"In theory, there's no difference between theory and practice.
In practice, there is."
This one is aimed at the visitor interested in prehistory ;-). It is a bit longish and if it's true that an image is worth 1000 words, then skip the reading and go to the end of the blog.
It is about LEI (leading edge inflatable, or tube kites) and foils, but not so much about the never-ending discussion on the relative merits of LEI and foils, but how it came about that two so different concepts emerged and how they are likely to develop.
If one is interested to guess where kites technology is going to evolve, we've got to look back at the evolution of other air and water sports such as hang-gliding, paragliding and windsurfing. So I hope I am going to be forgiven to digress a while from kitesurfing to talk about hang-gliding which was then (long ago), just as kitesurfing is today, my overwhelming passion.
The earliest hang-gliders consisted of tubes and polyester canvas (or even polyethylene sheet!), and had very low aspect ratio, which gave a L/D ratio (lift to drag, the equivalent to the glide ratio for flying things) of around 4. I can remember the front cover of the French magazine Paris Match picturing the "flight" around the Eiffel tower of a daredevil with a Greek name (not Ikarus), which must have been launched with the help of a winch. The article describes the contraption, inspired by Rogallo and called delta in French (and perhaps even in Greek!), which had a glide angle of about one to three. That figure disappointed me greatly, and I can clearly remember thinking the day such a thing will manage 4, I'll go for it. It didn't last long; the year after a guy called Bernard Danis offered deltas in kit made in his garage made deltas and they were reported to achieve a glide of 1 to 4 in good hands and perhaps with a bit of help of some friendly lift. I ordered one of the very first ones, and set out to learn with a couple friends of great experience (one of them learned at least one month before me and the other was Etienne Rithner, "the Swiss hang-gliding pope" as he was called later, who had actual flying experience (*) and who eventually made an industry of hang-gliding manufacturing).
As years went by the aspect ratio (the average span divided by the average chord, a measure of "slenderness" of a wing, a kite or a sail) increased significantly and the profile got to look more and more like a real airfoil, with actual thickness, by the gradual addition of a second skin and hiding the drag inducing cross bar inside. Consequently the L/D of flex wings approached the 1 to 12 mark of today’s best hang-gliders, and the sink rate lowered to 0.9 m/s from twice as much with earliest "delta-ploufs" (= sinking deltas). But more importantly perhaps, the "penetration", i.e. the ability to maintain reasonable L/D at higher speeds and thus not losing to much height in areas of downdraft, was improved to such an extent that cross-country flying became accessible to all.
Parallel to the flex wings, there had always been rigid hang-gliders designs; the great Otto being the first acknowledged one. They require, as opposed to the weight-shifting of the flexwings, aerodynamic controls to control yaw, whilst pitch often relied on weight shift as for flexwings.
10 years after the invention of hang-gliding, a small group of nutcases were throwing themselves from the cliffs of Mont Saleve (near Geneva) with square parachutes. Those guys quickly realised a bigger area would make the launch easier, and made basic improvements on their chutes. Laurent de Kalbermatten (another pope) was the first to realise the commercial potential of this new-born sport and was the first to manufacture on a grand scale very much improved gliders. The design of paragliders followed a similar path to hang-gliders towards higher performance by increasing the aspect ratio and optimisation of the airfoil.
Back to kites.
Tubes kites were improved from the early models, first and foremost by splitting the control lines into front and back in order to modify the angle of attack, and consequently, the lift (we kiteriders wrongly say "power"). The bar was a brilliant idea; it enables to control the kite's angle of attack (power) and the turning independently in an intuitive manner. It is the equivalent of the control bar of the hang-glider and the wishbone of windsurfing rigs. Early foils benefited the innovation; but as a matter of fact I am not sure which of foils or tubes were the first to use the “modern” 4 lines control bar.
Then came the pursuit of ever increasing performance, just like hang-gliders and paragliders, by increasing the aspect ratio. The difficult part of the improvement was that it should not be achieved at the expense of the handling. At that early stage, and regarding the turning ability and to a certain point the depower, tubes (LEI, or leading edge inflatables) had the definite edge, principally because of the fact they were maintained at their tips, they had to assume their characteristic C shape, whereas most foils (Peter Lynn kites being the exception) were “bridled”, meaning their shape was maintained across the whole span of the kites with a multitude of lines. This bridled concept had the advantage of a perfect control of the shape, approaching as far as possible the ideal airfoil (as paragliders), but they had the drawback of poor turning and depower, and are still suffering from that reputation in spite of the fact that a few year back, Flysurfer changed that state of matter with the improvement and introduction of new technologies giving as good turning speed and depower as the best tube kites.
Whilst foils were improving the handling, the tube industry worked on improving performance (power/sq meter) by “bridling” somewhat the kites so that they could become flatter (the Crossbow was the breakthrough), but as could be expected, it was at the cost of the handling. This is why most new-style pro riders generally prefer to stick to the C shape kites which were never supplanted by bow and hybrid kites for that handling reason. See http://www.inflatablekite.com/sitebow/BowHome.php for attempts by Legaignoux brothers.
Yet, strangely enough, the profile of tube kites remains as primitive as it was on day one! Still an un-faired tube and a single surface. Quite primitive really. There has been some, but un-sustained research done by a few manufacturers to add a second skin in order to reduce the LE tube’s drag. This will be the only way tube kites will ever be able to match foils kite’s raw performance. Some of the technical difficulties which will need to be overcome will be handling of course (a second skin adds rigidity to the structure), the trapped water should evacuate quickly, and the weight problem. Flysurfer had to overcome challenges which were just as problematic, but have the great merit to have stuck to the concept without despairing, and the results are Psycho4 (for handling) and Speed 3 (for sheer performance). The last and major challenge Flysurfer faces remains to get the tube guys to review their prejudice. Very easily done if we can get them to try!
I believe the tube industry sooner or later will react, either by starting producing foils (!), or by making tubes very similar to FSs in terms of aspect ratio and airfoil section. Time will tell if this view is correct.
Well. This turned out to be a foil versus LEI discussion after all. All my apologies.
The kite is only part of the game; the board part plays an equally important role and that could well be the topic of a blog to come.
Three cheers to anyone who got this far!
PS: Writing this blog made me want to fly hang-gliders again!
(*) Rithner’s experience included successful manufacturing and operation of a Lilienthal-like ski-launched glider, and a flight on a Tipsy Nipper ending hanging on high voltage electric lines.
is to this
is to that !
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