In 2013 I began developing single skin single line kites:
Kites that would be reliable enough in a range of winds to be useful as pilot and show kites at kite events but don't have sticks, inflated tubes or ram-air inflated spaces,.
It hadn't been done yet, was probably possible, and was a project I thought myself well suited to by resources and experience- somewhat hubristically as it's turned out.
I've now put some 1000s of hours into this and it's time for an accounting as to where things are at.
NOT MUCH PROGRESS!
Certainly not enough to justify the time and resources applied.
I've been a magnet for lost causes: Kite sailing, another developmental field I've put inordinate effort into, has sucked up a fair portion of many people's lives without achieving commercial success- though the spin-offs of kite buggying, kite surfing, foil boarding and snow kiting have been pay backs.
And yet another field I've been involved with; Stirling cycle engines (named after the Reverend Robert Stirling, who's first patent in the field dates to 1817) is similarly afflicted. Stirling engines have had billions of dollars (yes, really) spent on them by thousands of deluded engineers, but have so far only found minor niches such as desk top toys and as a marginally effective alternative to nuclear power for submarines.
Oh, and in my spare time I've become a sawmiller, operating the ex Hoberg family (Volker Hoberg is one of the infamous No-Limits kite team from Germany) vintage reciprocating sawmill which I brought to NZ 10 years ago and have now rebuilt. It cuts wonderful big slabs of exotic and special timbers for artisan furniture makers and wood turners, but takes as long as 45 minutes to make a cut that a modern bandmill can do in 15 to 20 seconds.
Single skin pilot kite (and monkey) by Robert van Weers
Hoberg family vintage sawmill at Ashburton
But back to the single skin (lack of) progress report;
Last week I had a reality check:
At this year's Nelson Kite Festival (number 29) the wind was so strong and turbulent that no maxi kites were flown until very late in the day, by which time I'd left to come back to Ashburton.
I tried flying some single skin pilot kites. Most of these were of the 1Skin style, which has been intensively developed through 50 or more prototypes and can be very useful in low to mid-range winds because of their excellent lift, high flying angle, and low cost. 'Out of the bag', none would stay up. In the strong gusty winds, they sooner or later dived off unrecoverably to one side or the other. This was neither a surprise nor a new problem, but I had thought it would have yielded to accurate construction, and relentless iterative development by now. It hasn't. But then again, as the ditty says; ."any problem worth attack will prove its worth by fighting back" and this is exactly what I signed on for.
Taking a step back, 'diving over' almost certainly has as its fundamental cause that the 'structure' of single skin kites (fabric), is unable to resist compression loads- any compressive load anywhere and they buckle.
This can be seen when conventional parachutes are towed behind boats as parasails. To gain height, the bridles on the side nearest the boat (called the leading edge) are pulled in a bit. This enables the parachute (and person) to rise to not much more than 30 degrees before the leading edge gets pushed in. When the pressure of air pushing on the outside is greater than the pressure 'captured' inside the parachute, the fabric buckles- because it has no capacity to resist compressive loads.
But to be useful, single skin kites need to fly at a line angle higher than 30 degrees- and this is the single skin kite challenge.
1Skins are shaped like an aircraft wing, except their lower skin extends only a short distance back from the leading edge; typically 5% to 10% of chord. This is aerodynamically efficient (that is, enables the kite to fly at a high line angle) and is the form used for high performance single skin traction kites.
But at high flying line angles, wind pressure buckles this type of leading edge inwards- which single skin traction kites typically counter with stiffening rods (weedwhacker cord or flexible carbon fibre rod).
For single line versions I'm restricting myself to what can be achieved without stiffening of any kind- with some thoughts that this might then feed back to single skin traction kites and kite sailing.
For 1Skins, when the leading-edge buckles in asymmetrically, which it always does sooner or later as wind speed increases, the kite dives off to one side. A second effect is that the wind pressure pushing on the leading-edge can buckle the fabric further back along the kite's skin- which has the same result.
1 skin 6sq.m showing leading edge buckling
1 skin 6sq.m with bungy and pulley bridle mechanism
Single Skin Pilot, prototype number 97
But there is a form of leading edge that is more resistant to crumpling- and better able to pop out again when it does. This is the bridle-supported leading edge used on the Octopus and Serpent style single skin show kites I've also been developing. The Octopus (not to be confused with the ram-air Octopus from 1990) was my (the?) first successful single skin show kite design: Successful as in flying reliably in a (very) wide wind range. They fly at a high line angle and don't suffer from the 1Skin's 'diving over', but have a problem which at first didn't seem serious: To fly satisfactorily their tentacles must be crosslinked with multiple cords. These provide necessary extra drag and stop the tentacles from tangling into big balls that take hours to untangle. I've not found any other way to stop this tangling, but these cross-links snag on things, damaging the kite and posing a danger to everything in the vicinity. An alternative form that doesn't have this problem is the Serpent, but these have a slightly narrower wind range because their single tail is heavier and doesn't have as much drag. They're improving at every iteration though (taking lessons from the Balinese Janggans), and at < 10km/hr to >50km/hr are adequate for general kite festival flying. It's especially satisfying that the 60m version flies at least as well as the 20m it was scaled from, while being 4 times as big by area. Fabric stiffness is not therefore a factor; a goal achieved.
Supporting this design are two developments which have general applications for single skin kites and indeed, for many ram-air inflated show kites:
The first is a bungy and pulley mechanism that automatically lets out some or all of the leading-edge bridles as wind speed increases to prevent leading-edge collapse and extend the kite's wind range without having to change bridle settings.
The other is 'aeolian' bridling. This is the use of braids rather than cords for a kite's rear bridles. When flying at high line angles, braids of suitable length, width and weight, flutter harmonically, which causes their drag to increase rapidly. By doing this they pull down on the rear of the kite when the wind is stronger, offsetting extra nose-down bridling that enables lighter wind flying. The result is a kite with wider wind range. A worthwhile feature is that when the flying line is horizontal, braid bridles lie in the wind direction so don't apply much if any extra pull to the rear of the kite. This reduces stalling and makes launching easier. But when the kite is flying high in strong winds, they hang transverse to the air flow in full vibratory mode- applying 10 to 20kg extra pull to the rear bridle points (in the case of a 60m Serpent), which prevents overflying and reduces leading-edge collapse.
20m Single Skin Octopus at Cervia 2017
20m Single Skin Octopus With Aeolian Bridles
60m Single Skin Serpent
So, single skin single line pilot kites are limited to light wind use at present, but there is now a single skin single line show kite that can hold its own at any kite event.
Will single skin pilots ever seriously challenge ram-air inflated styles for general use?
Two reasons to think they may:
Bridle supported leading edges from the Octopus/Serpent style can be grafted onto 1Skin style kites, which will address one of its problems (though there may be a few others lurking there too).
And, quite a few other kitefliers are engaged in this development now- their fresh thinking can only be helpful.
Making an all winds, tailless, single skin single line kite remains a formidable challenge.
But it is far from being a lost cause.
Peter Lynn, Ashburton, 1 February 2019
From 2014 to 2017 I ran a 'stream of consciousness' account of this development at:
which I stopped when some coherent theories began to form, intending to write up a retrospective analysis once I had a few more things nailed. I still intend to do this, only held back by concern that numerical modelling may soon get sophisticated enough to make my ramblings irrelevant.