Peter Lynn ARCHIVE
Forty five years ago I was a mechanical engineering student, intending, on graduation, to change the world for the better in some significant but as yet undetermined way.
Just like all of you are now.
Then I was seduced by kites.
Actually, I'd been keen on kites since a very young age- some of my earliest memories are about making and flying kites. My mother showed me how to make paste from flour and water and indulgently allowed me to make a big mess with bubbling pots, paper scraps, nests of tangled line, wood shavings and discarded sticks everywhere.
As an adult, my interest in kites re-ignited about 1971 and with all my newly acquired knowledge (graduated 1969) I expected to completely revolutionise and re-invent the world of kiting in 12 months or less- and then go on to do something useful and remunerative.
This didn't happen. I soon discovered that I knew a lot less about things than I had thought- and that kites were much more complex than I had realised- and that those who had developed the various kite styles that I'd known since childhood were much cleverer than I had assumed.
So, back down to earth I came, and ever since then have been trying to understand what makes kites fly and make some that do- with a few successes, but many more failures.
Since 1980, I've mainly worked on "ram air inflated" type kites. These are entirely soft with no sticks or pump pressurised elements, using only the stagnation pressure of the airflow to inflate internal spaces and cause them to take their form. This approach was initially developed by Dominic Jalbert (USA) in the 1950's and has spawned utility single line kites (usually called parafoils), traction kites (including the bridle-less Arc kites that I developed for kite boarding), sky diving 'chutes for jumping out of aeroplanes, paragliders for jumping off mountains with, and themed single line kites with, typically, sea creature and animal shapes.
I've spent some of the last 30 years designing and building large themed single line kites, including five very large kites, two of which have held the Guinness record for the world's largest kite. On Feb. 23rd in Kuwait a new Guinness record attempt kite that I've developed is to be launched for the first time. It is 1250 sq.m (my previous record holders were 635sq.m and 1019 sq.m). This new one is in the shape of a stingray (and its wingspan is 65m).
But by 1987 I had become particularly interested in traction kiting; the use of kites instead of sails for moving things by wind power. I thought then that kite sailing (for ocean going rather than dinghy sized yachts) could become a popular mainstream sailing activity alongside conventionally rigged craft within a few years. I was overly optimistic about this also; it hasn't happened.
Yet!
Kites have a long history and have probably been used for traction for most of that time. The inhabitants of Sulawesi (now part of Indonesia) had the necessary ingredients more than 10,000 years ago. There is a type of large leaf growing there that flies well as a (single line) kite without modification- and they certainly knew how to make lines to fly them with. They also had a reason to develop kite flying; for fishing. Kite fishing from canoes has been an activity there since before recorded history and their kites would have pulled the canoes along whether they wanted this or not. From linguistic and artifactal evidence, Polynesian migrations that started from that area more than 3000 years ago spread a kite culture. Hawaiian rock drawings and other evidence from Polynesia show kites being used in a purposeful fashion to pull canoes from island to island before Europeans arrived in those waters.
More recently, in the 1820's, George Pocock, a schoolteacher from Bristol in England developed kites for pulling horseless carriages and boats. He used large framed four-line controllable kites supported by smaller stable single line "pilot" kites (so that the larger kites could be 'fluttered' to reduce pull without crashing) and was quite capable of sailing upwind on land and water- and did both.
Kites as sails have considerable advantages; Unlike sails on masts, they can be attached to craft so as not to generate any heeling moment. They can also access stronger and steadier winds available at higher altitudes. And their speed through the air is to some extent independent of the craft's speed- so they can take advantage of this 'apparent' wind speed to generate much more pull for size than a sail on a mast can.
Why then didn't traction kiting take off in George Pocock's time or before? - and why do the overwhelming majority of yachts (water, land and ice) still use sails on masts rather than kites?
There are two reasons;
The first difficulty is that kites as sails are connected to the craft they are propelling only by tension elements. If the kite ever luffs (assumes a negative angle of attack) or the wind fails momentarily, the lines go slack, and the kite falls out of the sky.
The second is a disadvantageous aspect of the apparent wind advantage from above: Any kite with sufficient efficiency (measured by lift to drag ratio, L/D) to match conventional sails will occasionally accelerate to a speed through the air determined by the relationship: Kite velocity= L/D x true wind speed:- And will then develop 15 or more times the pull that it had moments before while flying steady state. This either: breaks the kite, the lines, the craft or the flier- or all of these.
As George Pocock knew 180 years ago, the answer, of course, is to develop some pull mitigation system- commonly called depower, reefing or sheeting.
It is possible to rig kites so that pull can be dumped by reducing the angle of attack to zero but retaining full control while in this mode is fraught.
A difficulty (but not the only one) is that kites that use the standard four (or sometimes three) lines attached to a bar or handles for steering and de-power suffer from 'soft' steering or even control reversal below about 3 degrees angle of attack. Alternative steering systems (such as the use of rudders) could solve this, if the large user base now established can be persuaded to accept change- but will they?
And, unfortunately, George's solution (flying a stable single line kite above the steerable kite to keep it in the air no matter what), reduces the overall L/D to an extent that makes such compound systems uncompetitive with conventional sails.
For minimalist kite traction activities such as kite boarding, kite skiing and kite buggying, kites are harnessed to the flier not the craft, so large amounts of de-power are not absolutely necessary. Kite boarding developed successfully as a sport for more than 10 years before effective de-powerable kites became available.
And the bridled 'foil style kites used for top level kite buggy racing still (2011) don't have any significant de-power capacity at all- because the performance loss from available de-power systems exceeds the gains from not having to be so precise with kite selection. (For different wind strengths, competitive kite buggiers will have a 'quiver' of, say, 10 different kites ranging in size from 2.5sq.m to 15 sq.m or more).
Fliers in these sports become adept at coping with brief periods of over-power during wind gusts by sliding or skidding a bit, or by jumping- and in fact for kite boarding, 'getting into the air' is often the goal.
However, for larger scale kite traction activities- like pulling yachts and larger ships- kites must necessarily be attached to the craft not the flier, and substantial pull mitigation systems are therefore essential. A 5-tonne yacht might require something like a tonne of kite pull in steady state upwind sailing in winds of 10m/sec to be competitive with conventional sailing craft. When 20m/sec gusts then come through (and gusts of twice the average wind speed are commonly experienced), without any de-power system the kite's pull will almost immediately boost to 4 tonnes- and to much more than this if the kite's apparent wind speed also increases. At this point the yacht's safety will, to say the least, be compromised- if the lines, the kite or the attachment systems don't break first.
Fortunately, by benefit of the intense development that has gone into kites for kite boarding (now perhaps a $100 million/year market) there has recently been substantial progress towards fully de-powerable kites.
By about 2006, "supported leading edge (SLE)" versions of LEI (leading edge inflatable) style traction kites developed by Bruno Legaignoux, achieved close to 100% de-power, albeit with substantial loss of steering and occasional luffing crashes when fluttering.
For ram air traction kites, substantial de-power is also possible.
One approach I've been working on for bridled 'foils is what I call 'adaptive profile' (AP) . By varying the kite's airfoil form (that is, changing the camber) precisely as the kite's angle of attack changes SLE/ LEI levels of de-power can be achieved. The limit with this system (as for the SLE/LEI's) is the drag coefficient of the kite when it's at zero angle of attack. But, to date, the maximum de-power I've seen from an AP system while retaining full control is about 60% - and there are still problems with heavy bar pressure.
Another is the "Slarc" system. In this, a standard four or three line Arc style traction kite is rigged to fly stably as a single line kite with the minimum possible pull, and then transitioned, at the volition of the flier, to full traction kite mode. There are some problems with behaviour in very light winds, but this system also shows promise.
Other designers use various pulley systems to change the bridling on 'foils in response to flier input. Gradual improvement year by year has taken this approach to nearly 50% de-power, and these kites now have a strong market share in some sectors (especially for kite skiing and kite mountain-boarding).
To what level of de-power each of these systems can be developed while retaining adequate control remains to be seen but even without much improvement from where they are now, there is a willing market because LEI style kite boarding kites are not very suitable for ground-based traction kiting activities such as kite powered mountain boarding, skiing and buggying. This is because their inflated envelopes are easily punctured by scraping along the ground. Except for water based activities therefore, ram air inflated traction kites seem likely to retain a dominant position even if their de-power doesn't improve much beyond 50%.
And for kite sailing (propelling large yachts), ram air inflated kite styles also appear to have an inherent advantage because of their easy scalability to 1000sq.m or so, which it's not currently clear that LEI's will be capable of. But if this market is ever to develop, nearly 100% de-power will be required- and the ability to stay up in very light winds.
Anyway, while working away at these challenging and interesting problems I've also made some things for kites to pull:
While working on kite boarding development in the late 1980's, I fitted wheels to a seat equipped with three water ski's that I was trying to get working on the water so as to enable development to continue while the lake was frozen- from which somewhat serendipitous event, the modern sport of kite buggying emerged- now an international sport with many thousands of enthusiasts. Most kite buggying happens on beaches and on dry lake beds (especially in the US and Australia)- and having yesterday been kite flying on the amazing Kutch salt pan (northwest India), I wonder why kite buggying has not yet become a popular activity there.
Then, starting in 1987, I filled the long grass in our backyard with some hundreds of attempts at developing the perfect kite boat. An eventual design, the "KiteCat", in it's current form by about 2005, has entirely satisfactory performance - speed, upwind course, ease of use, portability, and so on, but has so far failed to find commercial success. I expect that this failure is because kite sailing does not match the spectacle and drama of kite boarding, nor is it as easy to do as conventional sailing (with a mast to hold the sail up when there's no wind). That kites fall into the water during wind lulls , has so far been a terminal put-off for potential mainstream sailing customers.
And, from the first years of kite buggying, skis have been fitted to buggies in place of wheels for use on ice and snow. Unfortunately, this doesn't work that well because, for steering, the ski's have to scrape sideways a bit- which can more than double drag. Short wide ski's work a little better in this respect but have more drag for same area than long narrow skis when running straight. A solution I've developed is to use long narrow skis that bend laterally for steering. To enable skis that are wide relative to their depth to curve enough for acceptable turning radii, each ski is made up of multiple longitudinal elements that can slide relative to each other. The use I had in mind is a small but interesting niche; kite powered journeying across the Arctic, Antarctic, Greenland, and Siberia. Because sleds equipped with this type of ski (I call them KiteSleds) are able to carry all supplies on board, (adding to their ability to resist kite pull), performance can be better than kite skiing while pulling a sled, which most journeyers have used up until now. I'm not expecting any fortunes to be made from this though.
I have also continued to develop ram air single line utility kites.
Contrary to what might be expected, single line kite design is much more difficult than traction kite design. It's so difficult as to be virtually impossible in any conventional sense. I don't see any chance that NEW styles of single line kite will be able to be designed to any significant extent by quantitative analytical processes in the foreseeable future; trial and error requiring many prototypes and serial failures of pet theories seem to be the only path for now.
And there is no mystery as to why this should be so:
Single line kites have to pilot themselves- by a complex interplay between weight, inertial and aerodynamic forces - made all the more impossibly complex by the inherent variability of wind, turbulent flow and non rigid surfaces. Maybe it was physicist Arthur Eddington who said, "When I get to heaven, I have two questions for God; what's behind Quantum mechanics? And how does turbulent flow work? I don't expect there's an answer to the second."!
However, some principles and relationships that are of assistance in the development of single line kites are gradually becoming clearer to me- not yet to the extent of being quantitative rather than just qualitative, but getting closer.
And I'm really enjoying this- adding a few bricks to the edifice that might eventually become a definitive theory of single line kite stability while coming up with a (very) few successful new designs that may endure along the way- and getting to fly kites in exciting new places like Kutch and Ahmedabad as well!
And there is research now going on that could, in one hit, provide a solution to almost all the challenges I've so much enjoyed attacking as an engineer during the last 40 years.
Kite energy projects; the possibility of generating electricity at commercially competitive cost by various kite powered means, have attracted considerable investment in the last 5 years. As part of these projects, most teams are now working on rigid carbon fibre aeroplane form kite which are to be autonomously controlled. The practical problems they face are enormous- not least being how to ensure the safety of people on the ground below when there are large, tethered aeroplanes whizzing around overhead at 200km/hr or so generating enormous line and structural loads and completely at the mercy of fickle wind.
And the auto pilot systems are causing no end of grief. Confident assertions from many of these projects in their early days that this was a simple enough problem which would soon yield to good systems engineering have so far proven to be optimistic- though progress is undoubtedly being made. But even if none of these projects succeed commercially in this round, the data base and systems they develop is inexorably adding to the sum of things known about kites.
And I do expect that single line aeroplane style rigid kites with electronic controls could
soon be a reality for recreational kiting- even if they aren't developed to a sufficient level for commercially reliable kite electricity generation for some years yet.
Kites of this style achieve all the things that I dream of; they're ideal single line kites (capable of previously unachievable L/D's and of flying reliably in the widest possible wind range), and they are the best possible traction kites. With such kites, kite boarders will go higher and stay up for longer than even the bravest amongst them desire, and kite sailors will leave conventional yachts in their wakes on every point of sailing.
Bring them on I say!
Thank you for the opportunity to talk with you all and I hope your careers provide as many enjoyable challenges as mine has -but are attended by a few more successes!
Peter Lynn, Ahmedabad, India, January 14th, 2011.
