Dacron line strech?

[skydiverek 60]

I know Dacron lines stretch more during opening than Spectra/Microline (thus lowering the opening shock a little bit). My question is: how much does a Dacron line attached to canopy stretch during opening? One inch, 1/10th of an inch? How much?

Bart

[riggerrob 561]

Look in Poynter's Manual for Dacron stretch rates.
I suspect that the main reason Dacron-lined canopies open softer is that thicker, stiffer Dacron lines take a fraction of a second longer to slip through slider grommets. This logic is similar to the notion that a thicker rope equals slower rappelling. This is highly un-scientific, because slider performance is very difficult to measure.

[rigging65 0]

Bart-

Mil Spec. cord is tested for elongation under Federal Standard 191, using test method 6016.

The test method (in a nut shell) says you load a cord to 75% of it's rated strength, then measure elongation.

For Dacron cord tested under this method (which is what you would find on most canopies) you can have no more than 12% elongation. Spectra is 6% max.

Now, seeing as how you would rarely load a line to 75% of it's rated strength, you need to figure out line loads first, then extrapolate the elongation from it.

Suffice it to say, Dacron stretches twice as much as Spectra.

"...and once you had tasted flight, you will walk the earth with your eyes turned skyward.
For there you have been, and there you long to return..."

[skydiverek 60]

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Bart-

Mil Spec. cord is tested for elongation under Federal Standard 191, using test method 6016.

The test method (in a nut shell) says you load a cord to 75% of it's rated strength, then measure elongation.

For Dacron cord tested under this method (which is what you would find on most canopies) you can have no more than 12% elongation. Spectra is 6% max.

Now, seeing as how you would rarely load a line to 75% of it's rated strength, you need to figure out line loads first, then extrapolate the elongation from it.

Suffice it to say, Dacron stretches twice as much as Spectra.

Thank you for this great information!

[skydived19006 4]

I did a search on this subject and found this thread.

Now Vectran is supposed to basically not stretch at all, but in reality compared to Dacron and or Spectra how much does it really stretch. Also the big advantage of Vectran as I understand is that it doesn’t distort. How does Spectra compare to Dacron with regard to distortion? If bulk and drag are out of the equation is there an advantage to Spectra over Dacron?

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Bart-

Mil Spec. cord is tested for elongation under Federal Standard 191, using test method 6016.

The test method (in a nut shell) says you load a cord to 75% of it's rated strength, then measure elongation.

For Dacron cord tested under this method (which is what you would find on most canopies) you can have no more than 12% elongation. Spectra is 6% max.

Now, seeing as how you would rarely load a line to 75% of it's rated strength, you need to figure out line loads first, then extrapolate the elongation from it.

Suffice it to say, Dacron stretches twice as much as Spectra.

Experience is what you get when you thought you were going to get something else.

AC DZ

[mark 105]

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For Dacron cord ... you can have no more than 12% elongation. Spectra is 6% max.
...
Dacron stretches twice as much as Spectra.

12 = 2 x 6, but 112 <> 2 x 106.

The A-lines on a Sabre2 150 are 124" plus a little. With mil-spec Dacron they could stretch to 139," with Spectra to between 131" and 132." That's IF they're loaded to 75% of rated strength, which might happen in a really hard rogue (Bill Von spells "rouge" which is French for "red" which is weirdly appropriate) opening, but consider you've got 20 lines to spread the load to the links. After the canopy is open and you are in straight, unaccelerated flight, the load on each line is something less than 20 pounds, a fraction of the rated strength of Dacron or Spectra. Elongation (or lack) is a deployment issue, but otherwise not a performance issue for either material.

For Spectra, the problem is shrinking from grommet friction during opening. If it weren't for the bulk and drag, I think we'd all prefer Dacron because of its dimensional stability -- hence the continuing search for alternatives that's taken us past Kevlar to Vectran, HMA, and beyond.

Mark

[skydiverek 60]

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I suspect that the main reason Dacron-lined canopies open softer is that thicker, stiffer Dacron lines take a fraction of a second longer to slip through slider grommets.

That, plus as it was pointed they stretch more, and also provide more friction for the slider grommets (as compared to Microline).

Do you think that having thicker Dacron lines increases the chances of having tension knots or slider hang-ups (since the bunch of thicker lines will be trying to fit in the same size slider grommets, which are used with Microline)?

[mjosparky 3]

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(since the bunch of thicker lines will be trying to fit in the same size slider grommets, which are used with Microline)?

The Microline is trying to fit in the same grommets that they have always used for sliders, #8.
My Sharpchuter has 750 Dacron, no problems.

Sparky

My idea of a fair fight is clubbing baby seals

[skydiverek 60]

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The Microline is trying to fit in the same grommets that they have always used for sliders, #8.

Both Dacron and Microline can try fitting in #8 brass grommets OR #25 stainless steel grommets. BTW, #8 brass grommets are 2 mm wider when it comes to inside diameter (at least the ones PD is using), so maybe they are better for bulkier Dacron lines?

[mjosparky 3]

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The Microline is trying to fit in the same grommets that they have always used for sliders, #8.

Both Dacron and Microline can try fitting in #8 brass grommets OR #25 stainless steel grommets. BTW, #8 brass grommets are 2 mm wider when it comes to inside diameter (at least the ones PD is using), so maybe they are better for bulkier Dacron lines?

Are they SS or plated brass?

My idea of a fair fight is clubbing baby seals

[riggerrob 561]

For tension knots, the worst combination is old, fuzzy, dusty Dacron lines packed sloppily. If you do not get every single twist and kink out of old Dacron lines, they tend to knot.
Desert dust just makes Dacron stiffer and more difficult to straighten.
In the Southern California desert, only the best of packers could make Dacron-lined Strong tandem canopies open reliably.

[beezyshaw 0]

I think people are not making the proper distinction between "stretch" and "elongation". Stretch is the elasticity of a line, and the line returns to original length after deployment forces are reduced. Elongation is the more permanent lengthening over time from loads being imposed on the line. Big difference here. Dacron has the most stretch during deployment, and Spectra stretches virtually none. Spectra in skydiving applications will shrink due to friction heat, but in other applications (like sailing)Spectra is bad about "creep" or elongation when tightened with a wench.

[ryoder 1,408]

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For tension knots, the worst combination is old, fuzzy, dusty Dacron lines packed sloppily. If you do not get every single twist and kink out of old Dacron lines, they tend to knot.
Desert dust just makes Dacron stiffer and more difficult to straighten.
In the Southern California desert, only the best of packers could make Dacron-lined Strong tandem canopies open reliably.

Going from memory here;
I believe I saw a posting from Booth where he stated the field statistics he gathered from the RWS tandem program indicated Dacron-lined tandems with >300 jumps on the lines had 2X the malfunction rate of identical canopies with <300 jumps on the lines.

"There are only three things of value: younger women, faster airplanes, and bigger crocodiles" - Arthur Jones.

[billbooth 10]

That number is true.

[skydiverek 60]

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I believe I saw a posting from Booth where he stated the field statistics he gathered from the RWS tandem program indicated Dacron-lined tandems with >300 jumps on the lines had 2X the malfunction rate of identical canopies with <300 jumps on the lines.

Would this apply also to sport canopies?

[beezyshaw 0]

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Would this apply also to sport canopies?

Bill can jump in and correct me if I'm wrong, but I would say it does not apply to sport mains. One of things about tandem canopies with dacron lines is the fact that (on PD tandems at least) 900 lb line is used. That large line barely fits in the slider grommet, so when it becomes worn, and thus larger than when new, it lends itself to a higher malfunction rate. Sport canopies would usually be lined with 525 lb dacron, so the much smaller line wouldn't have the same consequences.

[billbooth 10]

I have no data about malfunction rates vs. age of Dacron lines on sport canopies.

[mjosparky 3]

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I have no data about malfunction rates vs. age of Dacron lines on sport canopies.

I guess that means tandem jumping is not sport?

I've got hundreds of jumps on canopies with dacron lines without having a malfunction caused by the lines. But then, I pack my own rig.

Sparky

My idea of a fair fight is clubbing baby seals

[skydived19006 4]

So, what type of line do the different manufacturers use on their tandem canopies as “standard”? I still have one Icarus tandem canopy with Vectran, and had decided to go with Dacron when I get it relined. Now I see that Dacron tends to get bulky with wear (even more bulky than it is new), which may lead to a slider hanging up. The pack volume and drag are not a deciding issue for me on my tandem canopies. What I would like is something that will stretch on opening, but not distort much, which lead me back to Dacron. What I’m really not interested in is increasing my malfunction rate (over 600 tandems and no malfunctions)!

What line do you prefer on your tandem canopies, and why?

Martin
Air Capital Drop Zone

Experience is what you get when you thought you were going to get something else.

AC DZ

[ryoder 1,408]

I think you may have intended to post your followup to BillBooth rather than me. I do know RWS/Booth use Dacron on their tandems. I don't know what the other mfgrs are using.

"There are only three things of value: younger women, faster airplanes, and bigger crocodiles" - Arthur Jones.

[skydived19006 4]

I basically just picked a post to reply to. Generally, I think Bill Booth knows a little more about skydiving gear than I do!!! If his preference if Dacron on his tandem rigs, that’s good enough for me!

Martin

Experience is what you get when you thought you were going to get something else.

AC DZ

[billbooth 10]

Tandem canopies are public transportation. Little old ladies and parapelgics jump tandem. Dacron lines give lower opening shocks. It's that simple. This might not matter much on a "normal" opening, but if someone make a small mistake packing which results in a super fast opening, the Dacron lines could save somebody's grandmothers life. I know Dacron is bulkier, but I've seen no evidence that Dacron lined canopies have more malfunctions. In fact, I believe the opposite may be true.

[skydived19006 4]

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Tandem canopies are public transportation. Little old ladies and parapelgics jump tandem. Dacron lines give lower opening shocks. It's that simple. This might not matter much on a "normal" opening, but if someone make a small mistake packing which results in a super fast opening, the Dacron lines could save somebody's grandmothers life. I know Dacron is bulkier, but I've seen no evidence that Dacron lined canopies have more malfunctions. In fact, I believe the opposite may be true.

Thanks Bill! That’s good enough for me. I had decided to go with Dacron, but the reference in this thread to a two fold increase in malfunction rate with worn Dacron lined tandem canopies concerned me a bit. I don’t have a problem with replacing line sets every 500 jumps or so, for us that’s only one line set a year.

Besides, my neck isn’t as young as it used to be either!

Martin
AC DZ

Experience is what you get when you thought you were going to get something else.

AC DZ

[riggerrob 561]

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So, what type of line do the different manufacturers use on their tandem canopies as “standard”?

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

Strong used Dacron lines on their F-111 tandem mains.
We had Vectran lines on a SET-400, but eventually replaced them with Spectra when they got "too ugly, too quick." Vectran was fashionable circa 2000, but has been replaced on sport canopies by HMA 2.
Now all of our SET-400s have factory-supplied Spectra lines.
Opening and landing performance on all Strong tandem mains deteriorate rapidly after 300 jumps on a line set.

Has anyone tried HMA lines on tandems?

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[masterrigger1 2]

Rob,
What in the world is HMA 2. I have never heard of it???
Also Vectran is still fashionable. We still put 500 + jumps on line sets even in the desert.

HMA has been put on tandems.. Strong is looking at it now.

MEL

Skyworks Parachute Service, LLC
www.Skyworksparachuteservice.com