Regarding the high-tech kite and paraglider fabrics, UV degradation, and other risks.

On Apr 4, 2013, at 4:38 AM, magnus@*****.*** wrote:
Hi! This is not so much a repair inquiry, but more like a question spurred out of curiosity. I'm a paraglider, and in our community it's a well known fact that the canopy is very sensitive to all sorts of things, such as unnecessary exposure to sunlight, crease, water (especially salty water), and dragging it along the ground. 

There are a LOT of theories on how to best store and pack your glider to keep it in good condition. Some of the newer gliders force you to pack the glider accordion-style, which some people say is bad due to the extra crease. Hell, some people even try to convince me to use a different packing method when storing my glider for the winter, compared to when its being packed for hiking or to be used within the next few days. 

I am not a kite surfer, but I sometimes fly in the same area. It always amaze me how the kites apparently have no problem dealing with salty water, and (from the looks of it) a lot more wear and tear. I also hardly ever see the kite surfers store their kites in UV-bags in-between flights. We paragliders always do that, to minimize UV-exposure when the glider is on the ground. 

My question to you guys is how does the fabric in a paraglider differ from a kite, and why is so much more sensitive to these things? Would it be possible to create a paraglider with the same material, without losing to much of its performance etc? I'm not entirely sure you are the correct people to ask this question, but with your unique view on both kites and paragliders, i thought I'd ask anyway! 

Thanks in advance, Best regards Magnus

In response to this we wrote:
On Apr 4, 2013, at 7:11 PM, Tim at WindFire Designs wrote:

Hi there Magnus,

This is actually a great question, and one that we have given quite a bit of thought to over the years.  We have repaired literally thousands of wings now, from everyone using soft wings in all kinds of sports.  Also, as I am personally a materials freak, and a collector of forensics knowledge via all the different types of damage we see, I have formed a rather large data set about everything you mention here.

I agree that paragliders fall under special scrutiny, but I don't agree that it's totally warranted in every area, and every case. Like any industry with enthusiasts, there are a lot of rumors and practices that may or may not have any basis in real data. That said, here is what we see as demonstrable fact.

The factors that kill paraglider cloth are not much of a mystery, and they are exactly the same as those which kill the fibers used in kitesurfing kites.  They are UV, and direct abrasion.  Creasing repetitively in the same place isn't great either, but it's mostly a secondary factor that actually relates back to UV in a way that I'll explain later.  

Direct abrasion, where focused point pressure is dragged across a set of fibers will be hard to avoid suffering damage from as a result.  However, free floating sand, that is not trapped between the cloth and another surface that's able to apply pressure against it, will not cause any appreciable problem for the fibers.  So as long as the sand is just a small amount, and is free to fly around and eventually fall out, i wouldn't freak out over it too much.  We see a certain level of sand in almost every wing we work on, and it doesn't seem to us to be the huge enemy everyone imagines it is.

Another obvious factor in damage, is simply where the breaking load on a fiber exceeds its rating.  This is obvious, and doesn't really merit a lot of discussion.  Don't pull on the fibers with greater force than they are proven via testing to rebound from.  

There is no question that sun exposure is to blame for most of the degradation occurring over the life of any fabric wing. Actually I don't agree that water, or even salt water, plays a huge role in the breakdown of high performance fibers.  Take Dacron for instance.  It's actually just a brand name for very pure polyester.  Polyesters, woven thinly, are what the canopies of kites and paragliders are made from.  This same fiber, albeit woven differently, is regularly used inside the body as a way to fix hernias.  As one must now ask, how can water, even water with electrolytes, be a huge factor on these fibers if they are used in the body?  The answer is that water is not a real risk at all.  These fibers are incredibly noble, and are actually used in their sheet and film forms to make normal plastic bottles that store all kinds of crazy chemicals, like solvents, and acids, and all kinds of things in very toxic industries.

There are a few secondary risks to water, such as mildew, dye transfer, and the extreme forces associated with dragging a submerged wing out of a body of water.  But wet fabric in and of itself should pose no real threat.  So what about salt? Well, it's true, that salts can crystalize after the water dries, and these micro crystals can then start to break down the coatings on the fabric.  However, usually it's UV that beats all of these factors no matter how much care is taken. Wet synthetic fabrics will change dimension while wet, but this is true also with changes in humidity.  People regularly fly soft wings when there is measurable humidity in the air.  Is that a factor?  Well it certainly interacts with the cloth, we can see this as dew forms on wings late in the day.  The water is there whether it's forming droplets or not.  Personally I wouldn't give it a second thought. 

These fibers are coated both before and after the weaving process.  In their coatings they use compounds just like you smear on your skin for sun protection, such as aluminum dioxides, and titanium dioxides.  The UV gradually starts to work on these compounds, breaking them apart each time the energy collides with the substances.  The reason that repetitive creasing is bad, is because it will start to form cracks in the coatings on the fibers.  These coatings then loose their ability to protect the fibers from the sun in those cracks, and leaves the sun to hit the fibers directly.  Again, there is some secondary risk to the fiber via the hinging effect that happens when you have intact coating with a crack in it, which then focuses the bending force on one part of the fiber, but this is not as big a factor as the UV, yet again.  

The good news is, paraglider fabric is in an upper-echelon of fabrics, and rapidly adopts, and drives all the latest advancements in all the available areas of textile technologies.  It's incredible stuff, and honestly much of the paranoia is just paranoia.  I know that every paraglider wing we touch is a cut above all of the kitesurfing kites on the market.  Also, they tend to wear much more evenly over the entire wing surface than does a LEI type kite surfing kite.  They wear more evenly because a ram-air foil is simply a more force-balanced structure than an LEI with rigid pneumatic structural components.  ie., the force map on a ram-air foil is beautifully even and free of major hot spots of stress.

To answer some of your questions more directly:

Making paragliders from kite cloth would actually be a downgrade in durability on all fronts.  Paraglider cloth is both stronger, and will last longer even with its lighter weight.  Paraglider cloth also has far superior memory after being deformed on the bias, because of superior chemistry of the coatings.  

The main differences in the care taken with the two sports seems to largely be cultural.  When kitesurfing started, nearly none of the people getting into the sport were even wind sport enthusiasts.  They just wanted a hook into a power source like a boat in the sky.  They didn't care how it performed, or why it worked, and the vast majority were certainly not sailers or kite fliers.  Of course this is a generalization, and I don't want to sound jaded, but 10-15 years ago, this is what we observed.  

Kites do tend to break down fairly rapidly.  It's rare these days for us to see kites from say, 2007.  The fabrics in kites do not age as gracefully as paragliders, and because less care is taken with the wings, they simply take more abuse.  Also, they spend a lot more usage time in proximity to land, and they are under less direct and immediate control of the pilot when they are near the ground than a paraglider is, thus they suffer more damage.  The fabrics used in kites can afford to be a bit heavier, and a bit cheaper.  The coatings are not as advanced, the weight of the cloth is a bit higher per area, and in general, people just tend to accept lower end fabrics because the performance and safety can simply tolerate a bit more leeway in kitesurfing. 

In regards to folding methods, there are many ideas, and when people spend a ton of money on something, they are going to make up all kinds of rationalizations and ways to add complexity to their habits.  This gives the sport something to talk about, and gives people a way to engage with the culturally-defined knowledge base that floats from person to person. Are there bits of wisdom in all the voodoo?  Absolutely, lots!  However, does it have to be followed quite as strictly as you might be lead to believe in a discussion forum?  Almost certainly not.  

Hey, don't form hard creases, and then crease on them again and again.  Subtly changing how you pack the wing over time is not a bad idea—it doesn't take much change to combat repeated folding.  Slavishly following every fold is not a great idea.  Sailors often promote the randomization of the pack.  All sails used to be systematically stuffed randomly into their bags to combat hard folds, most still are.  This actually would be a great idea for paragliders too, if it weren't for the LE.

New stiffeners in the LE of paragliders are very like sail cloth, and are often made of just that.  The main problem with wrinkling the heavier LE reinforcement panels that form the front of each rib isn't that it will weaken the cloth in any way, it's that the folds are difficult to straighten from the mild pressure differential alone found in a ram-air foil.  Thus the folds inhibit graceful inflation on launch if there are too many wrinkles in the LE.  Also, they now have rods, or other large (.080-.125") monofilament stiffeners in some high performance wings which afford the removal of some of the suspension lines (bridle).  They will prevent much creasing up there in one dimension, but as you stated, they will cause you to fold it very much the same way each time, which isn't really ideal.  The good news is stated above, that folds probably will not beat the rate of breakdown caused by other factors in the wing's life.

We see a ton of paragliders come through our shop for repair, and the ways they are packed are all very different.  I would wager to say that the users of these wings are all seeing very much the same lifespan of the wings even though their packing is so radically different from customer to customer.  It's difficult for us to form a habit of our own with packing.  This is because we recognize that the packing method is important to the user, so we attempt to copy their style of fold when we pack the wings for their return trip.  It's not easy to do well, but we try.

Wow, I wrote more than I set out to.  I often have this conversation on the phone, but I think I'll post this to our site somewhere if that's ok with you.

I hope it helps, and please let us know if you have any other thoughts.

Best,
Tim Elverston