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-   -   serious control system issue (https://vansairforce.net/community/showthread.php?t=163137)

jwilbur 08-12-2018 07:15 PM

serious control system issue
 
Doing my first annual. When checking the elevator control in the tunnel I found the jamb nut was loose about 6 turns. Upon further investigation I found that the F-1065 cross bar was scraping the safety wire which in turn had worked the jamb nut loose. This happens only when the stick is pulled all the way back and while all the back it moves left/right -- exactly what you do when landing.

I know for a fact this was not scraping when I finished the build. I believe the push tube just twisted enough to bring the safety wire straight up top. So this had to be fixed. My plan was to first remove the bearing from the assembly, loosen the other end and roll it to put the safety wire on the bottom so it could never interfere again. Then I was going to adjust the segments as necessary to prevent any lingering interference. .... Well, as soon as I started removing the nut/bolt holding the rod-end bearing, the safety wire fell out. The safety wire loop on the bearing side had broken and the safety wire was being held only by being compressed in the assembly. In other words, it was not doing anything at all.

This part in the RV10 is on-purpose designed to have the rod-end stick out far more than half the threads in order to prevent interference with the tube itself. The safety wire is meant to prevent the tube from ever turning and working loose. I know I built this to plans and had it checked by another builder at the time.

Based on this experience I'm not happy with this design. In my case the tube twisted enough to put the safety wire on the top where it was then the cause of the interference. Further, I believe the constant working of that safety wire back and forth during landing (especially when forward CG - like flying solo for the first 40 hours) caused the safety wire to work harden and break where it went around the rod-end. I'm not 100% sure what I'm going to do to prevent this from happening again, but I'm considering safety wire on both sides in such a way to prevent the push-rod from rotating in either direction.

I'm not special so I'm assuming I'm not the only one where this has (or is) happening. I suggest everyone have a careful look at this next time you have the tunnel exposed.

In my case I caught it while doing a control system check with everything opened up for my first annual. When I had the stick full aft and moved side to side I could hear something scraping. And that lead to this find.


From the plans. Page 39-10


What I found after disconnecting the rod-end bearing from the control assembly. When I first looked here I saw the jamb nut had worked loose. I tightened it by hand and kept looking at what was going on. When removing the bolt from the rod-end the safety wire revealed that it had broken.

Brantel 08-12-2018 07:30 PM

Not sure what the other end looks like since I don't have my plans yet but if it has no other constraints to prevent it, I would install extra nuts on the other rod end bearing so that the push tube could not physically rotate enough to unscrew that end. I would also consider putting a sleeve on that end between the bearing head and the nut to prevent it from unscrewing the other end as well. A low profile sleeve should clear F-1065 if the nut does.

It is hard to tell from the photo, but it also looks like the safety wire was twisted a bit tight. AC 43.13 states 6-8 twist per inch are acceptable.

Glad you caught this!

rvanstory 08-12-2018 08:02 PM

Thanks for sharing this with the forum. I am on this page of the build right now and will be very attentive to how this is done. If you have any further insights on possible cause, please let us know.

jwilbur 08-12-2018 08:16 PM

Quote:

Originally Posted by Brantel (Post 1280315)
It is hard to tell from the photo, but it also looks like the safety wire was twisted a bit tight. AC 43.13 states 6-8 twist per inch are acceptable.

You're right. Thank you. Way too tight.

Steve Melton 08-13-2018 07:17 PM

a loose jam and broken safety wire. I don't like it. recommend to verify the rod could not become free by turning the rod both directions until it hits the stops at each end. this is goofy use of safety wire. safety wire should be straight and not able to wind up around around tie rod end threads. add loctite to the jam nuts.

Scott Hersha 08-13-2018 08:27 PM

Sure seems like an odd use of safety wire to me too. The movement you are trying to prevent is perpendicular to the twisting moment on the rod end, so the longitudinal change on the rod end requires more turns in order to make that twisted safety wire too tight to prevent further movement, on a bearing that is already more than half way out of the tube - - by design..... sure seems unusual to me. I just haven?t seen this before, but apparently it usually works if installed correctly.

6 Gun 08-13-2018 09:37 PM

Jam
 
If the jam nuts are tight I don't see how it could move in the first place I have a RV6 and the plans just call for jam nuts why would the RV10 need safety wire and why would the 6 not ? Asking to learn.
Bob

BobTurner 08-14-2018 12:23 AM

Quote:

Originally Posted by 6 Gun (Post 1280606)
If the jam nuts are tight I don't see how it could move in the first place I have a RV6 and the plans just call for jam nuts why would the RV10 need safety wire and why would the 6 not ? Asking to learn.
Bob

Generally speaking, if the control rod rotates in service, one rod end unscrews, the other screws in. Most control rods have the rod ends screwed in more than 50%, so if tbe rod rotates in service the end going in will run out of threads (stopping further rotation) before the other end comes out. But in this particular application the rod ends could not be screwed in 50% of their length (due to clearance issues) so there was the possibility that if the rod rotated in service, one rod end could come loose. The safety wire was supposed to keep this from happening.

rvbuilder2002 08-14-2018 10:05 AM

Quote:

Originally Posted by BobTurner (Post 1280618)
Generally speaking, if the control rod rotates in service, one rod end unscrews, the other screws in. Most control rods have the rod ends screwed in more than 50%, so if tbe rod rotates in service the end going in will run out of threads (stopping further rotation) before the other end comes out. But in this particular application the rod ends could not be screwed in 50% of their length (due to clearance issues) so there was the possibility that if the rod rotated in service, one rod end could come loose. The safety wire was supposed to keep this from happening.

Exactly!

This type of issue needs to be analyzed with a bit of logical reasoning.......

RV-10’s have been in service for 16+ years. From the first prototype through many hundreds of amateur built examples (almost 900 and still climbing), and this is the first instance of this type of problem that I am aware of.
That is not a reason to be casual about the discovery, but I think it is a good indicator that design is sound and if executed properly, will produce the desired result.

In the condition inspection forums/seminars I give, I recommend that the inspection of the control system be systematic. Starting at the stick grip and working through to the opposit end on component at a time.
I.E., ball in rod end free to move but free of play. Jamb nut on rod end properly torqued. Proper engagement of rod end to push/pull tube end. Push/pull tube end properly riveted to push/pull tube, etc.

This is a good idea for every condition inspection. Including the very first one that you do (and have to make an entry in the aircraft maint. Records as having been done) before the inspector will issue an airworthiness certificate.

Do not lose sight of the purpose of safety wire. It is not to keep things properly assembled (for the majority of things safety wired on aircraft, they will stay tight just fine on there own if torqued properly in the first place). It is to add an extra level of safety in case things don’t stay the way the were intended to.

TimO 08-14-2018 11:52 AM

This is most definitely NOT the first time that this has happened on a customer built RV-10. Forgive the tone if that came off wrong, as I'm not being argumentative. This isn't the only forum, and I'm not sure which forum it was reported on, but I know its happened to at least 2 other builders. One of them I think it actually unscrewed all the way out. One of them is a good friend of mine.

I checked on mine immediately after he reported this and found that mine looked good, but I was a bit paranoid after hearing what happened to him.

So far, I have done nothing additional to mine, as it's built per-plans and working fine. BUT, if someone comes up with an improved design or some sort of anti-rotation method, I would surely be interested in it. Also, I would think that in this application, loctite would be appropriate, as well as the jamb nut.


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