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.
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.
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.
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.
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
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.
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.
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.
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.
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.
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
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
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:
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%
sheathed dyneema lines also undergo this process, but after the
second braiding operation where they have the sheath applied over
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.
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.
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!
Paraglider lines – backgrounder.
the reduction in lines?
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.
basic anatomy of a line.
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.
of the brand name
High Performance Fibres
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.
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.
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.
[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.
lines are a very high tech piece of manufacture, but needed to be
cared for to keep their strength up.
from Cousin on caring for your lines:
leave your paraglider near heat sources - in winter it could
be the radiator, in summer the locked boot of your car.
store a damp wing and avoid humidity when putting your wing
away. Don't dry a wing in direct sunlight - put it the shade.
drag your wing across the ground.
leave your wing out on take off for too long.
kinking your lines and don't knot or braid them for storage.
[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
a big shock [like a very big collapse] a line check will be
newly acquired but used paraglider should be subject to a line
your lines have heat shrink protection examine with great care
the end of the heat shrink - the edge leads to damage and fatigue.
stored paraglider that is not used still ages.
wary of ultra thin or unsheathed lines, especially on competition
gliders. They will require more care and are easily damaged.