Porosity Testing In Progress
Specifically: at 105 Alford Forest Rd, Ashburton New Zealand between the hours of 12noon and 12.30pm on Wednesday the 15th of October 2008, Tan Xi Bo and Wang did fail to prevent one blue ultra light 8m Pilot kite from becoming irretrievably treed.
The facts are simple, and can't be disputed (because I didn't give them an opportunity). During the morning I'd launched the said kite and tied it off. Mr's Tan and Wang, visiting from China, were flying some other kites in the same area (beside the kite factory) during this period. At 12 noon I went away for lunch, while they stayed and flew for a while longer. Shortly after I departed, the kite I'd left flying became irretrievably entangled in a tree. Clearly it was their fault - and they were late for lunch as well.
Craig (Hansen) later attempted to remove it by pulling hard on the line- but only succeeded in pulling out all the bridles and ripping the skin in multiple places. Such fragility is, unfortunately, an unavoidable disadvantage of using lightweight fabrics- 30gm/sq.m "Skysilk" in this case. In fairness to Craig, our usual 42gm high tenacity nylon with Si/polyurethane coating might have fared rather better. The Tree Kept A Bit
Since then it's been visibly deteriorating- thrashed by the strong winds we regularly get here (not infrequently greater than 100km/hr) and subject to relentless ultra violet (UV) radiation (typically five times Europe).
Flapping destroys fabric; even one day snapping back and forth like a whip in strong thrashing winds can destroy any known fabric (which is one of the reasons why kite energy projects are now tending towards aeroplane style kites made of rigid carbon fibre laminates). No fabric kite can survive even a week up a tree when the northwest wind blows like it does here.
Or so I thought.
Just two weeks ago, (after 14 months) and for some unknown reason, (I'm sure it wouldn't have been to show off) Stefan (Cook) heroically climbed the offending tree and retrieved most of the remains of the said kite (see "The tree kept a bit"). Treed Pilot Repaired
Unbelievably it's still flyable- as well as ever actually. Sure, I had to do extensive repairs and replace ripped out sections, but these were more from Craig's unsuccessful retrieval attempt than from either wind or UV damage.
I wouldn't have been more surprised if Santa Claus had emerged from our fireplace a few nights ago (not least because the chimney's just 150mm diameter).
So then, although fly-ability is the ultimate measure of fabric longevity, I then did some tests on this fabric to get some objective numbers for comparison.
The first test was for porosity. I have a Swiss made machine for testing this, called, obviously, a Porosimeter. This machine measures the time it takes for a fixed volume of air driven by weighted bellows to pass through a defined area of fabric. Porosity Testing Of Treed Kite
Rip stop kite fabrics are sealed by a coating, but this coating tends to progressively flake off as the fabric becomes fatigued. Coatings are by far the most sophisticated and technically difficult aspect of fabric making-they're what you're paying for in the pricier lines. It might seem paradoxical, but the more complete the coating, the less tear resistant fabric becomes. This is because if the coating completely fills the spaces between fibres and prevents their movement relative to each other than the fabric behaves as an homogenous material and loses the special ability that woven fabrics have to spread point loads out across many filaments. A perfect coating would seal the gaps completely and resist stretch on the bias while still allowing adjacent filaments to move- but there's no such thing as a perfect coating. In practice, the fabric maker's art is in the compromises between sealing (porosity), tear resistance, stretch, longevity and cost. Toray Chikara, a premium grade polyurethane coated 42gm/sq.m nylon generally tests, new, at between 3 and 6 minutes. After say 100 hours of flying in mid-range winds, a Chikara maxi octopus kite might test at 30 seconds around the head area but 1 second at the tentacle ends (they whip a lot). Single skin kites (1, 2 or 4 line) and open leading edge single line ram air kites will still fly (but soggily) at even 1sec porosity test. Open leading edge ram air traction kites lose their sharpness at, say 30sec test, but will still fly down to 5 or 10 secs. Closed leading edge (valved) traction kites like Arcs are out of business by say 20 seconds.
30gm Skysilk has an excellent polyurethane/silicon combination coating, but the basic weave weight is just too light to provide good support for this coating (light fabrics are more difficult to make- which is why they cost more). I would have expected it to deteriorate rather quickly but even after 14 months up the tree it still tested at between 1 and 3 seconds (depending on where the sample was taken from). This might sound pretty bad- but kites from other fabrics that have been treed here (a fair number) have all become un-flyable in just weeks. When I then tested a new piece of Skysilk, it reached 23minutes 36 seconds- and I've NEVER had anything above 10minutes before (except Mylar laminated fabrics that crack up completely after a few hours hard use).
The second test was for tear strength- I also have things for testing this, but they're from NZ not Switzerland, and called fingers. Tear strength is something that fabric manufacturers are rather loathe to release- in fact they not uncommonly deny that any such test is available. Instead, they make their products look good by quoting 'tensile strength'; which is measured by carefully and evenly loading up a strip of fabric until it breaks. For kite makers, (and most other users), this is not useful- kites generally break when point loads initiate tearing failure. The 'Kite Industry Standard' therefore is just to make a small cut in the edge of a sample piece and then see how much force it takes to develop this into a tear.
After 14 months up a tree, the Skysilk was still harder to tear than some low quality 42gm fabrics are when new off the roll- and a new Skysilk sample was about twice as tear resistant as the treed stuff. Partly this is because tear resistance is generally the flip side of stretchiness- and it's by being quite stretchy that Skysilk gets it's exceptional tear resistance, but it's still impressive. Treed Pilot Flying Again
Unfortunately, traction kites and sports kites do not perform well when built from stretchy materials. This is because, as for yacht sails, any wind or pressure induced shape change will generally make aerodynamic performance worse. Stretchiness isn't a problem for theme type single line kites though- or for relatively low L/D kites like Pilots.
By any standard, Skysilk's exceptional strength, durability and significantly lighter weight are a spectacular combination.
And it's resistance to fade, at least for this blue, is also beyond anything I've previously seen (the patches visible on the repaired kite are of new material from the original roll).
Congratulations Dominico (it's Korean maker), it's truly a miracle fabric.
And I guess I'd better thank the Chinese as well- without Tan and Wang's failure to cover my neglect, I might never have known this. I'll continue flying this kite during '10 to check for long term effects.
Ashburton NZ, Jan 1 2010
PS. I've never directly tested the heavier(42gm/sq.m) Dominico fabrics for porosity or flapping fatigue, though I know that they are more durable than other fabrics we have used- and their tear resistance is excellent. I will do these tests and report back sometime.