After Dieppe (Sept '16) to February '17.
This development project is becoming something of a saga, hopefully not of the type when the subject remains lost in the swamp forever!
Single skin pilots: Up to end '16, I concentrated on improving the LE. Many prototypes; starting with a shaped polystyrene model, stuck on pieces of fabric. The challenge is to estimate (guess) the spanwise camber of each cell at the leading edge, which is really difficult to do as this is also very sensitive to small changes in chordwise bridling. Most single skin traction kites deal to this by having close spaced ribs so that this spanwise camber and camber changes can effectively be ignored. 4Skins, and 1Skins have significant cell camber at the leading edge and the only path to clean shaping seems to be make, bridle, fly, modify,--- in repeating iterations. By 1Skin 79 (grey/blue) India (12 to 19 January) their LE's were very clean- and as part of this process, the lower LE had been progressively reduced, by the theory that indenting of the lower LE in stronger winds (lower angle of attack), was causing the upper LE to also indent, promoting superstability. 79 and successors show no indentation at all up to perhaps 100km/hr, their LEs hold shape. But they are still subject to superstability- and after going back and re-testing earlier models right back to the 30 series during January '17- it became clear that the onset windspeed that this occurs at has not changed substantially (around 40km/hr), even with LE improvements. Maybe later kites are more reliable and generally have a higher wind capability- the earlier models re-tested were all selected as being the "best of" for their time- but without doubt some early versions can fly reliably in winds as strong as any of the latest, up to 79. This suggests that there is a cause of superstability other than LE indentation or chordwise skin compression. For now, the most likely candidate is insufficient pendulum effect from the kite's weight (300gm) acting below its centre of lift, to overcome the large aerodynamic forces (30kg +) generated in strong winds. The obvious cure (apart from adding a tail, which shifts the centre of gravity rearward) is to move the kite's centre of pressure forward without also shifting its centre of gravity, but this is prevented by the necessity for having significant chordwise camber over the entire chord so as to prevent spanwise compressive creases in the skin (which also cause superstability).
Light wind flying: The 79 style 3sq.m 1Skins will accept shortening of the front 4 bridles by up to 80mm in 3 stages for light wind flying- the ranges being say <10 to 15, 12 to 20 and 15 to >40km/hr. In India it was noticeable that an earlier (40 series) 6sq.m 1Skin (ex Kaixuan) with 120mm shortening was not only able to fly in lighter winds than any conventional ram air pilots there (and pretty similar to Volker Hoberg's very lightweight plastic ram air pilot) but even by 10km/hr had substantial lift- enough to keep maxi show kites up when nothing else could. This kite was also marginally better in the very lightest wind than 1Skin 79 on the maximum shortening it would accept. This is very likely to be a consequence of the proportionally wider LE of the 40 series- which effectively decreases the kite's angle of attack by a few extra degrees (but promotes more LE collapse in stronger winds). The 6sq.m 1Skin also collapses the centre section of its leading edge before any shoulder distortion if left on more forward bridle settings as wind speed increases, flying quite reliably in this mode. When production at Kaixuan is upgraded to the latest series, it will therefore be sensible to continue production of the 6sq.m 40 series until a better light wind design comes along, if ever!
Before India, I had tried various reactive bridle systems on the 79 series in an attempt to eliminate the requirement to make bridle adjustments for different winds. I've often discussed the possibilities of this before, but know from long experience that this part of the swamp is particularly impenetrable, so I've been very reluctant go there, fearing I'll be lost forever. But surprise!: A lever bridle system fitted to 1Skin76 yellow (pretty similar to 79 with a reduced lower LE, just not quite as refined) worked immediately the best- noticeably superior to a pulley system with very similar geometry fitted to 1Skin78 (pink/purple). Flying side by side in India, wind rising from not enough to say 30km/hr max, 76 stayed up more consistently and moved around less.
Before India, had also tried a tried a version 1Skin 77 pink/purple with a porous rear section. The idea is to allow some pressure bleed in stronger winds so as to reduce maximum aero forces while not significantly effecting light wind behaviour. First attempt at this with fine mesh gauze was unsuccessful- required substantial wind to fly at all. Second attempt was also unsuccessful as the uncoated fabric used distorted during sewing causing just about the most left flying 1Skin I have ever built. Third try, stabilising the fabric with hair spray before cutting and sewing was successful. Light wind flying is not noticeably different, and it does seem a little less violent in strong conditions.
But none of these tries has lifted the wind range by enough for reliable upper end kite event flying so far (though the lever bridle is promising.)
Next attempt was to start a lower aspect ratio pilot kite series, called Singers (but in the same number sequence), basically by removing one centre rib from the 1Skin pattern- plus some consequent changes (every template is different). Singer 80 red is 2.25sq.m and has an aspect ratio of 0.85 (1Skin 79 is 1.08). Although this doesn't change the relative effect of the pendulum restoring moment at all (it's a function only of the distance between the centre of pressure and centre of gravity) it does improve recovery from any angular deviation because tip drag is closer to the kite's centre line. Singer 80 is therefore quite volatile, inclined to looping, but does not show signs of superstability in any conditions- not even in winds that are blowing branches off trees. To control this volatility, various tails were tried. The principle of tails, if they are to control volatility without exacerbating superstability is that they should be self-supporting (generating enough lift to support their weight), have as little drag as possible within this constraint, and be attached at a single point on the kite's centre line. In effect they provide damping and shift the kite's centre of gravity rearward while moving its centre of lift rearward as little as possible. For Singers, a fabric strip tail of around 30m length and weighing about as much as the kite itself works well. With such a tail, Singers 80, 81 (porous rear section) and 82 will fly reliably in VERY strong winds- in 1 case breaking the main line with something above 40kg pull I think. To reduce volatility a bit, 83 and 84 were then built, still with just 4 cells and 16 bridles, but a little wider in each cell for area 2.5sq.m and aspect ratio 0.93. (Aspect ratio,- span squared divided by area- is the strongest determinant of the volatility/superstability boundary for single line kites, higher aspect ratio kites are less volatile , more superstable). These are very pleasant kites, not requiring tails in normal conditions. With a lever bridle, 83 had some of the most reliable flying in mid-range gusty condition that I have yet achieved at the kite factory field with single skin pilots, very steady, with quick recovery from wind induced tilting, no signs of looping (volatility) or diving off (superstability). But when changed to a pulley bridle, it flew less reliably- though maybe this was a wind effect. Singer 84 (yellow/orange) is identical to 83 so as to enable side by side testing of different reactive bridle arrangements.
From 2nd Feb '17: In light variable northerly wind, (Northpark) even when flying Singers 83 and 84 side by side it's difficult to distinguish kite differences from wind changes. Indications so far are that differences between the pulley and lever bridle systems are probably in the details (especially bridle self-adjustment limits).
Limiting the amount by which the front bridles can be let out relative to other bridles seems to have a profound effect on stability. Allowing them to lengthen by say 25mm relative to the rear set, and having this change occurring at lower wind speeds appeared to promote quite wild swings to either side in light winds. The most likely explanation for this is that it's a centre of pressure migration effect- the front letting out moves the centre of pressure too close to the kite's centre of gravity for rapid response to wind gust induced perturbations. But by limiting their extension to zero (best for light wind flying), will strong wind flying be compromised?
If c of p migration from small changes in 'as bridled' angle of attack does have such significant effect on stability as this indicates- and this seems likely seeing as these are probably changing the relative length of the restorative pendulum by significant percentages (25% +/-) then total bridle length can also be expected to have significant effects- shorter for more responsiveness, longer for slower response. This accords with other kite stability experiences- for example; moving the line attachment point further from the kite is a known way of reducing volatility for ram air and sticked kites. Unfortunately, for single skin kites, shortening (to improve responsiveness) is not a realistic option because their skins cannot resist compressive loads. In stronger gusty north-west at Northpark, and after further detail refinements, Singer 83 with pulley bridle is very little different to 84 (lever) in crash frequency (short line) but seemed to use more sky to recover. Could this be from other minor differences? 1Skin76 yellow (with lever bridle) was if anything more reliable in the same wind except that it tended to hang to left or right- superstable- and eventually broke the lever. 1Skin 79 grey/blue was pretty good also- scarcely less fly-able. Best by far though, in these conditions was Singer 82 purple with one tail set.
Next test should be Singer 83 with fixed bridle against Singer 84 with lever bridle- it is easy and quick to immobilise the pulley. And then try with a tail- 30m x 200mm single strip rather than current double 80mm wide (as sewn) strips.
Octopus and Serpent: My latest pink 20m Octopus, made and bridled the same as the older blue one (which is such a reliable kite across the range) had never flown satisfactorily, after more than a year, many attempts and comparisons. It went to Dieppe in September, where I expected to get it sorted, but was in a bag lost by Southern China/Air France with the 55m Serpent, which I didn't get back until on the way home again. Before India I finally noticed 4 bridles in the rear body area that were approx. 50mm too short. This fixed it- which surprises me a lot because I have- now had- the view that these bridles are relatively unimportant.
35 m green Serpent: After very many reconstructions of the head leading edge and the tail- so many that the fabric was almost falling apart from needle holes in places- allowing some airflow to escape through the tail at the base of the head - as suggested by Volker Hoberg- and making these lateral slots into 4 scoops did eventually get it flying. Earlier attempts at providing some tunable drag at the tail/head connection had all failed. At one stage I added orange peel shaped scoops down the edges of the tail- more and more until eventually it became obvious, even to me, that adding ever more drag was not the answer. Although in some specific winds it would occasionally fly briefly, the tendency to dive off over to one side or the other rather than apex was basically unstoppable. For a long time I thought that the problem was just that I couldn't get the drag to act close enough to the head and that what I was seeing was typical superstability caused by having a tail that's too long. This effect can be seen whenever a long tail catches on something- the kite then slowly falls off to one side or the other. However, when I did get to look at this now-flying-OK Serpent from the side (in India) the scoop effect at the head/tail joint was just too unsightly- a big ugly protuberance where there shouldn't be one. Next try was to re-sew, again, with 4 longitudinal slits in the first few metres of tail. This does look OK now and is close to flying satisfactorily. It could still do with a bit more drag in this area, but its's possible that enough improvement can come from further bridle adjustment. What I have found during the many days I've spent trying different bridle arrangements on this kite is that it does respond quite well to the sides of the head being pulled forward to create a sled effect. I've also found that if the leading edge bridles are not at least 150mm shorter than the face bridles (which are all made the same length as a start point), the head compresses longitudinally into a series of ridges.
Still a work in progress!
On the more positive side, both the Serpent and the Octopus can now be trained satisfactorily- which is very useful in the limited sky space at most events. The top kite needs to always fly at a higher angle than lower kites. To this end, the 79 series 1Skins are best- or the 40 series 6sq.m in light winds. But if the top kite ever falls back or drops to one side, the lower kites will almost always wrap irretrievably around the top kite's line. Setting the lower kites for wind speed higher than they are actually experiencing also helps prevent them overflying. Ram air pilots do not fly at high enough angles to be useable above single skin show kites. I've now stacked up to 3 single skin show kites below a 1Skin pilot satisfactorily, and expect more will be possible.
Before West Java (25 Aug to 1 Sept 2016 to after Dieppe (8 to 19 Sept 2016).30m Serpent:
Very worthwhile developing because it has less tail snagging than the Octopus series- which are a nightmare because of the linked tails.
Yellow appeared to be flyable at Northpark, 22 Aug,, wind NW, fading. Seemed to need more tail drag, so added another 4 (?) central scoops to the 10 diagonal.. Then seemed to have too much tail drag (falling over to side in light), but also more LE collapse than 20m Octo blue. Why? - has to be in bridling and shape of head.
Tried sewing off 200mm crescent from top of head by theory that its pointy shape (compared to Octopus?) was a cause of LE collapse. This reduced bridles by 4 and did appear to reduce LE collapse, so I cut this extra fabric off. Now quite ugly head shape- 'ears' at shoulders. Sewed off all scoops, tested again at Northpark, seemed much better but was volatile unstable from mid-range up- test further at Dieppe, re-engage by progressively unpicking scoops until just enough.
At Dieppe, still no useful steady mid-range winds, still inconclusive. Refined LE bridle set a lot, slightly different to 20m Octo now, possibly better (more progressive). Does fly very well under 1Skin now- and in turbulent winds (predominantly offshore gusty) it was effective to let LE bridles off a further 80mm from 'standard'. Head looks quite good, curvature of top is not ugly like at Northpark last week. Opened one set of tail scoops.
An interesting observation at Dieppe;
This kite now had excessive chordwise collapse between each bridle set, really scooped and a lot of chordwise compression. Why? Shortening rear bridle set back to same as 'standard' (that is, all the same except for LE) made a dramatic difference. This implies that the problem was caused by shortening the head, and that the length of the head (relative to its width?) is critical below a certain minimum. How strange, consequences for 1Skins, traction kites, Octopus?
Volker Hoberg suggested (Ashburton 19th Sept) that a difference between the Octo and Serpent is that the Octo has slits between the tentacles to release pressure just below the head. This is definitely worth trying.
20m blue flew very well at West Java, most reliable fly-alone single line show kite yet (much better than Rays)- pretty close to 11sq.m Pilots, but has a quite narrow range for each LE bridle setting- too short causes collapse and crashing, too long causes falling off to one side or the other. Currently 4 settings in 75mm steps, shortest setting only for winds of 8km/hr to 15km/hr?! On longer settings, wind ranges are sufficient for wind that isn't gusty and turbulent. Need to develop either an automatic reactive bridle system or some shaping or other features to extend wind range without having to pull down and re-set bridles.
20m pink, new (4th Sept, Northpark) does fly but doesn't seem as reliable as 20m blue (or just bad wind?).
Bridling is the same within mms- amazingly similar- and 20m blue has had 3 replacement bridle sets (Weifang damage) while flying as well in each iteration.
LE of Pink is perhaps 50mm shorter around curve in total than Blue (5 cells each side of centre), and is more even.
Maybe Blue has stretched into more 'pocketing' in the cells. 3rd cell adjacent centre measures 36, 42, 37, 37 (38 average) for blue, 56.5 average diagonal. Pink is 39, 38, 36, 43, (38.75 average), 54, 57 on diagonals (55.5 average). Blue difference (measure of pocketing) is 18.5, 16.75 for pink. Not a lot- significant?
Check weight differences.
Blue's tails are somewhat damaged now, will have more drag- significant?
Taking centre top pair as reference, blue LE bridle differences are: 0, -160, -140, -170, -110, -50, +80. Pink is same within +/- 20mm.
Pink never arrived at Dieppe- lost bag, but Blue flew reasonably well in a range of conditions. Appeared to be volatile unstable in stronger winds, and did crash after some hours flying because of both too much and not enough wind on last Sunday, 2nd setting, falling onshore wind. With 1Skin 71 blue above and on standard setting it was then painted to the sky for the remainder of the afternoon. Need to fit thru-bridle to Pink?
Dirk's Boomers flew well in gusty strong conditions, and in the light. In strong winds, 67 black with turbulator (does it do anything useful?- maybe!) was noticeably better than they were when flown with 2 x 40m x 75mm tails. 67 black and 71 blue with their leading edges pulled in 80mm also hung up better than the Boomers in very light wind (only a Michel Gressier kite was still up- the 11sq.m Pilot would not hold). But the Boomers use a single bridle setting and no tails- clearly the flat aft TE of the Boomers (and earlier 1Skins) is a valid wind range extender, flapping notwithstanding.
The 2 x 40m x 75mm tails pull the TE of 67 up in stronger winds- and towards their end they are even more horizontal. These tails do seem to be effective in moving the kite's C of G rearward, improving recovery, reducing V instability, reducing SS, without deleterious effect except catching on things.
In very strong gusty offshore wind and heavy rain, 67 black became volatile unstable- even with tail. 71 blue (with tail) was stable in the same conditions.
Also, 67 black was noticeably more volatile than 71 blue during launching.
71 does have its front rib flares cut to align with fabric filaments (2 piece ribs) which required about 25mm longer bridles for LE stability.
71 also didn't appear to take up as much water- was noticeably lighter- so probably this is just a classic case of pendulum caused volatile instability.
Does 67's turbulator have an effect here?- I'd expect it to be opposite except for the weight difference.
Could 67's use of polyester bridles be a factor?- 71 has Dyneema bridling.
Though tending towards volatility, at Dieppe, Simon F's 0.75 sq.m. 1Skin flew well in stronger winds- looped easily without destructive LE collapse in winds that was causing the 3sq.m versions to show SS causing LE indentation. 1.5sq.m yellow was in lost bag, as was hybrid pink they would have been a useful comparative tests with this.
Also, 67 black appeared to be more resistant to LE indentation in strong winds than 71 blue (lighter, softer fabric), same bridling, same patterns (except for extra fabric at shoulder bridles on 71 to reduce creasing there).
Also, 72 grey (very soft stretchy Si coated 40denier ex OL Whale) barely flies at all in any wind- really bad LE distortion.
Suggests strongly that fabric stiffness matters:
To improve LE stiffness (or at least make it hold form better in stronger winds and at lower A of A);
1. Very small extensions of the 4 centre and first shoulder bridles can have a huge effect on SS in stronger winds. First tuning step it therefore to adjust these bridles until the LE does not indent- or at the least, indents equally.
2. A way to embed fine monofilament nylon in the leading edge fabric?- the idea being; something that makes this fabric stiffer while remaining packable and non-creasing.
3. Use polypropylene fabric (like used for bags and tarpaulins) for the leading edge piece and the top skin adjacent to the LE.
4. Use a laminate of fabric and chip bag plastic- this stuff is highly resistant to crushing, creasing, as everyone who has tried to tidy away the peanut bags given out on aeroplanes knows.
5. Shape the leading edge to the form it will take in strong winds at low A of A, not the semi-circular shape it takes in light winds. The strong wind shape is almost pointed at the front edge- so this should be a seam with stiffer or stiffened fabric to both sides.
6. Try bridling the LE (again)- will still need to be shaped to allow upward curvature between bridles.
7. Generally the more accurately the LE fabric can be shaped to reduce creasing the better- especially around the rib noses.
Reactive bridle approaches (also for Octo and Serpent);1. Pair front 4 LE bridles with rear 4 (or 6?) on a spring from main line attachment point. 2/1 pulley on front set so that they extend 60mm for every 30mm of rear set let-out?
2. Combine the front 4 bridles and extend to a point say 1m below main line attachment, have a sliding ring pulling this line and the line from all the remaining bridles together, spring loaded with a light spring to pull it towards the kite end and a heavier spring further down the kite line to pull this ring down, letting the LE bridles out as wind speed increases. Too much ring movement required for 80mm LE pull in? But does have some natural damping.
3. A, say, 3/1 lever with the 4 front bridles to one end, the main line to the other, a spring from the main flying line to the 1/3rd point on the lever to which all the other bridles are also attached. This is easiest to make and adjust (spring tension and leverage ratio0 but will cause bridle tangles, especially during over flying and pumping. This is the best approach for testing the reactive bridle principle.
4. Use a spindle with different diameter grooves taking, to the larger, the front bridle set, to the smaller, all the remaining bridles and with another groove for a stretchy to the main line, which also attaches to the spindle's axis. This would probably be less inclined to tangle than 3 above, is equally tunable.
5. A light "sticky" spring in the front bridle set- which extends promptly when they have a lot of tension on them (strong wind) but shorten slowly when the tension on the front bridles decreases- as discussed earlier.
6. Use a spring between the main line and the 10 centre rear bridles (with a self-adjusting ring pulleys between inner and outer rib sets to eliminate stepping). Then take a line from the tip and shoulder bridles through a pulley attached to the spring upper end to the front bridle set. This has the potential to be least subject to tangling.
All these reactive bridle systems suffer from not being angle of attack sensitive- at high A of A's the leading edge bridles should be shortened to reduce stalling but these systems, only reacting to total pull, will not do this. A consequence might be that stalling becomes worse on stronger winds. What would be better is a system that is A of A controlled - but the only reactive way to do this appears to be by exploiting that B bridle tension will increase relative to D as A of A decreases- or a powered sensing/servo system. Best would be a system that senses LE collapse and responds by letting out these bridles or pulling in on the rear set.
Shape changes to reduce 1Skin SS:
From Northpark and Wakanui before West Java, it was noticeable that asymmetric creases in the skin can also cause SS- even when the LE's hold form. At a given moment with 67 black hanging right, there was a visible crease in the right side. Moments later- after a jerk or 2, a crease would appear in the left side instead, and the kite would then hang left.
This suggests strongly that compression of the skin also needs to be eliminated, in the more forward sections especially. More chordwise camber accomplishes this- but also moves the C of P disadvantageously rearward. Does more spanwise camber also help?- maybe allowing chordwise camber to be reduced?. This could be done only in the rear section- perhaps by inserting wedges of fabric into each cell (or go to individual cell panels?) Another answer would be Octopus/Serpent style cells/bridling- but how to do this without losing keel area? Cut repeating curved sections into each rib?
Reflexive TE? - could be supported by cords or by a short top surface ribs. This moves C of P rearward though which is not desirable.
"Blink" stability as per Octopus appears to be a substantial solution to SS (but would it be for 1Skins without tentacles to move the C of G rearward/), could probably be contrived for 1Skins by using a very open net (fish net) strip, say 100mm wide , along every rib against skin.
Would making the rear 25% say of the main skins gauze reduce the pernicious effect of rearward camber without causing compressive creases? Would also act something like a turbulator.
1Skin variant designs;
Use Octo/Serpent style LE, cells and bridling with side keels only, 4 flares each side. Aspect ratio between 0.5 and 1.0. This eliminates the rearward C of P that derives from having to have enough chordwise camber to prevent chordwise compressive creases. Would also be simpler to make.
Another possibility is to use less bridles spanwise except at LE, maybe a triple sled or double. I did try a simple sled at the start of this program, might now have enough understanding to make it work. I expect problem will always be the LE and how to shape and support it well enough without it indenting at the shoulders.,
For next prototypes, seems indicated to try a 1Skin with polypropylene leading edge and skin front strip, one with a gauze rear skin section and one with fishnet rib edges.
More bridles, longer front bridles, different placement of front bridles, better fabric shaping, shorter lower LE, stiffer fabric, shaping for strong wind not light, stiffeners, ram air sleeves, higher A of A, different rib nose radius, top skin camber near LE, some active symmetrifier, plan form LE curvature, chord length relative to cell width (as noticed for Serpent), TE pull down, 'biplane' LE (like Peel prototype).
Tried preferentially collapsing the centre LE on 67 black- principle being to lengthen the shoulder bridles while shortening the centre LE bridles so as to have a net decrease in bridled A of A. The centre notched in while the shoulders stayed tight, causing the kite's planform shape to narrow markedly at the front (moving the C of P rearward) while also keeping the shoulders resistant to SS causing indentations. Results inconclusive- not obviously a clear improvement, but worth following up.
Test of 67 black, 23 July '16,
1Skins to July 20 2016K65 Red flew adequately in very strong southerly wind on 12th July, at kite paddock. Is it better than earlier versions though? Rear centre rib could be, and was, let out 25mm without causing leading edge collapse. B and C centre rib bridles were then let out to match, but some horizontal wrinkles still evident. A very long 75mm wide tail did make it somewhat more reliable; didn't seem to increase edging tendencies, which is interesting evidence. Built K66 Orange/pink from K65 patterns, refined. On 14th July this flew quite well in extremely strong gusty Norwest- stayed up for some time then took Craig, Gavin and I to pull down, maybe 30kg pull. Differences to K65 are minor- just flares changed to return all bridles to the same length again. At Wakanui, later on the 14th, flew very well, strong South long shore wind, reducing to light after an hour. Tip flares were let out 25mm without causing any noticeable shoulder LE change- shoulders are still very firm, some centre LE collapse in big gusts but only on the lower leading edge, never progressing up to the top skin. Some horizontal wrinkles, addressed by letting out centre rib A and B's and outer rib B by around 25mm. I'm still suspicious that the 65/66 design is inclined to going out to either side for a look sometimes for no good reason (that is, not because of wind changes), but they have much better recovery than earlier series, are quite volatile. In light winds they will figure eight, and can jiggle from side to side. In strong winds they will sometimes loop, which I don't recall earlier 1Skins ever doing regularly. Maybe extra volatility is because they have a bit less lateral area?
K65 Red and a set of orange cut-outs for another K66 sent to Simon Freidin in Melbourne on the 14th. Without extensive comparative flying against earlier 1Skins, it's not possible to be sure that 66 is an improvement (wrt diving over)- but so far it does seem so.
What is "diving over"? It's the strong inclination this style of kite -tailless single skin single line (SSSL) kites- have in stronger winds for heading off to one side or the other until they get into lower apparent wind speed, (turbulence or ground effect) or hit something. Although this may have different causes; see below, it is fundamentally a form of superstability (SS) in which kites are too slow in recovering from some angular perturbation. From here in this narrative, "diving over" will be called superstability (SS) with the understanding that SSSLs may have a unique form of SS.
Some recent observations:
The 6sq.m Orange 1Skin with the front centre 4 bridles shortened 30mm for light wind flying performed very well at Kelantan in winds up to 40km/hr- huge pull, no SS, some LE collapse, but no apparent adverse effects of this. At Chongqing this kite (standard bridle setting) successfully lifted a wet maxi Ray and Trilobite- at least 50kg average pull- but was probably SS at times (couldn't see because we were in cloud) in that the line occasionally angled out to the right. 1Skin 64 (lateral area disposed forward) also performed well at Kelantan- no sign of SS beyond requirement to tune straight. At Wakanui, 14th July, 1Skin 66 was either SS or the wind was erratic in direction (remnants of a Norwester but acting longshore from the south).
Letting the outer rear bridles, centre rib A's and B's and out rib B's by 25mm did seem to keep it from leaning off as much, and at Kite paddock, 15th July, on these bridles settings and with the front centre 4 shortened 20mm, performed well in mid easterly- slight right tendency, but good recovery. At Wakanui in stronger wind it was noticeable that the ribs all continued to flare out symmetrically even when the kite was leaning- which suggests that flow is not asymmetrical during SS periods. It's also perhaps true that this kite is straight and doesn't usually show any SS until it gets to high angle, low angle of attack- suggesting attached flow could be a cause.
POSSIBLE CAUSES OF SUPERSTABILITY IN SINGLE SKIN SINGLE LINE TAILLESS KITES.1. That the upward pointing moment of the kite's weight acting below its centre of lift is insufficient to overcome aerodynamic forces in stronger winds.
2. That the leading edge indents on one side or the other as apparent wind speed increases, causing the kite to dive off to that side, and does not "pop out' again until substantially lower apparent wind speed conditions are encountered.
3. That flow progressively attaches chordwise towards the trailing edge over the upper surface as angle of attack decreases/apparent wind speed increases, causing the kite's centre of pressure to move too far rearward relative to its centre of gravity to enable the upward pointin moment above to operate effectively.
4. That lateral area is disposed too far forward or too rearward, causing either the kites trailing edge (lateral area too rearward) to lift or its leading edge to dive (lateral area too forward) when the kite develops a lean to either side, making it unable to recover when a lean induced move off to either side initiates.
5. That TE convexity (in plan view) causes the rear area to become progressively inoperative (by flapping) as the angle of attack decreases in stronger winds) which effectively moves the bridle point for the area that is still contributing lift (the forward section) rearward, stalling it into superstability (SS).
6. That the included angle between the bridling in a chordwise sense causes compression of the skin (seen as span-wise wrinkles) as wind speed increases, and that when this occurs in the rear part of the kite the effect is as for 5 above.
7. That the cause is bridle stretch; one side of the kite randomly develops more pull, which stretches the bridles on that side by 25mm or so, which causes the kite to lean to the opposite side, which causes the stretched side to generate even more pull, in a positive feedback loop.
Comments on these theories:1. A 3sq.m 1Skin weighs 300gm and pulls more than 30kg in strong winds. 300gms acting with minimal leverage (the centre of pressure for single skin kites is necessarily further rearward than for ram air inflated kites, much closer to its centre of gravity) is all there is to correct any lean and point the kite back up. At some higher wind speed it seems likely therefore that this correcting moment won't be able to act quickly enough to correct a lean initiated move to one side driven by aerodynamic forces that are already 100 times the weight force by just 90km/hr. It's noticeable that recovery is slower for tailless single skin single line kites than it is for ram air inflated equivalents - and much slower than for framed single line kites that can be structured so as to put their centre of gravity relatively closer to the trailing edge. A prediction of this theory is that kites that are bridled more to the rear (placing their centre of lift closer to their centre of gravity) will recover more slowly from any angular displacement, and correcting for leading edge indentation effects, this is clearly true for SSSLs. Conversely, "bridling forward" is generally a cure for SS in ram air and framed kites, and does seem to reduce SS for SSSLs to the extent it's possible without losing leading edge form.
That tailless SSSL SS is at least to some extent a function of insufficient correcting moment is also supported by it's not being a problem for SSSLs with substantial tails (the 20m SSSL Octopus for example) except in very light winds when their bridle point is set to "standard" rather than "forward", which almost certainly has unrelated causes. Presumably the Octopus tentacles, comprising 75% of the kite's total weight, increase the restorative moment sufficiently to eliminate this type of SS.
What effect will scaling have on this cause of SS for SSSLs? Will a bigger 1Skin be even slower to recover and therefore suffer worse from diving over? Experience with the orange 6sq.m (which Volker Hoberg now has) at events this season doesn't suggest this. Ram air kites become more superstable (recovery so slow that they traverse to the edge of the wind before coming back up) as they are scaled bigger because their enclosed air mass increases as the cube of dimension and adds disproportionally to their rotational moment of inertia. Single skin kites are not subject to this (except entrained air?) so maybe it does make sense that their SS tendency will be independent of size? Need to check this by dimensional analysis.
2. Leading edge indentation definitely causes SS in SSSLs but that it's not the only cause is clear from two observations:
That 1Skin series with a ram air inflated leading edge sleeve seemed to be less subject to SS but not completely exempt.
And that 1Skins can be subject to SS even when there is no observable leading edge indentation.
3. This is a good theory- rollover type failure like the Dutch parafoil at Singapore in 1984. It's supported by the observation that 1Skin SS most usually occurs after the kite reaches a high flying angle (low angle of attack). Progressive attachment of flow over the top surface as the kite's angle of attack reduces would seem likely to be able to move the centre of lift back towards the kite's centre of gravity to an extent sufficient to reduce the effectiveness of the C of G's moment in pointing the kite upwards. However, reducing angle of attack will also move the underside centre of pressure forward, so the net effect may not be as significant. A test for this will be to fit a 1Skin with a turbulator to prevent full chord attachment over the top surface.
4. There is no doubt that having the lateral centre of pressure significantly forward of the centre of lift will cause SS (what I have called 'nose area instability' in earlier discussions). This causes a form of SS in all winds, including very light. Having the lateral centre of pressure too rearward will probably also cause SS in SSSLs, but if this manifests as it does for ram air pilot kites, will be a light wind problem only. From 1Skin 64 (lateral area disposed much more forward) it seems that there the current lateral area disposition is not too far forward, and from watching the way that 1Skin ribs bulge to either side, it's not too far rearward either (even when leaning severely, 1Skin 66 shows symmetrical ballooning of the rear sections of the ribs.
5 & 6. Considered together because chordwise compression is probably the main (only?) mechanism by which the rear section of 1Skins cease to contribute lift as angle of attack decreases (when wind speed increases) so causing the more forward section to become "back bridled", which can initiate or at least exacerbate SS. Average bridle length has, unfortunately, two contradictory effects on this: shortening increases camber but increases included angle, and vice versa. Check if lengthening bridles, while retaining original camber reduces SS.
The difficulty that addressing this presents is that single skin kites must have substantial chordwise camber to prevent chordwise compression , but chordwise camber pulls the kite's centre of lift rearward which is itself a prime cause of SS- see 1 above. An earlier theory /expectation was that progressive lift reduction of 1Skin rear areas as wind speed increases acted to prevent leading edge collapse (by effectively bridling thekite back as wind speed increases)- which was the main problem for the first 50 or so prototypes. Maybe this approach is useful but just needs to be backed off a bit. 1Skins 65 and 66 (better supported trailing edges) with much reduced trailing edge flapping and reduced spanwise compression wrinkles are if anything less inclined to SS than earlier convex trailing edge 1Skins. They also don't appear, as yet, to be subject to any more leading edge collapse than earlier versions, while retaining as least as good light wind performance. Why their leading edges don't collapse in stronger winds is evidence that theories to date are wrong, at least in part. 1Skin 66 currently has around 25mm longer rib centre bridles (centre rib's B and C, outer rib's B) than a 2.5m bridle radius would define- that is it is currently "overcambered 'by around
25mm- but has less chordwise wrinkles and appears to be less (or at least no more) subject to SS than earlier flatter versions.
7. Bridle stretch induced superstability; Ram air pilot kites have sometimes exhibited exactly this behaviour, and the conclusive solution has been to fit less stretchy bridling.
Is it happening to 1Skins?
To 25 June '16, including Kelantan, 1-6 June '161Skins: Built 1Skin 64 pink 28th May'16 with keel area moved forward to check, again, whether diving over is a function of lateral area distribution- or to what extent it is. Centre of pressure for this kite is on a line between the second bridles on the centre ribs. This kite is exceptionally smooth and its trailing edge tramps noticeably less than earlier 1Skins, probably because the rear bridles are closer to the TE. From the side while flying, it appears that the lateral area centre is significantly FORWARD of the lift point C of P. Have I moved the keel area too far forward now- will it be superstable for this reason (lateral area distribution either too far rearward or too forward can cause this)? Except, looking at it again, this side view was not accounting for outer keels, which are more rearward now (though smaller). Still does look to be too forward if anything. Flying at Wakanui and later, extensively at Kelantan, no sign of diving over, but wind was never much above say 30km/hr.
Could diving over be because when the TE flaps, there is effectively no lift from this area? Correct? If so, this will be causing the kite's c of p (for the remaining lifting area) to move rearward relative to the bridle point causing superstability (exactly what diving over appears to be)- and also having the useful effect of limiting leading edge collapse by preventing the kite's angle of attack from reducing too much.
Even though the lateral area disposition theory (above) hasn't yet been fully tested, this is a 'new' (didn't I consider this in earlier iterations- think I did but mainly from the LE stability point of view) theory, is worth checking out.
Built 1Skin 65 red, 23 June with all rear keel TE's steepened by about 100mm, full length shoulder bridles and outer keel smaller than pre 64 1Skins. Made completely new and refined templates for this kite also- in preparation for Melbourne workshop in August. Test flight in light wind, Wakanui 23June, flies very well, smooth, slightly volatile in minimum winds, need to be tested in stronger conditions before sending to Simon in Melbourne as a pattern.
At Kelantan the 6sq.m orange 1Skin flew exceptionally well in very light winds with the 4 centre LE bridles pulled in 50mm. It's VERY near a minimum wind match for the best light wind ram air pilots now, and as soon as the wind is sufficient to sustain it- <8km/hr? it's superior because it has so much lift. With the front bridles shortened this much, it wasn't distressed (no destructive LE collapse) up to 30km/hr at least. This kite has had some other bridle changes, so there are photo references in "Kite photos SSSLs" for when the next one is made- unless it's superseded by the 65 series.
20m Octopus SS
20m Blue flew exceptionally well at Kelantan, only just started to show LE collapse at 30 to 35km/hr with centre front bridle pair 2 knots forward, others 1 knot forward. Excellent angle, great pull, no stability issues, no leaning issues. Only one job remaining with this design; check that the production version replicas are as good.
50m Hybrid Serpent
Using the 6sq.m orange 1Skin as a pilot on this kite at Kelantan was an excellent choice because the 1Skin would keep the Serpent up in light winds with the Serpent bridle on the normal rather than forward setting. Without the pilot, on this setting the Serpent is inclined to fall off to one or the other side when the wind drops, which is annoying for adjacent fliers. The Serpent on its light wind setting flown alone needs to be pulled down and re-set for winds above about 20km/hr. Shifting the top kite attachment point forward to above the LE bridles definitely made for more reliable flying -less wrapping over failures.
30m Serpent green
It did fly for many hours at Kelantan in this mode but was increasingly volatile unstable with increasing wind speed- and this was with bridling fairly similar to the 'standard' 20m Octo bridle set, though with various minor differences (too many to specify in writing). Did try adding a midi drogue- attached close to the head was best- but this still caused unflyable superstability. Volatile instability (above) tends to suggest that allowing the head some degree of rotational freedom relative to the tail is unnecessary. Clearly it needs a little more tail drag- but not too much. Going to 4 slots widthwise (from 6), same number lengthwise, will increase drag by 50%. A better alternative (easier to make, better visually , more easily tunable) could be a series of diagonal scoops sewn to the tails underside edges near the head- tapering off by spacing and size by say 8m along - see design diary page 2226 for sketch.
Ashburton to May 11'1635m SS Serpent (Green): At Kaixuan, crescent 25 shaped bars were sewn to the tail to increase drag, but in a first test in moderate wind (Wakanui), it overflew, causing LE collapse- even with the LE bridles let out 50mm further than the 20m SS Octopus standard. After doubling the bars, modreate wind, Northpark, it dived off relentlessly. With a midi fish drogue to the tail end, it dived off even more.
Which suggested that the bar drag was now adequate to prevent overflying but too reaward.
After shortening the tail to half length and attaching the drogue close to the head, it did fly satisfactorily.
This is one of the clearest examples of "pulling the cat's tail' type instability (drag too far behind the kite, acting in a similar manner to when the end of a tail gets snagged on something, causing the kite to dive off to one side or the other inexorably.
I then made a new tail (purple, no green left) in 6 sections widthwise, with about 45 longitudinal slits, each 1.3m long. This slit section of the tail is about 18m long, with the remaining tapering section about 15m long and plain, single width. The joint to the head is cut with around 50mm cut back at each side so that most load goes thru the centre section, allowing the head some freedom to correct from angular displacement without having to haul the entire tail around with it. Other joints in the tail are also cut convex to put tension in the edges so that the tail holds its form widthwise.
Test fly; light northerly, Northpark; overflies a little, terminal LE collapse, but much closer to being satisfactory. The slits bulge too much so ties were sewn at their mid points- now twice as many but only 650mm long. The side whiskers were also replaced at this stage; 6/650mm slits per side, 8m total each. Tested at Wakanui on the 11th, light northerly, still just showing too much centre LE collapse. Need to find a way to increase drag unobtrusively and near to the head. Also seems useful to sew the whiskers into the main body to differentiate this design from the hybrid style. How to add a bit more drag?- some short crosswise slits seems to be the obvious answer- but how to do this without adding complexity and cost?
Chonqing ( 12 to 17May):The 20m SS Octopus (blue) was an excellent pilot 150m above Ray, Trilobite, Whale train in strongish wind and rain/mist on the first day. The key characteristic in this place (a fairly flat area roughly a km in diameter and 50 to 100 below the surrounding ridges) was to get high and have enough lift to pull up completely wet maxi kites until they could get into some wind. The Octopus was on the second bridle position from standard and was just verging on too much LE collapse at times. When the wind dropped, re-launch was successful twice but then became impossible because of the wet tails- at least another 20kg drag.
The 50m green Hybrid Serpent flew well on a much shorter line that the 20m Octopus on this day- it did come down a few times but less often, proportionally, than I would have expected given that it was not above the ridge turbulence. I didn't fly this kite on days 2 and 3 because its lines (70m/2000kg, 40m 6000kg Dyneema) were stolen before packing on the afternoon of the first day. This is an excellent design, needs to be retained even when the "pure" SS Serpents are fully developed, because it has a bit less pull for size and is slightly more resistant to sudden collapse in extreme turbulence.
I wasn't able to launch the 20m SS Octopus (still dripping wet) on the second day (and still raining) because of light wind at ground level and extra tail weight/drag.
But a 6sq.m 1Skin was launchable (even when wringing wet ), by running hard and this had enough lift once up (40kg +) to get a wet maxi Ray and Trilobite up, and keep them up, even when they lost inflation (frequently). No other kites, except Revs occasionally, flew successfully on this day. The 6sq.m 1Skin was on the second bridle setting and was still really stally on launch. It also sashayed from left to right in what looked to be volatile instability when the wind was in the lighter range during lulls, which it has never done before. Is this because it was wet?- a response to the higher ( 4 x's or more) pendulum effect and high angle of attack. It's amazing that there were so many spectators given that it was raining, cold and more often than not the lower kites were lost in the mist- the top kite always was.
Fairy Mountain (flying area at 1800m altitude) third day; middle wind, no rain:The 20m blue SS Octo at 150m above the Ray held the Ray/Trilobite/Whale train up all day. No re-launches after the first, which took an uninflated maxi UKS Fish up on the line. The Octo moved a little more laterally than most of the adjacent pilots, but in a narrow range, and it didn't get into any tangles that required attention. The Octopus's "blink" stability (the way that the underside leading edge collapses slightly causing the kite to re-centralise immediately a dive to either side begins) is unique to my observation, though similar in effect to "Rokaku stability (by which the camber of the faster side of a Rokaku increases because of the way their structure is formated, slowing that side down and restoring the kite to straight flying) . Most, if not all the conventional pilots flying (7 or 8?) did not survive the 3rd day without re-launches- basically because the mountain winds were quite turbulent, and when luffed or tipped they take a lot of height to recover. It was also noticeable that the show kites below the Octopus fared much better than those below conventional pilots, tending to loop and collapse more often because their lifters did not have as much pull nor fly at as high an angle.
Generally therefore, this event showed, again, that the SS's can be better pilots than current ram air designs and have opened a window for flying that has been closed until now- on the second day in the rain, no single line show kites did fly or could have been flown except under the 6sq.m 1Skin. As Craig said; they have turned our previously best lifting show kites (PL Rays and Andreas's Mantas) into line laundry.
Using a 4.5m 4Skin for buggying at Chongqing was also very successful- plenty of pull to go uphill and thru swampy bits and good control/stability/anti luffing downhill and when speed had to be scrubbed off. This design does not need much improvement (a reduction in the dreaded reverse flying tendency and a bit less sensitivity to brake tension) to be VERY good buggyng kites. For one session, the brakes were tangled thru the front bridles a bit- courtesy of locals who would try to launch and fly every kite left sitting around- and this was enough to completely prevent recovery from reverse flying (I didn't see what was causing this problem until later). Too sensitive!
Why do most (not NASA's or 4Skins) single skin traction kites, Boomers and 1Skins have a lower leading edge? The stagnation point (highest available pressure) is close to the centre of the leading edge, so any extension of the lower leading edge will reduce the available underside pressure significantly- which, top skin flow effects notwithstanding, must ALWAYS result in less secure leading edge form unless stiffeners of some type are in use. Is the usefulness of a lower leading edge merely to provide drag at lower angles of attack to prevent overflying/luffing? Would providing a reflexive trailing edge - so that at angles of attack approaching zero, the TE is pushed down- do the same job with less undesirable consequences (LE collapse). A reflexive TE would be angle of attack sensitive, whereas the current palliatives (lower leading edge for the 1Skin and hybrid Serpent, tail for the SS Octo ) are not, which means they will always have more than necessary drag in lighter conditions but not enough above some windspeed. The 4Skins use rapid flier response to stop overflying/luffing, but this is not available for 1 or 2 line kites.
Lower LE's do also seem to exacerbate LE collapse once it initiates- which seems likely to be true because the remaining still formed LE will be very marginal by then- easily collapsed by even the tiniest extra force. This could also explain why the "adaptive LE" used on the 1Skins and early model SS Octopus's is not as effective as expected- yes it does reduce stalling at low wind speeds and high angle of attack, but it also accelerates LE collapse once this initiates.
Using extra long bridles to the LE should also help push LE collapse up the wind range- and these bridles should be as small diameter as possible.
What is the "blink" stability mechanism? Can it be applied to 1Skins?
"Blink" stability appears to be a function of having the side leading edge bridles shortened so that one side collapses (folds in) just before the centre does. The effect of this is that when the kite begins to fall off to either side when the wind is strongish, the leading edge on the side it's moving towards collapses a little, which slows down or halts this movement (because of increased drag?). At this stage the flow is then asymmetric- because the kite is leaning but no longer traversing sideways. The opposite side leading edge- that isn't collapsed- then generates more lift than the side that is a bit, causing the kite to straighten up and centralise.
If this is a correct explanation, then to apply it to 1Skins will require that the side leading edges are bridled forward relative to the centre and that the keels don't prevent the required transverse flow. I have, until now, thought that the "falling-off" and "diving over" that 1Skins can be plagued by in stiffer winds, has been caused by one side LE collapsing in such a way as to pull the kite inexorably towards that side. The Octopus "blinking" stability theory above suggests the opposite- but I wonder if this apparent conflict is a function of rotational velocity? Could it be that the 1Skins, lacking a tail, get into a curving flight path of tight enough radius that the increased lift from the faster side overcomes their restorative mechanism (the weight pendulum). Nah, can't be true; 1Skins show some form of superstability, not volatile instability (of which the above is a classic description). And, it's their inner LE that stays collapsed during this- not the outer, faster, side
It's worth trying bridling the shoulders of a 1Skin forward to see if a similar "blinking" mechanism can be contrived.
It's also time to again try, (rather more determinedly this time) moving the centre of area of the 1Skin's keels forward, just to check that their "falling off' isn't just a lateral area problem, like it is, incipiently, for the 11sq.m Airbanner Pilots? (I need to fix this too, even though it's only an issue in very light winds).
Chengdu, Tongren, Nanjing, Weifang, Uiseong, Wuhan, Hancheng (to May 18 '16)
1Skins: No changes until Hancheng, having used them very little while taking every opportunity to use the 20m Blue Octo (ex Andrew Beattie) as much as possible, increasingly as a pilot. The latest double leading edge 3sq.m 1Skin (blue/purple) was stolen from our base on the field at Binhai on the 2nd day of Weifang, before it had been flown. At Hangchen there was a Chinese made version of a Boomer flying exceptionally well in very light winds. White with Chinese characters on it (see photos). I tried the orange 6m 1Skin in the same wind and it was nothing like as usable- even with continual pumping it could barely be kept up while the Chinese one was hanging up there tethered off. At this time no other kites were flying- various 11sq.m Pilots were being tried but couldn't hold, and even Helmut Georgi's Genki was struggling. The Chinese Boomer copy appeared to be much more cambered towards the leading edge, and it did appear to collapse its leading edge when the wind increased to 15-20km/hr. I tried pulling in the front 4 bridles of the orange 6sq.m. 1Skin by 60mm, and it did then fly as well as the Boomer copy- would sustain in almost no wind, and was, if anything, at a slightly higher angle. A bridle adjustment was then added to this kite (similar to those fitted to the 20m SS Octo before Weifang and the 50m Serpent after Uiseong). With the shorter LE settings, I don't yet know what the upper wind speed will be but doing this definitely provides a useful option for very light conditions; thinking ahead to Chongquing/Fairy Maiden Mountain.
The SS Octopus development (see below) is showing that a leading edge without any lower section is satisfactory in light winds, flies at a high angle, has a pretty respectable wind range- 9 to 15 on the forward setting, 12 to 50 on the second and 15 to perhaps 80km/hr on the back setting- and has excellent stability and reliability within the ranges. It's clear from watching the 1Skins flying that indentation of the lower leading edge is a cause of leading edge collapse and luffing- would they be better without this extra fabric? This also calls into question the value of the "adaptive" leading edge used on the first 4 SS Octos (and taken off the one that Andrew returned before Chengdu. The idea of the adaptive leading edge on the 1Skins- and earlier SS Octopus's- is to decrease the kites angle of attack at low flying angles so as to improve their light wind flying- the SS's archilles heel. This is an excellent theory, and does seem to work in practice, but the Blue 20m Octo is now flying in lighter winds than it did with the adaptive LE (albeit with alternative bridle settings, but maybe even on just the second position which has a pretty good wind range). Could it be that the lower leading edge flap is unnecessarily causing LE collapse when the kite flies at lower angles of attack 9stronger winds)? Does this need looking at again? I have a pretty seductive theory that the best possible single skin soft leading edge will be when the LE is exactly parallel to the wind flow (which is rising slightly in anticipation at this point so is not horizontal). Try scalloping (as tried on 4 Skins) again?
SS Octopus and Serpent.
At Nanjing (very light wind) the 20m Blue Octo did fly for a while, for one session as a pilot above the 50m Serpent,, and could be pulled up relatively easily. Tried pulling in the front 5 bridles on each side by 60mm and this improved light wind flying a lot- and almost completely eliminated "falling off". At Kaixuan before Weifang this bridle adjustment was changed to three sets of 4 leading edge bridles with 3 separate positions for each 60mm apart. The new yellow 34m Serpent was made the same and the 50m Serpent also fitted with an adjustable leading edge- about 75mm steps. At the Weifang event- sometimes rain and very strong winds, the blue Octopus flew pretty well with the 3 LE sets all to the second knot- about 60mm shorter, but did move left to right a bit when the wind was above about 30km/hr, and the LE would collapse causing it to dive or loop at above about 60km/hr. On the original full length setting, does tend to fall off in light patches but seems quite capable of flying in winds above 60km/hr. Does it fly a bit more stably when wet- seems to, as does the 50m Serpent. 41 of 56 bridles were cut at some stage late in the day, and the tips of all the tentacles were cut off- clearly by a Kevlar line. Fixed and re-bridled on the evening of the second day it flew well on the final morning- also still on the second setting- but did have some LE collapse causing looping in the strongest gusts (above 50km/hr?). A majority of the primary bridles were cut again- by Kevlar. The replacement bridles were made as primaries and secondaries, to reduce tangling potential, and with lighter line so that they will hopefully break rather than cause injury during any launching mishaps. At Uiseong, Wuhan and Hancheng this kite was really excellent- used as a pilot, very stable and reliable except when on the third bridle setting after wind increases. On the second setting it was barely showing any LE distortion in 30km/hr estimate.
The 50m Serpent with the LE bridles forward 75mm handled the strong wind and rain at Weifang very well, only collapsing and looping in a squall late on the first afternoon- when it took more than 10 people to get a 9kg Ray and 11sq.m Pilot down. The tail were ripped off in 2 places, probably on a sign. At Uiseong, Wuhan and especially Hangchen this kite flew exceptionally well- almost no falling off even in winds of around 10km/hr- is not far off being as useful in light winds as pilot kites.
The 34m yellow Serpent, at Weifang, would not fly with an identical bridle to that which works so well on the Octopus- very likely because of less tail drag. Changed after Weifang to having 25/75mm crescent shaped strips sewn across the tail it did seem to be much better when tried briefly in very light winds, but this needs to be tested extensively. An alternative way to increase tail drag will be to make the tail with 4 or more zig-zagging pieces, hemmed and sewn together so as to leave many longitudinal slits.
The 50m Serpent with the inflated LE collar is such a good flier- and seems to have a bit less pull proportionally than the Octopus, that this design should be retained even if the pure SS smaller version can be developed successfully. The tail should revert to the 3 piece with a 75mm gap between each, proportionally as per the earlier 34 m Serpent.
The general form of these kites can be extended to Chinese opera mask heads with staggly beard tails and many other designs.
The Octopus, Serpent and 1Skins are all clear of the bog and onto firm higher ground now- but how much height remains to be climbed before the peaks of these particular hills are attained?
Automatic bridle adjustment;
The fundamental issue is that the leading edge bridles need to be substantially shortened for good light wind flying but this causes LE collapse in stronger winds, and in turbulent or luffing situations- and prevents recovery after the kite has collapsed. This is almost certainly unsolvable except by using some sort of sensing system which triggers shortening of at least some rear bridles when the kite's centre of pressure moves too far forward. But there may be ways to mitigate some of this problem reactively. Splitting the bridles into 2 sets chordwise with 2 lines to the flier would of course enable active control of this- and might be a way to the lightest wind flying that is possible. A radio controlled servo system that can pull in or let out the LE bridles by the desired amount could also work.
A reactive system that should help would be to use elastic elements on some rear bridles- in stronger wind this would have the effect of reducing the kite's lifteing area and moving the bridle position of the remaining forward section rearward. This doesn't seem as likely to cause unrecoverable luffing as elastic elements to the front bridles would.
Another way would be to fit a slow spring to the LE bridles- so that with susteained load they lengthen by the required amount, with lower average pull they gradually shorten up.
A way to make a slow spring would be an elastomeric grease filled tube, sealed at each end, and rigged between the line attachment point and the main bridles. A constricting ring to which the LE bridles are attached that is also connected to the main line attachment point by an elastic element would enable damped lengthening and shortening of the LE bridles. This could be inside a braided line.
After Satun (4, 5, 6 March)And before Baoli (18, 19, 20 March) 2016
Received 20m blue/pink SS Octopus back from Andrew Beattie for comparative testing. Changed head to same shape as the 20m blue/yellow (removed the active leading edge extension and cut back the head top 200mm). In mid-range winds, no bridle changes from original, (Northpark) it flew well enough in light winds but with increasing VI as wind increased, some tendency to falling off in very light. With tentacle ends tangled VI reduced substantially. In strong winds, head LE pushed in a LOT- not OK (and more than in its earlier incarnation with active LE). Tried letting out front LE bridles by 50mm each, not enough improvement.
Blue/yellow: with leading edge collar inflated sashayed continually in all winds. Tried putting most tentacle drag thru centre 4 tentacles but could discern no difference. Tying the tentacles together at various positions from around 1.5m behind head only made a difference to sashaying when the tentacles ballooned out increasing drag a lot.
Blue/pink was then changed to the same bridling as blue/yellow- recorded in design diary 2188, 14 March 2016- and linked tentacles for more drag and to reduce tangling (note, gave more slack in link lines than for blue/yellow). The main changes noticeable were that the last row is out 140mm, the inner shoulders are longer by 100mm (typically) and the centre leading edge (4 bridles) are out by 150mm.Looks like up to 34 of 56 bridles can now be the same length. Strongish Northerly, Northpark: flew very well, quite similar to blue/yellow when it was in its most developed SS form (no collar). Some notchiness in upper LE, but a lot less than blue/yellow ever had in this mode. Re-cut LE to scallop each cell between bridles, now even less notchy, not quite good enough visually yet but getting there. Re-tested 15 March, Northpark, strongish southerly.
Blue/yellow: after this result with blue/pink, taped off (then sewed off) inflation- and it now flies almost exactly the same as blue/pink- that is rather well. Its LE is smoother than blue/pink, perhaps because of double skin and some residual inflation. This seems to be pretty conclusive; it's sashaying WAS because the inflated collar improved airflow, thereby improving L/D and thence increasing VI. That this VI didn't increase with windspeed is almost certainly because the dynamic LE indentation that was noted above as a source of stability and resistance to "diving over" damps the VI at higher wind speeds.
It will now be interesting to try a Serpent with Octopus tails and vice versa- make one of each with simple seams between head and tails.
From November 9, 20151Skins: At Wakanui, the kite factory field and the Nelson event (January '16), the Blue (no number ) 3sq.m with the tubular leading edge, K3/13 pink (flat ram air leading edge with single central inflation) and K3/14 red (flat ram air LE with centre closed and inflation to each side) all flew well and appeared to be a lot less inclined to diving over than single skin LE 1Skins. The blue ripped its LE at "A" bridles in a strong wind, was retired. 13 and 14 both seemed stally in light winds until the 4 centre "A's" were shortened 20mm, after which they were a little better in the light than previous versions, and quite a bit more volatile. Both flew fairly faultlessly at Nelson- maybe 2 terminal diving overs during the day, total. Both were also fairly straight without tuning and their LE's appeared to resist indentation much better. After the "A" shortenings their LE's were noticeably pushed under along their lower edge, but still held out well. At Pasir Gudang to Feb 28 '16 the wind was very strong for all 3 flying days (holding on to them was on the edge of not do-able, even with 4mm line. They both exhibited collapse of the LE at the outer ribs until the "A" bridles at these ribs was restored to original length (the centre "A's" were still OK at 20mm shorter than standard. 13/pink was used above the Ray, OLTrilobite, and Trilobite stack for one day but then started to lean right increasingly. When it eventually came down, there were 2 damaged bridles (cut except for a few filaments) After fixing these it was violently left, needs to be re-tuned. 14/red then took over and was handling the winds well- until it dived over catastrophically and was destroyed on a pavilion roof. Not good enough! But it is fair to note that conventional ram air pilots were not having any better time of it. None of the 15 or more flying survived a day without multiple crashes. One or 2 Boomers flying at low altitude did continue to do their job- albeit with very frequent re-launches.
It seems at present that the flat inflated LE's do reduce diving over but not completely- so what other cause (apart from really bad wind) is there?
Could it be a lateral area disposition problem- recalling similar behavior from prototype ram air pilots 10 or so years ago which was eventually pinned on too much lateral area to the rear, which when the kite is hanging off to one side or the other, prevents the rear of the kite from rotating downward to allow correction. Leong says that his 1Skin (number 42) is not as reliable as Danny's Boomer- but I didn't see either flying.
1 March 2016,Pasir Gudang: Re-built the remains of 14/red by removing the LE inflation and reducing the size of all the rear flares (including the outers), adding 6 more bridles to provide support. Tested in strong winds (70km/hr?) the LE collapsed terminally, even after letting centre "A's" out to original, and even after letting the 4 centre "A's" out a further 20mm and shortening 2 rear centre bridles 20mm had too much leading edge compression. Seems to have a lot more VU (volatile unstable) also. Needs to be tested in moderate winds and tuned so that the LE does not collapse.
Why did the inflated LE's of 13 and 14 allow shortening of "A's"- and why did they seem more stally compared to identically bridled single skin LE versions before this shortening? Having a more rigid LE explains the first part of this, but NOT the second.
And why does this latest single skin LE 1Skin prototype with more bridles and less rearward lateral area suffer from LE collapse even when bridled to a substantially higher angle of attack? VERY puzzling! That it is more volatile unstable is likely a function of less total lateral area.
1, 2 March 2016. Malacca, strong offshore, surprisingly smooth, except low down.
Tried reducing rear lateral area of 1Skin K25 yellow by adding a bridle to each side flare, taping off the tip (see photo). In strong gusty wind , this had the same response as 14/red above; leading edge collapse and much more volatility. Interestingly, the effect seems to be with the shoulders; when they are released a lot (K14/Red, K25 Yellow), not only the shoulders lose inflation, but the centre does also. This kite did have much quicker recovery- but is this reducing diving over?- it seems to head off to either side even more.
Both K13 Pink and K25 Yellow performed much better in original form- K14/Pink best in gusty strong winds close to the ground.
The only improvement from the curent standard (K25 Yellow for example) found so far is therefore the double skin leading edge, and it's as yet inconclusive whether a single inflation point or separated sides is best. K13 Red (split sides) was marginally better than K14 Pink (single inflation) in the extreme conditions at PG, but within the range of individual differences and tuning.
The side flares are currently providing a lot more of the general leading edge "inflation" than would be thought possible from the rib structure, and are also providing a fair amount of drag, which holds the kite back from ascending to a line angle where the angle of attack is too low for leading edge form to be sustained. Effec ts of disposition of lateral area are not clear- as yet too mixed up with other effects to establish.
To try next;
Transverse vents at leading edge - has been tried before, but many changes have been made since then; so it will be worthwhile checking this again.
Moving lateral area at the front- which will shift the lateral CP (centre of pressure) without reducing total lateral area.
An active bridle?- but optimal strong and light wind bridle settings need to be established first to set the range required.
SS Show kites from 9 Nov '15After Wuhan 2015 and before Nelson (Jan '16), the 34m Serpent was developed intensively. First stage; inflation, eventually improved to an acceptable level by moving the gauze inlets downward to roughly split the lower leading edge and adding extra bridles to speed initial inflation. Second stage was bridling, and the first step in this was to lengthen bridles progressively towards the rear while increasing camber (chordwise and spanwise in the uppper head area until the minimum angle of attack that could be sustained was found. The next step was to search for stable flight by changing leading edge and side bridles. Shortening the side rear bridles has the apparent advantage of creating stabilising keels but also pulled the kite's CP rearward to an extent that caused inexorable diving over. It's very difficult to get the CP into the upper half of the head let alone close enough to the leading edge to get useful recovery from the sides of the wind window in any wind. Shortening the leading edge bridles in the shoulder, and especially upper shoulder area was the only means found so far that achieves satisfactory flying. Stability is excellent in mid to strong winds- there doesn't seem to be an upper limit so far, this kite just becomes more stable (in the volatile sense) as wind speed increases without falling into superstability. Recovery from angular perturbation is excellent; neither too fast nor too slow. Flying angle is also excellent- at least as high as Boomers and 1Skins, much higher than pilots and other ram air kites. BUT, this kite still has an annoying inclination to fall off to one side or other in light winds rather than sinking back in the centre. Tried rigging the tail attachment so as to allow the head to pivot more freely, but couldn't see any improvement- but this needs to be tried again after everything else has been optimised- if it helps at all in light wind recovery, could be elastic so as to revert to full width attachment in stronger winds. Minimum wind is only slightly above conventional Rays, pilots etc but its mode of falling as the wind dies is unhelpfull. Nelson flying with this kite was excellent- perhaps only one unscheduled re-launch in 2 days. After Nelson, built a 4.5m x 50m version, one less bridle row spanwise, one more chordwise. First bridle set up was approximately scaled from the 34, then developed without further reference. Was found to be quite sensitive to the second bridle row - too short and the head crumpled, kite wouldn't fly. Too long and the collar inflation is poor. The collar was then re-cut to taper from the shoulders for better appearance (but did this make getting the CP forward more difficult?) The 50m has a central bridle row (no centre line bridles on the 34m) ansd single gauze (double on the 34). Eventual bridling looks quite similar to that on the 34m, perhaps the shoulders a little further forward (which might also reduce the light wind falling off for the 34). Launching is very fast and easy- huge apparent wind on the way up, just a little weaving (was none before LE bridles 4, 5 and 6 were shortened by 50, 100 and 50mm respectively to reduce light wind falling off. Flew very well in the gusty wind at Pasir Gudang (though neighbours didn't think so as it swept sideways at frequent enough intervals- but conventional pilots weren't handling the wind there any better and it was off the same anchor as a pilot, Ray, Trilobite, and Trilobite train. Malacca (strong winds) flying was also excellent, as was Satun. Pull is more than a ram air kite of the same head size, but manageable- did break 1500kg(nominal) Dyneema at Malacca and Satun- but evidence of cutting damage both times, and didn't break again after knotting. The tail on the 34m is in 3 sections with an about 75mm gap longitudinally between each. The tail on the 50m is overlapped (for a solid visual effect) and works as well, but actually people seemed to like the visible gaps more. A single piece tail with tensioned outer edges would probably work just as well, is quicker to make and would not catch trees, tractors and stuff as much- safer too. The 50m is a more developed and refined kite now than the 34m- will be the pattern. It's bridling system is easy to fit but does tangle more than the cascade set-up on the 34- split into say 4 or 6 bands chordwise to reduce tangling without adding complexity? The only remaining problem with this design is their inclination to fall off to one side or the other when the wind is insufficient (currently maybe 10km/hr?) This may be an inherent angle of attack problem which will be difficult to solve without getting leading edge collapse, but even a small forward movement of the CP position, if this is do-able, will help a lot, and the elastic head pivot idea (above) has theoretical promise at least Three sizes; 34m, 50m, 75m? Before Pasir Gudang, did try taping off the collar inflation- flies nearly as well, slightly more inclination to shoulder collapse (but this was fixed for the octopus below when it was tried as purely single skin by shortening the upper shoulder bridles by another 200 to 250mm). Without an inflated collar LE appearance is very notchy.
The 20m Octopus has not responded nearly as easily to inflation and bridling development as the Serpents have; in its original SS adaptive LE form the leading edge tended to fold under in mid to strong winds (top end was around 25km/hr) although the light wind flying was excellent (still failed eventually by falling off to one side though). Andrew Beattie called it "possibly the most dangerous kite I've ever flown"- the wiss. A procedure that requires care and respect is launching though; in mid andstrong winds the speed of launching is phenomenal; far too quick to allow an disentangling from the tentacles- this goes for the Serpent as well. On the other hand they haven't as yet shown any inclination to looping (because tight turning causes some shoulder or leading edge indentation which either disrupts the loop or causes collapse. When figure eighting, any dip below horizontal can, in light turbulent winds, cause head compression and collapse (Serpents and Octopus). This maybe be a source of "stability" as well as a cause of crashing (suspect as various incremental improvements are made that looping might become possible).
The inflated collar added to the blue 20m Octopus before Shenzhen 2015 was independent of the top skin and this flew well enough under a pilot (Shenzhen and Wuhan), was VU and dived over badly when flown pilotless. After the 34m serpent was eventually developed to fly successfully, a similar collar and bridling was tried on the Octopus, but after extensive development (before and at Nelson 2016), would not fly well alone and had terminal tail tangling. Flew at a noticeably lower angle than the Serpent in this format. After this, mainly at Northpark, linking the tentacles (necessary to prevent tangling, had tried careful tip tapering and Si coating to no avail) and pulling in the upper shoulders a A LOT eventually had it flying , but still with a lot of sashaying- which seemed NOT to increase with windspeed. It was then tested with the inflation taped off- and flew very much better, to the extent that the inflated collar was completely removed (and then the centre leading edge was cut back about 300mm to reduce leading edge collapse more and move the CP forward) , after which it flew the best it ever had- handling even strong turbulent winds on 15m of line beside the kite factory that most (all?) other soft kites would not have been have been able to- and excellent in the light also. To be considered is whether some of this improvement came from removing the adaptive leading edge? Mostly though it seems to have been from shortening the upper shoulders and letting out the lower edges (had been through the process of finding the lowest angle of attack it would accept without LE collapse, including letting out rear bridles progressively as far as possible). More and less spanwise camber was tried and some testing of more/less chordwise camber also. But the leading edge notchiness was pretty ugly, so for PG, Malacca, Satun, a new flat collar was fitted, after which it went back to sashaying (still fairly independently of wind speed). The only obvious explanation for this is that the collar improves airflow over the LE, which boosts L/D- hence increases VI (volatile instability). Backing off the upper shoulder bridles by 50mm or more did reduce the lateral oscillations somewhat, as did shortening all the bridles to half length (more spanwise and chordwise camber). A possibility is that making the head more free to pivot (recover) by taking the tail load centrally might help- worth trying. This Octopus flat collar is easier to make and cleaner looking than the Serpent's system - worth while trying on the Serpents therefore. If these two changes don't fix the sashaying, easiest next step may be to replace a good-flying Serpent's tail with Octopus tentacles- to isolate a lot of variables.
An interesting observation, applying to Serpents and Octopus both, is when any significant lateral displacement occurs, causing the kite to be leaning out to the side, the inner shoulder (nearest the true wind direction) is strongly "inflated", while the outer shoulder shows significant indentation. Clearly this asymmetry is aiding and may be close to being the entire cause of subsequent recovery. Simon's brief watching of this elicited, "no this is not happening", but after many many hours of watching, I'm sure it is. In very light winds the lift generated by the apparent wind is not quite sufficient to drive the kite up and out of this lateral displacement back towards the centre and the kite then collapses to the edge. This is almost like a soft kite equivalent of "Rokaku stability" (by which when one side of a Rok travels faster, frame geometry causes that side to camber more, slowing it down and damping any build up in VI (volatile instability). What will improve this dynamic for the semi circular head SS and collared SS single line kites?
Anything that moves the head's C of P forward should (by giving the lift component more leverage against weight and inertia holding it back from correction).should assist.
Allowing the head to pivot independently of the tail/tentacles also should- especially if this is made apparent wind sensitive- to operate less as wind speed increases (when they don't have a recovery problem). Adding tail weight or reducing tail drag will probably also help in this also. Would more tail weight- or moving the kites CG rearward- have deleterious effects in stronger winds though? There are ways to decrease drag as wind speed decreases - (elastic aperture drogues for eg).
From another perspective, anything that reduces the initial displacement/lean will also address this problem- causing the head to correct any angular displacement before the kite has displaced laterally to any significant extent for example (this is the theory that superstability is just very slow acting volatile instability- that correction doesn't occur until the kite is to the edge of the window).
The self correcting leading edge dynamic observable with the Octopus and Serpent SS and hybrid SS's is about flow not being parallel to the kite's centre line in this circumstance (no keels/or flares), so could be adversely influenced by using the outer edges towards the rear of the heads as keels to any significant extent (maybe explaining why this didn't work when tried early in this development). Disposition of lateral area has also to be something to consider (thinking back to noticing that excessive lateral area to the rear of pilot keels causes them to lean off and collapse to one side or the other in light winds and momentary lulls and similar possible effects now with 1Skins and Boomers). What consequences do considerations in this section have for smooth tail Rays- which have a not disimilar "falling off" problem in light winds?
After Mongolia Guangdong, Weifang, Shenzhen. 20 Oct to 1 Nov 2015.
Octopus and Serpent: The Serpent currently has much more curve-down on each side; more sled like than the Octopus (Blue ex Kaixuan). The Octopus also has less camber in the mid-top body area. Flown alone, the Octopus has more inclination to falling-off to one side in light conditions- making launching in light winds difficult. At none of these events, did it show any tail tangling tendency- and quite strong winds at Shenzhen, almost more than I could pull down- 3sq.m 1Skin Pilot.
The Octopus flew well straight away, had none of the leading edge collapse in stronger winds it had previously had. In a Wiefang test - strong wind very wind-shadowed area- it fly by itself very well, at least as reliably as the 1Skins, more reliable than Craig's 8sq.m pilot. It also flew well for very many hours with a 1Skin pilot at Shenzhen (light lifting to strong on shore). Craig thought it was volatile unstable, but I doubt this, did see some sashaying when the wind was light but this had no tendency to increase as the wind built- could be either a light wind (falling-off) effect or a pilot effect. It did seem to have less pull than before the ram air element was added.
For the Octopus, clearly the Serpent style leading edge cavity is lighter and simpler- but will it work as well? 320mm width is sufficient. Need to adjust the Octopus bridling to a bit more like the Serpent to stop the light wind falling off, and test how far forward the bridling can be pulled before leading edge collapse again becomes a problem.
Both the latest 3sqm (yellow) and the 6 sq.m (orange) ex Kaixuan were excellent- did good service at Guangdong and Shenzhen (though not used when the wind was very strong and gusty offshore on the second day at Shenzhen). Ron Spaulding and Andreas Agren both said the 6sq.m dived-off, twice, tangling them in the tight formation flying during the first day at Shenzhen. I didn't see this, but it's possible- or could have been interference from another kite or just a patch of turbulence and a slow recovery. At Weifang on the 29th- strong turbulent wind, both these kites were much more useful and reliable than Craig's red 8 sq.m. pilot- which was collapsing badly.
While at Kaixuan on the 29th, had a prototype 3sq.m. (blue, no number) with tubular ram-air inflated leading edge built. At Shenzhen this showed no sign of leading edge indentation (the presumed cause of diving-off) at all, and adds less than 5% by weight and cost. This kite did not have tuning lines and was progressively left wing so wasn't tested in the strongest winds, but is looking promising. Add tuning, test again- and try pulling the bridling forward to see if this kite will now accept a lower angle of attack.
And what if the indentation that presumably causes diving-off is asymmetric because once the kite starts diving off to one side the flow centre/stagnation point is displaced from centre towards that side?
To test for this, try a 1Skins with inflation at 1st rib each side and with no connection to the opposite side.
Could this also be a problem/solution for the ST Ray?
Wuhan 7th 8th November 2015
Octopus and Serpent; The Serpent bridling at the leading edge was mis-rigged in various ways from hasty re-attachments after the active part of the leading edge was sewn down at Weifang after Shenzhen. After correcting this to some extent, and reducing the amount of spanwise arch- which seemed excessive- the bridling was too low- would barely launch, even with a 1Skin pilot in the light winds. Looked good when it was up though. Leading edge still does not inflate well- better than at Shenzhen, but not now nearly as good as the Octopus (which has an inflation cavity independent of the top skin). Maybe shortening the centre front bridles will fix the Serpent's inflation and stalling; need to fly it beside the Octopus at Wakanui. Maybe there will be a useful advantage from adding an inflated central keel as well- for the Octopus also. The Octopus looked and flew exceptionally well and at a high angle using a 6sq.m 1Skin- wraps only happened when the top kite fell back and the Octopus overflew the top kite line. The tentacles never tangled if they started fairly clear. Sashaying does appear to be caused by having a top kite at a lower flying angle. There are definite hopes for these kites in stronger winds now.
For bridling; all the secondary's should be identical length, variations only in the primary's- which should all be cut to the same length and tied centrally- and 500mm x 2 longer to allow for length variation requirements.
These kites are currently excellent in their window, but that window still isn't wide enough at either end of the wind range for them to supplant conventional pilots. Their advantages are higher flying angle, huge pull and much easier launching and active flying. In the light to nothing winds at Wuhan, a 6sq.m 1Skin could immediately lift either a max Trilobite or Ray up to flying altitude uninflated -the only maxi kites that did get up enough to fly (briefly) on the second day.
What can be done to improve their light end? This is worth maximum effort as an even 10% improvement will likely make them the best of all pilot kites when the wind is alternating from nothing to very little- the usual conditions at China events- and not uncommon elsewhere.
Is there any way to make a reactive bridle that responds to stalling by reducing the kite's bridled A of A for just long enough to get the kite moving up again? Problem is that this is exactly what will also cause luffing - because the centre of pressure moves rearward in luffs and in stalls. Such a mechanism is possible if it's active- like a human pilot, but can it be done passively with levers, pulleys, springs and dampers? Theory suggests not because to prevent luffing the rear bridles must be shortened - which requires energy input. The active leading edge does seem to helping quite a bit to reduce stalling- if it's extended, will it help more without causing other issues? It is compatible with inflated element leading edges. Try this.
Weifang, 21st to 23rd July 2015:
Test flew 1Skins; K1 1.5sq.m, K1 6sq.m, K25 3 sq.m. (all with adaptive leading edges).
Main civic square in Weifang; almost no wind, then strong wind and heavy rain, then rain and no wind.
K25 3sq.m yellow and K1 1.5sq.m yellow appear to be satisfactory- didn"t try during the brief windy period, but their responses were as they should be. This 1.5sq.m (the first one) was very volatile until it had some line pull- then straightened up and was stable, as expected for a smaller kite. It will be interesting to see what upper range this kite has- if it will fly reliably in stronger winds than the 3sq.m"s, as I suspect, this implies that fabric stiffness is probably a significant factor at the upper end.
K1 6sq.m orange (the first 6sq.m), was flown straight-off without checking on a long line during the squalls to lift an ST Ray. In the strongest gusts it would eventually dive inexorable to either side (shoulder collapse)- and didn"t seem that keen to stay central except in very light winds. Is this a scaling effect (fabric stiffness as above?), or is there some scaling error or other influence. A bridle check showed the shoulder bridles to be all 60mm shorter than they should be proportional to the current 3sq.m"s- and this would definitely cause diving-off. Lengthen these and re-test. It"s also possible that its behavior was just very bad inner city wind- the square is surrounded on all sides by sky-scrapers- though none closer than about 200m .
SS show kites:The first sample 20m SS Octopus (blue) from Kaixuan looked generally OK but was terminally inclined to "diving off". Visually the bridling appeared to be significantly different to O2 yellow, but I didn"t have time to find the actual differences and correct them. They have also used bridling and cording line that is far too heavy - the bridles alone probably added a kg (to a kite that should weigh less than 3kg total). Easily fixed; and perhaps in future the bridles should be all individual rather than cascaded like for O2, S1 and this sample. This would be quicker to do and have many less possibilities for errors. Suggest cutting all bridles to the longest length, then marking in bands back to the shortest. This would allow the person bridling to quickly tie every line without further measurement- just by counting back the bands for each position. Left-over ends could then be cut off (the longest "tail" will be less than 500mm). Should some capacity to change the relative overall lengths, leading edge to trailing edge, be retained by keeping, say four secondaries at the line attachment point? Maybe there will eventually be "strong wind" and "light wind" settings, but I don"t have useable ways to do this yet for the Octo or the Serpent because far too many changes are required. The goal for now is; one bridle for all conditions- though at present, without changing anything for different winds, they"re a long way from this with less than the best achievable lightwind performance (still amazing though) and leading edge collapse from about 15km up for the Octopus, maybe 25 for the Serpent (at probably some cost to its light end).
Both O2 yellow 20m Octopus and the S1 green 35m Serpent flew very well when conditions were suitable- their ease of launching is phenomenal - quite unexpected- just let wind into any part of the leading edge and they "inflate" and launch themselves- provided they"re pointing approximately upwards. In light winds, pulling or running doesn"t help much, unless there is more wind at altitude- they either fly or they don"t. The Octopus was more susceptable to leading edge collapse than the Serpent, but seemed to require (very) slightly less wind- need to check this though, might have been because it was flying higher where the wind was stronger. The Octopus was also slightly inclined to diving over when the wind was zero- would slide off inexorably to one side or the other as it came down in the lulls. The Serpent didn"t seem to do this to the same extent- a function of its slightly greater spanwise camber? Developing the Octopus and Serpent bridling completely independently has been an excellent approach- it is clearly identifying the effect of differences.
Changes for the Octopus:Individual bridles as per above, taper tentacle ends and don"t have a cross hem (to reduce tangling), try increasing the active proportion of the leading edge for less leading edge collapse in stronger winds, and try one with Serpent bridling.
The Serpent had less leading edge collapse in stronger winds but did appear to be on the verge of volatile instability at about when its leading edge begun to push in. The lengthwise slots in the tail (to allow some airflow through so that the rear upper side of the tail does not become a big low pressure zone) have very successfully stopped the tail from folding over- and without the visuals being too detrimentally effected; a triumph for theory! Appearance is generally good- plenty of fliers wanting to buy- but sharper colour contrasts will be better. The eyes are too wide apart- one eye is partially obscured when looking from the side.
Changes for the Serpent:Individual bridles as for the octopus above, move the eyes closer together, try increasing the "active" proportion of the leading edge for better light and strong wind flying.
Generally both these new designs are a very good beginning- will find market niches I expect, but the challenge is to increase wind range without adding adjustment complexities that fliers will simply not accept.
Using a 1Skin pilot above the Octopus did help its upper range a bit (was excellent in Kelantan), but in the briefly much stronger and gustier Mongolian conditions it was not sufficient- its leading edge collapsed so extensively that the Octopus would wrap itself around the pilot line when recovering. I didn"t try a pilot above the Serpent at this event because it was generally OK- only came down twice in strong winds . Need to try it with a pilot, and also with a drogue. Best will be if a combination of active leading edge changes and bridling can push the wind range up without, at the least, costing lower end. They"re pretty good for such radical new designs already though, and will get better.
For the next style of SS show kite, to consider is that the Boomers, 1Skins, Octopus"s and Serpents all have approximately semi circular leading edges- is this contributing to these sucesses by comparison to the SS Ray and SS Flag which don"t (yet?) fly satisfactorily? Also, currently the SS show kites (Octopus and Serpent) are both upper range wind limited (though I haven"t yet tried a pilot with the Serpent or a drogue with either). The Boomers and 1Skins have better top ends but are not as good as the Octopus or Serpent at the light end (except if they"re actively flown). How to get more wind range? It seems that having lateral area stability (Boomers and 1Skins) helps the upper end (both the Octopus and Serpent appear to be volatile unstable in stronger winds- if their leading edges don"t collapse first). And possibly, tails help the lower end by preventing overflying and LE collapse when more forward bridling is used- though it"s more likely that the Boomers and 1Skins would fly in much less wind with more forward bridling, but would lose their upper end (which is barely good enough now). The Octopus and Serpent"s lack of upper range could be corrected by bridling back (which would prevent leading edge collapse), but uless taken to an extreme, they may then become volatile unstable without either longer tails or a drogue. Except that I did try a rearward bridling for the 20m yellow Octopus at Kelantan, and this did enable it to fly in much stronger winds (lost the light end though). And, the 10m pink O1 did fly in VERY strong winds in Vietnam, so perhaps it is just that the current bridling arrangements do not have wide enough ranges rather than there being any fundamental upper range limiting factor with these designs. Too much pull is also a consideration. More rearward bridling will increase pull- and they all have more than enough now.
For the next design, is there a show kite style that has some lateral area as well as some tail drag?
A caterpillar maybe (legs could provide lateral area)?
Weifang, 21st to 23rd July 2015;Test of K SS 20m Octopus number 1 (blue). after changing to individual rather than cascading bridling- and to July 21 bridle chart. Main civic square in Weifang, almost no wind (1Skins wouldn"t stay up). Flew very well, maybe a little more inclined to LE collapse than O1 20m yellow (but bridles are a bit different because of the change to individual lengths. Then tried with a mid sized fish drogue attached to the centre two tentacles. This clearly stopped leading edge collapse and extended the wind range a lot, but made it more susceptible to falling off to one side or the other in lulls.
The Serpent and Octopus bridling are quite different- up to 300mm in places- but both kites fly quite well. This suggests there is further improvement to be had for both styles as they aren"t fundamentally different kites (same heads). The Octopuses are slightly more susceptible to falling off than the Serpents and the probable reason for this is less central body spanwise camber (I still don"t really understand why this should be so). In particular for the Octopus, bridles in the centre top head area are in general 100mm shorter; lengthen these and re-test with and without a drogue.
At Kaixuan, made a 5m wide x 11m long seven legs per side SS Caterpillar (3m wide semicircular head that"s very similar to the Octopus and Serpent heads but with 1 less set of bridles spanwise). Not tested yet (no wind and still raining in Weifang) but looks promising because the legs, as well as providing lateral area (as do the mandibles), can be angled outwards (by re-sewing) to whatever extent is necessary to add drag for stability. Weighs 2.75kg with 45gm fabric and heavy bridles.
Mongolia, 15th to 20th July 2015;Three days flying; one period of an hour or two of turbulent mid-range winds, but mainly no wind except for occasional brief strong gusts. Generally thermals only, no consistent directon. Hagars mega kite (red diamond, 400(?)sq.m) never flew during this event (many attempts).
1Skins: Flew 62 pink as a pilot during the only mid range wind period. Seemed to be more volatile than usual and than conventional ram air pilots, with occasional strong movements off to the left, but generally OK. In the frequent lulls, it came down more often than other pilots, even on 90metres, but had much more lift than a 9sq.m Airbanners (which inverted fairly often as did other pilots there, including largish flow-forms), so was more useful. Actively flown it was a match for everything else when there was no wind- though one 8sq.m (approx.) inflatable lady bird kite was generally superior to everything else (except eagles), including the O2 yellow 20m Octopus . Responsive kites (which this lady bug was) can be kept up in almost zero wind by active flying using the standard eagle kite type reels- which are now often used in China for even quite large kites in light winds. Always Kevlar though- one of my 3.5mm Dyneema pilot lines now has 9 knots, and in this environment, re-splicing is a constant job with the 300kg Dyneema I use with the 1Skins.
The 1Skins are now fairly settled; no major changes required. The tuning system works easily and precisely- but not many other fliers seems to be able to use it. Some are clearly unaware of it"s existence as they still try to use bridle adjustments for tuning. The most useful improvement now will be extending the wind range- up and down. Extending the depth of the adaptive leading edge would seem to be the best opportunity for lighter wind flying and this wouldn"t effect anything else, so worth trying. Seeing how both smaller and larger 1Skins behave in stronger winds should indicate how best to reliably extend he upper range.
Ashburton, June 3rd to June 8th 2015.
Bridles are 60/1.5m long primaries paired to 30/3m secondaries plus one extra Dyneema pair to B centres for a top kite attachment, to 4/1.5m tertiaries. Centre body is let out nominally 250mm- as was found to reduce "falling-off" with O2. Because the primary plus secondary bridles are quite short (significantly less than on O2), the effective centre body arch is quite a bit more than 250mm. On the initial rigging, the rear two bridle sets (G and H?) were let out around 100mm and 250mm respectively, and by H there is very minimal body arch.
Wakanui, 8th morning,
Tow launching to get it up in zero wind, it caught wind at altitude and dragged me across the field yelling for help. Jenny declined to assist but did get Simon and John (Tan ) out to help. The three of us eventually pulled it back to the anchor, but they have asked me not to do this again please. As for the O2 yellow, S1 is a superior light wind kite. In stronger winds, the leading edge pushes in (never luffs though) and also as for O2, this works to stabilise the kite, particularly by the sequence in which leading edge push-in acts to assist recovery from dives. No sign of falling-off. Haven't yet tried it under pilot- but expect it will work as well in this mode as O2- that is perfectly.
It's unlikely that this first-guess bridle set-up is ideal. Lengths vary laterally by at least 100mm in places, and there are sewing asymmetries as well (eyes somewhat wonky) so obviously neither bridling nor shape is very critical for this design.
Excellent, nice bottle of Sauvignon Blanc awaits!
1 June 2015O2yellow sent back to Kaixuan with Tan Xinbo as a production pattern- will make 10m, 20m and 40m versions.
Kelantan 26th May to 1June
May 19th WakanuiLight to strongish gusty northerly, 5 hour session.
Immediate improvement; best flying yet, but still inevitably fell off to one side or the other in gusts. The impressive ability to recover from these may be because the kite gets down into lighter winds, or may be because it's the underside LE that collapses and transverse flow then lifts the head correctively. Whatever, the falling off behaviour will prevent this style of kite from being other than a curiosity unless fixed. (Does it still happen in very strong winds?- O1 suggests not, as does my current theory for cause- which is that the fairly stiff rotational connection between the tentacles and head reduces the ability of the head to correct angularly quickly enough, causing the kite to traverse to either side before correction is complete).
No noticeable improvement. But it will be the shoulder areas that cause this, not the rearward parts, and the shoulders need to have this curvature to hold pressure. What else can be done to offset this effect then? Tried flying at ground level and pulling on various bridles to see if anything reduced the sidling tendencies. Strangely, pulling in on both side rear 6 side bridles does seem to reduce the inclination to traverse- but is this just because this also increases the kite's overall angle of attack and makes it slower to respond to everything? Tried flying with these all shortened by 60mm; does behave differently , (like a lot more pull) but no inherent improvement in falling off (did seem quite volatile actually, would have looped on a long enough line). Back to longitudinal stiffness as a prime cause; a further cure would be to decrease the length of the outer tentacles, adding this to the centre ones- aesthetically OK too. Serpent, Sperm or Opera mask versions won't have this problem- it is Octopus specific. Is this falling-off behaviour just volatile instability, masked by leading edge collapse as the kite gets to either side of the window, so mimicking superstability? To test this, tried re-linking the centre 4 tentacles to add drag- but this made the behaviour worse (though could this have been because it also stiffens the connection between head and tentacles even though it was only on the 4 central ones). Checked this by also adding the spinsock again- worse still (but didn't try this with the tentacles unlinked, need to be sure about this). Then took the spinsock off and unlinked all the tentacles- and flying was as good as I've had yet. One last test; going back to the reduction in sideways tendencies from shortening the side bridles: Tried letting out all the centre head bridles by 250mm; reducing to 125mm for the second and 7th set, no change for 1's and 8's. This basically just cambers up the central area of the head. Very big improvement, sidling tendency appears gone, or majorly reduced- but wind was lighter, Much more centre leading edge collapse, but doesn't seem destructive in light winds, and 1's could be let out for stronger winds if this really is a general fix for falling off. A question is still why O1 behaves very differently; it's excellent in strong and very strong winds, but struggles in the light- and yet its various parameters, (including overall angle of attack) are basically the same. Best to take O2 as far as it can go now to get the best of this 2 boat program before doing more side-by-side testing- at Kelantan next week?
May 18th WakanuiLight to medium onshore:
Similar to the 17th Flew pretty much the same as yesterday- OK but reasonably frequent falling off. Tried very many bridle changes, but no substantial improvement except that letting 7 off by 50mm and 8 by 150mm did not seem to make anything worse, and did lower the stall speed a bit- already pretty good by any standards.
May 17th Ashburton AirportSmooth light easterly; flew sort of OK, very good recovery, but noticeable compression in forward 4 or 5 cells and has 01's tendency to fall off to either side and collapse the leading edge (or does this happen the other way around ?), but worse of anything. Pulling in on the more rearward bridles to reduce forward zone compression made it terminally dive over- wouldn't then fly at all.
May 17th Northpark EstateQuite smooth light northerly
Setting all bridles to the same length (the first set had been 50mm longer, as had 6. 7 was out 100mm, 8 by 150mm- pretty much scaled from O1) cured the compression in the top 4 cells and it flew pretty well, though still with occasional falling off and collapsing (usually to the right).
May 13th to 16th, Ashburton
May 12th, Wakanui beach:
Also tried shortening 62s shoulder bridle loops from 950mm to 900mm but this caused some diving over in the strong winds. Shortening all the bridles from 3.0m to 2.50m did eliminate some wrinkles and doesn't appear to have any other deleterious effects.
May 10th 2015 Wakanui Beach:
Ashburton, to May 9th 2015
A change that I began to suspect as a cause of poor strong wind performance while testing K6(grey) in the strong but smooth winds on the sea at Vung Tau was increasing the camber of the upper leading edge and flattening camber in the rear sections (from 53). These kites, that came to be called the '59' series, are generally excellent but repeatedly fail to perform as well as 52 in strong winds. (When it's "in tune", 52 consistently flies better than any others of the SSSL series in really strong winds.) The theory behind the 59 series was to move the kite's centre of pressure forward (by moving the max-camber point closer to the leading edge while keeping the total camber about the same) so as increase the distance between the c of p and the c of g- which provides the kite with a stronger inclination to point upwards. I also had a half-arsed idea that lowering the upper leading edge a tad should increase the air velocity and hence lower the pressure in this zone so as to sort of suck the kite upwards. But what I could see when K6 was at eye level in 50 - 60km/hr was that the leading edge form did not hold out that well. On the verge of pushing in, either one side or the other tended to buckle in first, pulling the kite off to that side. In gusty inland winds this is not apparent as the turbulence causes such leading edge effects to even out.
61 purple has the more forward max-camber point of the 59 series and an adaptive leading edge- and does fly tolerably well in strong winds- but to what extent is this a benefit of the adaptive leading edge? In strong gusty nor-wests here yesterday it clearly dived off to the side that was more pushed in.
May 9th '15 Very strong gusty nor-wests again, and 62 does seem to handle this better than even 52- and it's leading edge doesn't ever seem to compress asymmetrically like even 52 does eventually, a very good sign I think.
What's it like in light winds though? There is either too much or no wind here at present, and in zero wind it does seem to come up well on a tug- benefit of the adaptive leading edge, unless I'm conning myself again.
The test that will determine this is to fly side-by-side in a slowly declining wind with other kites that have known light wind credentials- Kelantan?
To confirm its strong wind behaviour, need to get smooth 60km/hr plus- because gusty conditions mask any leading edge weakness.
Ashburton, 28th April to 4th May
Completed sewing of the SSSL ray R1(purple) that I cut out earlier in the year- abandoning the parallel ribs and using 104 spaced bridles instead. First test flies are promising enough, but this style is going to be a big challenge to get to fly effectively tailless (wanting just an actual stingray style whip tail, no bucket tail or drogue). The leading edge needs a lot more pleating to get it smooth and R1 is strongly superstable in zero to light winds (try in stronger before changing too much?)- but it did stay up for short intervals when there was a bit of a puff. Seems very likely that I'll need to reduce the aspect ratio a lot to find the balance between superstability and volatile instability- how many iterations?
Adaptive leading edge description (see photographs of O1pink, R1purple, 60green and 61purple).
K4 (grey) was the first kite I built to test this theory (19th April '15) and it seemed to work exactly as I'd hoped- but "disappeared" in Vietnam before I could properly test it. By then I'd also used the same idea, but with a different leading edge restraint, for my first attempt at a single skin Octopus kite (O1 pink), and this seemed to work exactly as hoped also. Since arriving back in NZ I've built a prototype single skin Ray kite and SSSL60(green), with the Octopus style adaptive leading edges, and SSSL61(purple) with a fabric gusset restrained adaptive leading edge. I need to do a lot more testing in strong winds to be sure, but the adaptive leading edge system, at this stage appears to very hopeful as I search for a way to increase the SSSLs wind range through to what it needs to be if they are to supplant conventional pilot kites in general use.
I did think pretty hard about whether to publish this idea, or protect it, and was tending towards controlling it for a few days. Eventually I decided that it was just too generally useful to hold tight: Do I want to spend the next 20 years (if I'm around for that long) fighting off infringers and challengers, or should I just get on with making the best kites I can and getting them to the market at competitive prices?
Vung Tau 25th and 26th April 2015
O1(pink) with some extra tail tied as loops to increase drag did actually fly reasonably well in the strong onshore winds (5 + people to pull a Tattoo Ray down). This is a miracle kite- how many times have I tried such a radically new idea and have it work straight off? Only once before I think; the first PLT box kite (1973?) Since then everything has been difficult: New stuff (Peels, C Quads, Arcs, even the first inflatable Octopus and Ray, and especially the SSSL series), has always put up a tough fight- until now.
Further reflections after Valencia(sitting in a hotel with nothing else useful to do)
Another cause of hanging off to one side or the other is the tip flares having loose leading edges - one or the other billows out and holds the kite to one side. 50 and 52 have this problem I think.
Towards some useful principles for SSSLs (not in any particular order of importance as yet):
1. That a flat or slightly reflexed rather than cambered rear section does not prevent leading edge 'inflation' and provides for better stability, (especially recovery) and less hanging to either side; by pushing the centre of pressure forward relative to the centre of gravity. Flatter rear sections does make TE flapping harder to control though.
2. That shallow leading edges (relative to chord) make it easier to keep the leading edge from indenting in strong winds (and makes the effects of any indenting less likely to cause "diving-over" - but deeper leading edges provide better low wind performance (by decreasing the lower surface's effective angle of attack).
3. That it's much easier to avoid compression caused creases spanwise than chordwise.
4. That close spaced bridling in the spanwise sense make design of the leading edge much easier (can be just a straight panel), but that wider spacing is achievable with careful panel shaping- and is desirable by requiring less total bridles.
5. That there is an angle of attack, which when bridled for, will enable every SSSL to fly first try.
6. That as a rule of thumb, lines drawn from each bridling point at an angle of 30degrees to the bridle line should overlap at the skin on adjacent bridles to prevent humps and minimise creases.
7. That bias cut ribs with cording sewn only on a line extending the bridle direction, and only part way to the skin is the best way to spread bridle loads.
8. That anything more than a semi-circular leading edge reduces captured pressure in the leading edge- that is, extended lower leading edges are undesirable with respect to LE form. Advantages/disadvantages of scalloping?
9. That, independent of camber, longer bridles reduce chordwise creasing. Bridles length just greater than chord x's 1.5 is adequate. For convenience, flares and the shoulder cords are best dimensioned so that all bridles are the same length.
10. That pressure is maximum in the centre cell, reduces proportionally in outer cells and shoulders- hence cross-venting is not sensible, unless also valved (and probably not even then because flow rates could not be high enough to make a difference).
11. That LE form (pressure behind) can more easily be improved by shortening "B" bridles than by hooking the TE- effectively this is usually best done by lengthening "A's" by a little - but this also increases the overall angle of attack, so should be matched by letting the TE out a bit. Spanwise creasing- from chordwise compression does limit the use of this unless skin dimensions are also changed - and there are limits to how much is possible without increasing the chordwise number of bridles.
12. LE Sweepback is useful in that this pushes LE indentations away from the tips and towards the centre where they are much less likely to cause "diving over".
13. Some SS traction kites don't suffer from LE push-in even to 100km/hr and more apparent wind, so holding LE form across the required wind range is also possible for single line single skin kites- but is this compatible with the very low stall speed that SSSL's require in order for them to be useable?
14. Highly cambered sections- like the 4Skin and NPWs can have very low stall speeds (less than 5km/hr) but the rearward camber that these kites have probably makes across-the-range single line stability problematical- because their centre of pressure is not far enough in front of the centre of gravity to enable good recovery in the single line sense.
15. Adding stiffening rods (usually weed whacker cord)- or using stiffer fabric or stiffening patches can push the onset of LE compression further up the wind range, but fundamentally, using stiffening to do this will always just put off the problem- better, at this stage, to concentrate on getting more pressure in behind the LE- this is true also for SS traction kites.
16. Lighter fabric does not necessarily help light end flying- for the 42's, heavier fabric causes useful figure- eighting in very light winds which builds up their apparent wind enabling them to fly in lighter winds than kites made of lighter fabric do- everything else being equal.
Chongqing 4th - 7th Apriland Valencia 10th to 14th April, 2015
Chongqing wind was very light and intermittent and mostly the kites were invisible in the ground level cloud (event was on Fairy Maiden Mountain) , so not a lot of comparative testing was possible. Both 56 (purple) and 59 (pink) appeared to fly satisfactorily. These have flat rather than cambered rear sections- which does cause more flapping but also doesn't seem to exhibit the hanging-off to alternate sides that is a characteristic of 50 (blue) and 52(red) which have been the most reliable SSSLs until now.
Valencia had light to mid sea wind- not that steady though and rising to approx 40km /hr in gusts on the final afternoon (maybe a bit more at altitude).
No testing of 57 (green) which has the extra LE bridles- because I inadvertantly sent this on to Weifang from Chongqing, taking the earlier 54 (green) with the extended lower LE instead. 54 (given to Rolf Zimmerman) actually flew reasonably well at Valencia, even in the stronger conditions, but with quite ugly and noticeable folding in of the LE.
58 (blue), given to Ludowico (Italy) did seem to fly OK but wasn't used as a pilot during this event. It's extra leading edge part ribs don't seem to offer any useful improvement in leading edge form- but might do so if they are also bridled as for 57.
59 (pink) , which has a shallower leading edge, cut-back about 30mm clear of each front flare, was excellent- used as a pilot above the Tattoo ray both days, it did gradually lean left as the wind increased, but hadn't been tuned for stronger conditions and this leaning didn't appear to be of the diving-off type, but just a minor asymmetry. Pull was 15 to 20kg when I eventually pulled it down. The tuning adjustments now fitted are very effective; not too sensitive nor too insensitive. 5mm to 10mm makes a noticeable difference.
Above the ST Ray at Valencia, I used 50 (blue) then tried a 12sq.m Airbanners 3 bridle Lifter- which seemed to have about the same pull. Later on the last day, when the wind was strongest, the AB Lifter misbehaved (why?- I have previously flown this kite in stupidly strong gusty winds) and I went back to using 50, which was satisfactory but did seem to hang alternatively left then right? 59 does now seem to be better than 50 and 52 in stronger winds. 59 is good enough for production now.
Light wind: 47's are a match for any kites when actively flown, but come down 1 to 2 km/hr before the best conventional pilots when left tethered. Theory suggests (and observation tends to support) that deeper leading edges help light end performance. This would be because a deeper leading edge effectively reduces the angle of attack of the kite's lower skin. But deeper leading edges do seem to cause problems in stronger wind, by folding in and creating sometimes asymmetric drag, especially when they push in on the front flares. It should be possible to make a leading edge that folds back evenly and without contacting the flares as the kite's angle of attack decreases though.
Bernhard Dingwerth had a 42 series copy in white Icarex at Valencia that was clearly superior in light winds. He says, and I could see, that it was not just better than any conventional pilots but equivalent or even better than Genkis for example. I couldn't see what differences there were by just looking at it, but there must be some; need to measure it. He says it's no good in stronger wind though- so I wonder if it's just bridled to slightly lower angle of attack. At least I now know that superior light wind performance is possible from this style of kite.
To try at Weifang:
1. A deep leading edge for better light end that folds back in stronger conditions without pushing in on the flares.
2. The usefulness of extra leading edge bridles in stronger winds by testing 57(green), and maybe by adding another pair to 59.
3. The effect of cutting back the TE so as to reduce flapping (100mm to 150mm?)
SSSL Development from Chongqing, 4 April 2014.
SSSL 3 was reasonably promising but none of the attempts from then until SSSL13 were very hopeful (some did fly OK in smooth mid-range wind). SSSL4 to SSSL 12 were all pure sleds of one form or another (no central keel).
Some observations so far:
1. Using a lower leading edge seem to allow lower A of A without leading edge collapse- SSSL19 green?
2. It appears that pure sleds become too wide at rear, tapering in LE then causes collapse.
3. Or is the unsupported leading edge the problem- consider Robert van Weers's.
4. Supporting the LE in centre span (tried on some of the sleds) causes luffing at higher apparent winds.
5. For SSSL13 to SSSL19, left/ right tuning is very responsive to "c" sides.
6. Not having taper in at the LE (in plan view), seems to be critical to prevent tip collapse.
7. LE collapse always occurs at tips first- so far- even for SSSL"s extra centre panel camber near LE.
8. Later SSSL series are least stally when central keel bridles are at low A of A- but the keel then flaps.
9. Reducing the keel A of A causes the rear part to narrow- offsetting the A of A effect.
10. SSSL16 (shallow central keel) was clearly less stable than any of the series with deep keels.
11. There doesn"t seem to be any stability disadvantage in the central keel being shallower towards the LE.
12. SSSL19's lack of a lower LE (it has the arch section pieces) doesn't appear to have reduced LE stability.
13.Has SSSL 19"s centre panel forward camber helped extend the lower end range?- appears to have so far.
14.Tails don"t seem to help light wind stability at all- but maybe would in conjunction with reduced A of A.
15.Up to SSSL 19, recovery from dives is very slow- a function of C of P being very close to C of G?
16. What causes light wind /low line angle tip-overs?- is it fundamentally C of P falling below the C of G?
17. 4th April Chongqing, SSSL19 (purple) eventually flew reasonably in light wind; down after 9sq.m Pilot.
18. Low pull and line angle though;- would SSSL 18 (different keel camber) have been better?
Ashburton, March 11 to March 16 2015:53 (purple); Tried sloping the upper leading edge down (by 25mm at LE top marks). Was almost volatile unstable in light winds, and did luff once or twice without recovering. No noticeable improvement in LE compression at all. Appeared to have a significantly lower light wind threshold, but dived over worse than 52 et al. Then tried 54 (green) with leading edge back to original height but parabolic rather than semi- circular and using a full 2 piece LE with widish lower surface. Eventually this flew reasonably well in mid-range gusty winds, but the lower leading edge appeared to push in a lot in gusts, distorting the front flares.
Then re-built 52 (purple) to the same parabolic leading edge profile as 53 but with scalloped lower leading edge. This appeared to be a substantial improvement- though probably still not back to as good as 50 (blue) in strongish winds (but 52 hadn"t been extensively tuned at this point and is also a bit huckery with all its sewn in pieces).
55 (pink), a cleaned up version of 53, was wildly off (asymmetric LE collapse) when first made. Letting the shoulders out in steps to an eventual 50mm (try this for 53 and 54) helped a lot, but noticeable tension creases at the outer rib leading edges appeared to be initiating LE collapse (mid-range gusty southerly with rain).
Later on March 16; added 60mm Vs to outer edge of leading edge just outboard of the outer ribs. In gusts the shoulders then showed flappy "ears" about 200mm out (worse than they had beem before the 60mm Vs were added), so took 25mm out ditto 250mm further out which corrected this.
Tip flares were still bulging out between the rear shoulder bridle and the tip bridle though, so took two V"s of 2x12mm out, reducing this a lot. Very much improved LE appearance and flying now: LE holds out quite well in up to moderately strong winds at least (say 20kg max pull).
These changes have been made to the 47 series patterns, but 55 needs to be tuned a lot more and tested extensively against 50 and 52 to be sure that they are actual improvements.
Observations and questions 17 March"15 :
Letting out D"s on all ribs by 25mm doesn"t seem to have any noticeable effect on LE form, though it does promote creases at C which can become destructive.
Pulling in on B"s DOES seem to improve LE form- and without any increase in propensity to stalling- but this makes creases at B which can also be destructive if they are big enough to initiate general chordwise compression in strong winds. 1. Using spanwise arch and more shoulder/ tip bridles is the traction kite solution to eliminating spanwise creases without adding more bridles. But would anhedral effect SL stability? Sleds CAN be SL stable, but arched down shoulders have been unmanageable in earlier prototypes, so this needs further testing .
2. Letting out tip bridles eventually causes shoulder collapse- which is a much more severe cause of diving over than compression of the LE at the ribs.
3. Did 53's first iteration when it had substantially more "nose down" (and a shallower LE) show reduced separated flow (less flapping) as expected from Satun notes? Perhaps, probably even; It did seem to fly at a higher angle and something seems to have reduced the skin flapping (rear of ribs still did flap though).
4. More "nose down" did improve light wind performance but caused worse LE collapse in stronger winds. But if it was combined with an appropriate A of A increase? Why hasn"t adding camber in the forward 20% of chord been useful yet? This shouldn"t have C of P consequences and should reduce spanwise creasing at A and B.
5. Using even more LE sweepback seems likely to be beneficial for reducing the effects of LE compression- the theory being that the shoulder areas become easier to hold out as they sweepback more because the external pressure is progressively less than stagnation pressure . Also, if the part of the leading edge that is fairly transverse to the wind direction is only in the centre, when it does compress it has less effect on causing the kite to dive off to left or right. Pointier noses are a suggested direction therefore.
6. What effect does trailing edge shape (in plan) have? Is the "Flowform" style concave TE useful stability-wise? At the least, a straight or concave TE should reduce flapping- and this change could be combined with a pointier nose while retaining the current ribs- for 42s and 47s.
7. What is the optimal LE depth relative to chord? Too much and its lower edge just folds in, too little doesn"t seem to have as much rigidity. 55"s parabolic LE form does appear to be an improvement in that this kite seems to fly at a higher angle, especially in stronger winds.
8. Would using LE bridles rather than keels/flares reduce LE compression? Should do, at the least because there would then be the minimum possible rearward pulling component of bridle tension (is this really true?). This would also reduce the drag effect when the flare LE"s are folded back by LE collapse.
9. Or should the flares overlap the leading edge a bit so as to support the ribs to nearer their leading edge? With scalloping, moving the leading edge of the front rib flares further forward wouldn"t seem to have any other consequences anyway- just cut the leading edge pieces back a bit to suit.
10. What is the optimal slope angle for the front rib flares?
11. Wrinkles and creases in the leading edge, especially at the shoulders, act as an initiator of LE collapse. The outer rib LE area on 55 can be cut better- but current templates should be better already than 55 is (and how does 55 compare with 50 and 52 in this respect? )
12. Using heavier hard finish Dyneema bridling is MUCH better- never tangles and won"t get cut as easily as various SSSLs did at PG-Malacca-Satun either.
13. Call the 47 series "One Skins"- retain the name "Boomer" for the 42s?
20 was from 19, no significant changes (was it light weight fabric?), given to Orlando in Uiseong Korea (April 2014?) to see what he can make of them.
21(green) had minor changes from 19, but no lower leading edge.
Quite a bit of testing, but has not yet (October 2014) flown as well as 19, which does have a lower leading edge- for now this has to be assumed to be significant.
At Uchinada (May 2014) 19 flew well as a pilot above a Ray +- was really excellent, steady, better angle than any of the pilots and much more pull relative to its size- pretty much the same as a 9sq.m Airbanners.
This is hopefully a harbinger of what may be possible from the SSSL approach but at Uchinada there was a soft steady sea breeze- just enough to keep above 19's stall point, not so much as to cause LE collapse on that bridle setting- (above about 30km/hr). When the wind dropped, the 9sq.m Airbanners and 19 came down at about the same time- though I strongly suspect that the 9sq.m has a generally lower light wind threshold than 19- because the SSSl"s weak point currently is their inability to recover from a stall.
After this sequence of events, tried a 3 cell with higher aspect ratio- 22 (red),
22 also went to 3 bridles per cell- 12 total (earlier series are all 4/cell, 12 total) tested at Wakanui in Sept, initially with chronic outside LE collapse. Sewing in a pleat to choke A side flares by 50mm was a substantial cure for this.
Taiwan (Sept 2014). Appeared to be much more stable off the launch- goes straight every time, not as loopy at high angles of attack as the lower AR versions, but also had an edging type behaviour- would not stay at the apex for long, would always eventually fly off to one side or the other and crash- maybe a bit less inclined to do this in stronger wind than light. 22 also appears to have a creasing problem on the outer cells especially at B, from point bridle loads- a consequence of having just 3/side- but maybe also because B flare is too small (B outers are out about 75mm further than A and C). Just before Taiwan, sewed out pleats beside the outer flares to reduce overall camber. Unfortunately I accidentally made one of these adjacent to the centre cell instead, so consequently 22 did not get a good test in Taiwan. After unpicking both these pleats it didn"t seem to fly as well- suggesting that less camber is indeed the right direction. It did still fly in mid-range winds, still inclined to edging though. Took 21 (no lower leading edge) to Taiwan also- wouldn"t fly very well, I expect not nearly as well as 19 would have there, even after some bridle changing.
19 was in Mongolia with Craig and Gavin as a pilot while I was in Taiwan - did they use it?
After Taiwan, re-cut the outer panels on 22 to make the tip choke more accurate- and sewed pleats in the outer cells adjacent to the centre flares to generally reduce camber. Wakanui test looked promising- definitely holds its form well now, quite resistant to tip collapse, but still edges.
Shenzhen October 2014, very light wind
19 (purple) wouldn"t stay up- stalled and collapsed, neither would 22 (red) for any length of time- would apex then eventually dive off to one side or the other with a serious crease at B outer. At a school demo, we could tow launch and run 19 around- definitely more inclined to ground loops than 22 is on launching, but does stay in the middle while flying
To 22 Feb 2015Tuning adjustment; an adjustable cord on the outer seam just rearward of the shoulder bridles works very well in every situation tested so far (see photo). 10-20mm of pull-in has a noticeable effect and none have yet needed more than 30mm total (48 blue above). Have retro-fitted this to 47, 48, 49, 50, 51 and as original to 52 (red 45 gm). 50 is light blue 30gm, stiff fabric, 51 is 30gm gold, soft.
These all had a noticeably ugly notch at the shoulder bridle, which has been cured by using a Vee line (160mm spacing at fabric edge) here rather than a single cord. This doesn't take any longer to make and preserves the "all bridles the same length" standard- has now been incorporated into the templates.
52 (red) weaves noticeably in very light winds when it is wet- suggests that it may be possible to get back this weaving by small changes (but what- further decrease in AR?).
Looking at leading edge compression, which seems to be the main remaining problem:
A fix is higher angle of attack, but, guessing, every 15mm of trailing edge pull-in costs a 1km/hr increase in the minimum wind required. 30mm does substantially stop all LE compression though.
How much improvement will be available from detail refinement of the leading edge form- more or less scalloping, larger or smaller diameter (currently approximately 10% of chord), more or less length of lower leading edge, shaping other than semi-circular? The leading edge shape is unlikely to be optimal now, so there is likely to be some improvement available.
The leading edge bridling is also likely to be less than optimal at present.
Cross venting makes leading edge compression worse- deflection of ribs shows that the centre cell has the highest pressure, and as it also most subject to leading edge compression, bleeding anything away from this cell will make its leading edge compression worse. This does suggest than an even more pronounced "pointy nose" plan form may have advantages.
It will be interesting to see how they perform at Pasir Gudang (next weekend) then Malacca and Satoon- an excellent real-world test, especially PG's gusty and difficult winds.
17 Feb 2015.Wakanui, stiff easterly. 20kg + pull. 48 (blue, horribly asymmetric fabric) is becoming increasingly right as it's flown more. With the leading edge fairly evenly inflated, it apexes straight but once up there , gradually edges off to the right- sometimes to the ground. Tried B outer left by 30mm, C outer left by 25mm then B outer shortened, C outer lengthened. Couldn't correct it completely- in stronger gusts it still edged right. Tried right shoulder out, and in, and lengthening B outer right 9which eventually caused right outer leading edge to collapse. No useful effects. A theory is that every one of these changes has 2 conflicting effects - like that B outer de-cambers that side, but also improves leading edge inflation on that side and etc.
Tried shortening all bridles by 1m- seems beneficial, less creasing everywhere and no noticeable negative flying effects - but strong wind only.
Tried stretching leading edge of left side tip flare- and this had a dramatic effect straight away- immediately flew almost straight, even with every other bridle restored to original length. How to make a tuning adjustment for this?
And noticed that centre ribs leading edge bridle causes noticeably less skin creasing, so for 51 on, have changed outer rib templates to same leading edge angle as centre rib leading edges.
Feb 15 2015
49 is an excellent kite, very straight - but haven't tried above 30km/hr yet and this was smooth beach wind- hasn't required any tuning yet except evening up the leading edge.
I'm now pretty confident in stating the cause of the intransigent and annoying 'diving over' behaviour that these SSSL's are plagued by: And it is pretty fundamental- not the sort of problem that can be solved by a frontal attack I think:
By requiring (as we should) that these new single skin single line kites fly in winds as low as 7 or 8 km/hr (around half the minimum flying speed of latest generation single skin traction kites- except the Skin- see below) they are pretty much doomed to leading edge collapse in stronger winds. And when leading edge collapse onsets it starts asymmetrically - which drives the kite over to one side and becomes unstoppable except by reducing the apparent wind speed enough to allow the leading edge to pop out again.
A way to deal to this is by interminable tuning. With enough patience, the effects of asymmetric LE collapse can be neutralised, even up to 100km/hr. But only until something minor- like packing the kite away overnight- causes the fabric to buckle in a slightly different pattern, requiring re-tuning.
A first permanent solution was to bridle rear sections so that the trailing edge buckles upwards and flaps in higher wind speeds- causing the angle of attack of the remaining operative forward sections to take a slightly higher angle, which keeps the leading edge 'inflated'. This has the disadvantages of destroying the fabric quite rapidly and being very noisy (Gavin complains when they fly above his bedroom all night).
The second answer was to make the leading edge section that is subject to this collapse narrower (spanwise) while making the recovery moment the kite can generate larger by going to lower aspect ratio. In effect the leading edge still collapses asymmetrically but the kite is less bothered by this so goes on about its business regardless. (47 series).
The third was to reduce the lower leading edge dimensions by 'scalloping' to capture pressure that can hold the leading edge out at a few degrees lower angle of attack. This also stops the leading edge from pushing in on the front flares causing them to deform asymmetrically. Scalloping helps a bit- and is consistent with the performance of the Skin, mentioned above, which has a lower minimum flying speed than any of the latest generation single skin traction kites, from which it differs by not having as much lower leading edge.
Another palliative is lighter fabric- stall speed is a function of wind speed squared, angle of attack, profile, and kite weight. 49 is in 30gm fabric- noticeably better too, weighs just 286gm ( 45 gm/sq.m fabric versions are 100gms more).
Another solution would be reactive bridling- but the devil in this is very much in the detail- I expect that using elastic or springy elements without damping would cause local luffing in some wind circumstances- how to make a light weight and compact "slow bungy"? Could be at the bridle point rather than on individual bridles.
For now; maybe explore increasing the scalloping depth.
And yes, you may not suggest adding reinforcing materials to the leading edge area.
Ashburton Feb 9 2015 Morning.
Stupidly strong southerly with squalls. 45 is handling this very well- sitting up their shivering and shaking, but centrally and reliably, (notwithstanding that it removed the toilet vent from the kite factory roof while settling in).
Unfortunately it took 5 or 6 adjustments to get it to this state- yesterday in strong winds it was just slightly right wing with B outer left shortened by about 12mm, this morning it was strongly left and to centralise, B outer left is back to original plus B outer right shortened by about 8mm. This is insanity.
Andreas Fischbacher reports that 42 (7 cell) flies well in the light but amongst building (Schenzhen) suffers from inability to recover whenever it heads in some errant direction- also my experience.
Afternoon; built SSSL46. Green, 5 cell (4 ribs), 18 bridles, aspect ratio 0.76, about 1,25sq.m. Flew straight off: Light steady SW with rain. Relative to 45, 46 has a flatter nose, more convexity by plan view in the trailing edge and has the trailing edges of the wingtip flares and ribs cut back 75mm to see what this does to trailing edge flutter. Seems to be much less creased in the skin, and doesn"t flutter at all in this wind (so what!). Recovery does seem much improved, could easily loop back out of a dive by the look of it, but it doesn"t seem to figure eight much at all in the very light conditions.
Is this evidence for a theory I"ve long had: That when the ratio of the distance between a kites centre of pressure and its centre of gravity, and its rotational inertia is lower than a certain value, recovery from any perturbation is so slow that the kite will crash to one side or the other before recovering (a form of what I call super stability). And if it"s above a certain value the kite will recover almost instantaneously from perturbations. But if it"s between these two values, recovery can build into those series of figure eights culminating in looping out that I call volatile instability. I"m trying to think of any kite experience I"ve had that voids this theory but if the effects of angle of attack, wing tip drag stabilisation and lateral area (keel) stabilisation are allowed for, I can"t currently think of any. I recall making an exactly square test kite in the "70"s (flown as a diamond) and weighting it with 5/8" Whitworth nuts to see what effects moving its centre of gravity had. Without any nuts it was volatile unstable, spun out immediately. Adding a nut to the rear end of its spine made it even more so. Placing a nut at the nose end of the spine caused it to fly half decently (no tail). Nuts on one wing tip or the other had surprisingly little effect- caused a bit of a lean in light winds but almost none in stronger winds. But 3 or 4 nuts to the rear end of the spine did stabilise it (as above, just one made it worse). I never tried the effect of adding nuts to the both wingtips to check the effect of rational inertia, should have.
Anyway, perhaps the SSL 42 series, when flying at high angles of attack (low angles of attack occur only with high line angle and in this situation they"re on or below the critical minimum), are in the critical zone between these 2 values- and that the eighting this series exhibits in light wind is damped by their very considerable lateral area (which is approx. 100% of their flat area) in stronger winds. That they oscillate by a few degrees quite rapidly in winds just above their minimum would support this- because it takes some angular displacement before lateral area applies much resistance to rotation in light conditions.
If 46 becomes the path from now, how do I get back the eighting that is probably the key to the higher aspect ratio 42 series flying so well in light winds? Moving camber rearward would do it, as would getting the centre of gravity more forward somehow- but both of these things will cause the very problem that going to lower aspect ratio has cured (slow recovery).
Ashburton 8 Feb 2015
Light turbulent northerly turning NW and building to 40km/hr(?) at 5pm, 50km/hr gusts at least at 5.30pm. 45 with pleats in the keel trailing edges and just forward of each rear bridle stayed up reliably (90m) from 10 am to 7pm, even with more rearward camber than any previous SSSLs- suggests that lower aspect ratio is advantageous. 21kg pull measured at 5pm. In the gusts, does tend to hang off to left or right- seems to depend on which way it"s pointing when the gust hits- which supports asymmetric leading edge collapse as being the cause of this behaviour. Flaps a lot in the gusts, but no more than any 42 series 7 cell.
This kite is flying well enough today for all winds use by other people- but will it stay in tune for tomorrow and so on?
Tuning 45: Doesn't seem to respond to shortening rear outers but does responds very precisely to adjusting B outer- which changes the camber on each side at about half cord. This is the same tuning system as used on standard pilot kites- works by making one side less powerful relative to the other, and is largely independent of angle of attack. (Trailing edge adjusters can only work if angle of attack is always greater than the critical 5 degree "control reversal" range).
One observation is that the ribs all deflect outwards while the kite is flying, showing that there is more pressure in the centre cells than the outers (why is this?). Even though they have significantly less pressure behind them (from the outwards deflection of ribs) the shoulders on this kite are solid- don"t push in at all, whereas the centre cells do- suggests that it"s therefore best not to cross vent because the centre will lose pressure from this, and push in even more.
By the theory that I think cross venting will make things worse, but I know shit, so it"s worth a try; made 70mm dia holes cut concentrically in every rib nose. Can"t tell if it better, isn"t worse. Maybe if anything, slightly better- but the nose folds back enough in strong winds to at least partially close them off- and ribs still deflect outwards to about the same extent.
Another observation: in stronger winds; line angle seems to reduce from, say, 60degrees max to 45 degrees. This is almost certainly because of leading edge distortion in stronger winds- which suggests this is most likely the cause of the superstability.
Volker has been trying a hanging weight rigged to bridles for active control, isn"t sure of the effect as yet- says it"s too bloody cold where he is (0 degrees). I haven"t tried this approach for 10 years or so, but recall from then that the problem was keeping the weight force independent of inertial forces generated as it swings around. Boomers are pretty steady kites though (because they"re partial to superstability not volatile instability), so this has a good chance of working- especially for larger sizes that respond more slowly to everything.
Try scalloping the lower leading edge- by the theory that seeing this is pushed in now in strong winds and at high line angles, there is more pressure on the outside here than on the kite side, so cutting this area away should pressurise the underside of the kite more. Best to do this on a new prototype though- keep 45 as a reference.
Try scaling up to 3 sq.m (45 is just over 2 sq.m), Need to make a new template set soon anyway as there are now a lot of camber and rib length changes that are better not added later by various pleats. This also keeps the current templates unsullied for 42 series kites- which I may yet have to re-boot from
Ashburton 7 Feb 2015
Andrew Beattie has been nagging me try adding weights to the trailing edge as a potential cure for the super stable behaviour. I told him that this would be rather like farting in church- some benefits, some costs, and predictable consequences. Anyway I did try this today; 137gms (28% of total weight- kite is 350gm standard) to 44"s TE. Behaviour is as expected; does recover quicker, might now loop, stalls much more often in light winds- drops rapidly backwards- LE stays inflated better (from A of A increase). Testing so far does tend to indicate that the super stable behaviour (which name covers a multitude of sins) is reduced (incipient norwester here at present; moderate swirling gusts alternating with calms and 90 degree+ shifts). Not that adding weights is a sensible way to achieve this end, but it does confirm that lower aspect ratio (45 Orange) which does the same thing without costing lower end or causing looping in light winds at high A"s of A is a useful direction.
But an alternative theory for why they are so inclined to super stable "death dives". What if the noticeable crumpling of their LE whenever a gust strikes is making for behaviour like diamond style fighter kites? (Hata, Indian etc). This type of kite is volatile unstable in light apparent winds (like when line is let out), but strongly super stable (SS) in stronger apparent wind (like when line is pulled in). Their SS is caused by the frame bending allowing the skin forward of the bow to distort upwards to create drag- and drag at the front where it has the effect of pushing the nose around in such a way as to cause the kite"s C of P to move rearward, reducing or even reversing the pendulum effect.
If this is the correct explanation then reducing aspect ratio won"t help that much- what will be needed is some way to stop the leading edge crumpling (especially asymmetrically) in stronger winds. The obvious way to do this is by increasing the kite"s overall angle of attack (A of A), - but this will adversely affect light wind performance and exacerbate stalling- a cure that won"t be worth the cost as this light end is currently their main advantage.
Further 45 testing; was right wing then became treed, which has pushed it left (like for 44, must be something about Spanish Firs I expect). Now pretty central. Has luffed, has tripped going backwards, but neither to a worrying extent. Has looped, which is promising - is it flying at a better angle than the 42 series? Seems to be. Doesn"t seem to eight too much in no wind- but need to try constant 4km/hr-6km/hr wind like at Dubai.
Tried sewing in 2x 12mm tapered pleats 150mm long to every trailing edge panel on 45- see photo- to reduce trailing edge flapping in stronger winds. Dramatic effect: causes kite to loop uncontrollably during launching in light winds- and to show quite extreme volatile instability as it reaches for height. Once at apex it"s relatively stable but if tipped or caused to drop back down the window, generally dives off to one side or the other and crashes (still light to moderate turbulent winds). Max pull was up to 10kg at times- which seems a lot for the wind speed- suspect hooking the trailing edge has increased pull a lot too. Trailing edge did stop flapping, but has a compressive wrinkle about 200mm forward.
As a check, tried sewing 2x 12mm pleats in all rib trailing edges (see photo). This pulls the kite"s trailing edge down by much less than using pleats in the panels themselves, but there was a shadow of the same effect on flying: definitely more volatile, loopy at high A"s of A but little or no noticeable effect when at apex. It did reduce trailing edge flapping a lot- but I suspect the price by way of exacerbating super stability will be too high. The theory on this is that anything that moves camber rearward reduces the moment of the kites restorative pendulum: (c of g stays in the same place, centre of pressure moves closer to it), but both these tests are pretty strong indication as to how little margin there is for moving the camber rearward from where it is now.
Is it possible to add more camber nearer the leading edge as an offset for adding more at the rear, even a little bit? Probably this has already been explored adequately in the earlier series, and would just cause a vanishingly narrow wind range again.
Summary to today: the challenge is to develop a single skin single line kite that works across the entire range from less than 8km/hr to more than 100 without alteration (should I drop this back to 70?). Not an easy goal; but already tantalisingly close- I have had one or two SSSL"s in the last few weeks that were good enough to do this on some days (but would very likely fall back into sin by the next day). But it"s rather like tuning a soft bowed Nagasaki Hatta to fly straight in a gale; when it will distort into an arrow shape with the leading edge sitting up at almost 90 degrees to the wind. I need a way, analogously, to stiffen the SSSL"s bow so it doesn"t distort as much in strong winds- but without harming light wind performance. This assumes that there is enough pendulum effect, tiny as it is for these kites, to allow this- but that some of these kites have achieved this wind range on (rare) occasions says that there is.
Ashburton 6 Feb 2015
Made 45 Orange with one cell less (now 6 ); aspect ratio 0.95 (42 series are 1.1), 22 bridles. In strong gusty SW behind the trees, does recover much better than the higher AR versions- might even manage a loop on 60m lines now (42 series definitely can"t- they go straight to wherever they"re pointed). Will 45 be unstable in light winds? Surely with so much lateral area it will not "eight" too much?
Ashburton 5 Feb 2015
44 Purple. (A 42 style to be used as a reference for the next series) In a strong gusty northerly it was suicidally right wing off the sewing machine, ditto the next day in a strong gusty southerly. 15mm shortening of rear outer two ribs didn"t entirely correct this. Then it had a near death experience with an abies pinsapo, after which it became moderately left wing. Letting out B" in both centre ribs by 15mm and letting out the wingtips by 25mm (which respectively moved the camber forward and reduced wingtip drag) definitely made this kite less inclined to SS, as they should do 15mm on the outer right hand rear rib bridle then cured its lean- central now, but will it still be tomorrow?
Took 2 x 10mm tapered pleats from ribs just forward of rear flares, and 2x 12mm from tip flares ditto to reduce flapping in these places- which it has, and seemingly without other adverse effects.
19 Jan 2015
42 orange (After Nelson); 3.0m bridles; cambered centre and 2nd ribs 40mm by a vee between B"s and C"s, also added 15mm to A"s centre to reduce nose collapse. Light to strongish gusty easterly at 105. Span-wise creasing is noticeably less than for earlier prototypes. This one"s pretty good. "Natural" camber for 3m bridles is 60mm for 1.2m between A and D so 40mm shouldn"t create any more camber than the bridles are requiring, but should decrease rib flutter- which it has. Noticeably less flutter in rear panels, and sometimes flies at a very high angle. Do they now have a greater tendency to "roll-over" instability (a consequence of progressive attached flow over the cambered top surface as A of A decreases)? Not sure about this yet, but later prototypes do seem to have an apparently increasing problem with sensitivity to tuning- they can dive off unrecoverably to one side or the other in gusts.
Shortening D outer on the preferred side by 15mm +/- does seem to be effective tuning in stronger winds.
One possible solution to "roll-over" could be a flap strip on the upper surface at around 1/4 chord. Another idea worth trying could be to make the rear 25% of the skin in flag type porous fabric; would be heavier to reduce super stability, porous to improve stability in stronger winds, and more resistant to flapping fatigue. Goa, Pune and Dubai, to Feb 1 "15. SSSL43 had a religious experience at Goa which wasn"t good for it, but has since been rehabilitated I think: http://youtu.be/LY4yyWOxiVM (Peter Rieleit"s video). The SSSL"s performed very well at these events; were excellent in zero to light wind because they are so easily pumped. Fun too- I have worn through the skin on some fingers from doing this- haven"t done this for 25 years or so. They"re really good pilot kites, not least because they fly at a good angle , have heaps of pull and weigh so little- but mainly because they are so stable. However, while I was away, they did not perform well at:
Flight for Life, Ashburton 31 Jan "15, nor at New Brighton the week before. Both of these events had strongish winds, some turbulence, but I"ve flown SSSL's for many days in the worst winds we get here (both Norwest and Southwest) with great success- and these are winds that no one else around here is willing to even attempt to fly in. They"ve definitely been better than the pilots I"ve flown alongside for comparison in these extreme conditions.
So what was the problem I wonder?
Almost certainly it"s a tuning drift problem- 39 flew the best of all the SSSL"s at Nelson on the 17th, 18th Jan when the wind was upper range moderate at times- it was very central. But at Dubai in wind that was lighter than Nelson this kite was strongly right, though nothing had been changed. It had been OK at Goa and Pune. They seem to change from day to day. I expect therefore that this is what happened to the kites I had left with various fliers for Ashburton and Brighton, and that they either hadn"t known how to tune them or just didn"t get around to doing so. I have to find a way to make them at least stay consistent from day to day- but how? If the problem is the extreme ratio of pull to weight that they achieve (tiny weight pendulum effect relative to aero forces), then this is likely to be very difficult. One approach may be to try reducing AR- this will improve recovery and make them less sensitive by increasing the length of the restorative pendulum relative to the inertia and aero force moments that the weight forces must act against but will they then lose the ideal level of figure eighting they now have in light conditions?
42 green: had to lengthen centre A"s by 15mm to reduce LE collapse, but then flew well at Nelson - a bit right - and earlier kites, back to 34, noticeably flew as well or better in the quite strong gusty onshore winds there - with some exceptions. 39 gold 30gm soft fabric was a stand-out- very straight and seemed to get better as the wind became stronger.
15th Jan 2015
SSSL41 pink; Made all new rib templates with difference between rear and front flare depths all ribs, now 50mm (was 40mm)- to shorten front bridles matching SSSL40.
Light gusty easterly; does fly OK but appears to be significantly SS caused by quite noticeable creases at B and C (less) during gusts: noticeable chord-wise compression.
Let out all B's by 20mm, chord-wise compression much less now flies much better, reasonably determined right wing though- as is 39 and 40.
Tried shortening all bridles by 720mm to increase camber (after restoring all B's back to original length) Almost as good as with full length bridles, B's lengthened, but crease at B again- so let out B"s by 20mm again and it"s probably now flying best- does seem more volatile, less inclined to stall in gusts.
After some more testing of this version- need to try it in strong wind, 40 also- and if the increased camber hasn"t caused instability of some type, should try pleating ribs to match camber again, see if this can now help. Damaged in a tree, repaired; restored bridles to 3.1m, not worse, maybe better.
10 Jan "15
Built SSSL 39 (gold 30gm). - Very minor template tidying from 38.
Flies straight off but strong right (lightish easterly) Still to the right after shoulder bridles made the same- and still a little right with LH shoulder 15mm longer.
Doesn"t appear to be any less inclined to hold LE shape than heavier fabrics, and no more (perhaps less?) creases in the main skin. This bridle line is very stretchy- even after each bridle has been individually tensioned to 2 or 3 kg, there is still a lot of settling to come. Weight is 250gm complete.
In stronger winds, terminally right wing- had to lengthen LH shoulder and 3A 20mm, shorten 3B 20mm, lengthen 2B and 3B 20mm to get almost straight- is this stretchier lighter fabric inherently more likely to distort asymmetrically?
There are now 5 of the larger 26 bridle SSSLs in good flyable condition, each one significantly different in some respect- maybe best to get some hours at events on these before too much more development- need to check light end for eg. It is tempting to try hooking the TE on 38 or 39 though, either by way of TE tightening or rib pleats.
Is also now possible to establish some formulas for spacings and lengths.
Cutting out- 30minutes to 45minutes? All sewing took just less than 2 hours. Bridling, total, including pre-stretching; 45minutes.
Light Norwest, slight drizzle, flies really well straight off- LE does seem better and shoulders aren"t as notchy, but there is a bit of a flare out in LE of tip flare. Quite right wing in stronger gusts- but haven"t checked bridle symmetry yet.
Seems to be flying at a bit higher A of A than previous- quite a bit stiffer and a little slower to respond to line pull in the light - is this because the leading edge is now 90mm longer, increasing forward area, so moving C of P rearward? Tried shortening all LE bridles by 15mm and it does now feel more like 35-39.
No noticeable tendency to nose collapse yet- nor to instability- and this will have increased camber so decreased wrinkling in forward area, which was noticeable for 39. Need to check this in stronger and lighter winds. Does now (15mm shortening of all A"s) behave in no wind more like 34-39, that is, lower leading edge does fold in a bit during acceleration. Cut away LE of tip flares- a lot less flare out now, changed template to this.
9 Jan "15
Tried 2. By sewing 2 x 12mm pleats in rib and tip TE"s of 37 purple. Quite strong easterly, 20kg + pull, does seem to be flapping a bit less - and does seem to now fly at a higher angle in this wind (or is the reason that it"s now flying straight?)
38 red; as for 37 but with rear bridles (ribs and tips) moved rearward by 85mm (other bridles spaced appropriately. Flying 38, strongish N E; initially strongly left wing, after making bridles symmetrical is straight- but responds very well to tuning ONLY with shoulder bridles- +20mm on left shoulder was enough to cause strong left, when corrected, kite is central.
TE tramping a LOT less severe than earlier versions, probably better than modified 37 above. Because 37 mod did reduce tramping and doesn"t appear to increase instability, suggests could do this to 38 in addition to moving rear bridles rearward- which didn"t seem to cause any additional creasing anywhere either.
6th Jan "15.
Built 37 (Purple) shortened as for 36 green but with more even spacing of bridles, especially on outer ribs (35 has big gap from B to C) Flies exceptionally well with reduced TE tramping in strong winds (still a lot though) after 8 or 10 hours heavy wind flying, quite noticeable wear marks in TE panels.
Tuning: was initially left, then caught in a big tree, damaged pulling out. After repair, strongly right. Tried very many things over 5 hours+ continually. Pulling in left side 3/B and 2/B,C and letting out right side mirrors did almost correct the lean but in stronger winds it re-appeared. Max 30kg pull (gusty easterly).
Tried releasing left TE bridles - but didn"t work at all.
Then tried letting out left LE on shoulder and 3A- works immediately and precisely, no other bridle asymmetries then required.
Is it a necessary A of A modifier that allows the kite to hold LE inflation at higher wind speeds, as per above, or is it just inadequate TE support.
If the latter, fixes could be:
1. Shorten TE a little to create some hook- works for sports kites
2. Shorten TE of ribs so as to do the same.
3. Move rear bridles rearward by 50mm or so.
2nd Jan "15
This one, SSSL 35(yellow) is the best yet- flying reliably (came down twice in 3 hours, each time because an unsettled bridle had stretched out 20mm- (used soft Dyneema not hard finish) on a short line in a strongish easterly/southerly- trees are thrashing a bit. Has a LOT of pull- not really possible for me to hang on to it on 2mm line. There is a serious crease at C bridles centre, none at A or B or D- but trailing edge flaps loudly in strong wind, compression collapse. This seems to be having effect of moving the C of P of the functioning (forward) part of the kite rearward which ensures that the LE stays inflated. Definitely an effect worth keeping if it"s doing what I think it"s doing- working like a reverse spring bridle.
SSSL 35 has 26 3m long bridles, weighs 360gm without line (45gm acrylic coated fabric).
Built SSSL 36 (green), same as 35 but centre ribs shorter by 150mm, 2nd ribs by 120mm, outer ribs by 80mm- 2.5sq.m effective lifting area.
SSSL 36 flies OK- quite right wing, more creases on right rear, does tramp trailing edge but not as much as 35- does it fly as well in stronger wind?
SSSL 36 compared to 9sq.m Airbanners (latest model) Pilot:
Light gusty intermittent easterly at 105 AF Rd, 4th Jan "15
36 has more pull on average, holds a higher angle, doesn"t wander off to the sides as much (quicker recovery from perturbations), seems as reliable or more reliable for same height, but presumably won"t have the upper end. Light end seems equivalent but this needs testing in smooth unobstructed gradually dying wind.
7pm same day, More north easterly now, 30km/hr?:- Trunks on main pines are moving noticeably. 35 came fairly straight as bridles settled in- some tendency to SS in strongest gusts, either way. Angle same or only marginally better than the 9 Pilot in this wind, pull around the same for both- 20kg to 25kg. Pilot is much less stable (but 70 m out as compared to 80m for 36), collapses, inverts and usually recovers.
Lots of TE tramping on 36, but the LE holds form very well in the strongest gusts- strongly suspect that this is because of C of P shift deriving from TE panels disengaging from lift? Much less centre panel creasing than for 35.
All the bridles the same length exactly still seems to be better across the wind range than any other variant- even just 15mm +/- on a few bridles.
1 Jan "15
Re-sewed one side"s ribs (except one nearest centreline) to get lateral symmetry, restored bridles to original- all same length. Now flies as well as 32, and slightly left if anything. Does compress chord-wise and LE bends under a bit in strong winds, but recovers exceptionally well and remained flying when 12 sq.m Airbanners Blue was crashing every 30 minutes or so - and 34 was on half the line length- strong easterly.
Began templates for larger version; scaled x 1.5 chord-wise, x1.4 span-wise- all flare depths scaled 1.5x
30th Dec 2014Built SSSL 34 (Red), quite strongly right wing, didn"t fly anything like as reliably as 33- but had let out centre B"s by 15mm to eliminate crease at B- which it did.
27 Dec 2014:Why does 32 (green, 4 Dec 2014) collapse while 23 (yellow, 5 Nov) doesn"t?
1. 23 has chordwise choke, 32 doesn"t have.
2. 32 is larger than 23 (10%?)- fabric stiffness effect?
3. 23 has compound triangular sewn on flares, 32"s are one piece scalloped.
4. 23 has marginally greater leading edge tuck.
5. 23 is flatter in profile- test by de-cambering 32.
6. Is it progressive collapse? That 23 collapses in such a way as to remain flying? 32 folds to one side.
7. Bridle overall length? No, checked this by trying much longer and much shorter on 32.
8. Fabric type? 23 is lighter, softer- so how could this be?
Wakanui, 27th Dec 2014 Strongish NW
23 stayed inflated, flew marginally, LE crumpled back a bit but not destructively, TE panel tramped. 32 not much behind- but LE would eventually crumple enough on one side to cause a dive -off. No TE panel tramping (bridling?) and much more noticeable creases at 2nd and 3rd bridles. Became noticeably compressed chord-wise compared to 23. Tried with cambered ribs and without, same bridling, no sig difference- maybe a little less inclined to collapse chord-wise with the rib camber removed.
For 33, try same as 32, same chord and ribs and bridles but 10% less span; by theory that there is a span-wise as well as chord-wise max bridle spacing.
Built 33 (Orange) on 28th, flew 29th at Wakanui and 105, strong gusty easterly.
Definitely the best SSSL yet- flew well immediately with initial bridling (all the same length), not even checked for symmetry. Much less lateral wrinkling than 32 or 24 . LE stays inflated better too.
There is no other explanation for this improvement than that span-wise bridle spacing matter- narrower cells, with closer spaced ribs hold better shape. There is likely to be a formula for this- perhaps some product of span-wise and chord-wise spacing is approx. constant - though deeper flares do appear to help as well.
Took 19, 24, 25, 26, 30 to the beach and tuned/tested them: 20+ km/hr north easterly (not from the sea).
The only one of these that looks promising is 24 (yellow, 24 bridle plus 2 extra) made in Weifang, Quite high aspect ratio, every cell is choked a bit, very curved leading edge. This kite did look promising at Xiamen but was very distorted at the LE of the outer ribs.
29 November 2014After flying at the beach, added a fabric wedge to 24 to eliminate the distortion and added an extra bridle on the LE between the outer rib nose and the rear outer flares. After some more bridle adjusting, this kite has become an excellent flier- was as good as any of the pilots and lifters at the 30 Nov Domain flying day - when the wind was south to east (every minute) quite strong (difficult to hand hold a 12sq.m Lifter) and VERY gusty. In current mode, 24 is very flat in the profiles- almost reflexive. It is exceptionally stable, not quick recovering but doesn"t tip over so doesn"t ever need to recover. Pulling in B on an outer rib (there"s A B and C on this rib) works for tuning - 5 -10mm has a big effect in strong winds. In the strongest conditions the trailing edge tramps- but it continues to fly. In even stronger this will probably cause super stability- was just starting to at about 40 or 45 km/hr.
See discussion and photos in December Newsletter.
On 2 Dec, made a development of 24;single panel with scalloped ribs (4, 4, 3), slightly lower ( by 10%?) AR, slightly shorter lower LE and it"s made from a separate curved piece, not an extension of every panel as 24 was. A bridles come straight from the LE lower edge, do not overlap 20mm as for 24.
In initial testing it was fairly cambered and zoomed around a lot- would loop, but more inclined to dive to the left and crash. After letting off TE about 25mm, pulling in B"s and C"s about 20mm and letting A"s out 20mm, started to look and fly somewhat like 24- though isn"t yet as flat in the TE and appears proportionally narrower at the TE. This kite will be a bit wider than 24 on account of not having chord-wise panels with seams, so maybe AR is higher than I intended.
25 Nov 2014 Ashburton;
Made #30 green 4 x 4:
Reduced lower leading edge to what 29 had been modified to above, but didn"t add cross venting.
Added flares back to the centre A"s and reduced centre B, C and D flares to 100mm shorter than for 29 and earlier prototypes - to move the lateral area C of P forward as much as possible, and to reduce the flopping of the centre ribs which may have been contributing to edging.
Added flares to outer B"s and reduced outer C"s and D"s by 25mm also to move lateral C of P forward (see in red above). Kept outer A"s cut away so as to not have shoulder collapse.
First test (gusty easterly behind the trees) looks promising- definitely recovers now, much less tendency to edge, maybe a little volatile- so make bigger or higher AR? Recovers well from stalling- probably the best yet.
Quite a tidy smooth kite and appears to be able to retain inflation at low A of A. Need to test and tune in smooth wind and try very light wind.
Up to 2 December 2014
Extensive testing of 30 in easterly at 105 and at beach- can"t get it to stay up reliably, even with a lot of bridle adjustment. It"s OK for a while but as soon as it tips to one side more than about 60 degrees it dives over to the ground and doesn"t recover. It is good in light winds, doesn"t have the tendency to tip over whenever it drifts backwards that 19 has.
Ashburton Nov 13 2014
Tried a 4 bridle per cell 7 cell developed from 23 (no number), did fly quite acceptably in a gusty northwest, but also seems to have a hang-up mode when it sits at lowish angle, stalled. In light winds it tends to edge quite badly.
Orange, 3 x 8 (no number), developed from this was not improved in the edging- what"s causing this?, but does seem to be able to accept quite forward bridling without LE or shoulder collapse. It"s interesting that even without chord-wise choke, the LE shoulders do stay well inflated.
25 (purple 3 x 6l) had a lot of flying and development but is plagued by this edging problem- when it gets perturbed by more than about 20 degrees it inexorably dives off to one side or the other and crashes. Tried many things, but nothing seemed to make much difference.
26, (yellow, 3 x 6) same basic patterns but small changes to the shoulders and some camber in the ribs. A lot of testing with this in mainly gusty winds, no useful improvement.
27 red, 3 x 4) reduced span-wise to 3 cells (12 bridles total). Did fly for periods in gusty easterly, especially after adding another bridle set between A and B. Very sensitive to tuning and with the dreaded non-recovery when flying sideways
28 orange, developed version of 27, with 4 bridles/cell (16 total) small changes to bridle points and with a larger lower leading edge- not significantly better - is quite volatile so;
29, purple, same as 28 but with a 300mm wide centre panel (28 is 225mm) many hours of testing with this in gusty easterlies. Initially it wasn"t nearly as reliable as 28 but reducing the LE return by 30mm- 25mm helped somewhat, as did cutting away the central ribs between bridles to reduce pocketing. A theory now is that the edging is caused in part by transverse flow. When the kite is lying on its side towards the edge of the wind, it functions as an airfoil with the rear outside upper edge generating more lift than the front outside upper edge- which causes the rear to tip up and the kite to slide/dive off to that side. If the dive gets steep enough, the kite will loop and recover, but on a short line and in the tree wind shadow it more usually crashes to one side or the other. Shortening C on one side is an excellent and sensitive tuning system- but this kite is extremely sensitive to tune- if it is set to not deviate more than a degree or 2 either way it will stay up for periods, but more usually hangs off a bit then gets into a death dive.
Tried cutting 2/75mm dia. vents in each rib near the LE- did this reduce edging?- didn"t make it worse anyway, but top skin now has more bumps.
Tried cutting away the rear outside bridle triangles to reduce area at the rear relative to the front and in this form 29 does still fly about as well as before this- but is suffering from poorly supported rear area- develops creases, makes flapping noises and hangs low- or crashes.
Summary: it now seems likely that cutting away the front flares from #25 on, which did improve the shoulders a lot (in the absence of choke) but had the initially unrecognised consequence of shifting the lateral area rearward which seems to be a cause of edging/tipping. A solution may be to return to using larger area front flares in the centre ribs only (and go back to 3 bridles per cell in the centre?) while continuing with using the cutaway front flare at the shoulders and 4 bridles/per. It may be that once this edging is solved, a return to 7 cells might be possible (and helpful if volatility is a problem for the lower aspect ratio versions).
Xiamen 9th Nov 2014:
Light onshore wind building to 35km/hr by 5pm.
Tested 23 in light wind mode; did narrow LE a lot and was inclined to edge terminally because of shoulder collapse, but seemed quite close to being flyable.
Xiamen Nov 7 2014
Strongish wind off the sea- 30km/hr.
23 green; leading edge smooth and completely solid (seems fairly forward bridled too).
Trailing edge (actually the rear 25% of the kite) loose and flapping-and pulling in the 4th row even to stalling didn"t stop this- why?- not enough (or even negative) chord-wise camber which will need to be corrected by letting B"s and C"s out, not by shortening D"s?
Flying off the bridles, seemed to be a bit volatile.
Quite a bit of pull- but I"d say nothing near as much as 19 would have had in this wind- but very much doubt that 19 would have flown- or that its LE would have stayed solid.- check this.
24 yellow: LE solid but notchy, and sides need an arch pleat- very loose.
TE is solid too- doesn"t flap at all like 23- the skin does seem more cambered than 23.
Flying off the bridles was towards super stability- and noticeably right wing (bridlings on 23 and 24 are still the fairly random first guesses from Weifang.
Directions; Need to test now in very light winds- check if stall collapses are a problem with these designs also.
Generally a pretty stunning success- What combination of: Lower LE, LE being deeper, having ribs closer together, and less camber is the source of this?
There doesn"t now seem to be much doubt that SSSLs will be successful kites.
The next "pilot" version could be some combination of 23 and 24 maybe- 23 with higher AR or 24 with slightly lower AR and the LE panels shaped better. 24 has the virtue of only 24 bridles (32 for 23). Could try releasing the D bridles on 23- and if it flies OK (will need to shorten A"s a bit to compensate for C of P shift) then try cutting the rear ¼ off. This increases the AR, and reduces bridles to 24.
And make a new test kite in the shape of a Ray"s wing; could standardise on shallow ribs, making bias cut strips, slight scalloping, bridles at about half the current spacing and say 4 or so different depths, which could then be used for the different chords required on show kite wings. The primary bridle set could be in chord-wise sets, all the same length (perhaps the LE bridles should be singles). Strips like this would probably have a better appearance than the current triangular flares. There might be some possibility of just using close spaced bridles, no ribs at all- like NASA "spines".
Xiamen 7 Nov 2014.
Very light to no wind.
23 does seem like it will fly without the rear quarter- all D flares. With D"s released rear was still not very solid. LE and shoulders are very solid though- better than 24, which has excellent centre but creased and collapsing shoulders. Did play around with 24 extensively- flattening off to see how far forward it can go. It"s a lot, especially when D"s are released until just short of getting a crease towards the front of the D flares. If the LE is pulled down too far it dives off to one side or the other irretrievably sometimes- presumably because one shoulder or the other is collapsing more (has this appearance). With the flattest profile it will accept without creases (perhaps 50mm to 100mm camber chord-wise), and at the most forward bridling it will fly without too much narrowing of the LE span-wise or indenting. It"s just volatile unstable in the lightest wind that it will fly in but steadies up if there"s 10% more. With a V tail, was quite reliable. It would fly in wind as light as the latest 9sq.m Airbanners but only when continually pumped.
Tried taking a duct tape 2 x 15mm arching pleat from the shoulders- did improve appearance but didn"t by itself reduce preferential shoulder creasing. It recovers well from lulls- drops back, sometimes turns quite a bit sideways, but then zooms off to the side and comes back up in a long curve. I did briefly try 19 in the same conditions- 24 is noticeably easier to launch, is more forward bridled for the same or better LE stability, and recovers much better from lulls. 24 does still exhibit the same volatile behaviour which steady"s down as it gets to a higher angle- and in more wind- but it has it to a lesser extent.
Tested 21 with the larger B outer flares- still wouldn"t stay central- always dived off to one side soon after getting to apex. Didn"t try different bridles though.
Next prototype: a 22 with higher aspect ratio and maybe only 3 flares/cell (they need to be slightly larger than for 23 anyway to reduce its volatility. Should the TE be concave or convex?
Weifang, Nov 4 2014:
Made 24 yellow: 7 cells span-wise, semi-circular shape (curved leading edge) 24 cascaded bridles (span-wise rows of 4, 4, 6, 8), approx 5% lower LE, but this tapers to no leading edge in each outer cell (LE in this cell is very swept back. approx 1.3 m span, 1.3m chord, perhaps AR 1.2. There is around 5 degrees choke in every cell. As for 23, no camber at all as cut- but 2x6mm pleats in the centre 5 cells to accommodate arch.
The thinking behind this design is that if sweepback does prove beneficial, this is the type of shape that will used. Their difficulty is that the rib to panel connections at the high sweepback sections are difficult to shape right so as not to wrinkle- and difficult to sew (the more the sweepback the trickier this is). Maybe making the LE of individual span-wise strips like for the 19 would make this easier- maybe only necessary for the outer more swept back cells.
24 was also bridled very far forward for first pull up test outside factory door. Appears to fly, no LE or tip collapse apparent, but has major wrinkling the 3rd rib LE each side caused at least in part by having paired 3A and 3B bridles; should have used only span-wise pairing..
Weifang, Nov 3 2014;
Made; 23 green: 7 cell, parallel sides, with matching convex LE, concave TE to maintain equal chord, 1.2m span (nominal), 1.3m chord, 4 bridles/cell, 32 bridle total, about 5% lower leading edge with 6mm x 2 of pleat per cell to match bridle camber. All cells span-wise are to have equal bridle lengths in 3 cascading sets. Concern is whether LE tips will hold form without choke- but the outer cell sweepback is around 45 degrees, so this will be a test of how much sweepback contributes to tip stability in the absence of choke.
It has no camber at all as cut- completely flat skin LE to TE.
First pull up test at workshop door, appeared to fly OK on first (guessed) bridle dimensions, which are very forward. No noticeable tip or LE collapsing.
Weifang, Nov 2 20141. Is 22's edging behaviour an aspect ratio effect (super stability) or is it just point bridle load at B outer?- make B outer flares larger and re-test at Xiamen.
2. Choking the tip panels by 50mm over 250mm (approx 1m chord) definitely reduces wingtip collapse.
3. Tapering off the lower leading edge to the flares at the tips and in the centre seems to have worked well- doesn"t seem to reduce the effectiveness, provides access for sewing on the bridle without a lot of stuffing around.
4. It"s tempting to try an Arc style SS again- flat choked centre panel, 2 long flares per side (so that A of A can be changed), but I"m anticipating that if such a kite stalled it would not easily re-inflate- would hand clap like ram air arcs only maybe worse because of having no structure to initialise the opening.
5. Flat multi cell fully bridled SS"s after the style of Michel"s 4 line SS traction kites seem to be able to fly at lower A of A"s without stalling or collapsing than the Skin style. Maybe the significant measure of A of A is a line from the stagnation point at the LE through to the trailing edge- and using bent down TE"s as the Skin and current SSSL"s both do may not allow a lower A of A - but just moves the overall C of P rearward, putting it closer to the C of G and maybe exacerbating instability approaching the stall.
6. The current inability of SSSL"s to recover from stalls may be just that when they fall backwards, one side rear collapses preferentially causing the kite to tip to that side even to 90degrees or more- from which re-inflation and recovery is impossible in the very light conditions that there must have been to cause the stall in the first place.
7. But is there any solution to this except just reducing the stall speed? What if the outer rear flares were made with reverse choke?- though this would also cause them to push in when flying normally- but maybe there"s enough pressure back there to resist this or at least so as this wouldn"t cause collapse. Or what if the SSSL could be caused to drift back with reducing A of A (that is, glide back) rather than at increasing A of A as now (which is a caused by a combination of long bridles and TE camber?)
8. Try a flat, hour glass shaped, AR 0.75 (19"s AR seems to be too low for best stability and recovery, but 22"s is too high?), multi cell 5 bridle/cell SSSL with no TE pull down and a 10% (+/-) lower leading edge. Say 7 cells (8 sets of bridles span-wise), Flares 60degrees included angle or even a bit more tapered?
9. Flare choke could be applied by adjustable cords between the rear edges of the outer A flares- and this might also have a useful feedback function in stability by choking the side with the least bridle load on it (bridle tension decreases on the LE bridle as that tip begins to collapse). This could also work for the rear flares reverse choke- would this cause the rear to narrow or collapse?.
10. Another approach might be to make in the form of a laterally symmetrical trapezium, narrow LE, with a row of flares down each side, all angled so as to pull outwards- maybe the front flares could have more choke than the rearward. This shortens the leading edge a lot and maybe would work with bridles only to the side flares- try 4/side to start with- and overlapping.
11. Another approach; try making from a single sheet with folded over LE like the centre section of a NASA, bridled to each tie, 4 (?) side flares all bridled and many centre bridles directly to the skin- sew on cording for attachment points.
12. These style of single skin wings are going to work very well on show kites - much better than ram air wings do because they don"t have any tendency to overfly, whereas ram air wings do. The only way to stop inflated wings from them wrapping themselves around the body or head is by extreme choking and/or angle of attack- (Rolf"s dragon, Simon"s Toothless and pegasus, Andreas"s birds and etc) I think I included a claim for single skin wings on show kite wings- ray, birds, dragons, butterflies etc in the 2013 single skin provisional patent application, but if I didn"t, then publishing this should constitute disclosure and prevent anyone closing such uses off.