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Shortening Spreaders on Troopers for Strong Wind

Trooper history

The original kite that ultimately became the Trooper was a light wind kite with long, thin ¼" dowels for wing spars. I called it the Zephyr. Its plan is on page 9 of my first book of drawings, below a design based on a Jack van Gilder delta modified by Bob Ingraham (A.K.A. founder member and editor of "Kite Tales") to become the A.K.A 10th Anniversary Delta, and modified further by me. This was in 1974.

Having acquired two lengths of a new ¼" fiberglass rod, and wanting to try them in a kite I put them into the Zephyr. At that time I hadn't yet started using the proper mathematical formulae for calculating towing points, so by coincidence this kite had a towing best suited to strong breezes, and also by coincidence I made spreaders quite a bit looser than I would later for light wind kites. All the delta plans I'd seen to that point in time had relatively long wing spars, too. By that I mean long in proportion to the overall length of the leading edges. All this together made a good strong breeze delta. It had straight trailing edges, no scallops, and no flaps.

About 10 years later I resurrected the design, putting a double-walled pop-fin on it just for the extra strength. The then "Tempest" had wood spine and spreader, with a mini-fin on top to stop spreader flexing and breakages. (Spreaders bend and break from the buckling load applied at the pockets by the wings - the same as if you were to push down on a long rod hard enough to make it bend.) The original towing point was revised only to take into account the scalloped trailing edges. The spreader clearance was reduced some.

I wasn't satisfied with the design's landings, and improved this finally by shortening the wing leading edge spars. This meant the kite didn't feel quite as solid in strong winds, but not far from it. It became the Trooper. The Tempest could be scaled up to about 10 foot span, but not so the Trooper. The difference was the length of the wing spars. The biggest ones needed the longer length of the originals (but were no easier to bring in for landings).

Originally, Tempests had 5/16" Ramin dowel spines, and 3/8" spreaders, later changed to 7/16." The current Trooper has a fiberglass center spine to lower the center of gravity, and an over-engineered carbon spreader that's lighter than the original 7/16" wood. But the fit is not as loose as the original Zephyr's.

Switching from wood to carbon for spreaders means the supporting Flex-Stop is no longer required, which is just as well, because fitting carbon spreaders is quite a bit more difficult. Wood is easy to trim, so fine-tuning the fit is straightforward and quick. Not so for carbon! Where a sliver of wood could be sawn off, carbon has to be filed, because the cutting tool is much thicker than a mini-hacksaw blade - thin slices aren't possible. (And then there's the extra factor of an allowance for the second end cap.)

In addition, humidity affects the fit, so that Troopers in different parts of the world fly differently if it happens to be significantly either drier or damper than it is here when I fit the the spreaders.

Strong breeze performance

Once a design is finalized, the only variable left to consider is the fit of the spreader. It has a direct relationship with the range of winds the kite flies in. For light winds, spreaders are a tight fit, and vice-versa. The tighter the fit, the more lift is generated, increasing performance in light winds. But stability suffers. With zero clearance a delta would not be stable. It would most likely just spin, unless there was a lot of extra drag somewhere, and it probably wouldn't handle well, if it flew at all. With a small gap for high efficiency, lift can be maximized, but any wind beyond a very low threshold causes a sudden uncontrollable dive. So there's a compromise. Elsewhere on this site I explain this in some detail.

When the goal is purely strong breeze performance, the spreader clearance will want to be as deep as possible without ruining the kite's ability to fly altogether. As mentioned above, the original Zephyr had a very loose spreader, and so the next section will show how a Trooper's spreader can be shortened to match local conditions of humidity, and increase the top end of the kite's wind range.

Measuring the spreader drop

Only two measurements are involved, neither of which is the spreader's actual length. Follow the steps below, and then plug the values into the simple equation that follows. This will give a pretty good picture of where the kite stands, and whether the spreader can indeed be shortened. Old kites may be pretty loose already.

Step 1
(Not to scale)

Step 2

To calculate the percentages take the drop measured in Step 2 and divide by the distance from the nose in Step 1, and express as a percentage.

(Step 2 ÷ Step1) x 100

Example: if the nose-to-spreader measurement in Step 1 is 28.75 inches, and the gap from Step 2 is 4 inches, the percentage is (4 ÷ 28.75) x 100 = 13.9%.

When sparring up a new Trooper I normally aim for about 15½ to 16%. 18% should be noticibly better in stronger breezes. On the first Tempest the actual figure was 13.1%, but my notes say "up to 20%."

The original Zephyr's gap was 19.9%.** That's really loose by my current standards, but it did fly in strong breezes.

**note: the Zephyr's spreader attachment was 1.08" closer to the nose.

Use this technique to get an idea of what to aim for, and what to expect. Although it's a general rule, it's better than shots in the dark.

Adjustable spreader fit

Shorten the spreader and make some little pocket spacers to take out for the worst winds.

There are many ways to make spreaders adjustable. Adjustable loops, beads on strings, elastic bungees - I had one once with a telescoping center section with an internal spring (it wasn't as good in practice as it sounds). They can lack precision and long-term durability, and require parts unavailable to me.

My personal favorite, the one I use, is simple. Make pairs of small dowels (trimmed with a pencil sharpener) and push them into the spreader pockets so that they lie across the bottom. Using the "gap" method to determine the required diameters for different percentages, sets of dowels could be made to cover a range of different spreader tightnesses, though in practice all you'd probably need is one set. The spreaders butt against the dowels. They actually improve the pockets, too. They add longevity. Thin spacers could be carbon rod (for strength) instead of dowels. As long as you don't loose them, and they don't have sharp edges, they're pretty much Murphy's Law-proof (well, they could be lost, but not in flight). Something like 19 or 20% without spacers, and 14 or 15% with them, might be a good setup.

The flexible spreader, or why I prefer stiff spreaders

More than 20 years ago I designed a 90º fringed delta around 27" fiberglass spars 1/8" diameter. It had a 3/16" wood dowel spine, and although the spreader is as physically short as it can be on 90º delta geometry, I felt (at the time) that 3/16" dowel was too thin and that ¼" was a bit too heavy. So I tried a spreader made from the same 1/8" fiberglass as the spars, and the result was interesting and instructive, though I never made much use of it until the R4 bird-scarer. (I didn't get many orders for the "Cub", so I never re-ordered 1/8" fiberglass.)

Everything must have been optimum on that design, because the spreader didn't buckle too much or bounce uncontrollably like a car suspension with worn out dampers (mis-named shock absorbers). It sat glued to a spot in the sky in spite of gusty conditions - the winds here are rarely steady. It was content to remain where it was, and wouldn't climb as high as a regular delta with an unyielding spreader. The spreader, along with the wing spars, just "gave" with the wind, in tune with the wind, but never climbing overhead.

Unfortunately it's almost impossible to find a spreader that's both flexible and in tune with the kite's natural frequency, so that it doesn't get into some wild oscillation, but rather allows the kite to ride out gusts while staying in one place. The ideal kite would be a 90º design, with the longest possible spars, so the spreader is as close to the nose as possible. The aim is a short strut, but there aren't a great many choices available to me. Fortunately near-rigid spreaders are readily available. They resist buckling, and don't try to share the job of flexing with the wing spars.

 

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