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Stretching lineset back to trim

 


EOCS  (C License)

Dec 28, 2012, 3:30 AM
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Stretching lineset back to trim Can't Post

Has anyone heard of taking a Spectra lineset and stretching it back into trim? this is something ive heard about and have a friend who is jumping a canopy with the lineset stretched and he said it was like a new canopy.

Does anyone have experiences with this good or bad?


phoenixlpr  (D 3049)

Dec 28, 2012, 3:59 AM
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Re: [EOCS] Stretching lineset back to trim [In reply to] Can't Post

I have seen seen this kung-foo once done.
I can't remember any difference in flight.
There is a better trick out there: get a new Dacron or Vectran line-set instead.


EOCS  (C License)

Dec 28, 2012, 4:06 AM
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Re: [phoenixlpr] Stretching lineset back to trim [In reply to] Can't Post

I totally agree, just the idea of the practice seems kind of radical. i would think it could decrease the strength of the lines. I did ask PD about it and they had this to say.

''What little you may gain will then be lost again right away as the lines continue to heat shrink. My best guess is that he is not going to accomplish anything beneficial.''

I asked about it possibly weakening the lines but there was no comment on that. makes sense as its not something they recommend doing at all...


GLIDEANGLE  (D 30292)

Dec 28, 2012, 9:27 AM
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Re: [EOCS] Stretching lineset back to trim [In reply to] Can't Post

i was taught how to do it by Brian Germain.

When I tried it on my Spectra line set, the results were unimpressive. i suspect that my technique was poor.

I suggest that you ask him for more info.


adumbelk  (C 39695)

Dec 28, 2012, 10:49 AM
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Re: [EOCS] Stretching lineset back to trim [In reply to] Can't Post

The amount of noticeable change will depend on how many jumps your lines have, and how fast the openings are.

I jump a monarch 155 and it can spank like the girl next door. After about 4-500 jumps on the lines I was getting way too many rough openings and was told stretching the lines might help. After I did that she opened like a dream again (well, as soft as she can open).

At the club I jump at, 7hills, we have a hook screwed into one of the wall support beams. I unhooked the risers and linked them to the hook and was able to pull on the lines with my entire body. The whole process took me about 1.5-2 hours, and I was dripping sweat by the end, but it made a big difference. Before I stretched them, they were uneven by as much as 1.5 inches in some areas, and afterwards they were all the same length. I have some pictures of the before and after that I will try to find later today.

The way I did it was to put a metal bar through the end of the lines where it attaches to the canopy and pull on it for a good minute. After a couple of lines were stretched, I realized I could lean my entire body back to stretch them without fear of the lines breaking.

If you are uncomfortable doing it, talk to your rigger, but if you are up to the task it can really make a difference for the last hundred or so on the lineset.

Edit: Attached the picture. I can't remember which lines these were and unfortunately I don't have an after shot (I wanted to make the first load) but you can see how out of trim they are. Honestly, if you have the money on hand it will probably be easier to get it relined, and if you are looking for improved flight characteristics you might not find much by doing this. But, if you are feeling ambitious and are comfortable with the idea, then give it a try.


(This post was edited by adumbelk on Dec 28, 2012, 11:01 AM)
Attachments: 2012-07-04_08-52-28_982.jpg (177 KB)


Premier ianmdrennan  (D 25821)
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Dec 28, 2012, 10:49 AM
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Re: [GLIDEANGLE] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
i was taught how to do it by Brian Germain.

When I tried it on my Spectra line set, the results were unimpressive. i suspect that my technique was poor.

I suggest that you ask him for more info.

Reline the canopy.

If your car needed an oil change would you try and 'fix' the oil, or would you just do what needs to be done and change it.

I'm constantly fascinated as to how some people are looking for ways to cut corners on an article designed to save your life. I'd imagine you would frown on aircraft operators doing the same thing - why do it to yourself intentionally.

Ian


(This post was edited by ianmdrennan on Dec 28, 2012, 10:52 AM)


sundevil777  (D License)

Dec 28, 2012, 11:00 AM
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Re: [ianmdrennan] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
In reply to:
i was taught how to do it by Brian Germain.

When I tried it on my Spectra line set, the results were unimpressive. i suspect that my technique was poor.

I suggest that you ask him for more info.

Reline the canopy.

If your car needed an oil change would you try and 'fix' the oil, or would you just do what needs to be done and change it.

I'm constantly fascinated as to how some people are looking for ways to cut corners on an article designed to save your life. I'd imagine you would frown on aircraft operators doing the same thing - why do it to yourself intentionally.

Ian

The use of crappy analogies is not needed. What would be useful is a study of the result of stretching. Does it affect the strength or some other property? Perhaps it is a very reasonable thing to do and the fear of some ill effect is unfounded. I'm not saying that I'd try it, but if others are willing to do some experiments, I'm open to the possibility of good results.


Premier ianmdrennan  (D 25821)
Moderator
Dec 28, 2012, 11:46 AM
Post #8 of 73 (4387 views)
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Re: [sundevil777] Stretching lineset back to trim [In reply to] Can't Post

Whatever blows your hair back.


(This post was edited by ianmdrennan on Dec 28, 2012, 11:48 AM)


dragon2  (D 101989)

Dec 28, 2012, 11:48 AM
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Re: [EOCS] Stretching lineset back to trim [In reply to] Can't Post

Well Brian Germain stretched the lines on my vengeance once, and it def improved the canopy for a bit (eventually I had to reline it of course).


Deyan  (D 322)

Dec 28, 2012, 12:49 PM
Post #10 of 73 (4346 views)
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Re: [EOCS] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
Has anyone heard of taking a Spectra lineset and stretching it back into trim? this is something ive heard about and have a friend who is jumping a canopy with the lineset stretched and he said it was like a new canopy.

Does anyone have experiences with this good or bad?

This trick it's working only if you start stretching the lines from day one. What I mean is that if you have a new canopy and you stretch the lines every 30-50 jumps, you will have a good result. If you have a canopy with 4-500 jumps and you stretch the lines the trick would work for 5-10 jumps only.

My canopy has about 300 jumps ( Spectra 725 lines). Right now it's off trim by 2/3 inch. I'm stretching the lines twice per season ( which I found useless because the time I need for that is 45 min. to an hour and that mean I've spent 6 hours to keep the lines in trim. For the same time I can build my own lineset)

I can't comment on the strength of the lines. So far so good . I'll let you all know if I break a line .

After all, it's not worth the trouble. It takes time and if done wrong, it will give you funny openings or the canopy will not fly straight ( depend of the size of the canopy of course).
If your line set is new and you know how to do it , and you are willing to spend the time....go ahead. In any other case, reline is the answer .


Martini  (D 23756)

Dec 28, 2012, 4:29 PM
Post #11 of 73 (4248 views)
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Re: [EOCS] Stretching lineset back to trim [In reply to] Can't Post

I was also shown how to do this by Brian Germain. The difference it made on that canopy was remarkable. I've done it on several canopies, the results depend on how out of trim the canopy is. (more out of trim shows more benefit from stretching) The effect lasts remarkably long and the lines can be stretched more than once. Paraglider lines lose trim for a different reason but many pilots stretch their lines routinely 1-2 times per season.

Personally I believe that stretching Spectra parachute and paraglider lines 1-2 times per season is a good idea, keeping the lines tuned up between changes due to wear makes perfect sense as a maintenance routine. Parachute lines take me 30-45 minutes depending on condition. I don't worry about trim charts, the lines won't stretch past their original length. I'm primarily trying to attain symmetry, also working to stretch outside lines the most since they shrink the most due to slider heat.

Note that stretching does not work on HMA and Vectran. As always YMMV.


popsjumper  (D 999999999)

Dec 28, 2012, 6:08 PM
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Re: [sundevil777] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
What would be useful is a study of the result of stretching. Does it affect the strength or some other property? Perhaps it is a very reasonable thing to do and the fear of some ill effect is unfounded. I'm not saying that I'd try it, but if others are willing to do some experiments, I'm open to the possibility of good results.

I'm with Ian on this. Everyone get off your cheap ass and do the right thing. You are seeing worst case here. People doing stuff with no factual back-up other than somebody's opinion. And no, just because it was Germaine doesn't make opinion any more valid without factual back-up.

We hammer students to never get involved in that flaky business and here we go doing it ourselves. Great example for them, eh?

And I am also with you in this: Leave it for the test jumpers. They will provide the answers whether or not the practice is safe enough and provides any value-added.


EOCS  (C License)

Dec 28, 2012, 6:30 PM
Post #13 of 73 (4195 views)
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Re: [EOCS] Stretching lineset back to trim [In reply to] Can't Post

Quite a few replies here with different opinions. Ive personally no need for new lines or stretching but am interested in the topic more then ever now.. I too would like to see some tests done to see if it actually changes the strength of the line. i personally dont have the equipment for it. Interesting idea and im hoping someone comes along with some data. Does anyone know if this is an old practice or something more recent. i would think the longer its been in use without problems the more reliable it is.

Thanks


Martini  (D 23756)

Dec 28, 2012, 7:31 PM
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Re: [popsjumper] Stretching lineset back to trim [In reply to] Can't Post

Here's a little fact: My lines are vastly closer to stock trim after I stretch them than before. This particularly applies to symmetry.

Safety-wise I am not aware of an increase in line breakage due to this practice. Broken Spectra lines seem uncommon even on badly worn linesets. If line breakage isn't a problem and openings, flight and landings are improved by better line trim then I would say safety is improved by using this technique. The only potential issue I see is brake lines snapping during a swoop, that would merit some study to see if there might be some vulnerability.

Value added? For 30 minutes of free labor (you can do this during a weather hold or whatever) your lines are in far better trim.

You're right about one thing, people are mostly too lazy to do this.

(edited to correct "break" to "brake" Wink )


(This post was edited by Martini on Dec 28, 2012, 9:20 PM)


talon2

Dec 28, 2012, 9:58 PM
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Re: [Martini] Stretching lineset back to trim [In reply to] Can't Post

yep ,when my tires go bald I rotate them .....what a crock


Martini  (D 23756)

Dec 28, 2012, 10:01 PM
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Re: [talon2] Stretching lineset back to trim [In reply to] Can't Post

Terrific analogy Crazy


pnuwin  (D 112233)

Dec 29, 2012, 1:47 AM
Post #17 of 73 (4073 views)
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Re: [Martini] Stretching lineset back to trim [In reply to] Can't Post

Are there any videos or tutorials for stretching spectra back into trim? How did you hang onto the end of the line and how much weight did you put on it?


Martini  (D 23756)

Dec 29, 2012, 9:49 AM
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Re: [pnuwin] Stretching lineset back to trim [In reply to] Can't Post

No tutorials that I know of. It would be nice if Brian spoke up here, he has by far the most knowledge on this technique. This subject isn't new, there are posts going back several years. I use as much weight as I can by sitting or kneeling on a carpeted floor and leaning back. I am careful not to stress the attachment point at the canopy fabric. I found that I could hold on better without pulling on canopy fabric by inserting a rod through the finger trapped line loop so the rod rested on the attachment material. I don't know if this is "recommended" practice but it seemed the best solution to hanging on to the line.

I use a different method on my paragliders, using a 40 lb weight suspended by a pulley I stretch each line to the same pressure for the same count (like 10 seconds). There seems to be less controversy in the PG world about this, not everyone does it but its pretty common.


timgaines  (C 107228)

Dec 29, 2012, 10:33 AM
Post #19 of 73 (3946 views)
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Re: [EOCS] Stretching lineset back to trim [In reply to] Can't Post

Not a rigger, but Spectra is basically a form of polyethylene. To make it into a line it gets drawn out into long thin fibres. During this process the long polymer chains that make it up essentially get 'untangled' and get arrange neatly in order (think more like uncooked spaghetti compared to cooked spaghetti).

When it gets heated (due to friction of the slider running down the lines) this gives these polymer chains energy to start tangling themselves back up. These tangled polymers take up less space than untangled polymers making your line shrink.

Once this has happened the quoted tensile strengths for the spectra has dropped along with its limits for elastic and plastic deformation.
Will stretching this new material will make the line substantially weaker than the original line? Yes.
Will this new, stretched line be within the tolerances of skydiving? Well it seems like people have done it and it works... there are comments that people in the PG community have done it but the big stresses for a parachute (opening shock) don't really happen much for a PG...


Martini  (D 23756)

Dec 29, 2012, 10:47 AM
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Re: [timgaines] Stretching lineset back to trim [In reply to] Can't Post

Quote:
Will stretching this new material will make the line substantially weaker than the original line? Yes.

Good to have info on this. What is your source? How much is Spectra weakened by heat shrinkage? How much further weakening is caused by stretching the shrunken line?


hookitt  (D License)

Dec 29, 2012, 10:49 AM
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Re: [Martini] Stretching lineset back to trim [In reply to] Can't Post

Personally, if I had known about rotating bald tires, I would have rotated them my entire skydiving career.I used to put a lot of jumps on Spectra and just dealt with the crummy trim. I believe it would have helped me. I would say the reason few people knows about it, is that many people will damage their own equipment so its not even brought up.

Edit. plus I'm sure there are rules that people do not want to break about the manufacturer's recommendations.


(This post was edited by hookitt on Dec 29, 2012, 11:02 AM)


phoenixlpr  (D 3049)

Dec 29, 2012, 11:41 AM
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Re: [hookitt] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
I would say the reason few people knows about it, is that many people will damage their own equipment so its not even brought up.
I'd rather say Spectra is going out of fashion, HMA and Vectran are used on more and more canopies.


Martini  (D 23756)

Dec 29, 2012, 12:54 PM
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Re: [phoenixlpr] Stretching lineset back to trim [In reply to] Can't Post

Are you implying that my Sabres lined with Spectra aren't fashionable? Shocked


Martini  (D 23756)

Dec 29, 2012, 1:07 PM
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Re: [ianmdrennan] Stretching lineset back to trim [In reply to] Can't Post

Quote:
If your car needed an oil change would you try and 'fix' the oil, or would you just do what needs to be done and change it.

If your guitar was out of tune would you change the strings? More than likely you would tune the guitar by adjusting the strings many times before deciding to replace them.

In any event canopy suspension lines are a unique case, awkward analogies don't mean much.


popsjumper  (D 999999999)

Dec 29, 2012, 1:09 PM
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Re: [hookitt] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
....I used to put a lot of jumps on Spectra and just dealt with the crummy trim...
Funny you should mention that.
I had a new line set put on and the rigger told me that the old set had so many jumps on it and was so far out of trim that I would have to re-learn flying and landing.
LaughLaughLaugh


popsjumper  (D 999999999)

Dec 29, 2012, 1:12 PM
Post #26 of 73 (1002 views)
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Re: [Martini] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
Quote:
If your car needed an oil change would you try and 'fix' the oil, or would you just do what needs to be done and change it.

If your guitar was out of tune would you change the strings? More than likely you would tune the guitar by adjusting the strings many times before deciding to replace them.

In any event canopy suspension lines are a unique case, awkward analogies don't mean much.
Speaking of awkward analogies.
LaughLaughLaugh

Hmmmmm. I can't remember the last time my life and well-being depended on guitar strings. But then, I'm an old fart and conservative so take it from there.

Do the right thing and buy or make a new line set.


(This post was edited by popsjumper on Dec 29, 2012, 1:12 PM)


pnuwin  (D 112233)

Dec 29, 2012, 2:53 PM
Post #27 of 73 (987 views)
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Re: [phoenixlpr] Stretching lineset back to trim [In reply to] Can't Post

Quote:
I'd rather say Spectra is going out of fashion, HMA and Vectran are used on more and more canopies.

HMA and Vectran have their own disadvantages.

HMA is very sensitive to UV light. It loses a lot of strength when exposed to the sun for relatively short periods of time. According to PD, it also loses its strength when experiencing peak loads during a hard opening for example. Both HMA and Vectran are less resistant to wear.

Spectra is low bulk, very resistant to wear and nice to pack with its naturally slippery feel. If we could solve the trim/shrinkage issue, it would be my preferred line type.


Martini  (D 23756)

Dec 29, 2012, 3:53 PM
Post #28 of 73 (972 views)
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Re: [popsjumper] Stretching lineset back to trim [In reply to] Can't Post

Quote:
Do the right thing and buy or make a new line set.

The right thing according to who? Linesets don't simply go from "good" to "bad" as you are obviously aware. They begin to go out of trim gradually and start the process from jump #1. If better trim quality can be safely maintained by a simple procedure then that is the right thing to do.

Have you ever tried this procedure? Do you have evidence that it has no merit or that it is dangerous? If not then your opinion isn't particularly useful and contributes nothing to this discussion. I'm not a rigger, I claim no special scientific knowledge. But I have used this technique on three of my own canopies with varying degrees of success and no adverse results. I was shown this process by a man who has many years of canopy design and construction experience, this experience gives him a great deal of credibility. So continue to be a conservative old fart naysayer or instead maybe take a more open-minded approach. Either way, blue skies Pops.


popsjumper  (D 999999999)

Dec 29, 2012, 6:13 PM
Post #29 of 73 (951 views)
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Re: [Martini] Stretching lineset back to trim [In reply to] Can't Post

Quote:
Do the right thing and buy or make a new line set.

In reply to:
The right thing according to who? Linesets don't simply go from "good" to "bad" as you are obviously aware.
You are right. I am aware. Did you have a particular reason for re-stating the obvious?

In reply to:
If better trim quality can be safely maintained by a simple procedure then that is the right thing to do.

Keyword "IF". You seem to think this has been tested and recommended by manufacturers of any type. Run it through the mill and see for sure. As it stands, you have no basis in fact.

OK you just want to argue.

In reply to:
Have you ever tried this procedure? Do you have evidence that it has no merit or that it is dangerous?
Nope on both counts. And, like you, I have no evidence that it does have merit.

The fact that I don't have, nor do you have, evidence one way or the other is why I recommend "buy the damned lineset". What makes you think I should do this on YOUR recommendation? There's already info out there that says it weakens lines. You choose to disregard that?
And, until this idea gets formally tested, I will NOT try this "procedure".

My contribution, because you will not see it, is your method has not been tested for safety. If you want to be one of those who will blindly try anything anybody tells you to do, have at it. I would ask that you don't lead our young jumpers down that path, thank you.

There are no Gods in this sport. Everybody is subject to error, regardless of past history. Anybody that tells you to do something they dreamed up just may be wrong...regardless of who they are but you wouldn't know it.

Again...Buy the damned lineset and be done with it. The activity has not been tested for safety validity that we know of as yet.

Good day, sir.


(This post was edited by popsjumper on Dec 29, 2012, 6:15 PM)


Martini  (D 23756)

Dec 29, 2012, 7:09 PM
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Re: [popsjumper] Stretching lineset back to trim [In reply to] Can't Post

Your position then is that my canopy should be jumped until it needs a new lineset and I should just "Buy the damned lineset and be done with it". That's a really fascinating statement since you have no idea of what canopy we're discussing, it's condition, the number of jumps on the lines or their condition and trim or the performance degradation of this canopy if any. You couldn't possibly know these things because the canopy in question is purely hypothetical. I'm in no way insisting that you use this process, that's your decision. I believe that this is a useful topic and that the OP asked a reasonable question. If your contribution here is that this technique is untested then we allready have agreed that no formal testing has been done. Since I have not had nor heard of any issues I'll continue to use this technique. You do your thing, I'll do mine. Continuing to insist that I or someone else buy a new lineset is dogma.

By the way I do have evidence that this idea has merit, that's why I use it. The evidence is simple: I end up with lines that are measurably more in trim than before. One canopy in particular demonstrated markedly improved openings after the lines were brought closer to spec trim. Is this scientific proof? Obviously not. Am I concerned about safety? Of course. If I see evidence that stretching lines weakens them to a degree where they are unsafe then I'll reconsider, so far that's been anecdotal. Doing casual internet research on that subject has brought no result. Since line strength degrades inevitably and as a factor of many environmental conditions it would be interesting to see the effect of line shrinkage and stretching.


(This post was edited by Martini on Dec 29, 2012, 7:27 PM)


kuai43  (C License)

Dec 29, 2012, 11:03 PM
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Re: [Martini] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
Your position then is that my canopy should be jumped until it needs a new lineset and I should just "Buy the damned lineset and be done with it". That's a really fascinating statement since you have no idea of what canopy we're discussing, it's condition, the number of jumps on the lines or their condition and trim or the performance degradation of this canopy if any. You couldn't possibly know these things because the canopy in question is purely hypothetical. I'm in no way insisting that you use this process, that's your decision. I believe that this is a useful topic and that the OP asked a reasonable question. If your contribution here is that this technique is untested then we allready have agreed that no formal testing has been done. Since I have not had nor heard of any issues I'll continue to use this technique. You do your thing, I'll do mine. Continuing to insist that I or someone else buy a new lineset is dogma.

By the way I do have evidence that this idea has merit, that's why I use it. The evidence is simple: I end up with lines that are measurably more in trim than before. One canopy in particular demonstrated markedly improved openings after the lines were brought closer to spec trim. Is this scientific proof? Obviously not. Am I concerned about safety? Of course. If I see evidence that stretching lines weakens them to a degree where they are unsafe then I'll reconsider, so far that's been anecdotal. Doing casual internet research on that subject has brought no result. Since line strength degrades inevitably and as a factor of many environmental conditions it would be interesting to see the effect of line shrinkage and stretching.

+1
After all, you can't trust those infernal horseless carriages anyway, muh horse's always got me where I'm-a goin', dag-nabbit! Wink


Premier ianmdrennan  (D 25821)
Moderator
Dec 30, 2012, 1:00 AM
Post #32 of 73 (880 views)
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Re: [Martini] Stretching lineset back to trim [In reply to] Can't Post

If this was considered a good idea, manufacturers would offer a "retrim stretch" option...after all, there'd be money to be made.

Just saying Wink

I don't believe anyone is arguing against the improved opening (BECAUSE the lineset is more in trim), but the bigger questions is: At what cost?

Ian


(This post was edited by ianmdrennan on Dec 30, 2012, 1:01 AM)


popsjumper  (D 999999999)

Dec 30, 2012, 5:41 AM
Post #33 of 73 (840 views)
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Re: [Martini] Stretching lineset back to trim [In reply to] Can't Post

OK you are missing or denying the point so have at it.

-Canopy type has nothing to do with it so why interject that?
-Reasonable question? Yes, of course.
-I have evidence that skydiving won't kill me. I haven't been killed yet. Hey! It's the same reasoning you are using.
- I certainly hope that the evidence you see about the strength issue is not breakage on a HP landing.
-Pretty much all canopies will demonstrate better openings with lines in trim...nothing new there.
-I have no vested interest in your well-being...the sport does.

I am not trying to make you change your ways. Simply pointing out the fallacy of your reasoning. You are welcome to continue in your vein. Again, please do not propagate that type of reasoning to our youngsters.

Go ahead and reply. I'll not reply to you again. Let the thread proceed as it will.

And again, Good day, sir.


popsjumper  (D 999999999)

Dec 30, 2012, 6:00 AM
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Re: [kuai43] Stretching lineset back to trim [In reply to] Can't Post

Understanding the history of the sport and the problems encountered and overcome will help you understand the reasons why the old-timers are trying to teach you what they know.

Trying to help the youngsters avoid making the same mistakes others have made in the past is getting to be a thankless job. Kids apparently want history to repeat itself. I sincerely hope you do not fall into that fallacy.

Those who stick around for a while always eventually come to find that they were off-base about much of their "youngster" ideas. Knowing and understanding the history will help speed up that precess.
Talk to your guys at Orange....listen to what they have to say.


timgaines  (C 107228)

Dec 30, 2012, 6:24 AM
Post #35 of 73 (828 views)
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Re: [Martini] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
Quote:
Will stretching this new material will make the line substantially weaker than the original line? Yes.

Good to have info on this. What is your source? How much is Spectra weakened by heat shrinkage? How much further weakening is caused by stretching the shrunken line?

Physics/Material Sciences/Engineering 101

The exact tensile strengths, I am struggling to find, mostly because a lot of this information isn't freely distributed.
There is a paper here but you cant view it unless you are a subscriber or you buy it but some of the figures are visible.
http://www.sciencedirect.com/...ii/S0142941802001277

Look down to figure 12 which is the closest scenario to this. Increasing temperature reduces it's Youngs Modulus and Tensile Stregnth. No material is 100% elastic in it's properties (it won't go back to exactly how it was when you started) so constantly heating the lines 100's of times will weaken this. No weak analogies required.

Once you then start stretching your now weakened fibres, that remaining tensile strength has to be distributed over it's new length ie If it was 10 metres long and had a tensile strength of 100MPa but you now make it 20 metres long it only has a tensile strength of 50MPa.


Premier ianmdrennan  (D 25821)
Moderator
Dec 30, 2012, 7:42 AM
Post #36 of 73 (813 views)
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Re: [popsjumper] Stretching lineset back to trim [In reply to] Can't Post

Quote:
I have evidence that skydiving won't kill me. I haven't been killed yet. Hey! It's the same reasoning you are using.

Even outside of the context of this thread, this is an incredibly important statement for skydivers to realize. I've been reading a spectacular book "Deep Survival" (that I recommend ALL skydivers read btw) and while it has nothing to do with skydiving there are a lot of things that help make sense of why we learn, and do, certain things. One topic covered is "Negative Reinforcement"....essentially: Doing the same thing 1000 times with success is not indication of a good behavior but rather that the right set of conditions haven't occurred yet to highlight the errors in assumption (Paraphrased).

Ian


(This post was edited by ianmdrennan on Dec 30, 2012, 7:43 AM)


AggieDave  (D License)

Dec 30, 2012, 8:35 AM
Post #37 of 73 (789 views)
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Re: [ianmdrennan] Stretching lineset back to trim [In reply to] Can't Post

Quote:
Doing the same thing 1000 times with success is not indication of a good behavior but rather that the right set of conditions haven't occurred yet to highlight the errors in assumption (Paraphrased).

To be more direct, I would phrase it "doing the same thing 1000 times with out consequence is not an indication of good behavior, but rather that the right set of conditions haven't occurred yet to highlight the errors in assumption."

Our sport's history is littered with examples that highlight this in many different ways. From gear design, gear modification, to aircraft operations and maintenance, skydiver decisions and judgement.


Martini  (D 23756)

Dec 30, 2012, 8:55 AM
Post #38 of 73 (783 views)
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Re: [timgaines] Stretching lineset back to trim [In reply to] Can't Post

I can't see the data but thanks for the research. Is the reduction in strength described while the material is still heated or after it has returned to ambient? BTW I would be surprised if heating/shrinking/stretching didn't degrade tensile strength, the question is by how much especially in the skydiving environment.

Also note that the lines will never be stretched beyond their original length. What's more is that the shrinkage/stretching is relatively small by percentage. For example if an "A" line on my Sabre-120, which is 278 cm long, shrinks by 10 cm (which is nearly 4 inches) that's less than 4%.

I'm not disputing the likelihood of tensile loss, only interested in whether the loss is of significance in lines that are incredibly strong and not typically highly loaded.


Martini  (D 23756)

Dec 30, 2012, 9:06 AM
Post #39 of 73 (778 views)
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Re: [popsjumper] Stretching lineset back to trim [In reply to] Can't Post

We can certainly agree to disagree to some extent. Here's the original post:

"Has anyone heard of taking a Spectra lineset and stretching it back into trim? this is something ive heard about and have a friend who is jumping a canopy with the lineset stretched and he said it was like a new canopy.

Does anyone have experiences with this good or bad?"

My response to his question is yes I have experience with this and it has been good.

So the breakdown becomes a matter of functionality which I have proven to myself and safety which is not proven except by default. My perception is that this is a low-risk behavior.

And I agree that you and I have no need for further argument.


sundevil777  (D License)

Dec 30, 2012, 9:36 AM
Post #40 of 73 (768 views)
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Re: [timgaines] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
Once you then start stretching your now weakened fibres, that remaining tensile strength has to be distributed over it's new length ie If it was 10 metres long and had a tensile strength of 100MPa but you now make it 20 metres long it only has a tensile strength of 50MPa.

From what do you make this conclusion? The fibers are not like a ductile metal.


Martini  (D 23756)

Dec 30, 2012, 9:45 AM
Post #41 of 73 (774 views)
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Re: [ianmdrennan] Stretching lineset back to trim [In reply to] Can't Post

Ian, I don't believe that manufacturers will support line stretching. There's money in selling linesets that most skydivers can't install themselves vs. selling a service that any jumper can easily perform and would never buy. Lineset replacement is unavoidable and stretching the lines may or may not extend the useful life of a lineset but may give better performance during that time.

"Deep Survival"........what a terrific book. "Doing the same thing 1000 times with success is not indication of a good behavior but rather that the right set of conditions haven't occurred yet to highlight the errors in assumption (Paraphrased)." is a pretty well established philosophy though, we are always surprised when bitten by our family dog.

Like anything that involves risk, swimming in uncharted waters should be done thoughtfully. That is not to say that we should never attempt to experiment or change at all. In this case I believe the risk to be low enough to justify the benefit. I perceive the risk of causing a line to break to be small while receiving a perceptible benefit in improved line trim. I feel fortunate that this option was shown to me and I choose to use it. It's interesting that skydivers readily accept Spectra shrinkage without wondering if there is a remedy, it's reasonable that they would be wary of an unfamiliar one.


AggieDave  (D License)

Dec 30, 2012, 12:11 PM
Post #42 of 73 (751 views)
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Re: [Martini] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
Ian, I don't believe that manufacturers will support line stretching. There's money in selling linesets that most skydivers can't install themselves vs. selling a service that any jumper can easily perform and would never buy. Lineset replacement is unavoidable and stretching the lines may or may not extend the useful life of a lineset but may give better performance during that time.

So let me get this straight, you believe that no one will tell you what you want to hear because it is a vast conspiracy to pad the profits of the canopy manufactures?

Right...


sundevil777  (D License)

Dec 30, 2012, 12:48 PM
Post #43 of 73 (741 views)
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Re: [AggieDave] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
In reply to:
Ian, I don't believe that manufacturers will support line stretching. There's money in selling linesets that most skydivers can't install themselves vs. selling a service that any jumper can easily perform and would never buy. Lineset replacement is unavoidable and stretching the lines may or may not extend the useful life of a lineset but may give better performance during that time.

So let me get this straight, you believe that no one will tell you what you want to hear because it is a vast conspiracy to pad the profits of the canopy manufactures?

Right...

A former canopy mfg/designer did already tell him - not what he wants to hear, but that isn't what you want to hear.

The mechanism of the lines shrinking may indeed lend itself to stretching with no ill effect. I definitely do not think this is so unlikely.


(This post was edited by sundevil777 on Dec 30, 2012, 12:50 PM)


JerryBaumchen  (D 1543)

Dec 30, 2012, 1:06 PM
Post #44 of 73 (732 views)
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Re: [timgaines] Stretching lineset back to trim [In reply to] Can't Post

Hi tim,

Quote:
tensile strength has to be distributed over it's new length

I might not be the brightest Mech. Engr. to get a sheepskin, but it has always been my understanding that tensile strength is a function of the area of the material and not the length.

Just what I have learned, I 'think,'

JerryBaumchen


AggieDave  (D License)

Dec 30, 2012, 1:22 PM
Post #45 of 73 (726 views)
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Re: [sundevil777] Stretching lineset back to trim [In reply to] Can't Post

Edit:

I realized that I really don't care. Do what you want with your canopies, get your linsets to "last" 1000 jumps. Good luck.


(This post was edited by AggieDave on Dec 30, 2012, 1:28 PM)


julio_gyn  (A 705)

Dec 30, 2012, 2:20 PM
Post #46 of 73 (712 views)
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Re: [Martini] Stretching lineset back to trim [In reply to] Can't Post

I know nothing about line strength, but the lines will build up heat from the top down on the lines, making me think the lines will be shrinking more on the bottom. and whenever you stretch it again you can't control where it will happen, so it will be weakening more some part of the lines (maybe bottom?) while the stronger (top?) will stretch less....

and again, I'm just following the thread and trying to see the scenario on real life....


5.samadhi

Dec 30, 2012, 2:32 PM
Post #47 of 73 (706 views)
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Re: [AggieDave] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
In reply to:
Ian, I don't believe that manufacturers will support line stretching. There's money in selling linesets that most skydivers can't install themselves vs. selling a service that any jumper can easily perform and would never buy. Lineset replacement is unavoidable and stretching the lines may or may not extend the useful life of a lineset but may give better performance during that time.

So let me get this straight, you believe that no one will tell you what you want to hear because it is a vast conspiracy to pad the profits of the canopy manufactures?

Right...
Thats a little dramatic Smile
I think he is trying to say that you won't get support from manufacturers on this topic (although waiting for BG to chime in?) because they have a financial bias that strongly predisposes them toward being conservative on this issue. Not saying they are being malicious or purposefully deceptive, just that the money bias they have toward the issue is muddling their subjectivity.

Does that make sense to you? That is how I took his reasoning (you were not being charitable to him and were attempting to label his line of reasoning as illogical because you disagree with his idea? you are bored? WHatever).


sundevil777  (D License)

Dec 30, 2012, 2:37 PM
Post #48 of 73 (703 views)
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Re: [julio_gyn] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
the lines will build up heat from the top down on the lines,

How do you make this conclusion?

In reply to:
whenever you stretch it again you can't control where it will happen,

"Control" may not be possible in the strict sense, but it might be that the areas that shrunk are where the stretching will occur. It depends on the mechanism of how the fibers shrink, and what happens during stretching.

The lack of interest by some to want to know is, interesting.


Martini  (D 23756)

Dec 30, 2012, 3:26 PM
Post #49 of 73 (684 views)
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Re: [AggieDave] Stretching lineset back to trim [In reply to] Can't Post

Well Dave, if that's what you heard then it's only because that's what you wanted to hear. Right...

Do you have something useful to say or just want to stir the pot?


AggieDave  (D License)

Dec 30, 2012, 3:28 PM
Post #50 of 73 (682 views)
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Re: [Martini] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
Well Dave, if that's what you heard then it's only because that's what you wanted to hear. Right...

Do you have something useful to say or just want to stir the pot?

I did and had a long post written out about it, then realized that I really don't care and people will continue to do what they want regardless. Hence the previously shown edited comment.

Do what you will and best of luck for it.


Martini  (D 23756)

Dec 30, 2012, 3:44 PM
Post #51 of 73 (1385 views)
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Re: [julio_gyn] Stretching lineset back to trim [In reply to] Can't Post

I think I get what you're thinking, that the slider starts with zero velocity/zero heat at the top and builds heat as it travels downward. The first part is somewhat correct but max velocity is not necessarily at the bottom of the lines because the slider may slow down considerably before hitting the risers. Also the slider may reach its max velocity very near the starting point depending on how the canopy opens.

As far as stretching goes, all of the stretching should take place in the shrunken areas because those areas are shrunken due to "curling" of the heated fibers. The unaffected fibers are straight and won't significantly stretch. Spectra lines are considered non-stretch just like HMA or Vectran. The problem is that Spectra isn't non-shrink like those fibers.


Martini  (D 23756)

Dec 30, 2012, 4:11 PM
Post #52 of 73 (1377 views)
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Re: [AggieDave] Stretching lineset back to trim [In reply to] Can't Post

Too bad, you often post good stuff. I'm not sure why you, as well as others, have chosen an adversarial position on this topic. And I disagree, people can be influenced to change what they do. If John LeBlanc or Bill Booth supported this technique it would be the norm. If they came out against it the discussion would be over. I don't expect that either of them will publicize an opinion so it is up to us to discuss the subject and if possible study it to reach a sensible conclusion. But if you really don't care then that's OK too, I expect at this point most people don't care either. But this isn't like discussing an RDS that will only ever affect a few, if stretching was a generally accepted tool it could affect many.

I didn't invent this process and I don't mean to deliberately promote it but I think that it merits discussion. I also believe that since I have used the technique and have given it some consideration that I have something to contribute here.


AggieDave  (D License)

Dec 30, 2012, 4:25 PM
Post #53 of 73 (1370 views)
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Re: [Martini] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
I didn't invent this process and I don't mean to deliberately promote it but I think that it merits discussion. I also believe that since I have used the technique and have given it some consideration that I have something to contribute here.

The short version is that line design has changed and what we expect out of our canopies has changed. For a good number of people in the sport, gone are the days of dacron and even spectre lines. What you're left with are line designs that not only don't respond to attempts to stretch the lines, but lines that could even be damaged by the forces their unknowing owners applied due to reading a handful of posts.

Also, the large majority of people reading this thread have zero idea how to measure line trim and could only line up the line groups and see that some don't match. Just because the outside A-lines don't match the inside A-lines doesn't mean that any of them are wrong, it depends on the canopy. Then you're dealing with the cascades and even some sections that are mixed line types (like on control lines).

Basically in the hands of 80-90% of the modern fun jumper population, trying to "stretch" lines will cause nothing but grief. The other 10-20% will either not read this or will already know the answer.

With those newer design line types, jumpers are much better off replacing their linesets a little before they notice wildly bad openings or have lines break. Marat gave us an extreme example of that when his canopy loaded from a thermal after rounding out of his dive, obviously his case is beyond what the average jumper would accomplish; however, how many "average" jumpers have you personally seen at a DZ jumping X-braced canopies with HMA lines? Pulling the spectre lines on your Sabre1 210 didn't really do much in either direction besides maybe temporarily clean up an opening. Back when folks were jumping in the neighborhood of 1:1 it really didn't effect much either way. Now in the world of 1.5+:1 for a weekend jumper, things could be exaggerated to the point of injuring someone (through a violent opening, for example).

Your response would be that this technique is for line types that would respond to such methods, how many of your average jumpers in the world, scanning through DZ.com can even name more than a couple line types? How many of them know exactly what is on their canopy?

That is why I'm opposed to a general speculation thread like this and I'm generally opposed to this technique.


Martini  (D 23756)

Dec 30, 2012, 5:37 PM
Post #54 of 73 (1357 views)
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Re: [AggieDave] Stretching lineset back to trim [In reply to] Can't Post

Thanks for the thoughtful and well written post.

In my first post I wrote that HMA and Vectran do not respond to stretching but you're right, there are many who can't distinguish Spectra from string cheese.

The stretching I referred to was on a Sabre 120 (not 210) loaded at 1.6-1.7, it was deliberately done to clean up twisting openings. The lineset was not only out of trim lengthwise but more significantly was badly asymmetrical. The stretching adjustment had the desired effect and lasted about 100 jumps, I did it again to good effect but after that I had it relined.

While use of this method probably should be something that is either taught or performed by a competent rigger some people seeking do it on their own would be good with a clear set of instructions and video specific to their canopy type. While I appreciate your point of view regarding potential pitfalls I would prefer not to disregard a tool that I, as well as others, have found to be useful. I have never attempted to give detailed instruction on this technique and wouldn't except with qualified people and on the same canopy type. I'll always be willing to discuss this topic though, thanks for contributing more depth to the subject.


sundevil777  (D License)

Dec 30, 2012, 6:19 PM
Post #55 of 73 (1349 views)
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Re: [Martini] Stretching lineset back to trim [In reply to] Can't Post

Another thing that should be kept in mind is that the stretching that has been applied, to my understanding - how it has been described, is well under the load that a line should easily withstand. So, why should applying a load so much under that rated load cause anyone to assume bad consequences would result?


(This post was edited by sundevil777 on Dec 30, 2012, 6:19 PM)


Martini  (D 23756)

Dec 30, 2012, 10:33 PM
Post #56 of 73 (1329 views)
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Re: [sundevil777] Stretching lineset back to trim [In reply to] Can't Post

Two reasons.

Stretching could result in breaking or at least thinning some of the fibers, in fact that seems very likely. Not all fibers are at the same stress level, the line is not perfectly homogenous to begin with and heat/shrinkage will most certainly not be uniform.

Some degree of makeup may be a factor similar to steel with grains, crystallization, work hardening, micro tears etc. In other words shrinking and stretching might end up with material that has physical properties that differ from the original.


BrianSGermain  (D 11154)

Dec 31, 2012, 10:47 AM
Post #57 of 73 (1275 views)
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Re: [EOCS] Stretching lineset back to trim [In reply to] Can't Post

PD has not tested this in any way, therefore their oppinion is as uselful as someone you meet on the bus. It is unwise to advise beyond your data.

I have developed this method over many years, and I have data to support the conclusion that this is a hugely valuable technique. It does not last forever, but the shrinking is a long slow process, and it takes as long to back out of trim as it took to get that way in the fear place.

Heat shrinking is only part of the dimensional change on a spectra lineset. My original article in parachutist magazine on this topic is what PD is basing their information on, but I have learned more since them. It appears that heat shrinking is not sufficient to cause the magnitude of the change that we are seeing. Lack of loading must therefore be a significant aspect of the difference between the outboard and inboar lines. It is a combination of these two forces that causes a canopy to become "out of trim". Therefore, the direct loading of the "unloaded" lines, including the brakes, changes the parachute significantly, and for a useful duration.

I find that most canopies with 500 jumps on it has a differential of 3 inches or so. This can be completely irradicated with this technique. That is a singnificant difference both in statistics and in useful practice.

Regarding all the guessing about stressing the lines I have no evidence to support the conclusion that stretching the lines weakens them in any significant way. Controlled test results are coming shortly.

-Brian


(This post was edited by BrianSGermain on Dec 31, 2012, 1:03 PM)


Martini  (D 23756)

Dec 31, 2012, 10:53 AM
Post #58 of 73 (1269 views)
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Re: [BrianSGermain] Stretching lineset back to trim [In reply to] Can't Post

Quote:
This can be completely irradiated with this technique.

Then I'm never using the technique again, radioactivity scares me. Wink


wolfriverjoe  (A 50013)

Dec 31, 2012, 11:07 AM
Post #59 of 73 (1263 views)
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Re: [BrianSGermain] Stretching lineset back to trim [In reply to] Can't Post

Thanks for the input, Brian.

Has anyone done any strength testing of the stretched lines?

Has anyone taken lines that were stretched several times before replacement and then pulled them until they broke to see if there is any significant loss of strength?

I really can't see doing this to a material without losing some of the strength, but I'd like to know if it's enough to cause concern.


BrianSGermain  (D 11154)

Dec 31, 2012, 1:04 PM
Post #60 of 73 (1239 views)
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Re: [wolfriverjoe] Stretching lineset back to trim [In reply to] Can't Post

As I mentioned, there is no evidence of a significant reduction in the tensile strength of the line post stretching.


julio_gyn  (A 705)

Dec 31, 2012, 2:50 PM
Post #61 of 73 (1218 views)
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Re: [sundevil777] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
In reply to:
the lines will build up heat from the top down on the lines,

How do you make this conclusion?

well, I assume the slider will accelerate as it start its descent...

In reply to:
whenever you stretch it again you can't control where it will happen,

"Control" may not be possible in the strict sense, but it might be that the areas that shrunk are where the stretching will occur. It depends on the mechanism of how the fibers shrink, and what happens during stretching.

The lack of interest by some to want to know is, interesting.
hum... I'm not against any way of doing it, just curious about the thread....


julio_gyn  (A 705)

Dec 31, 2012, 2:59 PM
Post #62 of 73 (1214 views)
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Re: [Martini] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
I think...max velocity is not necessarily at the bottom of the lines because the slider may slow down considerably before hitting the risers. Also the slider may reach its max velocity very near the starting point depending on how the canopy opens.

ok, i got it.. so the slider hits its vmax and then (maybe by the friction generated) it slows...

In reply to:
As far as stretching goes, all of the stretching should take place in the shrunken areas...

I think the fiber is a "plastic" and the place it shrink will get "harder" and deform or restrict the stretching to more "softer" areas...

Sorry if I'm having a hard time to understand all this, and causing any trouble to the thread..


masterrigger1  (D 14167)

Dec 31, 2012, 4:07 PM
Post #63 of 73 (1195 views)
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Re: [BrianSGermain] Stretching lineset back to trim [In reply to] Can't Post

Brian,
I hate to disagree with you, but I do on several points.
Quote:
....
but the shrinking is a long slow process,...

It is not always slow.
Spectra can start into a transformation state with a temperature as low as 190 degrees.
As the the rate of heat rises, so does the shrink rate.

If you have a traditionally slow opening canopy, the rate may be as low as an inch per 175 jumps. This would be an ideal rate of shrinkage.

But then again, if you have a few fast openings where the heat was let's say ten fold the nominal number, then you would have increased and rapid shrinkage.

This does happen BTW. I have seen canopies with just a few jumps have as much shrinkage as a canopy with hundreds of jumps. It just depends.

This is one of the main reasons that we should all move away from Spectra as a main stream line material. It is a moving target of sorts.

Quote:
Heat shrinking is only part of the dimensional change on a spectra lineset. My original article in parachutist magazine on this topic is what PD is basing their information on, but I have learned more since them. It appears that heat shrinking is not sufficient to cause the magnitude of the change that we are seeing. Lack of loading must therefore be a significant aspect of the difference between the outboard and inboar lines. It is a combination of these two forces that causes a canopy to become "out of trim". Therefore, the direct loading of the "unloaded" lines, including the brakes, changes the parachute significantly, and for a useful duration.

As I have probably written more on this subject (line materials) in these forums than anyone else (including yourself), I would have to say that I could not disagree more.
First Spectra does stretch, but it will return to it's original length when unloaded or the weight removed.
You can verify this over and over again on the line table....

The dimension changes that we see are due to the slider /line friction. The heat is the only culprit, plain and simple.

I absolutely do not agree with stretching the lines.

This is why:
The lines vary in shrink rate along the length of the line. This is due to the friction increases and decreases during the opening.

If you look at the carriers (twisted bundles) of the line weave, you will notice size and shape differences along the length of a used line.

Also some carriers will be locked down with one another and others will be free.
Stretching the line with these carriers locked down is a really bad idea.

Plain and simple... just get a line set that does not have these issues to begin with!!

For those of you that do not know me, I have for years been working on different line mediums to replace Spectra used on parachutes. We have had great success over the years.
In that time, we brought out the first competition swoop lines, coated Technora, coated Vectran, and enhanced braiding of both materials.

The result is that most manufacturers (including PD) have started to follow in both Icarus' and Skyworks footsteps with regard to these line materials.

This is a good thing IMHO...better flying canopies for everyone!

Lastly, modifying the trim is well.....a modification.
...and who can modify parachute things....legally?Wink

Cheers,
MEL


popsjumper  (D 999999999)

Dec 31, 2012, 4:38 PM
Post #64 of 73 (1185 views)
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Re: [BrianSGermain] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
As I mentioned, there is no evidence of a significant reduction in the tensile strength of the line post stretching.

Are you willing to post your data, testing methodology and conclusions? As yet, with no back-up to your statements, we just met on a bus.

Why has this not been passed on to canopy manufacturers?


(This post was edited by popsjumper on Dec 31, 2012, 4:43 PM)


skyjumpenfool  (Student)

Dec 31, 2012, 7:22 PM
Post #65 of 73 (1155 views)
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Re: [Martini] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
Do you have something useful to say or just want to stir the pot?

I do love to stir the pot...

When the lines become out of trim enough to affect the canopy's performance, I like to call that a "wear indicator". Time for new lines. Streaching may or may not work? Fact of the matter is, I don't care! Lines are cheap when my ass is on the line (punn intended). Cool

You do whatever you like. Shocked


Martini  (D 23756)

Jan 1, 2013, 12:12 AM
Post #66 of 73 (1135 views)
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Re: [skyjumpenfool] Stretching lineset back to trim [In reply to] Can't Post

Your approach isn't stirring the pot, it's pretty much the norm. Things aren't really that black and white though. Lines going out of trim are like boiling the frog, it usually happens so slowly that you only realize the trim issue when the performance degrades to a clearly unacceptable level. By occasionally stretching the lines you may enjoy a much better overall performance. You may not care, that's fine it's your canopy. I believe stretching lines is a good plan so I do that. It's not just a case of taking horribly out of trim lines and resurrecting them to save a few bucks. But as you say, do whatever you like.


popsjumper  (D 999999999)

Jan 1, 2013, 2:02 AM
Post #67 of 73 (1128 views)
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Re: [skyjumpenfool] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
In reply to:
Do you have something useful to say or just want to stir the pot?

I do love to stir the pot...

When the lines become out of trim enough to affect the canopy's performance, I like to call that a "wear indicator". Time for new lines. Streaching may or may not work? Fact of the matter is, I don't care! Lines are cheap when my ass is on the line (punn intended). Cool

You do whatever you like. Shocked

THIS^^^^


craigbey  (C 31991)

Jan 1, 2013, 7:34 AM
Post #68 of 73 (1070 views)
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Re: [EOCS] Stretching lineset back to trim [In reply to] Can't Post

The first person who responded to your OP described the technique of stretching lines back into trim as "kung-foo". That was an appropriate choice of words.

Spend some more time with different experienced riggers and they might share with you some of the voodoo magic they have used on their own gear.

These techniques may be tricks that such experienced riggers can employ on their own gear or the gear of others jumpers if they are fully aware of the nature of such modifications. So I guess it's good to see some technical discussion of what could happen when you stretch Spectra lines back into trim.

I can't say whether stretching Spectra is good or bad. But I do know of a couple of other types of materials that work a lot better.

Wink

This sport is not cheap. Don't try to make it so.


Martini  (D 23756)

Jan 1, 2013, 9:57 AM
Post #69 of 73 (1034 views)
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Re: [masterrigger1] Stretching lineset back to trim [In reply to] Can't Post

I glad you're posting on this subject, right now it appears that you are the only one with scientific knowledge. I'd like to ask a few questions, I really have no agenda, naturally I'd like confirmation that the technique I've been using is sound but you have already stated that you are against line stretching and I'm quite open minded.

Can you describe the amount of tensile loss as a function of stretching heat-shrunk lines? I know there are several variables but a general idea would be useful if you haven't graphed your testing.

I've never subscribed to the theory that unloaded lines shrink, that would mean that they shrink just sitting on the shelf. Since paraglider lines also shrink but aren't subject to frictional heat, do you think that exposure to sunlight is causing shrinkage? I'm aware of the increased complexity of PG lines (sheathing, mixed materials etc).

I have three Spectra-lined Sabres, one with nearly new lines, the others with decent but slightly out of trim lines. I don't know if non-Spectra linesets are available but in any case I'll be jumping the current linesets for a while. Do you believe that I'm putting myself at risk by stretching these lines to some degree? BTW these canopies have consistently butter-soft openings.

I do own one HMA-lined canopy, it's my preferred line material. Unfortunately it's a smallish crossbraced canopy and not my favorite for wingsuiting which is what I generally do. No perfect solutions here barring purchase of another canopy, I like the Sabres too much to replace them, never mind the cost.

Thanks for contributing here, we all benefit from more reason and less speculation.


masterrigger1  (D 14167)

Jan 1, 2013, 11:00 AM
Post #70 of 73 (1010 views)
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Re: [Martini] Stretching lineset back to trim [In reply to] Can't Post

Quote:
I'd like to ask a few questions, I really have no agenda, naturally I'd like confirmation that the technique I've been using is sound but you have already stated that you are against line stretching and I'm quite open minded.

There are a couple of reason that I am against stretching the lines.
As someone that has been in the Non-Destructive Testing field for 30 some odd years, It is my belief that doing this procedure is taking the material into the plastic stage for sure,
Taking load bearing material, that is still to be used or placed back into service, into this state of deformation is not good.

Line material can be subject to five states.
1. non-loaded state
2. Loaded state
3. Elastic
4. Plastic
5. Compromised, broken or breached state.

To get Spectra, that has gone through a shrink cycle, back to a longer dimension than it currently has, there is only one state that will do it. That would be the Plastic phase or state.

As you can see from the list above, you are right next door to the Compromised state.

How close is the question at hand.
If you have 550 Spectra, you can rest assured that it probably is closer than say if you had 725 Spectra.

Quote:
Can you describe the amount of tensile loss as a function of stretching heat-shrunk lines? I know there are several variables but a general idea would be useful if you haven't graphed your testing.

It would vary so much that I would not even know where to start.

As I stated before, there are areas along the line length that have more shrinkage and damage than other areas of the same line.

When stretching the line, the areas where the bundles are locked down will tend to go Plastic first, causing further loss of load capabilities.

Think salt water taffy here....
The more plastic you go, the less tensile strength you will have.

Quote:
Since paraglider lines also shrink but aren't subject to frictional heat, do you think that exposure to sunlight is causing shrinkage? I'm aware of the increased complexity of PG lines (sheathing, mixed materials etc).

Remember that Spectra can start shrinking at about 190 degrees Fahrenheit.
I have seen a canopy that was left sweltering in a hot car for a good length of time have this type of shrinkage. It was a canopy with less that 15 jumps on it BTW.

Quote:
I have three Spectra-lined Sabres, one with nearly new lines, the others with decent but slightly out of trim lines. I don't know if non-Spectra linesets are available but in any case I'll be jumping the current linesets for a while.

PD does have alternate line types available and we also have them here.

Quote:

Do you believe that I'm putting myself at risk by stretching these lines to some degree?

I think I have this covered just above a couple of paragraphs, but yes to some degree.

The problem is that the A-B measurement is pretty critical to one another along with the brake set measurement. Get anyone of these two wrong and you could have serious issues.
Just saying....

Happy New Year,
MEL


Martini  (D 23756)

Jan 1, 2013, 2:18 PM
Post #71 of 73 (975 views)
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Re: [masterrigger1] Stretching lineset back to trim [In reply to] Can't Post

Thanks Mark,

It's great to get some insight into this subject. I didn't really expect that you had done a series of tensile tests on stretched lines but having an expert opinion of the mechanics is helpful. I would have expected that loading the shrunken lines into the elastic state would have been sufficient to "uncurl" them into extension. I have also seen and heard of canopies with serious trim issues after relatively few jumps, it's a great indicator of how heat sensitive Spectra is. Also nice to hear that alternative line types are available even for the Sabre, I'll look into it if I need new linesets.

Happy New Year to you too.

Paul


base283  (D 15343)

Jan 1, 2013, 8:29 PM
Post #72 of 73 (928 views)
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Re: [EOCS] Stretching lineset back to trim [In reply to] Can't Post

FWIW, I am a parachute rigger, I work on paragliders also. and line stretching works IMHO on the polymids as opposed to aramids like kevlar, Dacron seems to be the most receptive. It has to do with the molecular structure. Aramid molecules connect head on whereas Polymids are angled giving it an elongation property. I had an asymmetrical opening on a BASE Canopy that knocked the lines out of spanwise trim by over an inch and stretched them back out. But this was a basic 7 cell design based to something similar to a Django Pegasus canopy. If one is dealing with a critical profile I would reline it. Remember the Nova canopy? The profile was very sensitive to trim variations.
Some of the class 3 (expert competition paragliders) are relined after only a few flights. more to do with line wear (0.6mm upper gallerie cascades) though but it negates the need to stretch them out. A lightly loaded 7 cell wont be affected much but a highly loaded swoop canopy could fold on ya more easily. The effect is exponential due to the speed/wingloading in the equation. Ian does have a point but a student Manta wont care but sometimes you have to trim them due to the regulations of rigging in some countries. I hope this helps.
take care though.
space


jonathan.newman  (D 30644)

Jan 3, 2013, 9:43 AM
Post #73 of 73 (800 views)
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Re: [EOCS] Stretching lineset back to trim [In reply to] Can't Post

I just got done "stretching" a stilleto 135 back into trim. It involved measuring line trim, picking out the bartacks of the offending lines, and moving the larksheads to the right length. It's back in service with smooth, on-heading, and non-spinning openings. Flies like new.

As an aside to this thread, I don't think the lengths will change much more as the worst lines have already shrunk by 3 - 5 inches. I'm just glad that PD or whoever cut the last lineset left enough line in the fingertrap for me to do this .

I just want to throw this out as a labor-intensive alternative to physical stretching. I believe as MEL and other engineer types said that deforming a line by restretching will either be temporary or damaging. But we're talking about 20+ lines rated at 550 to 1000 pounds each. You can degrade that a lot before you start seeing breakage.

There is room in this argument for all of us to be right.

What I'd like to see is someone at a rigging loft cut a line set from pre-shrunk spectra (bake it).



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