June  2018

Which sounds like a patent claim description, but it's not. This is free; adapt it, copy it, use it as much as you like, and welcome. The problem is simple enough: How to make a self-adjusting bridle for frameless kites so they don't have to be pulled down and adjusted every time the wind changes.

For framed kites, bridles that adjust automatically in response to wind changes have been around for a long time and were a feature of the cellular kites used for lifting instruments in the late 19th and early 20th centuries. In simple form (diagram 2) the rear bridles are spring loaded so that the kite's angle of attack decreases as wind speed increases, causing the line pull to increase at a rate less than the square of the wind speed (which otherwise defines the line pull/wind speed relationship). Frame flexing can also be used for this purpose (deltas and bowed kites for example), but kites that don't have a way to do this won't fly higher than a km or so unless the wind is constant and doesn't vary with height- which doesn't describe anywhere I've ever flown. We've all seen kites let out until their line is dragging on the ground- after which they don't get any higher. This happens because, with no way to limit pull increases as the wind strengthens, line strong enough for gusts has too much drag for the kite to lift at other times. The ideal kite for an altitude attempt would fly at a consistent high angle and have constant line pull across the entire wind range, and the nearest any kite has ever approached this ideal is probably the cellular kites designed by Rudolph Grund for the Lindenberg Observatory in Germany. These used a system that changed the angle between the front and rear cells, which very effectively controlled the increase of pull in stronger winds and also enabled more lift in lighter winds than is achievable with a simple spring bridle. Their altitude record for a train of kites, of 9,740 meters, set on 1 August 1919, still stands.

Grund 42m Lindenburg Kite hangar 2005

So why won't spring loading the rear bridles do the same job for single line ram-air (soft) kites? One reason is that most soft kites are stable for only one bridle geometry; change any bridle by more than a small amount and they lose most of their wind range- or won't fly at all. But there is hope; this used to be true for steerable soft kites, until 20 years of intensive effort by very many designers eventually cracked the problem. Latest ram-air inflated traction kites with substantial de-power (by way of active bridles) are what's driving the amazing developments in kite foil-boarding. But don't hold your breath waiting for single line soft kites to become less sensitive to bridling. For starters, traction kites don't have to be auto-stable as they have a person steering them all the time. And most single line soft kites are theme kites; their purpose is to look like something, and there often aren't options for extending their angle of attack range without compromising appearance. And then there is their lack of rigidity. With framed kites, pulling in or letting out a bridle tends to change the angle of the entire kite. For a soft kite it just pulls in or lets out a small area.

Diagram 1

Single line soft kites have only recently achieved wind ranges that framed kites have enjoyed since pre-history: Invented by Jalbert in the 1950's, until well into the '90's, even soft kite styles that weren't compromised by appearance generally flew well only in mid-range steady winds. When the wind dropped below 15km/hr they stalled and fell from the sky and in stronger winds they generated insane pull and tended to loop out. In this early development period the usual way to achieve stability was to bridle for quite high angles of attack (Doug Hagaman's excellent 1980's parafoils for example). But gradually, starting with pilot style lifters, soft kites have been developed that will fly stably when bridled to low angles of attack- so as to fly in lighter winds and not pull too much in strong winds. Some themed soft kites are now equal to the best pilot styles and are challenging even specialist light weight framed kites to be last down when the wind drops. Smiley Faces, Rays by PLK and Andreas Fischbacher and Mr Ma's Trilobite, are notable examples. These perform very well pilotless at show kite events where the challenge is usually at the light end, while still being able to handle strong winds provided lines and anchors are up to it.

Diagram 2

But for kites that are required to either lift payloads or fly very high, low angle of attack (called flat) bridling is not the entire answer: Such kites do need flat bridling when the wind is very light, but as soon as there is adequate wind, the bridling angle needs to be increased to generate as much lift as possible. And when the wind increases to 'more than enough', the bridle angle needs to be reduced again so as not to break the line. For this, active bridles are required- and one such is shown in diagram 3: As the wind speed increases from zero, the front (weaker) spring is first to extend, increasing the kite's angle of attack. When the wind then increases further, the rear (stronger) spring progressively extends until the bridling returns to a low angle of attack setting.

Diagram 3

PLK uses half of this system on OLO Octopus kites (diagram 4). Octopus kites were our second completely soft theme kite design (1990, Rays were the first in 1988). To prevent looping type instability, their front bridles are rigged with bungies so as to lengthen as the wind strengthens- the kite doesn't then have to be pulled down and adjusted whenever the wind changes. This gives them good enough light wind performance but more pull than I'd like when the wind is up. Fixing this will require an acceptable way to make them stable at lower angles of attack in stronger winds. ('Acceptable' to me means not just adding drag. I know, picky!). Unfortunately, this type of spring bridle system can't be used with most styles of soft kites because, unlike the Octopus, they have neither sufficiently robust leading edges nor long heavy tentacles to prevent the nose suddenly folding down when the wind momentarily drops. Called luffing (when a kite's angle of attack becomes negative, causing it to dive forward), this sets the minimum angle a kite can be bridled to. Luffing is not a trivial problem, especially for soft kites, because it usually also causes the inflation opening to close, deflating the kite and making recovery even more unlikely. I've been looking for a self-adjusting bridle system that doesn't trigger luffing for 30 years.

Diagram 4

The problem is that frameless kites, and especially the single skin types, are very sensitive to front bridle lengths: If these are any longer than absolutely necessary, the kite's light wind performance will suffer. But if they are even slightly too short for the wind at any moment, they'll luff uncontrollably- and spring loading exacerbates this: Say a diagram 5 kite is flying stably in stronger winds with the front spring stretched out- the kite's leading edge being held out and up by wind pressure. Whenever there is a momentary lull or down-draft, the spring reacts immediately by contracting, which causes the kite's leading edge to fold down- and instead of a kite there's now a disorganised heap of fabric falling from the sky. I've tried 'slow' springs (devices that extend fast but contract slowly), separate and closed-off leading edge inflation, mechanisms to lengthen the front bridles whenever a luff seemed imminent, and even a few attempts at angle of attack sensing via centre of pressure migration - including one using lever mounted bridles, which did sometimes seem to work. I have even considered using sensors, servo motors and a power source. This would work, but similar systems were generally unsatisfactory when used for steering pilot kites, because the weight of batteries and mechanism seriously degraded the kite's light wind flying, and battery life caused the kite equivalent of electric vehicle range anxiety. Finding an answer became even more imperative since I've immersed myself in single skin kite development, as single skin kites are always closer to stalling and more susceptible to luffing than any ram-air kite.

Diagram 5

At last a solution, whew; (diagrams 5 and 6), and it works perfectly, changing the front bridle length only (well not only, but near enough to) in response to the amount of tension on the main bridles, so doesn't cause luffing. Spring rate, length, and limiters are used to arrive at the precise dynamic response which works best for each kite style. The only problem I've had so far (10 or so versions) is with the bungies used on a 6sq.m 1Skin version stretching out- which can be avoided by using purpose built helical springs when I'm sure the parameters are optimised.

Diagram 6

Unfortunately, I can't claim the development of this mechanism as evidence of either cleverness or persistence: If I was even half clever it would have been working years ago, saving myself and other kitefliers a heap of grief. Nor is it the inevitable fruit of persistence, as I'd pretty much given up looking, having concluded (wrongly as it turned out), that what I was trying to do was not possible without a power supply. Luck then? To what else can I credit an unbidden middle of the night idea that, for once, was still a good idea in the light of day?

Single skin Serpent and Octopus with auto bridling Berck 2018

It doesn't mitigate line pull at the top end so is not useful for high altitude kites, and it's not necessary for the few designs of soft kites that are fixed-bridle stable across the range (Pilots and Rays for example). But it will improve the light wind flying of all other ram-air kites and works best of all for single skin kites, eliminating all bridle adjustments for Serpents, Octopus's and 1Skins- that until now required up to 5 different settings.

Peter Lynn, Ashburton, New Zealand, June 1st 2018