Following last month's update on single skin single line (SS) kites, I'm really boring into their development (ha ha- obsessions are never boring to the obsessee). With the benefit of a few days thinking time while getting to and from Kuwait for an event that was cancelled because of Corona virus fears, I've realised that my recent focus on show kite versions has caused me to neglect pilot kite options. So, I'm changing direction a bit.
But to re-cap SS show kite development so far: Octopus SS's are excellent, but unusable at events, 60m Serpent SS's fly well in mid-range and stronger winds, and SS Rays are looking promising.
Octopus SS's are the best flying SS design I've found so far, with excellent stability across the widest wind range, but their tentacles tangle. I've tried slippery fabric, soft fabric, stiff fabric, finely tapered ends, blunt ends, unequal lengths and cross links. Of these, only cross links work, but they snag on everything within range, damaging tentacles in the process. That tentacles on small framed Octopus kites like PLK have made many thousands of, don't tangle, is entirely a function of size. In larger sizes, single layer uninflated fabric tails ball up into nightmare tangles that take hours to undo.
Serpent styles fly well enough to be useful at events and look great. The 60m sizes are exceptionally stable through to any wind speed that I've had the courage to fly them in, but smaller versions are volatile unstable above about 50km/hr, eventually looping out as wind speed increases. All sizes fall off to one side or the other when the wind drops below about 10km/hr.
This does not make for friendly relations at major events. It happens because the wide tapering tail prevents the head from rotating independently when it needs to. This doesn't seem likely to be curable for the Serpent form.
Rays are a not a new style, with the first attempt more than 5 years ago. It was irretrievably super-stable (inexorable leaning off to one side). I've recently found that this was because of its high aspect ratio (AR, width/length) of 1.5. Lower aspect ratio versions (AR around 1.0) do fly quite well, though to my view their appearance is not as authentically Ray-like, so I'm now working to push their AR up as far as possible. I haven't yet developed an autonomous bridle to widen their single-setting wind range, but expect this will be more challenging than it has been for semi-circular leading-edge styles such as the Serpent.
Trying for fly-alone single skin show kites at this very early stage understanding of the genre may not have been the best approach. By way of analogy, even after 40 years of intensive development by many people, there are very few successful fly-alone ram air inflated show kites. Of my own designs, I'd only class the Ray (1987) and Octopus (1990) as being fundamentally reliable enough to fly pilotless at events. Martin Lester's Spirit range, Claudio Capelli's Cherubim, Mr Ma's Chinese Knot and Volker Hoberg's Heart and SFT also meet this standard to varying degrees, but there aren't many others (excluding kites like Ma's Trilobite and Martin's legs that are essentially just open leading edge parafoils with decorative features). Most ram air inflated show kites require pilot kites, at the least to enhance stability and extend wind range. Also, for comparison, I would rate current styles of single skin show kites as about where ram air show kites were in the 1980's/90's - while show kite events now, require a higher standard. Yes, successful fly-alone single skin designs will eventually evolve, but many thousands of hours, a lot of creative thinking and a fair dollop of luck will be needed before this happens.
An immediate answer therefore is to fly single skin show kites under pilots. This is a challenge, because of the general rule that pilot kites must fly at a higher angle than the kites below them. SS kites routinely fly at >60 degrees whereas most ram air pilot kites fly at around 45 degrees, and those that fly higher aren't reliable over the wide wind range (<10km/hr to >70km/hr without adjustment) that current international kite festival flying requires.
Going back to framed pilots is one possibility, but structural scale effects limit the size (and lift) that such kites can achieve while retraining acceptable light wind performance.
Which puts single skin pilots back in the frame. But after 7 years, more than 100 prototypes and by far the greatest sustained effort I've ever applied to any goal excepting kite sailing, I've so far failed to get any to fly well enough for general use.
Problems I've come up against:
Wind range: Single skin single line kites have very high lift coefficients (pulling maybe 3 times as much as a ram air kite of similar dimension) and they are more aerodynamically efficient (flying at a much higher line angle), but they require multiple bridle settings to cover the necessary wind range. This problem has been perhaps 90% solved by the development of automatic bridle adjusters - though the last 10% is always the most difficult. Autonomous bridle adjusters let out the leading edge as the kite's angle of attack decreases. There are two current systems: One links to increasing kite pull. This is a 'false' input when the kite is stalled at high angles of attack (like at launch) because the leading edge should then be pulled in not let out. But it is surprisingly effective, enabling a single setting range of 10km/hr to more than 70km/hr for Octopuses and 60m Serpents. The other uses braid bridling to the kite's trailing edge, rigged to let the leading edge bridles out as the drag on the braids increases. Called 'aeolian bridles', they have their minimum drag when the kite's surface is square to the wind, so are an excellent proxy for angle of attack. With both of these systems, detail design is crucial. So far, for the 60m Serpent, 10 or more bridle settings have been reduced to just 3; 7km/hr to 12km/hr, 9 to 20, and 10 to >70 (approximately). Perhaps with very careful design, one setting may be able to cover the entire range- a required feature for successful pilot kites.
Boomers and 1Skins suffer from 'diving over'- a tendency to dive off to one side or the other which increases in likelihood with increasing wind speed. This has been tracked down to compressive buckling in the skins and leading edges which, when asymmetric, causes the kite to fly off to one side. Skin buckling was eventually overcome by minor changes in shape and bridling. Tails also reduce chordwise buckling by the simple expedient of pulling on the kite's trailing edge. Leading edge buckling of single skin single line kites that don't have bridle supported leading edges has so proved unsurmountable making them unsatisfactory except in smooth mid-range winds. A question is why this is so, seeing as successful single skin steerable kites and paragliders use 1Skin type leading edges? There are two reasons. Firstly, when steerable SS wings buckle, the operator instinctively increases tension on the brake lines to recover them- which can't happen for autonomous flying single line kites. Secondly, single skin power kites and paragliders use stiffeners (typically weedwhacker cord or dental wire) to form their leading edges and to spread bridle loads. To date in this quest, I've avoided using stiffeners so as maximise my understanding of how to hold shapes solely by pressure differences. Nor do people who have tried stiffeners in 1Skins report this as a cure for diving over. I expect that to completely eliminate buckling, the entire leading edge would need to be quite rigid.
*Leading edge buckling is therefore the major remaining barrier to successful single skin pilot kite development. Fortunately, Octopus, Serpent and Ray style SS kites with bridle supported leading edges fly in every practical wind speed without ever 'diving over'. When their leading edges do buckle in, supporting bridles quickly pop them out again.
An additional requirement for all-purpose SS Pilots is that they must also be able to fly tailless. This is because tails cause a problem when launching from boats and jet skis for Tow Shows- and single skin kites have a major advantage in this application because they 'inflate' and fly immediately. For over-the-ground uses, a Serpent head with a short ribbon tail would be a satisfactory pilot kite if the current 3 bridle settings could be reduced to just 1.
Two tailless possibilities; a 1Skin layout with bridle supported leading edge, and a sled style with bridle supported leading edge and centre (see photo). One or both of these may eventually succeed, but the bar is very high because not only do they need to fly at a higher angle than ram air pilots, but they also have to hold this angle more reliably because SS show kites can collapse unrecoverably if a kite above them drops to a lower angle- a daunting requirement.
As I said when I started this SS project some 7 years ago, at least I know what I'll be doing for the rest of my life. I wasn't wrong!
Peter Lynn, Ashburton, New Zealand, February 29th 2020
Note: From now on I'm thinking to put this newsletter out through PL Kites only when the subject has some kite relevance. Newsletters with other topics (and the kite ones) will be posted to peterlynnhimself.com as always.