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Paraglider lines are a major modern technical achievement. Your life could be hanging by a thread, or that may be the view of the people who just asked you what you are flying at your local site. "It looks thinner than parcel string/ fishing line!" they exclaim. All the non flyers see is something that looks like very thin string or possibly dental floss, but paraglider strength when new is getting pretty good these days. Some gliders have recently resisted up to 16 G when tortured on either the DHV or Aerotests load vehicle, and the credit for this goes to the cloth, the lines and the way in which they are finished and connected. In the past few years paraglider pilots have talked a bit about the cloths their gliders are made from but hardly at all about the lines that connect them to the glider itself.

A modern paraglider has between 300m and 450m of line, in several cascades going up to the sail. A paraglider has no rigid structure, so the problem for the designer is to evenly spread the load via a number of connection points to the glider, but also to minimise the amount of line used to cut down on drag. Gliders used to have one line per cell, combined together lower down in the cascades to cut down on total line consumption, but after diagonal ribbing came into common use in the mid 1990s it was then possible to place lines every two or three cells, again combining them lower down but with a huge reduction in drag. The reduction in A lines from 5 per side down to 2 or 3 meant the lines have to be much stronger than they had been previously. The amount of line used in the modern intermediate glider has almost halved since the introduction of diagonal ribbing, but with the huge paradox for the line manufacturers that the market now demands much stronger, thinner line, which costs more to make, but having achieved that for the PG manufacturers, those manufacturers will then buy much less line for the same number of paragliders made.

Understanding the process that leads to the finished paraglider lines we have helps a lot in caring for them. The cost of a replacement set of lines can call into question the life of a paraglider, as an older one may end up beyond economic repair.

We were invited on a factory visit to Cousin Trestec, one of the largest manufacturers of line for paragliders in the world. On a factory site in the Northernmost part of France near Lille, adjacent to the canal that marks the Belgian border, this 150 year old family owned business makes high tech lines and ropes for a variety of specialist applications. These include yachting ropes, climbing ropes, various ropes and lines for Mountain rescue and other emergency services in addition to paraglider lines. The site is Europe’s biggest braiding machine park, with over 60,000 spindles working away! Cousin Trestec has a turn over of ten million Euros and is part of the Cousin group, owned by the Cousin brothers and Jacques Ferrant.

Paraglider lines are made using rope construction techniques. The most basic rope construction technique involves twisting the fibres together to form a strand. Braiding, a more advanced technique involves the individual strands being passed over or under each other in a mechanised process to form a line or rope more like that in Fig. 1 (right). Paraglider lines are made by this process of twisting and braiding. An unsheathed line is produced as a single braid. If the line is sheathed, the outer is then braided around the core in a second process. Twisting and braiding improves strength and makes for a line that is easier to handle and that remains together.

There are a large number of brand names for the high technology fibres used in lines, and therefore it's important to understand which names are similar and to make the comparison with other brand names of the same base polymer. The outer sheath is made from polyester, and in the early days of paragliding the whole line was made from braided polyester. Factors contributing to the end of polyester as a main load bearing material were its stretchiness and its low strength compared to modern materials. It's used for the outer as the outer only accounts for 10% of the total line strength. There are now two main materials used as fibres in the core, these are Dyneema, a high modulus polyethylene, and Technora, Kevlar or Twaron, all brand names for aromatic polyamide or aramid. From now on, we will refer to the two main fibres simply as Dyneema and Technora, as Technora is the brand that Cousin uses in their finished products. It's easy to tell the difference as a sheathed Dyneema line has a white core, whereas the Technora line will have a browny yellow centre to it. Other possible materials are Vectran and Xylon. Vectran is a liquid crystal polyester with very low stretch characteristics but high weight compared to Dyneema. It is still in use and specified by a few PG companies today. The other fibre, PBO or Xylon has fallen out of use due to its extremely poor UV resistance.

Dyneema has high strength, low weight, low stretch, very good UV resistance and is very good at resisting fatigue and bending damage. Dyneema is lighter than water, which can be a major asset in sailing and kite surfing, as the line will float on water. On the down side, it’s not that heat resistant, [softens at 144ºC and melts at 165ºC] and suffers more permanent elongation than Technora, although Cousin have a combined post braiding heat and stretching process that reduces elongation quite significantly. This process, whose details remain a closely guarded secret, needs to be done under very tightly controlled conditions to have the desired effects.

Technora [an aramid similar to Kevlar] is very strong, more so than Dyneema, has very good heat resistance [it doesn't burn or melt] and very low stretch, where again it bests Dyneema. Technora is five times lighter than steel on an identical strength basis. On the minus side, it's heavier than Dyneema, less resistant to UV and fatigue and bending damage. At present the push in development is to get Dyneema to the point where it has negated all its disadvantages compared to Technora.

However, just like the choice of materials for a glider sail, paraglider manufacturers don't use one material for lines, they tend to have a variety of materials in use. We might have a mixture of sheathed Technora, sheathed Dyneema and unsheathed Dyneema in a typical DHV2 or 2-3 glider.

Now we know what the yarn is made of, let’s have a brief look at the processes that Cousin Trestec applies to the material coming into their factory.

The filaments of yarn arrive in bobbins, and they are checked at this phase for quality and continuity. The bobbins are loaded onto the braiding machines. These produce a core, or the finished braided line in the case of the unsheathed lines. To produce a sheathed line, a second braiding process is required where the core has an outer sheath braided over the inner. Both braiding processes are continuous and result in a long length of the line being wound onto a bobbin.

The newer style unsheathed lines are dyed and coated with a polyurethane compound, which improves UV resistance and the way the lines handle, including making them less likely to tangle. This produces a very different end product to the unsheathed lines seen at the turn of the century.

A second process for the unsheathed Dyneema lines involves stretching them under very carefully controlled conditions, of which a closely controlled temperature and stretch rate are part of the cocktail. This process results in an increase in strength and a reduction in permanent elongation under load, and also reduces the diameter of the lines. This new process has some interesting results:

Stretch of the line under a 12kg load reduced by 78%
Breaking strength increased by 19%
Abrasion resistance increased by 10%
Bending resistance increased by 9%
Diameter reduced by 5%

The sheathed dyneema lines also undergo this process, but after the second braiding operation where they have the sheath applied over the inner.

The finished line then has samples taken from it, which will then be subjected to a number of tests to ensure the quality of the product. Tests include a steady loading to failure, shock loading and the DHV bending test where the line is subject to 5,000 bends of 150 degrees each way before being tested for load resistance again.

The lines with all the necessary information on the bobbin to allow them to be traced back via all processes to their origin then leave the factory for the paraglider manufacturer. This traceable quality control is part of the process required for the ISO 9002 certification for quality held by Cousin.

The finished lines are amazing. An unsheathed line made from Dyneema with a diameter of 0.66mm [yes, barely over half a millimetre] boast a breaking strength of about 56 kg, and after 5,000 cycles on the DHV test would still hold out until 54 kg. If we go up to 1mm [1.12mm] the strength has increased to 172 kg, with failure after the DHV aging test reduced to 153 kg. The figures for sheathed lines are still pretty amazing, but they include a less strong polyester coating, so their figures are lower. Even then, strengths of about 128 kg for a 1.1mm sheathed Dyneema line are the order of the day. The sewing process at the paraglider manufacturers, along with the small diameter of maillion the line passes around reduce this strength somewhat in real life, so the lines will be specified by the PG manufacturer with this in mind. Looking at gliders like the Airwave Magic 3, they have managed to sustain +16G on the test rig, so the sums done by the PG designers are correct!

 



Fig.1. Reprinted with permission


Paraglider lines – backgrounder.

Why the reduction in lines?
Paraglider performance has improved massively since the first retrimmed jump chutes were flown off the French Alps in the mid 1980s. A good deal of this improvement in glide and sink rate has been achieved by drag reduction. Compared to the early 1990s, line consumption is down on the average intermediate by about 40%. Line thicknesses have come down from 4mm on some early retrimmed sky diving chutes to the 2.1/1.6mm commonly seen on the modern intermediate. Total canopy airtime has extended from tens of hours to hundreds and maybe even thousands, in the case of some test gliders used by manufacturers.

The basic anatomy of a line.
For the majority of pilots, a line has two distinct parts, an outer sheath, primarily for protection, and a load bearing inner core. For competition pilots or sometimes for the upper cascades in a canopy, the manufacturer may specify unsheathed lines, usually to reduce drag. The down side of this is a reduced resistance to UV degradation and abrasion, but abrasion is not usually a problem for the lines in the upper cascades as it is unlikely they will touch the ground.

The Chemistry bit

Material table.
Brand name
Material Type
Owner of the brand name
Dynema
High modulus polyethylene
DSM High Performance Fibres
Spectra
High modulus polyethylene
Honeywell Performance Fibers
Technora
Aromatic polyamide/aramid
Teijin Ltd
Kevlar
Aromatic polyamide/aramid
DuPont
Twaron
Aromatic polyamide/aramid
Teijin Ltd
Vectram
Liquid crystal polyester
Celanese Acetate LLC

Aramids are from the polyamide family of organic compounds, which also include Nylon. They are comprised of polymers, which are very long chains of repeated organic units called monomers.

Polyethylene is simply another name for polythene. The molecules in the compound used to make Dyneema are much longer chains than the stuff that’s used in the bags supplied at the supermarket to take your shopping home in. Polythene bags have a lot more branching on their molecules, as well as shorter chains. This results in lower intermolecular forces, and hence lower tensile strength.

Polyester is another long chain organic compound, but with an ester as the basic building block. It has found widespread use in fabric for clothes, as well as free flying applications like Dacron and Mylar [hang glider sails] and Teijin Tetoron, a fabric widely used in the early days of paraglider manufacture.

Polyurethane [PU], the coating used for the newer unsheathed lines is a plastic, and helps protect the lines from UV and abrasion damage, as well as improving the way they handle.


The lines are a very high tech piece of manufacture, but needed to be cared for to keep their strength up.

Advice from Cousin on caring for your lines:

  • Don't leave your paraglider near heat sources - in winter it could be the radiator, in summer the locked boot of your car.
  • Never store a damp wing and avoid humidity when putting your wing away. Don't dry a wing in direct sunlight - put it the shade.
  • Don't drag your wing across the ground.
  • Don't leave your wing out on take off for too long.
  • Avoid kinking your lines and don't knot or braid them for storage.
  • Manoeuvres [ears, spirals, b line stalls and any form of acro] accelerate the ageing process and weaken lines. Frequent use of these manoeuvres will require that you accept the consequences - more frequent line replacement.
  • After a big shock [like a very big collapse] a line check will be needed.
  • Any newly acquired but used paraglider should be subject to a line check.
  • If your lines have heat shrink protection examine with great care the end of the heat shrink - the edge leads to damage and fatigue.
  • A stored paraglider that is not used still ages.
  • Be wary of ultra thin or unsheathed lines, especially on competition gliders. They will require more care and are easily damaged.

The author of this article, Steve Uzochukwu (UK), is a Freelance Broadcast Technician, free-flyer of different aircrafts and occasional writer for Skywings and Cross Country magazines.
You can visit his website: www.steveu.org


Skywings:
www.bhpa.co.uk/bhpa/skywings


Published: January 31st, 2007


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