Peter Lynn

July 2008

Engineering: what you do if you can't do art.

Actually I've misquoted this aphorism; it should be the other way around.

Or should it?  I had it the other way but then got to thinking about Claudio Capelli's Cherub kite (developed in collaboration with Marco Casada)  This is not only great looking (which it should be of course, Claudio being an artist and all that), but it flies great, which is rather in the realm of engineering.  Soft kites are difficult enough things even when they don't have to look like something as well as fly well.  The Cherub not only flies well it flies VERY well- up there, pilotless, at an angle usually reserved for kites aerodynamically uncompromised by a cute baby face, ridiculous angel wings, and such an endearing little willy. (Cherub Photo)

Cherub and other kites

Congratulations to Claudio (and Marco), it's the best ram air themed kite ever I reckon (run close by Rolf Zimmerman's Dragon (below), which only loses by requiring a somewhat incongruous drogue), but damn, how embarrassing!  I mean to say, bridges aren't designed by artists (though that sub species called architects might contribute to their visual appearance), aeroplanes aren't designed by artists either.  So how come the design of kites (of the single line type, an important distinction), is still in the realm of art- and black art at that?

Rolf Zimmerman's Dragon

Well it's simple really- or rather it's not simple.  Engineers need formulas and principles, but for single kites there aren't any, or at least not enough to give an analytical approach any advantage over intuition.

I can't do art it seems, but I should be able to do formulas- so it's time to stop being a whining sooky and get down to doing engineering.

Actually I've been working away at this job (defining why kites fly) since the early '70's- mostly by closely observing kite behaviour then trying to think it into some universal model.  Like for classical Greek cosmologists, this approach mostly failed, so I've fast forwarded to the age of reason and am now trying reductionism and experimentation.

For the last few years I've concentrated on basic four cell parafoils, isolating one thing at a time to make some sense of cause and effect; things like aspect ratio (width / length) cell opening area, lateral (flare) area and it's longitudinal distribution, camber disposition, centre of lift (defined by bridle position) and so on.  There seems to be progress, but before rushing into print, the developing theories need to be tested against a variety of other kite types.

As a first step I've been applying them to the development of Slarcs, (single line Arcs) for the dual mode project (see April '08 Newsletter).  It's working.

Getting Arc style kites to fly well single line has been challenging, even though most kite surfing style Arcs will fly as single line kites without modification.  For the dual mode concept to work though, they need to fly exceptionally well as single line kites- that is, docilely and reliably and without sashaying all over the place even when on a short line.  The major problem has been that Arcs are just fundamentally too efficient, so take every opportunity to accelerate to maximum speed whenever displaced from steady state flight.  For the first few years of Slarc development I attacked this by building a succession of dedicated prototypes (with much appreciated help from Vlieger Op in den Hague and Makani Power in San Francisco).  More lately, as principles developed from the experimental / reductionist approach to kite stability have filtered through, I've learnt to rig basically any model of kitesurfing style Arc kite to fly well on a single line.  In fact they now fly better than purpose built Slarcs ever did.

F-arc 1600 flying as a Slarc

Scorpion Slarc

This has had some unintended consequences.  The goal had been dual mode kites, but it's obvious now that these kites also have a stand-alone role as single line kites.  It's not just that their angle of flight is superior enough to excite the most discerning observer, but their stability in strong and turbulent winds is now good enough to make them my pilot kite of choice in these conditions.

A limitation is that they are heavier for the same projected area than conventional parafoil style pilot kites, so need a bit more wind to get started and will fall first as the wind drops.  But when there's wind enough (about 15km/hr at present), this is not a problem.

One obvious other application is for kite altitude attempts.  They will be ideal for this; VERY high L/D, excellent stability, excellent strong wind tolerance, and pull that increases much less than with the square of wind speed.  10,000m's seems to be a reasonable goal.

Best of all, they should be cheap.  In anticipation of interest, I'm having some left over old stock from den Hague sent to NZ, and in every kite surfing country, there must be lots of perfectly serviceable earlier model Arcs whose on-the-water days are over. 

For rigging; I'll make bridle details available for each style and size as I refine them.

Peter Lynn, Alcochete, 30 June '08




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