Peter Lynn ARCHIVE
History: The 1Skin Saga; late 2014 to mid 2018.
Using the uniform camber principle (no trailing edge pull down), an SSSL with semi circular shape and 7 keels was soon flying- followed by 5 cell 1Skins and 4 cell Singers.
While retaining the same leading-edge construction as earlier designs, the use of chordwise camber instead of trailing edge hook-down provided a much wider wind range. Their excellent lift to drag ratio (L/D) gave them a 60degree plus flying angle and the usual pull/size advantage that single skin kites have over ram air designs gave a 3sq.m Boomer lift equivalent to a 10sq.m or larger ram air pilot. At this stage it seemed that intensive detail development would eliminate their remaining problems, which were:
A tendency to lean to one side or the other rather than flying straight.
Requiring multiple bridle settings for different wind speeds.
And sometimes diving over unpredictably when flying in winds above about 30km/hr.
I spent much of the next 3 years developing and refining the Boomer/1Skin/Singer style SSSL's, building close to 100 prototypes.
The leaning problem was addressed by reducing aspect ratio. 5 cell ISkins are much less inclined to leaning and 4cell Singers have the opposite problem, tending to weaving instability. The relationship between aspect ratio and leaning, slow recovery and weaving is explained more fully under SSSL Observations.
The need for multiple bridle settings was largely solved by the development of automatic bridle adjusters.
This is covered in Automatic Bridles.
Diving over has proved to be much more intractable.
The following are the possible causes of 1Skin diving over that I considered (in no particular order)
1. That the C of G is too close to the C of L to provide sufficient restorative moment as the wind strength increases: SSSL's are very light. A 3 sq.m 1Skin weighs just 300gm- which is all the restoring force that is then available when the kite leans over- and in a decent breeze will pull 40kg and more on the line. Additionally, SSSLs generally have their C of L's at 35% chord or more (by comparison with sticked and ram air kites at 15% to 25%), which doesn't give the small weight force much leverage to pull the kite straight when it leans a bit. It's easy to imagine the tiny restorative moment available being overwhelmed by aerodynamic forces that can be 200 times as large.
2. Bridle stretch. When a kite flies on any sort of a curve, its outer wing tip generates more lift than its inner tip by a square relationship. This creates more pull on the outer bridles and if they stretch significantly will cause the kite to dive over even more because of positive feedback.
3. Asymmetric fabric stretch. Kite fabric when heavily loaded, as it is by SSSL's in strong wind, often does not stretch symmetrically because fabric's warp threads don't have the same spacing as the fill threads and they are often not at right angles to each other.
4. Migration of the kite's centre of lift towards the rear as it's A of A reduces in stronger winds. This is a speculation that as the kite's A of A approached zero (high flying angle), flow was attaching progressively to the upper surface, rather than separating in a turbulent wake, causing the kite's centre of lift to shift back behind the kite's C of G and causing it to dive over and fly downwards.
5. Chordwise buckling. When flying, 1Skins often show spanwise crease lines. These are sites where further buckling is likely to occur under compressive loads. If these develop at all asymmetrically then they can, and eventually will cause diving over.
6. Fabric creases. For strong wind flying, 1Skins generally require re-tuning for straight flying after being packed away. This seems likely to be because the creases from tight packing are in different places each time and have an initiating effect on where compressive buckling occurs when flying.
7. Leading edge buckling (indenting) in stronger winds and at lower A of A's
Addressing these possible causes of 1Skin diving over in order:
1. Insufficient restoring moment because their C of L is too close to their C of G. That they are so close together is a consequence of the camber that SSSLs have to use in order to prevent compression buckling. That this does contribute to diving over is undoubted.
A first attempt to get the C of L further forward was to let off the rear bridles by 30mm or so, and this definitely helped, though it also allowed the trailing edge (TE) to buckle upwards in stronger winds which caused a lot of flapping and wrecked the fabric after a few hours flying.
The next approach was to reduce aspect ratio (AR)- a move from the 7 cell Boomer to the 5 cell 1Skin. Reducing AR has the effect of moving the C of L and C of G relatively further apart, giving the restoring moment of the kite's weight more leverage. This was taken a step further with the 4 cell Singer. Singers become WU in strong winds but because of their slow recovery (from having a lot of lateral area) they usually don't have enough space to recover and the end result is the same- a crash.
The next attempt was to try kites with porous rear skins. The idea behind this is that leakage through the rear section would shift the centre of lift forward while keeping the C of G in the same place. A disadvantage was that the effect seemed to be greater in lighter winds- the opposite of what was optimal. At a small cost in light wind flying, they did seem to be a marginally better in strong winds, perhaps because of having more drag at the rear keeping the skin tight, or it could have been the desired C of L shifting effect- or maybe I was just being hopeful.
An obvious way to increase the distance between the C of L and C of G is to add weight to the rear, but doing this increases the kite's A of A, which moves the C of L rearward, so there isn't as much gain as may be expected- and besides, I've foresworn adding weight (except for testing theories). A better way to shift the C of G rearward is to add a tail and ribbon tails do improve the strong wind reliability of 1Skin type SSSLs. Their disadvantage is that they are additional weight to pack (if air travel is necessary), can catch on things (like little children and adjacent kites) and make launching over water more difficult- which limits the usefulness for towed displays.
2. Bridle stretch. I tried different weight polyester bridles (stretchy) and Dyneema bridles (not so stretchy) on various prototypes but couldn't be sure of what effect this was having. Diving over didn't decrease dramatically, even with hard finish Dyneema. There are also the chordwise effects to consider. Because there is more tension on each flare bridle than on any leading-edge bridle, bridle elasticity could be expected to exacerbate any luffing tendency in strong winds, by pulling the leading edge down (relatively). I have seen the deleterious effects of stretchy bridles on larger ram air kites but not yet unequivocally on 1Skins - but still tend to use Dyneema bridles, just in case there is something disadvantageous going on.
3. Asymmetric fabric stretch. Every 1Skin I made flew differently and required some tuning for straightness, even after bridles had been re-set to exact lengths after a settling in period. This can only be because of sewing asymmetry and fabric asymmetry. These could be contributing to diving over, but my sense of this is that other things are causing the diving over while the kite's basic asymmetries just decide which side the kite is eventually going to dive off to. Even perfectly tuned 1Skins can still misbehave in strong winds.
4. C of L migration at low A of A's. To test this, I tried flow separators at around 1/3rd chord on the upper surface. There was no noticeable improvement.
5. Chordwise buckling. As explained earlier, provided a kite's camber is of lesser radius than the bridle lengths, some small component of bridle tension is available to keep the skin unwrinkled- in which condition it's best able to resist compressive (buckling) loads. But as using more camber causes the C of L to move too far towards the rear, there's a fine balance to be struck here (see 1 above). Another way to reduce chordwise buckling is to apply some chordwise tension to the fabric by adding drag at the rear of the kite- like by adding a tail or tails.
Doing this by hooking the trailing edge has been ruled out but hooking the trailing edges of the outer flares could accomplish the same thing (and is used on later SSPs). However, 1Skins that have substantial tails still sometimes crash out in strong winds. So, though asymmetric chordwise buckling definitely is a cause of 1Skin diving over, it's not the only cause.
6. Fabric creasing. Softer (non-creasing) fabric would be an answer, but such fabrics are stretchier and less dimensionally stable, so would cause other problems. Basically, that this is a cause of 1Skin asymmetric flying, tells me that the design is too sensitive. Successful kite designs need to have a margin of reliability sufficient to cope with aging, minor damage and small left/right differences.
7. Leading Edge Buckling. Wind forces are inevitably going to push the leading edges of SSSLs in when they are flying at low A of A (high line angle). This is because the leading edges of SSSL's must extend down significantly or the kite will stall whenever it has to fly at high angles of attack - like in light winds. But this buckling-in doesn't seem to prevent 1Skins flying reliably in moderate winds, and even sometimes in very strong winds (I have at times managed to keep them up in winds of above 70km/hr for short periods). The question was therefore, why do 1Skins sometimes cope with stronger winds and sometimes don't- and is this a leading-edge problem or something else?
As a way to maybe answer this, I tried fitting double skin leading edges to 1Skins, with the space between ram-air inflated (I know, I know, cheating- and I tried the same check on an Octopus and Serpents too). The results were equivocal; definitely more reliable, but not a complete cure.
Simon Freidin (Melbourne) tried another idea to reduce leading edges indenting; Air dams. These are fabric dams across the kite's cells positioned so as to 'bounce' some pressure back to the kite's leading edge, in order to keep it 'inflated'. They do seem to do what they are supposed to do to an extent. This is something that could probably be usefully modelled in a virtual wind tunnel to optimise dam placements. 1Skins fitted with air dams still sometimes dive over when flying in strong winds.
Conclusion;
Asymmetric leading-edge indenting IS a primary cause of 1Skin diving over, and nothing tried so far is a sufficient cure. Occasional satisfactory strong wind flying is almost certainly because on these occasions, LE indenting just happened to be symmetrical enough not to make problems.
But contributing to this effect, the keels prevent crosswise flows which on other SSSLs can help leading edge indentations to pop out again. LE's of 1Skin type SSSLs rarely if ever recover from indentations once they begin to dive over.
But leading edge buckling is not the only factor.
The high lateral area ratio (keel area exceeds lifting area) makes for slow recovery which often causes an attempt to recover to intersect with the ground before it succeeds.
And the tiny size of the restoring moment no doubt gets lost in the very large aerodynamic forces in strong winds.
When the wind becomes strong, the 1Skin style of kite will sometimes dive off to one side or the other, taking other kites in the vicinity down into a big tangle. The causes are a various but seem to be inherent to the design.
By 2018 I had formed the view that the problem was not solvable for now and I shifted effort to other styles.
Coming back in 2022 for a new look, 10 or so representative 1Skin style kites were tested in strongish wind.
Most were not very good- hanging over a bit even in light wind without adjustments and variously dived over in stronger conditions from asymmetric leading edge buckling.
The ones with ram air inflated LEs were generally quite reliable in lighter winds but are not noticeably more reliable in stronger winds.
SSSL 79, Jan '17, blueish 1Skin is outstanding. Even in strongish wind it shows no creases and has only dived over once during recent testing, unfortunately while not being watched, so of unknown cause. 79 has aeolian rear bridles and no auto bridle system.
SSSL 95, Oct '18 purple 1Skin is also quite good- a few more wrinkles than 79 but seems similarly reliable. 95 doesn't have aeolian bridles but does have an auto bridle.
SSSL 84, Feb '17 yellow orange, a Singer is also excellent. Wrinkle free, flies exceptionally well in light winds and
yet to be tested in strong except that it became WU when a tail was attached in strongish.
After more testing maybe either 79 or 84 could be copied and developed further.
An obvious change to try is reducing rear outer keel area, because this may be contributing to diving over.
Another change could be to try decreasing the keel depths (adding more bridles to keep creasing down)- to check whether excess lateral area is contributing to diving over.
Is a cause of diving over that their lateral area is too rearward? This could be tested by cutting down the rear outer keels and adding an extra bridle.
SSSL 84 (yellow orange Singer), Feb '17 becomes WU in stronger winds when a ribbon tail is attached. Without the tail , will fly relatively stable with some weaving in the same wind.
27 Jan '22, Singer 84 in strong gusty southerly. No LE collapse or even indentation as far as visible, but zoomed all over the place, crashed every minute or so. With 4 mauve tails, would stay up but still a bit wild. Had ferocious pull- more than SSP 14 or 15
Peter Lynn 2024
