Crash
GONE WEST
Nikiski Alaska
Mark Reply
Mark
I believe my method for makng nicopress splices is both legal and as strong as the one you describe. The "typical" installation as described on the "FAA Methods and Techniques" manual is just that -- typical, and not mandatory.
In the section on thimble to eye splices they describe the "3-1-2" sequence of pressing, and the label under the diagram describes it as the "typical" procedure. I read that as not necessarily the "only" way to do it. They do require that your nicopress tool be in good condition and pressing fully. They also require that a sleeve gauge pass over the presses to make sure they are pressed all the way. These two items are required as I read it.
My A&P /I&A (30 years rebuilding Cubs) reccommended the "2-3-1" crimping order I use and since he oversees my work and signs off my plane as airworthy, I will use his crimping order. I came up with the cut off procedure for the tails. I also form the points of the thimble (as you do) so it will go up into the sleeve about half the length of the points. If you press the middle first, then the thimble end, it pushs the sleeve too far up onto the thimble in my opinion. I have also seen it cock the loop off to one side.
I do thank you for giving my son an idea for a science fair project called "The Weak Link in Aircraft Control Cables". We used a portable 4,000 pound shop crane and did some pull tests on 1/8", 7x19 galvenized control cable with a eye-to-fork turnbuckle, thimble, and nicopressed sleeve as is used on a PA-18 aileron horn.
See photos at:
ftp://ftp.supercub.org/upload/Crash1
(password is cub)
We did the first pull test and found the weakest link in the connection was the eye end of the turnbuckle; it snapped before the cable broke. We then nicopressed the ends without a turnbuckle installed to see if the end of the cable would slip and open before the cable snapped. I did one end with the "3-1-2" sequence like you describe, and the other end using the "2-3-1" sequence I described in my previous post.
As the tension increased with each pump of the handle, I started to hear strands of the cable break but nothing appeared, then POW. The crane almost did a back flip as the cable snapped. Upon close examination I found that the "Mark Darth / FAA 3-1-2" splice started breaking strands down inside the nicopress sleeve and snapped with only a few strands sticking outside the sleeve. The "2-3-1" splice had all the cable and showed no strands broken, hmmm.... If this was Thanksgiving and we were pulling the turkey's wishbone, you just lost.
I know this is not a conclusive test, done in a laboratory, and I should do it several times to prove my point. It tells me, though, that your theory is not necessarily correct about my splice not being as strong as yours. Either way, it's a moot point because the splice isn't the weakest link anyway. I don't think the sequence of compressing the sleeve has anything to do with the strength of the splice. The weakest points in a PA-18 control system in my opinion are, in order:
1) The aileron horn to aileron connection. The aileron horn (riveted to the aileron with aluminum rivets) will pull out of the aileron before anything else would let go.
2) The turnbuckle ends - they will snap before the cable will break.
3) The cable, which will break before the thimble loop and nicopress sleeve will slip or open up (at least with the "Crash Splice").
P.S. I measured the length of the sleeves on your "3-1-2" crimping method and my "2-3-1" crimping method and they were the same length when finished. Take care. Crash
Mark
I believe my method for makng nicopress splices is both legal and as strong as the one you describe. The "typical" installation as described on the "FAA Methods and Techniques" manual is just that -- typical, and not mandatory.
In the section on thimble to eye splices they describe the "3-1-2" sequence of pressing, and the label under the diagram describes it as the "typical" procedure. I read that as not necessarily the "only" way to do it. They do require that your nicopress tool be in good condition and pressing fully. They also require that a sleeve gauge pass over the presses to make sure they are pressed all the way. These two items are required as I read it.
My A&P /I&A (30 years rebuilding Cubs) reccommended the "2-3-1" crimping order I use and since he oversees my work and signs off my plane as airworthy, I will use his crimping order. I came up with the cut off procedure for the tails. I also form the points of the thimble (as you do) so it will go up into the sleeve about half the length of the points. If you press the middle first, then the thimble end, it pushs the sleeve too far up onto the thimble in my opinion. I have also seen it cock the loop off to one side.
I do thank you for giving my son an idea for a science fair project called "The Weak Link in Aircraft Control Cables". We used a portable 4,000 pound shop crane and did some pull tests on 1/8", 7x19 galvenized control cable with a eye-to-fork turnbuckle, thimble, and nicopressed sleeve as is used on a PA-18 aileron horn.
See photos at:
ftp://ftp.supercub.org/upload/Crash1
(password is cub)
We did the first pull test and found the weakest link in the connection was the eye end of the turnbuckle; it snapped before the cable broke. We then nicopressed the ends without a turnbuckle installed to see if the end of the cable would slip and open before the cable snapped. I did one end with the "3-1-2" sequence like you describe, and the other end using the "2-3-1" sequence I described in my previous post.
As the tension increased with each pump of the handle, I started to hear strands of the cable break but nothing appeared, then POW. The crane almost did a back flip as the cable snapped. Upon close examination I found that the "Mark Darth / FAA 3-1-2" splice started breaking strands down inside the nicopress sleeve and snapped with only a few strands sticking outside the sleeve. The "2-3-1" splice had all the cable and showed no strands broken, hmmm.... If this was Thanksgiving and we were pulling the turkey's wishbone, you just lost.
I know this is not a conclusive test, done in a laboratory, and I should do it several times to prove my point. It tells me, though, that your theory is not necessarily correct about my splice not being as strong as yours. Either way, it's a moot point because the splice isn't the weakest link anyway. I don't think the sequence of compressing the sleeve has anything to do with the strength of the splice. The weakest points in a PA-18 control system in my opinion are, in order:
1) The aileron horn to aileron connection. The aileron horn (riveted to the aileron with aluminum rivets) will pull out of the aileron before anything else would let go.
2) The turnbuckle ends - they will snap before the cable will break.
3) The cable, which will break before the thimble loop and nicopress sleeve will slip or open up (at least with the "Crash Splice").
P.S. I measured the length of the sleeves on your "3-1-2" crimping method and my "2-3-1" crimping method and they were the same length when finished. Take care. Crash
