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Kites don't all behave uniformly the same. Some are strictly utilitarian; some are powerful workhorses; some are as light and delicate as feathers; some are racy. A super-light delta with long, thin and bendy wing spars simply can't stand up to harsh conditions and vice-versa. You can't simply fly any delta in any wind. They may all look outwardly similar, but design differences determine the operating envelopes for each. If a single design did it all, there would be only one design in the catalog!
Different regions have smoother or rougher winds at different times of the year and it's a question of zeroing in on the right combination of kite and line for a given set of local circumstances. A cardinal rule for successful kite flying is to always, always, choose the right kite for the wind. When I was a kid it was common knowledge that if you wanted to fly a kite in strong winds you needed a box kite. Experienced kite flyers take a selection of different kites with them whenever they go out flying. That's how they make it look easy. They choose the right kite, present it to the wind, and let the wind take it. No fuss. The wind does all the work.
It's not always easy to predict in advance conditions on-site or aloft, so you should have at least a light wind kite and a medium or strong wind kite. They don't have to be deltas, but they could be. I always have a strong or medium wind kite (or both) along with me just in case.
One of my customers has over 30 of my kites in all types and sizes. He keeps meticulous, detailed logs for each kite and every flight, including weather data from three local airfields, flight duration, and beautifully hand drawn 3-D sketches of interesting flight paths. At the end of each year, he totals up the hours flown for each kite(!). Even though he lives in London, where the winds are a lot gentler than here, he found that the kite with the most air time (over 300 hours per annum) was the Trooper, my strong wind delta - more than double the number of hours of the second most flown. If simply not flying when the weather's not ideal isn't an option, you need a selection of kites to cover a range of winds.
Planform is discussed elsewhere.
Scalloped trailing edges have less drag - for the highest flying angles, quicker turning, and livelier handling - by sacrificing some wing area.
Trailing edge drag stabilizes. Deltas with straight trailing edges are more stable; lifting area is maximized.
Deltas with flapped or fringed trailing edges are steadiest of all, smooth flying, and able to hang in the air in lulls.
Nose angles: in general, narrow (90 to 96 degrees) nose angles give a wider wind range; while wider ones are primarily for light winds. Deltas with narrow nose angles are more reliable and stable. (Not to be confused with aspect ratio.)
The most crucial design feature is the towing point.
Towing point position determines wind range, efficiency, handling, stability, rate-of-climb, and the amount of "belly" in the line. There is an optimum position (50%; see below) that gives excellent pull combined with the best overall handling and stability. Because it gives the most pull, it gives the highest possible flights at the best angles with the least belly in the line; it is strictly for light winds, though - because it puts maximum load on the kite, it's most likely to over stress the kite as winds increase. (Kites for strong winds need to be designed differently, to cope with a more complex cocktail of forces.)
At the opposite extreme is the forward towing point. The towing point is positioned as far forward as it can be before the kite becomes too unstable to fly. Here the pull and, therefore, stresses on the kite, are the least. Rate of climb is fastest, but the kite reaches its service ceiling soonest, that is, it starts to drift downwind sooner, rather than climbing further, and might not "hold" once it's been worked up into a light wind. Because the pull is less, there will be more belly in the line in light winds, but since kites with forward towing points are for strong winds, this point becomes moot. Deltas with forward towing points are extremely useful. Though the pull is reduced, in strong winds there will be plenty of pull regardless; and they can also still manage to fly in light winds, albeit at lower angles. I know of two famous kite designers whose deltas are exclusively forward towing point designs. They are good kites for the average person to fly in practically anything, but the ripstop delta was evolved as a light wind specialist design; forward towing point deltas can be too much like hard work in light winds.
In-between towing points are used on some of my kites. The R-series kites, for instance (R4 Ranger, R5 Rustler, R8, R10), have heavier frames to cope with fresh winds, with a moderately forward towing point; the frames cope with the breeze, the towing point doesn't sacrifice too much in terms of pulling power or handling. The XFS Delta has a more forward towing point on a light wind frame. The towing point in this case reduces overall pull, so the kite can fly in higher winds, while the wing can still catch lift in light winds.
At this point you might be thinking adjustable towing points seem like a logical thing to do. The Shifter Fin is an option on some of my designs,
those with a frame that suits it. I can design them for other designs, too.
Shifter Fins are simply a refinement of an old experimental method where the best towing point is an unknown. There is a short adjustable on a short extra stick along the bottom of the fin. The refinement is in the mathematical calculations of the possible extremes of adjustment, so the fin can't easily be set at an unstable position. This range is usually 18% (forward) to below 50%.
I have been asked why I don't put a couple of extra eyelets on either side of the towing ring on my fins, as other manufacturers sometimes do. By now you will have guessed I locate my towing points precisely, mathematically, for very good reasons, and a point to either side would be wrong. Small deltas only fly on the one correct towing point. Conversely, on big deltas, where there is a much wider range of possible towing points, a small horizontal shift of a half inch or so either way wouldn't make a significant difference. Another thing is that in order to stress the fin as a web, nice and flat, the forces are carried from the towing ring along the edges of the fin; if the fin is to be tensioned evenly, the flying line has to be attached at the point of the fin, not to one side of it, where it would mess up the structural integrity of the fin. I used to make a dual-towing point fin; it works very well on big standard deltas with stout frames. The second towing point falls halfway up the front edge of the fin, and there are reinforcing strips to carry the loads through the fin so it remains flat. I used various methods to stop the untensioned part of the fin from flapping. Some kite makers go as far as putting two separate fins on deltas, one for light, one for strong winds. The problem with all these is that a light wind delta really should have a light wind frame, and vice-versa; the strong wind one would be better with a strong wind frame. It all gets a bit untidy.
(But add V-cells or a tunnel keel to a pair of delta wings and a bridle becomes a necessity! There's no design challenge here as far as figuring out the towing point is concerned - the problem is structural: the spreader struts end up being overly long.)
The towing point (picture) is defined as the point along the center spine vertically under the tip of the fin, where the flying line is attached. It determines a kite's pull and therefore its efficiency in light winds (or otherwise). It also contributes to stability. The towing point combined with frame stiffness determines a kite's wind range. Most of my kites have a light wind towing point position. This is illustrated in the chart below. Kites with TP values smaller than 45% have a tolerance for stronger breezes. Some of these kites have stiff, heavy frames combined with moderately relaxed towing points; one has a light wind frame combined with a somewhat more relaxed towing point; and one has a very relaxed towing point. This is the mark of a strong breeze delta.
"Relative" towing points are a means of comparing towing point positions on a variety of differently shaped kites as if they were a percentage along the spine of a standard delta.
Solid colored kites are best for distant visibility. Black shows up best in the far distance against any daytime background. Dark orange is next best for visibility, followed closely by red and bright pink. Bright white is highly visible in a haze-free cloudless sky. Purple shows up well in any conditions, as does royal blue and, to a lesser degree, green. Light greens and blues show up against a cloudy sky, but disappear otherwise, except when the light catches them. Pale yellows and greens can be washed out in strong light.
If you already have line, that will dictate the size range of kite(s) you're after. If you don't have line, you might consider an assortment of smaller deltas covering a range of winds rather than going for, say, one large fancy specialist delta suited only to a narrow range of winds. Smaller deltas cope with a wider range of winds and fly just as high as bigger ones, with the added advantage of quicker recovery from upsets. Without the tedium of handling line under heavy tension, you're free to enjoy flying the kite. And a basic reel can hold a great deal of light line. Proportionally lighter than bigger versions of the same design, even conservative-looking small deltas can have very high angle, high altitude capability. For those conscious of the official height restrictions for kites, a mere 60 meters (about 197 feet) in the UK, small deltas offer the experience of flying high simply because they look higher than bigger kites do at the same height. It is in fact quite difficult to tell the height of a lone kite. The smaller it appears to be, the higher any kite looks. Thus small kites give the impression of being higher than they actually are, while very big kites don't look as high as they are. When bystanders enquire as to the height of a kite (as they will), it is often more helpful to simply give the kite's span, to put it into perspective.
It's a considerable advantage of small kites that a great deal of light line can fit comfortably onto an ordinary fishing reel, conveniently mounted on the end of a fishing rod.
More big kites
As mentioned earlier, the key to successful, trouble-free kite flying is to choose the right kite for the wind, whatever size of kite. The size of a kite has little to do with its wind range: it's the towing point and frame stiffness. In other words, all sizes of the same design will have pretty much the same wind range. (Actually, the smaller versions of any given design have better wing loadings because of the way frame densities increase as kites are scaled up. Bigger versions of any given design will be harder to stabilize because increased mass means more inertia, or momentum, so it becomes increasingly difficult for the aerodynamic forces to correct a big kite when it needs it. That's why carbon frames are useful in larger sizes of certain designs - to redress the mass problem. But the wing spars also act like the balancing bars of high wire walkers, and so there are trade-offs in switching to carbon wing spars in deltas: the big heavy kite may be preferable as an all-round flyer!)
Normal day-to-day winds exert a wide range of forces. Designing a single delta to perform well over the spread of winds from light to strong is almost impossible, and involves trade-offs and compromises. As a rule, the more radical the design, the narrower its wind speed band and vice-versa. By all means fly high nose angle, high aspect ratio deltas in ideal conditions. Light wind flying - surfing a gentle ground wind searching for a boost, grabbing barely perceptible puffs of lift, hunting for thermals and riding them, spiralling overhead - can be the ultimate kite flying experience. But if you want or need a wide wind range, go for a conservative design. Look for designs covering wind ranges like medium to fresh, or light to medium. And be wary of designs advertised for light to strong winds. There is such a thing as a jack-of-all-trades and master of none.
For load lifting, the same rules apply: choose the right kite for the wind as well as for the job. But bear in mind that a light wind kite, regardless of how big it is, may not generate much pull in a light wind. Ideally, use a medium-wind design and fly it in a steady medium breeze. If the wind picks up, a large kite can become a serious handful, and indeed may require more than one pair of hands. The stronger the wind, the less need there is for a big kite, but strong winds also be choppy and turbulent, swinging a camera rig all over the place. Also, as the pull quadruples for each doubling of the wind speed, in a variable breeze the payload can go up and down more than you might have been expecting. (The pictured kite has been in constant almost daily use for over four years.)
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