An annoying problem finally identified and cured

Ever since I’ve been making large ray shaped soft kites (from 1987) they’ve been afflicted by a tendency to fall off to one side or the other. And it’s not just my ram air inflated ray type kites that have this disease, Volker Hoberg’s SFT and Andreas Fischbacher’s Manta Ray sometimes show the same symptoms – they probably caught it by getting too close when they were hanging out here.

In 25 years of trying to find a cure for this affliction- I’ve gone through more bright and shiny new theories than there are trees falling in the forest with no-one to hear them. In darkest moments I’d begun to think that the entire lifetime edifice of kite stability theories I’ve constructed was of no more value than completely random guesses and was feeling uncomfortably like as weather forecasters and people who write horoscopes must regard themselves in moments of self honesty- if they ever have any.

When watched closely, what seems to happen is that the leading edge on one wing or the other loses inflation; gradually crumpling up and pulling the kite to that side. Especially when the wind is very light this will be terminal, but in stronger winds, the kite will sometimes then recover and repeat the sequence but to the opposite side. Ray style kites with lots of tail drag are less susceptible than the later smooth tail styles- but they all do it. With a mega version about to be started, this most annoying behaviour had really been giving me sleepless nights.

From the beginning, a puzzling aspect was that it appears to only afflict larger versions- smaller models are immune, even when dimensions are exactly scaled. A standard maxi bucket tail ray will rarely fly pilotless for long periods without coming down with a bad case, whereas midi versions of the same kite fly on forever, apparently uninfected.

Ah-ha, why could this be?- a clue! Something to do with fabric stiffness?- which has an appreciable effect on the structure of a small kite but less effect as kites are made bigger. Reynolds number effects (don’t ask!) and non scalar surface roughness are the only possible other candidates but they just don’t stack up- so it HAS to be fabric stiffness- there’s no other sensible explanation

Which pointed to some sort of structural irigidity being the primary cause of the problem

I’d made it this far by the late eighties, but then wrongly seized on poor inflation as the perpetrator- leaving the real villain to continue it’s serial offending until a road to Damascus moment during the Xiamen Festival in November.

In defence it’s fair to say that poor inflation does contribute to errant behaviour of this type. Careful positioning of the inflation point(s) and the use of flap valves to prevent egress during lulls does help.

But even with perfect inflation the underlying behaviour always still lurked- just waiting for the most inconvenient moment to strike.

OK OK, no more suspense:

I now know that the actual cause is fabric shaping that leaves the leading and trailing edges loose while the centre chord area alone resists the spanwise component of internal air pressure. And it happens because we all took what seemed to be the easiest way to make these kites:-Sewed up two essentially flat lozenge shaped pieces of fabric, one for the lower skin, the other for the upper, then sewed them together around the edges and added a tail eyes and mouth.

And to visualise why this causes there to be looseness at the leading edge, think of a heart shaped helium filled message balloon. They’re made from two flat pieces of aluminised Mylar film that are then sealed around the perimeter and inflated. All those little creases that then appear transversely across the edges are exactly what our problem is.

Or take two A4 sheets of paper, tape them together around the edges and inflate them. Transverse creases will appear across the edges, especially in the centre of each long side- these are the only way that flat sheets can adapt to taking a convex shape.

Instead of using two basically flat skins for the upper and lower surfaces (albeit with shaping to lift the tips), we should have been taking care to make them conform to the three dimensional shape that they take when the kite is inflated- perhaps even adding a bit of extra fabric through the centre chord area to ensure that as much tension as possible is taken where it matters most- along the leading edge.

Simple isn’t it!- and so easy to fix seeing as we have complete freedom to use many and varied shaped fabric pieces when making the upper and lower skins- they don’t have to flat, but can be made to conform to any three dimensional shape we choose.

But does it really work?

When I belatedly realised what the cause of the problem was (while watching a smooth tail TT Ray) during the Xiamen festival, two tie cords rigged along it’s trailing edge to pull each side in by 0.5m or so immediately smoothed out the leading edge- and the kite then flew central all the time: Problem solved.

So why did this take 25 years?

Good question.

The only answers that readily spring to mind are, incompetence, a slow brain, and fundamental lack of understanding.

And now the alert reader (if there are any still following this except Volker, Andreas and Robert van Weers*), will be thinking:-Yes this explains why the leading edges looks wrinkly and un-inflated on these kites, but how come their trailing edges are tight?

This is because, by air flow and Bernoulli’s theorem, there is always less pressure difference between the outside air and the internal space at the leading edge of ram air kites than at their trailing edges. Any looseness at the trailing edge migrates to the leading edge and adds to the woes there.

And why does looseness at the leading edge cause such bad kite behaviour- even when inflation pressure is good? It’s because looseness in the leading edge doesn’t remain equally disposed to either side of the kite. Sooner or later a small wind shift or whatever will cause all the looseness to accumulate along one leading edge while the opposite leading edge becomes tight. With one leading edge smooth and tight, the other loose and draggy, the kite will inexorably fall off to the high drag side.

Peter Lynn,

Ashburton, Jan 1 2010.

*Because Volker and Andreas will be reading on to see if they agree with me or not and whether there’s anything useful for their designs in this, and Robert will be telling me by now that he never fell into this trap in the first place with his diamond shaped ram air kite (which could potentially have the same problem) because he’s not a Luddite and used a 3D program for skin shaping.