SSSL update from Feb 19 to May 1 '17

Octopus;

There was no change to shape or bridling during this period (Feb 19 to May 4 '17).  Octopus's remain the best flying SSSL's - better in some ways than even the latest 1Skins in their ability to deal with strong winds without sometimes diving off to one side unrecoverably.  Light wind flying is also impressive, but the tail links are a pain the way they catch on things - and in their propensity to tangle.

20m blue (the original one that was really flying well by Kelantan '15 and that was used and abused in the violent conditions at Weifang in 2016) and the later 20m pink, broke away while flying at Satun in 2017, went a km or so, and landed in the forest canopy.  Disabled as I was by then (PHN), Thai army recruits had pulled and cut them from the trees before Volker arrived on the scene.  Blue was totally destroyed- half the tentacles remained in the canopy, pink was substantially damaged (tentacles ripped or cut off in 10 places, bridles all cut off unevenly), but was eventually reconstituted and re-bridled. Pink flew very well in a subsequent test (Northpark) and in train and independently at Cervia (to May 1 '17).  Beneath the purple and green 30m Serpents at Cervia during the first weekend , the head would not stay open in winds above about 20km/hr.  After replacing the 6 rear bridles with 12mm white tape, the head then kept its form for the remaining days , sometimes in winds to 50km/hr and more.  This is graphic proof of the efficacy of the Aeolian bridle system (see 1Skins below).  It will be interesting to see if these vibrating bridles have also extended the wind range for each bridle setting for SS show kites, as I expect they will have. 

High drag rear bridles were suggested by Volker Hoberg.  The principle is that their drag increases more than with the square of the wind speed when they start vibrating at about 15km/hr.  By pulling in on the rear of the kite, they act to automatically reduce leading edge collapse in stronger winds.  Cleverly, they also don't begin to have much effect until the kite is at higher flying line angles (because at lower angles they are nearly parallel to the wind), which means that they do not cause the kite to stall, as any permanent (or adjustable) shortening of the rear bridles would do.

Another change made to 20m pink at Cervia, and to all the other SSSL show kites, was to replace the through line with 1000-1500kg (nominal) Dyneema  stitched to the crossed reinforcing lines where it passes through the head and tied into the main line attachment knot at the other end.  This enabled stacking of up to 4 SSSL show kites in moderate winds - the pull from kites above does not pass through the lower kite's bridling at all, is completely independent. This system is now to be used on all the show kite SSSL's (Octopus, Serpent and Centipede so far, Chinese opera mask next). 

Another addition that all the SSSL show kites need is a take- down line.  To the centre LE should work for the Octopus and Serpent, but maybe the Centipede's needs to be to one side of the head, and this might be best for all of them..    

Serpent:

30m Green: In early Feb, I had made a tail with 4/3m slots, 50mm wide, just below the head.  This was flown at Pasir Gudang and Satun (Feb 19 to March 3 '17) in train with the blue 20m Octopus and pink 20m Octopus, but wasn't tested by itself until, briefly, at Satun. In light shifty winds, it flew quite well- very little low wind superstability, but was volatile unstable above about 15km/hr.  A 30m x 100mm wide heavy green ripstop tail tied to the Serpent's tail end was enough to stop this volatile instability up to mid range winds at least.  Later in March, the tied-on tail extension was replaced with 2 side whiskers, each about 10m long, and it appeared to fly about as well when tested at Northpark- not as wide a wind range on each leading edge bridle adjustment as the Octopus's though. 

Later in March and early April, I made a purple 30m Serpent, all new dimensions.  With whiskers and 3/50mm slots. This was superstable at (Northpark).  I then removed the whiskers, re-sewed the slot panels, adding one more, overlapping all edges (centre panel to back) and cut 50mm camber into the seam between the lower end of the panels and the beginning of the single-width tail.  This was to allow the head some rotational freedom- to reduce superstability.  Tested at Northpark before Cervia, this was successful- it now flies well, better than the 30m green above, but still not quite up to the Octopus- I need to refine the seam camber and maybe add a bit more tail drag.  (It generally flew at a higher angle than the pink 20m Octopus at Cervia, which suggests that its tail still has less drag total than the Octopus's tentacles). This Serpent's head shape is superior to the 30m green and to the Octopus's and in particular, is amazingly resistant to wrapping around the line when flown in train or under a pilot, and even when it does wrap, it very often eventually comes clear without intervention.  Thru-line positioning is excellent, rarely shows a leading edge notchiness when overflying or/underflying.  Bridling (53 at present) may be economised to 47 by removing the last row (braids shifted to next row up), if this can be done without causing chordwise compression.  This would decrease the head size relative to the tail drag, maybe give it a higher wind range.

At Cervia 1May, strong wind (sand blowing everywhere), the 30m Purple's tail repeatedly twisted up then untwisted - 10 to 30min cycle, the 55m green hybrid Serpent's tail never twisted even once (it has 2 slots full length, the 30m has 4 slots for 3m only, at head).   

Centipede:

Originally made by myself and helpers at Kaixuan in early 2015, it refused to fly satisfactorily until extracted from the kite archive in early April '17, and modified it to the Octopus/Serpent style leading edge.  Originally it had a loose extension as per all early SSSL's- the "active leading edge" idea.  The aim of this was to have greater leading edge extension for light winds that automatically folded back in stronger winds (lower angle of attack), but so far the lower leading edge flap has always tended to cause premature folding in of the entire leading edge in stronger winds (tried on 1Skins and Octopus's ).  The new leading edge was cut to the same width as for the Serpent and bridled with centre leading edge lengths to; head bridle length less 150mm. 

Tested at Wakanui in mid April, it flew better than expected under a pilot, especially when all the feet bridles were let out by around 1m.

At Cervia, after further bridle refinement, it showed considerable and surprising promise.  Just possibly it is capable of flying alone now.  There don't appear to be enough rows of head bridles (chordwise) to support a 150mm edge depth at the centre (the 30m green Serpent show unacceptable chordwise compression if the bridles were reduced by just one row).  Two additional pairs of body bridles were added (to the outer edges) and this made a big difference to head inflation and to the transition from head to body. Perhaps the rear head could be made more sled-like.  The feet flare out and hold form reasonably well (and probably provide about the required amount of drag for single line stability), but the first pair need careful refinement of shape and line rigging- at present, in strong winds, the leg rigging distorts the eye shape..         

1Skins:

2015 6sq.m style ex Kaixuan.

These are excellent light wind fliers when the front 2 centre bridle sets are made adjustable (shortening) by up to 4 steps of 60mm.  The centre rib set should always be shortened by one knot more than the outer rib set for best performance. At Satun the yellow 6sq.m  I've been using since late 2016 began to misbehave- dived over and would not stay up in any wind on any bridle setting.  I noticed that Malcom Hubbard's one was also misbehaving- and it had been flying well at Pasir Gudang the week before.  Solution was to shorten all the rear bridles by around 30mm to compensate for fabric stretch (the rear of these kites flap in stronger winds, and though they are specifically aimed at providing a high lift kite for extremely light winds, even in light conditions they will probably stretch out and require occasional adjustment for this). 

It is particularly noticeable that this model collapses its centre front before its shoulders (which, when viewed from the side have a noticeably higher angle of attack). This allows the kite to continue flying without diving to one side or the other because of asymmetric leading-edge collapse when the wind strengthens to more than the bridle setting chosen could usually support.  This approach has now been adopted for the latest 3sq.m 1Skins (from 76).  At Cervia, the yellow 6sq.m performed exceptionally well in light conditions.  There are ram air 'foils that will stay up in wind as light, but not even 20sq.m 'foils have as much lift or fly at as high an angle. A 2x 15m heavy 100mm wide fabric tail has about the same cost to minimum wind speed as 1 or 2 knots adjustment in the bridle.  This tail does make the kite less volatile in all winds- which is a disadvantage in very light conditions if lift is the criteria rather than staying within a narrow piece of sky.  The reason that some kites, including SSSL pilots are volatile when flying at high angles of attack (low line angle)- a big advantage in light winds because this generates apparent wind- is that the moment of the weight force acting at its centre of mass, below the kite's centre of lift, is greater at higher angles of attack (it's a function of the sine of the angle of attack).  

The  6sq.m 1Skins of the 2015 style are still as good or better than the latest 3sq.m 1Skins in light winds- and this is very much a consequence of their shoulders being bridled at a higher angle of attack than the centre cells. They will fly, not prettily, but without too much inclination to leading edge collapse caused  superstability in mid-range winds, even when set to the lightest wind setting.   Current 3sq.m 1Skins are getting very close to as good, but it's not yet time to drop the 2015 style.

3sq.m 1Skins:

From 76 yellow (the first with the reduced and refined leading edge) they are all flying very well in light to medium winds and the random diving-over in stronger winds appears to be somewhat reduced,  76, 77, 79 and 86 have all been fitted with adjustable bridles for the bridles to the fronts of the centre 4 ribs in 4 steps of 40mm each.  At Pasir Gudang and Satun (no strong winds but very changeable and turbulent) they performed well as pilots for a Whale and 2 Rays (inflatable's) and for a 55m hybrid Serpent, 2/20m SS Octopus's and a 30mSS Serpent.

Ram air pilots definitely move around less and rarely dive off to either side, but the 1Skins have the advantages of much more lift and a higher-flying angle (essential for the Serpents and Octopus's.  However, for the average kiteflier they probably still require too much attention and knowledge (tuning for left-right and bridle adjustments for different wind strengths).

After Satun, they were progressively fitted with ribbon rear 6 bridles (Volker Hoberg's suggestion).  These begin to vibrate by 20km/hr of apparent wind, pulling in on the kite's rear so as to increase the angle of attack in stronger winds (but they also move the kite's centre of lift rearward, which may be exacerbating superstability). Because of their angle to the kite, these ribbons do not start to have much effect until the kite is near apex, which effectively allows more forward bridling during the climb phase, reducing stall propensity.

There was extensive comparative testing and development of 76 yellow, 77 pink/purple with porous rear skin, 79 mauve/grey, and 86 black Si, at Cervia from April 21 to May 1 in every wind from nothing to very strong (it was difficult for me to get enough foot traction on the sand to run them down with a karabiner).

One approach, eventually adopted for all of these kites was to increase their shoulder's angle of attack (by letting out the front shoulder bridles and/or pulling in the flare bridles).  This was from seeing the excellent resistance the 6sq.m 1Skins ex Kaixuan have to diving-over even when their centre leading edge is very noticeably folded inwards.(from being flown in strongish winds while on light wind settings). 

77 pink/purple with porous rear skin eventually became the best light wind kite of this series.  When on 3 x 40mm for the centre front bridle pair, 2 x 40mm for the 2nd rib set, it almost matched the 6sqm yellow on its lightest setting and had an uncanny ability to recover by itself just above the ground.  79 mauve-grey and 86 black were close to as good in the light conditions, especially 86.  76 yellow had a lever bridle and 79 mauve-grey had a pulley bridle until it was conclusively clear that the adjustable bridle system was superior.  Although the lever bridle and pulley bridle systems are very effective in enabling recovering from stall (by momentarily forward bridling the kite when stall occurs), they can both cause sudden unrecoverable luffing collapse in very turbulent t conditions. This never happened even once to 76 yellow at the Delhi event in January but has happened once or twice since.  Limiter lines to prevent the kite's angle of attack increasing above a "normal' setting were added to both the lever and pulley systems, and this overcome an inclination for the bridles to 'back' enough to initiate superstability (centre of pressure too far rearward).  A limiter line was also used to prevent the bridling going too far forward on the pulley system (the lever systems don't require this).  Different length levers were also tried and the longer ones (300mm) appeared to behave more like the pulley system.  The lever systems have  so far always been a bit more reliable than any pulley systems, probably the lever system allows so many options to shorten and lengthen various bridles by different amounts.

With all these kites (76, 77, 79 and 86) rigged with adjustable front bridles and ribbon rear bridles, at Cervia, all of them flew straight, or to one side a bit as desired without moving around in up to mid range winds and more.  In very light winds, 86 and 79 were much more volatile than 77 when flying at high angles of attack (a desirable characteristic provided it doesn't occur to extreme - dipping below horizontal when traversing- by building apparent wind).  79 is almost too volatile in this mode.  Bridling is not identical in all of these kites, but follows a common theme-  precise comparison might now be useful in establishing some reasons for the differences in performance.

On May 1 in strong winds 77 (centre set on 40mm, outers on 0) leant one way or the other pretty much randomly, flew straight only half the time or less.  86 (on 3 x40mm for centre set, 2 x40mm for outer rib set) flew very straight until at some wind strength its leading edge crumbled, and it went into a wild terminal dive (but this was at 40km/hr plus when on a very light wind setting).  79 (on 3 x40mm, 2x40mm as for 86) sustained right through to the strongest wind, when it took 2 people to pull it down- it was sometimes leaning right by then though. 

During heavy rain, a standard fabric 1Skin has been seen to became terminally volatile unstable- looping like crazy, but whether this always happens is not yet known.  86 black was built with silicon fabric to test whether this effect can be mitigated, which hasn't yet been properly tested- 86 is very quick to launch again after falling in the sea though.

It appears that these kites are behaving better in strong winds when on light wind settings than they do when bridled for full leading edge 'inflation'.  Could this be that their centre of pressure, being further forward on light wind settings, acts as a preventer of 'diving -over', outweighing the deleterious effect of some centre leading edge collapse?  The ribbon rear bridles also cause rearward migration of their centres of pressure in strong winds, which will be exacerbating superstability tendencies when the kites are already "bridled back".

They have noticeably less pull when on light wind settings also and less pull makes stability easier to achieve.  This is true because aerodynamic forces which increase with the square of the apparent wind speed can overwhelm a kite's weight, the only force available to point the kite upwards. 1Skins weigh around 0.35kgs (much more than this when wet), while their line pull commonly exceeds 30kgs in reasonably strong winds.  At this ratio of 100/1, it's asking a lot of the weight force to keep a leash on the very much greater aerodynamic forces (lift and drag).

There's a lot of information from this flying to digest, towards the goal of all-wind flying.  Certainly, the ribbon bridles do seem to be effective in broadening their wind range (worth at least 40mm on the front bridles?). Tails seem to be sometimes useful in turbulent conditions but it's notable that 79 flew best of all in the strongest wind and it was tailless.

Just what are the causes of the tendency of single skin pilots to violently dive off to one side or the other when the wind gets stronger?  After 3 years being plagued by this and many attempted cures, it's still happening- maybe not as often, and at a somewhat higher windspeed, but it's still the single characteristic that is preventing their general use as lifters.

I have no doubt that one cause is asymmetric collapse of the leading edge- this can be seen to happen and its effect is also immediately apparent; a dive off to the side which is more pushed in.  1Skin 76 and after, with their narrower and refined leading edges, are much less susceptible to collapse when flown with 'standard' length bridling, but the now general use of shortened leading edge bridles for better light wind performance has taken all of this improvement.  Post 76 kites now generally show just as much leading edge push-in as the earlier series (but they are MUCH better in light winds and still have at least the same max-wind capability, albeit with 2 adjustment steps.  How much of this improvement has come from ribbon rear bridles?

Another cause of 'diving over' that I am almost completely sure about is chordwise skin collapse- I have watched 1Skins flying centrally, then, as the wind increases, develop an asymmetric spanwise crease at anywhere between 20% and 80% of chord.  This then causes the kite to dive-off to the side that is creased.  Post, about, 50 series 1Skins, don't do this at all- or very rarely, because from then, the span wise camber was increased to keep some chordwise tension in the skin. This almost certainly exacerbates the centre of pressure migration problem discussed above and below though. 

An alternative way to reduce chordwise compression without increasing the camber is to lengthen all the bridles- but very long bridles can also have deleterious stability effects- as I first noticed with the double Serpent in the early '80s and later with inflatable Rays. It is possible to use long bridles in the chordwise sense while having shorter bridles span wise, but very long chordwise bridles can also have stability consequences, by making the kite less able to change its angle of attack without also changing line angle.

A fundamental cause of this 'diving-over', the cause that best fits the definition of superstability (slow  but inexorable diving over to one side or the other), is that the centre of lift of the more cambered airfoil that post 76 1Skins use, is likely to be close to 50% of chord at very low angles of attack, may even be more than 50% momentarily.  The 1Skin's centre of mass is slightly forward of 50% chord- and if there is ever a negative distance between the centre of lift centre of gravity, then single line flying becomes impossible. To address this requires for the aerofoil section to have more camber towards the front- and shortening the front centre bridles does exactly this- which might explain why 77, 79 and 86 all showed a counter-intuitive inclination to fly more reliably in strong winds with their front bridles shortened 80mm or more, and their leading edges folding under, than with standard bridling and perfect leading edge form. 

Therefore, a possible direction is to find ways to allow the leading edges to fold under without this distorting the ribs, or occurring asymmetrically. 

Using pulled-down leading edges with dams to ensure 'inflation', is another option, as is even narrower leading edges. Where does an arms race between narrowing the leading edge to reduce collapse while pulling down the leading edge for better light wind flying lead to?

Another approach may be stretchy rear bridles instead of ribbons (opposite effect but may be a similar result).  I expect that if stretchy rear bridles are to be used these should only be for the centre cells as experience indicates that shoulder collapse always causes diving over, and that ribbon bridles have been shown to be effective in preventing shoulder collapse,

Singers:

Almost a complete failure so far; At Cervia to May 1 '17, 76, 77, 79 and 86 1Skins were more reliable in mid to strong winds, and 79 mauve - grey 1Skin, tailless, was better than any of them in the strongest winds.  The 1Skins are always much better in light winds. After the initial tests at the kite factory, Northpark and Wakanui, Singers were not much used at Pasir Gudang, Satun, or Cervia until the last day.  Test flying during the week at Cervia was equivocal; they stall more easily than the 1Skins and  tend to being volatile unstable.  A few hours stuffing around with bridles in every wind from light to medium strong didn't discover any magic settings, though Simon Freidin's did fly better in mid-range winds without a tail than any that I had there (4 in total). With tails they do fly quite satisfactorily provided the wind isn't too light.  I surmise that with the very strong winds they were initially developed in at the kite factory paddock, the extreme turbulence caused them to appear to be better than they actually are.  The wind there was so violently changeable that any dive-off was soon saved by either entering a lower windspeed zone close to the ground, or by a beneficial wind shift. At Cervia on 1May '17, when the wind was very strong off the sea (40km/hr at ground level, assumed 60km/hr at kite altitude -pull approximately doubled from ground level to full height- and comparatively smooth), they performed badly.  Extensive testing and adjustment of 82 pink/purple with porous rear skin, could not get it to fly at all tailless (not for more than say a minute anyway) and with 2x15m 50mm light ripstop tails it was little better. Using 2x15m 75mm heavy ripstop tails was quite a bit better, but no sustained flights over a few minutes and a tendency to dive around relentlessly.  It was noticeably less volatile at apex, but sooner or later would slide off down into a higher angle of attack zone and become relentlessly volatile again. It was then tried with the rear centre bridle off; from the rear, the centre 2 cells was noticeably billowing up.  In this format 82 was more volatile unstable than any SSSL pilot I've ever seen flying. Even with a heavy tail it looped uncontrollably. This idea came also from Volker who found that one of his SSSL's flew quite well with a broken rear centre bridle.  It is worth trying on a 5 cell 1Skin, a design that is not inherently volatile unstable- and this could also provide useful insights into the likely effect of stretchy rear bridles.

Singer 84 yellow/orange with a lever bridle (and nose-up limiter) was the best Singer in these conditions- With the heavy tail it would hold its apex reasonably well and usually had to be pulled down for changes to be made, (rather than waiting for a convenient crash).  It would also fly reasonably well without the tail in this strong wind.  I then tried it without the lever bridle and there was no noticeable difference (short test though)- but the wind was strong enough to obviate the lever bridle's usefulness in stall-prevention. I have wondered whether the lever bridle also has some beneficial stabilising effect by reducing the kite's angle of attack in momentarily volatile situations, but apparently not.  During this lat test, the wind suddenly stopped- going from strong to nothing in less than a minute, so there was no opportunity to try it without the tail and without the lever bridle.

The original theory for reducing to 4 cells (from 5 for the 1Skins) was to decrease aspect ratio so as to make the kite more volatile and so less super-stable.  However, if the driver of superstability (diving over to one side or the other) for the SSSL kites is more to do with their centre of lift being inconveniently rearward or leading edge/skin compression, than being to do with the weight pendulum's leverage over wingtip drag, then their flying behaviour is explained.  Their leading edges do hold shape better than for the 1Skins though, and they never observably dive off because of some asymmetric leading-edge collapse, everything they do is explainable simply by their being too volatile.  Maybe an increase in aspect ratio without increasing the number of cells might be worthwhile trying (wider cells and shoulders). 

Simon Freidin's 0.75sq.m yellow 5 cell with7dams and a reduced lower leading edge (scaled from 1Skin 65) flew the best of all SS's in the very strongest wind at Cervia, tailless.  It was very quick responding, darted around a lot but almost never crashed (while on an about 20m line) These kites with interior dams (their leading edges remain 'inflated' all the time, never fold under) do seem not to have the terminal diving-over behaviour of the 1Skin series but don't perform as well in lighter winds, so far, and fly at a slightly lower line angle.  Simon's 1Skin 65 red, to which he has added dams rather mimicked the behaviour of his 0.75 sq.m yellow above- it recovered from perturbations very rapidly, not at all like the 1Skins, Singers or Boomers.  This suggests that the dams do have some fundamentally beneficial effect.  There is definitely something worth pursuing in this approach therefore, especially if their advantages can be retained while also recovering light end performance and higher flying angle. It seems plausible that the front dams have the effect of moving the kites centre of pressure forward- which is highly desirable.  How many dams are actually necessary, and how wide/?  What function do the dams towards the rear of the shoulder cells perform?  In effect, dams have the same function as the inflated leading edges used on the current Kaixuan  3sq.m 1Skins in keeping the kite's leading edge 'inflated', but the dams also work to shift the kite's centre of lift forward, at some lift/drag cost.  On the other hand, inflated leading edges have the disadvantage of trapping sand and water. 

Tails:

Tails have the effect of exacerbating stalling during take-off (before the tail becomes self-supporting) but once a tail is self supporting (tails incline at an angle of attack that is exactly self-supporting) it has three effects:

Its drag provides some downwind pull, causing the kite to fly at an averagely lower flying angle than it would without the tail.

It damps volatile instability (when the kite moves out of alignment with the tail- which will always align almost exactly with the apparent wind- the tail drag acts to pull the kite back into alignment). 

And, the tail's weight assists the kite's weight to point the kite up- which reduces any superstability tendency the kite may have.  I'm currently uncertain as to the quantitative relationship of this last, although there is an observable effect qualitatively- I need to look into this.

Chordwise camber:

Is there any way to get the 1Skins (and other SSSLs) to hold shape chordwise except by camber of at least the bridle radius length? 

The SS show kites use pocketing in each cell (bridles at each of 4 corners), assisted by the cording pulling in a bit to make these pockets more 'shaped'.  But they still require chordwise camber equal to the radius described by bridle length, and even then its noticeable that, for example, the green 30m Serpent would not hold chordwise form with even one row of bridles less because of major chordwise compression.

As was found in the early stages of this development, using more spanwise camber (sled like shape) does not seem to help either- a flat sheet bridled only by 2 opposite edges and with no chordwise camber compresses up into creases (This needs to be checked again- does spanwise camber reduce the need for chordwise camber by even a bit?).

The early series 1Skins had chordwise camber in the front 60% (approx..) were flatter in the rear 40%.  This did seem to work in that some of those 1Skins would fly in wind as strong as the best do now.  But the rear skin flapped madly, quite rapidly destroying the fabric- and annoying users who then tried all sorts of methods to stop the flapping (Volker's gauze add-on for example).  Could chordwise camber in the front 50% and spanwise camber in the rear 50% work?  Needs to be tried.

There is one way to get a flatter rear section that is definitely achievable: Using drag pull on the trailing edge to hold the rear section tight against compressive collapse.  Could this be just a scooped trailing edge?  Up or down?