control rod ends tested had 57% or the strength of an Aerospace quality part. A .3125 (5/16) Heim brand aircraft spherical rod end has a maximum static load of 2,500 pounds and a .250 (1/4) has a maximum static load of 1,700 pounds. As I calculate it for the "inferior rod ends" that would be a maximum static load of 1,417 pounds for the .3125 and 952 pounds for the .250. In my opinion that is more than up to the task.

                            I also read in the report a confirmation of the previously articulated "the control rod had fractured by overload in bending".

                            In the RAFs I have inspected some had badly corroded spherical rod ends that showed evidence of a lack of lubrication. In my opinion these aircraft are not airworthy.

                            If I owned an RAF gyroplane I would replace the spherical rod ends because I like quality parts in my control system; not because they are not up to the job.

                            If I were to pre-flight an RAF and found the control rods clean, showing signs of maintenance and not binding I would not ground the aircraft because I was not familiar with the rod end manufacture. I would expect and inspect a reasonably comprehensive aircraft and engine log books as part of a good preflight inspection. Incomplete log books to me suggest a more comprehensive pre-flight inspection is needed.
                            Regards, Vance Breese Gyroplane CFI http://www.breeseaircraft.com/

                            Comment


                            • #15
                              Then are you actually alleging that the only way a control rod end can develop a crack is from having been bent?


                              As I calculate it for the "inferior rod ends" that would be a maximum static load of 1,417 pounds for the .3125 and 952 pounds for the .250. In my opinion that is more than up to the task.
                              That assumes that the steel itself holds together well, a more dubious prospect regarding material with "high concentration of manganese sulphide 'stringers'".


                              If I were to pre-flight an RAF and found the control rods clean, showing signs of maintenance and not binding I would not ground the aircraft because I was not familiar with the rod end manufacture.
                              Ah, but my question posed to you specified OEM RAF rod ends, thus you are familiar with the rod end manufacture, at least by reputation.

                              ______
                              I would not fly an RAF with its original 3/8" control rod ends at all, and the only exception I'd possibly make is
                              if I'd removed and inspected every one.

                              If it still had its original pair of 1/4" rod ends in the pitch rod, I'd replace them with Auroras before flight, period.

                              Regards, Kolibri
                              PP - ASEL (Piper 180, C172, RV-7A), SP - Gyro (Calidus, RAF), soloed in gliders; checkride soon

                              Wasn't happy with my RAF's pitch instability, so I installed a Boyer H-Stab to my great satisfaction!

                              "My expectations: disclose the truth I need to fly safely, and act like you truly care about quality. Anything less is greed or laziness."

                              Comment

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