May 2004

Newsletter, May '04

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 entourage of female familie and friends that kept me in line by crikey, l-o-n-g lunches at Benito's every day notwithstanding. The femmes were not at all slow to notice when any  male accidentally glanced towards 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 Trepannier ( though they would say that they are just riding a popular wave) to pejoratively re-name our Flag kites as Pillows and, our Quilt kites as  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 edging 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 kite fliers 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 and tend to lean off to an edge in stronger winds 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 proportionally heavier and especially adding weight to the trailing edge area, will 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 and the aerodynamic correcting forces from the rudder effect of a longer body are less.  This is why Quilt kites (aspect ratio 1.0) are easier to design in larger sizes than Flags (aspect ratio 1.5).  For the pilot kite design, 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 one-  I'll try the design changes this developing theory suggests 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 reducing their aspect ratio as they become larger is not an option- The span to chord ratio of a flag is 0.63+/-.  Anyway, the experience gained from 30 years spent developing theories as to why kites don't fly will have it's way even against a seemingly intractable problem such as this- and it has.  As from a few weeks 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 still showing a little residual volatility even at 50km/hr- and volatility is easy to solve, (as is attested by the general smugness of small kite designers)- for large soft kites, superstability is the bete noir.

For the last day at Cervia,  the first time in 5 years at least,  I flew entirely without pilots (had to, they were all sold ), 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 for the entire afternoon.

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.  The cure was to remove every second bridle along the kite's leading edge- and I have absolutely no idea  why  this worked.  A pyrrhic victory indeed if it's killed 30 years of theory.

Oh well,  back to the drawing board- except I can't remember where I left it..

And, a message for  Scott/Robert:  Natt 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".

Excepting, barely, the 8m Pilot,  Flags are flying better now than any inflatable kite I've ever designed, with Quilts a close second -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 Swindon (England) tomorrow