The delta kite was conceived in the mid-1940s by a Texas architect named Wilbur Green. Deltas are easy to assemble and fly - there is nothing to adjust. They fly to unusually high angles, and owe their popularity to this plus their ability to fly in lighter winds than ordinary kites. The design is versatile, enabling the development of deltas with different planforms, different stiffnesses, and varying degrees of efficiency (for different winds). The two wing spars are not joined at the nose which allows them to move move independently with a self steering action that allows the kite to ride the air currents with a charming, lifelike quality.
Delta kites are non-rigid structures that require a certain amount of wing flexibility (this is crucial), and this plus the fact that the fabric is non-porous means light wind deltas must never be launched in breezy weather. There are deltas expressly designed for moderate, fresh or strong breezes for flying in these conditions.
Most commercial delta kites rely some extra drag along the trailing edges for stability. Some use a fairly flexible spreader, for a bouncy ride (at lower angles) in all sorts of weather.
I fly kites by hand, for the full experience: the feel of the wind through the line, guiding remote kites at high altitudes, finding lift, riding thermals - I don't tie kites down; you may have to move quickly to avoid someone else's line, or respond to a dive from a gust. For me, kites with a lot of drag (or a lot of flex) lack sparkle; they can be pretty dull to fly, especially in light winds. Some are fine in brisk breezes, but flying in windy conditions is devoid of any kite flying challenge, and it's always more chore than recreation. My own preference is for less drag, which allows steeper flying angles, and stiff spreaders, which gives crisp, responsive handling. But the kites end up being less good in rough weather. Scalloped designs do not have that extra stabilizing drag, so more designs are required, each one specifically tailored to excel in narrower bands of wind speeds.
There are a few basic design philosophies which apply to different delta designs for different wind situations. The simplest and easiest is the light frame, for light winds, with a light wind fin and any sort of trailing edge. Then, for stronger breezes, a light wind frame can be combined with a fin with a fresh breeze towing point, which means the towing point is further forward so the kite generates less pull at any given wind speed. Light weight, light wind kites can have less drag (i.e. scallops), for high flying angles, or extra drag (i.e. flaps or aprons), to hang in the air in lulls.
Another line of thinking is to use stout (but heavy) frames for fresh winds. Such kites may need extra drag for stability, usually as flaps on the trailing edges. With their towing points shifted forward as well, they are fine kites for fresh breezes. But, as the delta is primarily a light wind kite, the flying qualities of such out-of-character types can suffer from the inevitable compromises involved, making the designer's task unexpectedly difficult. One obvious approach might be to make the stiff wing spars from a lighter material, permitting flying in light wind as well, but while this sounds great on paper, it isn't easy to actually do in the real world.
A delta for strong winds is almost a contradiction in terms, but there have been a few noteworthy ones. I know of some made with porous fabrics, and those of the late Tony Cyphert of San Diego were said to have been great for strong winds (I wish I knew more about them). My own best strong wind design came about not from a consideration of frames and towing points, but from an eclectic combination of new ideas with an older design that worked. The frame is neither a light wind nor extra heavy one, but the towing point is as far to the front as possible.
In the course of attempting to develop the ultimate or definitive light wind delta kite my family of designs has grown to include not only many light wind deltas, but also, out of necessity, deltas for fresh and even strong breezes. The towing points on the fins are normally set at a mathematically determined optimum position (most critical on small deltas) that is best for light wind. A design shift forward from this position amounts to detuning: pull is reduced for breezier conditions, distorting stresses on the kite are reduced - the kite produces less pull at the same wind speed.
Light wind deltas can be very delicate. They are structurally flimsy; which means they can easily get bent out of shape in a gust. Therefore, practical deltas need to be stronger than all-out light wind ones. The challenge is to design deltas strong enough for moderate minute-to-minute variations in wind yet light enough, and controllable enough, to excel in light winds and thermals. This is achieved through a process of scaling down a series of prototypes for a given wing spar material. Successively smaller deltas with the same wing spar diameter are each stiffer than their predecessor. The process ends as the design becomes too difficult to fly due to stiffness and/or weight. The final design represents a compromise between lightness and strength that gives good handling qualities.
This process, however, does not guarantee a good fresh or strong wind delta. A strong and hence relatively heavy frame is desirable for such types, but deltas become increasingly unstable as the design process leads to successively stiffer frames. They also become unstable as the towing points are moved further forward. Thus both logical factors for strong wind deltas - stronger frames and less pull - unfortunately each lead to instability, and when added together present the designer with some real difficulties. One approach is relaxed (i.e. forward) towing point fins combined with light frames, while slightly less relaxed towing points coupled with stout, heavy frames is another.
When I began making deltas, they were all based on existing designs. My favorite was Bob Ingraham's AKA 10th Anniversary delta, because of its development history. This kite featured a special trailing edge - a flap of material with a series of slots, called by its inventor a "slotted apron." This made the kites very stable, but I had also made a few other delta designs which lacked this feature, and it soon dawned on me that I preferred the livelier flying of those with less drag. I started making kites without the apron. They had snappier handling and in side-by-side tests would go to steeper angles. Less drag became a priority, yielding higher performance at the expense of what I considered at the time to be an excess of stability.
I had been following hang-glider developments, and soon after reading about the first one with a scalloped trailing edge, I saw a lovely 100 degree nose angle delta kite on Parliament Hill in London which had a basic scallop. It was made by the brilliant and innovative London kite designer Len Pradier, who developed among other things the familiar (and much-copied) seagull kite and the first multi-cellular honeycomb-style box kites.
After I'd made a few experimental scalloped designs using adjustable bridles, my kite guru Professor Harold Alexander let me on to a little secret. He and John Loy had been using a convenient mathematical tool for determining towing points on sled kites, and now deltas. It freed me from the standard delta format and allowed me to design fins for deltas with scalloped trailing edges. I made a pair of otherwise identical deltas, only one was scalloped and the other had trailing edge flaps. In side-by-side testing I found I preferred flying the scalloped one. It moved more freely, turned more smoothly, and climbed faster. It was better for the light winds I flew in. Scalloped trailing edges appealed to me. (Of course the kites had to be very precisely made, and the penalty for less drag is less stability in gusts.)
I pushed the scallop to its limits, going deeper and deeper, and also did the exact opposite: I added wing area using one or more battens per wing. This was ultimately a dead end. While adding lifting area, complexity was also compounded. Battens had to be carefully fitted, they were easily lost in the field - they were a nuisance. But primarily, the kites behaved in curious ways.
They had violent and sudden stalls, tightly spinning, wing tip down, winding the kite line around the kite much like a spider winds a web around a captured fly. The extra wing area merely flapped about uncontrollably when attempting certain maneuvers. Eventually I came up with a single-batten design I liked, but it was still not worth the added complexity.
Meanwhile, a strong wind design eluded me. I preferred flying scalloped, high angle light wind deltas, so that's what I designed. But where I now live the winds are not as smooth and are generally stronger than they were in London's Kensington Gardens where I started out.
Bear in mind deltas are really light wind kites; but unfortunately not everybody seems to realize this, and there have been some cases of light wind deltas permanently damaged through abject neglect of this simple fact. I felt it was not only a challenge, but my duty to produce delta kites, if at all possible, that could fly in box kite winds.
So, I resurrected an old design of mine, scalloped it, gave it a new fin, and this became my faithful fall-back kite (the Trooper) for bad weather. It still is. But there was still a medium-wind gap to fill, and the next kite to fill it was the XFS. This design has scalloped light wind wings with a forward towing point fin. Having the towing point moved toward the nose means it pulls less, and less stress is placed on the wings as the breeze increases. I test flew the prototype a full year, on 50lb. line. It's not a bad approach but I felt it was biased toward light winds.
I'd long ago given up on the idea of simply putting stiff frames into deltas; it only made them twirl, spin or otherwise impossible to handle. But it was time to go back to square one and consider stabilizing stiffly framed deltas with some sort of additional drag along their trailing edges.
I was looking for something with a heavy duty frame. To counteract the tendency to twirl, I went back to a flapping apron on the trailing edge. This became what I call my "R" series. They feature stout frames, moderately relaxed (or shifted forward) towing points, and trailing edge flaps for stability. Perhaps I'm just getting old, but I find their solid, unshakeable stability very reassuring. Gone is the anxiety that any minute now a gust might come up and give me trouble. Sure, they tend to fly out rather than straight up, but that's easier on the neck.
Of course I couldn't resist scalloping one of them, which led to my very popular Wildcard design, but my next project is perhaps going to be something along the lines of the first aproned deltas I ever made. I am still looking for that elusive all-purpose delta. The idea goes something like this: strong yet light frame - probably carbon - with stiffness compensated by the flapping trailing edge - plus either an adjustable bridle or perhaps a dual-towing point fin. It's not quite as easy as it sounds; the first ones have been a little too unstable even with the flaps. Handling improves by scaling up, but the spreaders are weakened. The eventual size has to fit available stock sizes of framing materials while still giving good handling, yet not be too weak for stronger breezes.
Deltas are not only technically interesting to design, they're also some of the most interesting kites to fly yet evolved. No other kite in history can glide like a delta under full control and few can fly so high and to such steep angles in light winds. Their performance can be spectacular. They can be guided into and through unusual high altitude wind forms and deliver surprisingly high levels of fascination and wonder.
It's surprising just how good a good kite can be!
The frustration adults often experience when (or associate with) flying kites is mainly due to inferior design and/or construction. Bear in mind commercial kites are made to sell, not necessarily to fly (with thanks to The Popular Recreator, 1872-3). Mine are, at least I try. I make each one as if I were making it for myself. They're built with aircraft precision, thoroughly tested, durable, and, with nothing to adjust are very easy to assemble and fly. All spars are removable, so they're also easy to repair and modify. On the negative side, their flexibility means they can be vulnerable to gusts; hence the need for a range of kites for different winds.