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Peter Lynn

May 2004

After the best ever Berck event in France, for the last week I've been at the Cervia Festival on the Adriatic coast of Italy- in company with an enterage of female familie and friends that kept me in line by crikey, loooong lunches at Benito's every day notwithstanding. The femmes were not at all slow to notice when any male accidentally glanced towards any of the lekker Italian babes lolling on the beach there. However, this keeping in line business seems to be a one way thing, said female entourage being not at all slow in working out the Italian for "I like your Speedo's----*". Not all their subjects were Italian stallions either, there's almost no limit to the posing that some well know kite fliers will do to get their photographs published. See attached.

And, while on the subject of pillow talk, there's a scurrilous plot, ably led by Scott Skinner and Robert Trepanier ( though they would say that they are just riding a popular wave) to re-name our Flag kites, my favourite new design, as Pillows and, even worse, Bags.
I've been working on the flag design for about a year now, had immediate success with the smaller sizes but came up against the usual single line soft kite stability problem as they scaled up. This problem is easier to describe for our pilot kite designs because it manifests particularly clearly. In the 2 sq.m size, pilots are almost uncontrollably volatile- wild oscillations building rapidly to terminal looping even in light wind. In 4sq.m size they can be excellent but still tend towards the volatile as wind speed increases, especially if heavier fabric is used. The 8sq.m size pilots are nearly perfect kites- insensitive to bridling, not oversensitive to fabric weight or stiffness and neither volatile unstable nor subject to edging or superstability in strong winds- at least until 60km/hr or more, when all sensible large kite flier have retreated to the bar anyway. By 16sq.m's, Pilots are inclined to edge or hang off to one side as wind speed increases. I've always called this type of instability "superstability" though this term covers a multiplicity of sins. One thing that superstability isn't is assymetry in manufacture or bridling- of course it's true that perfect symmetry can delay the onset of superstability, but a useful analogy is balancing a triangle on it's point- even when perfectly made and balanced, sooner or later something will cause it to tip one way or the other. Our job as kite designers is to make kites that behave like triangles resting on one side, not a corner- so that when tipped a bit they settle back quickly to their original orientation.
The reason that soft kites change from volatile to superstability as they are made larger is very likely to be that the mass of air contained within the kite increases disproportionately as area increases. The Megabite has 635 sq.m of lifting surface and contains roughly 2 tonnes mass of air. a Trilobite of 1/100th of this area contains only 1/1000th of this air mass. When a kite is tipped a bit by turbulence or a wind shift, the time it takes to restore it's orientation determines how far to the edge of the wind window it will fly before this correction happens. Because of the huge mass of air they have to swing, very large soft kites take so long to correct from a perturbatuion that they can fly right to the edge of their window before adequate correction occurs. A kite's weight and where it is centred can have a big effect on this correction time also- making a kite heavier and especially adding weight to the trailing edge area will help to conteract superstabilty, as will bridling the kite more forward (effectively moving the centre of gravity back). But, the largest effect is aspect ratio- how wide a kite is relative to it's length. High aspect ratio kites tend to superstability because more of their contained air mass is further out from the centre of rotation. For the pilot kite design therefore, if the 2sq.m size was made wider but shorter it would be less volatile. For the 16sq.m size, making it longer but narrower would reduce the superstability problem. We have only recently started to make very small pilots and have so far made only one larger than 8sq.m, but I'll try these design changes as soon as there's a break from continual festivalling.- it's a tough life!
The Flags are a challenge with respect to superstability because changing their aspect ratio as they become larger is not an option- The span to chord ratio of a flag is 0.63+/-, anything very far from this looks silly. Anyway, 30 years of gradually improved understanding and the developing theory of why kites don't fly will have it's way even aganst a seemingly intractable problem such as this- and it has. As from about a week ago, the big flag edging problem is solved- at least to 8m and I'm sure to much larger sizes because the current 8m (span not area) is now still showing a little residual volatility even at 50km/hr- (and volatility is an easily solvable problem, it's superstability that's the bete noir).
For the last day at Cervia, the first time at a festival in 5 years at least, I flew entirely without pilots (actually I'd sold them all), using Flags and Quilts of different sizes instead- many big kites shoulder to shoulder in a tiny space and nary a problem sufficient to interrupt lunch.
Yep, it's pat on the back time, wallowing in self satisfaction I am after 12months/1000hrs of frustration beating my head against this problem.
Well, not really, what I did was to remove every second bridle along the kite's leading edge- and I have absolutely no idea why it worked.
Oh well.
And, a message for Scott and Robert: Nathanial Herreschoff, famous 19th century yacht designer, said with respect to yacht aesthetics: "anything that's consistently in front of you starts to look beautiful after a while".
The flags are flying better now than any inflatable kite I've ever built, pilots almost not excepting So, S and R- , you'll come around.

* I never did quite catch the second part of this - sounded to me like "--and will you take them off", but this can't be true of course.


Peter Lynn, Den Hague,

May 2 '04 - off to the Swindon festival in England tomorrow.

 

 

 

 

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ASHBURTON 8300
NEW ZEALAND

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