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turnbuckles: tension vs threads

dbhill916

Well Known Member
Hi folks,

How do I reconcile the recommended tension (35-45#) and the admonishment to have no more than 3 threads showing on the turnbuckles? When both cables are tightened to 45 # (using a calibrated tensionometer) I have about 5-7 threads visible.

Should I over tighten (> 45 # tension) or leave the tension at 45 # and have too many threads showing?

Thanks in advance,
-dbh
 
I would be more concerned about threads pulling out with so many showing. I'd tighten to three threads (which is what I did 3 1/2 years and 350 hours ago). No issues yet.
 
It was at this point we decided to have A&P IA assist us. He brought his tensometer with him. Remember that the tension is Variant upon temperature. If I recall, he worked each end until the 3 threads were showing.
 
The RV-12 stabilator cables are manufactured with a certain length tolerance. Some customers happen to get cables that just meet the short length spec. When all short cables are installed, the turnbuckles can not be tightened with the required threads showing without exceeding the proper tension. This happened to me. I returned a cable to Van's and requested a longer one. Problem solved.
 
David, I think there is a pretty good possibility of something else wrong if you have that many threads showing with the correct tension. I would check everything related to the cables from the control sticks to the stabilator. I note in your blog that earlier you had a problem with getting the cable routing correct. I suggest checking the routing again and everything along the cable route from fore to aft.

A question, do you have the extra threads showing on both ends of both turnbuckles?

BTW, your blog is very nice. Good luck with the fiberglass work. It is not so bad once you get used to it!
 
David, I think there is a pretty good possibility of something else wrong if you have that many threads showing with the correct tension. I would check everything related to the cables from the control sticks to the stabilator. I note in your blog that earlier you had a problem with getting the cable routing correct. I suggest checking the routing again and everything along the cable route from fore to aft.

A question, do you have the extra threads showing on both ends of both turnbuckles?

BTW, your blog is very nice. Good luck with the fiberglass work. It is not so bad once you get used to it!

As it would happen, I just discovered that I have Friday off work. Now I know where I'll be spending it. :eek:

To answer your question: yes, both cables have too many threads showing at both ends of the turnbuckles. I'll get back to this thread after I verify correct routing or find some other problem.

Thanks for the compliment on the blog. Your observation of my own comments just made the effort of keeping it up to date worthwhile.
 
routing good; tension still too high

BigJohn pointed me in a good direction of verifying that the cables were in fact tracking through the correct holes, since a simple mis-routing would very easily generate the differences in lengths that correspond to my current dilemma. I really had hoped that was the problem.

Alas, I could find no routing errors by inspection. As an aide to identification, I temporarily moved the rudder cable far enough forward such that the plastic sleeve (aft end) was within the space spanned by F-1207B and where the turnbuckles are located. Having positively identified the rudder cable as being in the correct location, I turned to the stabilator cables themselves. I removed the turnbuckle and pulled the upper (right) cable back to the pulley and verified that the slack rear cable was in the upper holes of F-1207B and F-1206A and that the empty holes were in fact the upper holes in F-1204A and F-1204D. This confirmed to me that the cables were in the correct runs.

The upper (right) forward cable was re-run back to the turnbuckle and I verified that each hole had only 1 cable in it. The F-1227 seat ramp cover was re-installed and the cables were re-tightened such that only 3 threads showed on each end of each turnbuckle. The resulting tension was off the scale of my tensionometer, approximately 100-120# by extrapolation. The resulting feel of the stick was far too stiff. The turnbuckles were loosened back to 45# tension on each cable. The upper cable has 8 threads showing (both ends) while the lower cable has 5 (both ends.)

I'm reaching the conclusion voiced by Mich48041, that I may simply have short cables. If that is the case, replacing them would be a haphazard approach since the replaced cables may also be too short. I would like opinions on using a longer turnbuckle to achieve both tension and adequate thread count.

Specifically, MS21251-B5S (provided) is 2.25 inches long, while a -B5L is 4.00 in length. The extra length of the -B5L would not result in encroachment of the barrel against the flare on the cables when the cables are positioned properly. This was verified by removing 1 cable from each turnbuckle, placing the turnbuckle at the point where 5 threads were showing, then advancing it 21 turns (7/8" at 24 threads/in) and verifying no interference. The longer barrels would result in having all of the threads from the cable termini within the barrels, thus 0 would be showing, and 0 is not more than 3.

Does anybody see a problem with this solution that I'm not seeing? Any other suggestions on sources of error with my build?

As always, thanks in advance,
-dbh
 
David,

I too struggled with getting the appropriate tension and thread showing ratio. I ended up sending my short cables back to Vans and getting new ones. This worked and took out all the frustration I experienced trying to get what I had to work.

It was the first time I had to adjust cables with required tension and turnbuckles with required threads showing. I kept thinking I was doing something wrong, the installation, the routing, something!

It was a conversation with someone at Vans and their immediate willingness to send other cables which helped me decide to give this change a try. Everything went together as shown in the plans and I was happy with the results now for a little over 2 years.

At least with the RV12, I am not big on re-design or moving away from the plans and what was designed. I'm not saying the different turnbuckles may not work, but I would try the cable change first so you could stick to the components in the plans. MHO
 
If the new longer turnbuckles had no threads showing after installation and proper tensioning, you would have no place to go in the future when the cables stretch a bit. I wouldn't do that. Get new cables from Vans.
 
sounds like it's time for me to put in a call to Van's next Monday. Thanks for all the feedback, gents.

-dbh
 
update

I called Van?s last Monday and spoke with Gus. He arrived at the same conclusion that the cables were just too short due to statistical variation and bad luck. He had not heard of / thought of longer turnbuckles and said there was no reason not to give them a try and was curious about how they might work, so I ordered a pair from Aircraft Spruce as an experiment.

Unfortunately, as predicted by BigJohn (that's twice in a row!), they were too long and I can?t get the proper tension on the cables. (Not to mention being able to adjust later when they creep after being in service for a while.) Cutting the turnbuckles to be a bit shorter is logical, but definitely strays into the realm of jerry-rigging rather than doing things correctly, so that idea remains an idle thought.


OK. Back to plan A. Just got off the phone with Joe and he?s sending me out two new pair of cables. Joe said I can send the old ones back for credit against my account, and thus don?t have to pay up front for the replacements. (Good thing?$430 for the pair!) If the second set still doesn't fit right, then I'm going to pay for BigJohn's plane ticket out to OH and ask him to finish figuring out what I'm doing wrong :eek: !
 
Success!

I finally had enough time to re-install the cable sets that Joe sent me three weeks ago. I tried just the rears (easier) first, but still too short. Pulled off the seat pan, removed the pulleys, replaced the front cables and reassembled everything. Perfect. Only 0 or 1 threads show when tension is 45 # at 62 ?F. Original cables are in the mail back to Van's.

Thanks for all the advice, gents.

-dbh
 
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