April 2009

Of Soft Kites and Male Organs.

The males of most mammalian species have a baculum (penis bone).  Human males don't, which could be regarded as a regrettable deficiency (except by the dysfunction industry- and the internet, which would be much smaller without the traffic it generates).  In some social situations though, I imagine that a baculum would be quite unhelpful.

Many aspects of ram air inflated kites are similar- both helpful and unhelpful.

Principally, the freedom to work in 3 dimensions without using rigid elements provides huge creative opportunity, while the tricky aerodynamics of non-rigid ever changing shapes is undoubtedly the main reason why soft kites aren't even more ubiquitous.  Probably this handicap is not so much intrinsic though but just because soft kites haven't been around for long enough yet.

Accepting that framed kites have been with us for possibly 10,000 years, their current diversity of form is not surprising.  Incremental development during the diaspora from an original invention in South Asia somewhere, some leaps of pure genius, and the occasional lucky accident, have all contributed to styles that now range from the simple Loko loko leaf kite that most probably started it all, to the (reputedly) 1000 year old Vietnamese Dieu Sao (see accompanying photo; of tailed versions not the pure style), an expression of pure aerodynamic elegance by any standards, (and a kite that I could scarcely believe would fly - except that it does), through the cellular kites of Rudolph Grund (photo) to modern kite artists such as George Peters working with carbon fibre and ripstop fabrics.  By contrast, soft kites have had little more than fifty years of development since Jalbert's and Rogallo's original insights.

But progress is happening; driven by traction kiting in its various forms, and by the growing demand for large, themed, single line show kites.

These newsletters for January and March have described a theory of single line kite stability which I fondly hope will be a useful development aid.  Whether a theory based approach will produce better results than just randomness remains to be seen though- as evolution demonstrates, it's not necessary to have any understanding at all of the 'why', just trying many variations and culling failures gets the job done.

Following are some explanations of the theories proposed, by reference to a specific kite.

I first made a soft kite in the shape of a Ray in 1988 (see photo).  In the 21 years since then I've put more time into developing this kite style than the total for all the other single line kites I've made. But while the current PLK Ray is a successful design, it's still not as good as it should be relative to the many thousands of hours it's taken.

Having now poked my head above the poppy field with claims to a useful theory of single line kite stability, I'm also abandoning all caution by testing these claims in public with a clean-sheet Ray design- and with rather ambitious goals: Closed leading edge (makes stability more difficult to achieve- see the March newsletter), no keels, no tail drag, realistic appearance and movement, and stability across the entire wind range from threshold to too-much without adjustment.

Could be luck- maybe I've just been fooled by randomness, but this new Ray is flying well with excellent realism and movement already- after just 2 months and 150 hours (I wouldn't have been discouraged by 6 months and 1000hrs).

But it's doing something strange- true to theory- but strange never the less.

You may skip this next bit if you find technical stuff boring, but I promise it's not complex or intimidating. 

As per the January and March newsletters, kites 'know' where 'up' is by their weight force acting at a point below the lift forces they generate.  This creates a pendulum, the axis of which points the kite to the sky.  However, instability is still the default setting for kites, even when this pendulum is operating, because various dynamic effects from aerodynamic interactions with the weight force generally cause them to sashay around the sky and loop out - taking neighbouring kites with them, which causes relationship problems amongst kite fliers similar to what would probably occur if human males were baculum equipped.

The second principle of the kite stability theory I've proposed is that the forces driving instability derive from aerodynamic lift while the stabilising forces derive from drag- so therefore, changes that increase lift relative to drag will tend to increase dynamic instability.

I thought it might be possible to replace the tail drag of the current Ray form by increasing its aspect ratio (span relative to chord) and by curving the wingtips upwards so that they generate drag rather than lift (and also show realistic movement).  The principle behind this (increasing aspect ratio ='s decreasing dynamic instability) works by moving stabilising elements further from the rotational centre- rather analogous to tight rope walkers use of balancing poles.  It was described in the January newsletter, but a graphic (and 1000years earlier- according to locals) example is the Vietnamese Dieu Sao, (and the Balinese Pelukan- are they related?).  For these kite styles, as for Genki's, dynamic instability is also controlled by increasing the span relative to chord (to an extreme degree in the Dieu Sao case).  I salute the unknown long ago genius kite maker who first found this not-obvious relationship.

Weirdly, the new Ray flies at an almost negative angle of attack- nose down!- while its tail hangs trails behind at a positive angle of attack.  This is completely the reverse of every other kite I've ever made before, or seen, I think.  Other single line ram air inflated kites and most (all?) framed kites fly with their body at a higher angle of attack than their tail-(see the accompanying photo of the new Ray flying beside an Octopus).  

I'm not quibbling though- it flies!- and I think you'll like the appearance and movement as well.  It still has a problem at the light wind end- until there is enough wind for complete inflation it tends to fold a wing under- but this only costs 1 or 2 km/hr relative to the older style Ray (which is an excellent light wind performer), and I expect that the new style will surpass the old even in this respect eventually- because it's lighter for the same area.

With this high aspect ratio soft kite design showing so much promise, and with Martin Lester's Spirit, Pierre Fabre's Space Man and the Capelli/Casada Cherubs excellent performance at the other end of the design spectrum, we can expect that the space between these extremes to eventually be filled by soft kites as well.

So soft (baculum deficiente) kites may one day rule the skies!- Sorry George.

Peter Lynn, Vang Tau, Vietnam, 29 March '09

PS Single line kite business seems to be anti-recessionary- PL Kites Ltd have huge demand pressure for show kites-unprecedented.  The chauffeur driven sewing machine they've kindly put at my disposal for the last few years has understandingly been withdrawn for the duration, so I'm back to driving myself.

And, the new 'peterlynnhimself' website fires up from 1 April- April Fools Day.

Robert van Weers, (webmaster), decided this was an appropriate date for some reason.