December 2008

Single Line Arc s for you.

We all need just solvable problems- too easy and there's no satisfaction, too hard leads to frustration and a sense of failure.  By these criteria, trying to make Arcs fly really well as single line kites has been an ideal challenge for me- and it's something useful to do during kite festivals when everything else is up and flying.

The lure of this project is not just possible applications- superior pilot kites for one, a new kite altitude record for another, but also that single line Arcs are an excellent tutorial in how kites fly, what little things make all the difference to stability and why.

After five years and an embarrassing number of purpose built prototypes, earlier this year I stumbled on a way to rig just about any old Arc so that it will fly rather well single line -better than any of the purpose built ones ever did actually.  So much for a planned incremental development program, but hey, when something works, why fight it!

And this is where you can come in:

We've just hauled 10 years worth of left over Arc style kite boarding kites out of the dark corners where they had settled, quite a few hundred kites, all of them now suitable for single line conversion, and I want to give them away - well sort of.  I'm not just looking for good homes for these kites; I'm also hoping that development from now can be collaborative.

But before going on (don't I ever sometimes, especially about the scourge of Greenism and the great climate change scam), perhaps it should be explained as to how come we have all these kites lying around:                                                              Since tentative beginnings in the '90's, kite boarding kite development has been rapid, so it's often been expedient to stop selling residual stocks of superseded models when new designs come in rather than disrupt the market by discounting them out.  This generally amounts to less than 1% of production though, so doesn't get noticed much until storage space becomes an issue, which it just has.

The rigging system I now use is a good enough- some re-rigged standard kite boarding Arcs already fly more reliably in very strong and turbulent winds on a single line than any other kites I've flown- but it's not perfect, there is still a problem to overcome:

Which is that the Arc's superior L/D (lift/drag ratio), why they fly at such high angle, also makes them fly very fast.  Maximum speed, for all kites, is equal to the true wind speed times the kite's L/D.  When launched into a 20km/hr wind, a kite with L/D of 6 can accelerate to 120km/hr as it climbs- at which speed, its pull is theoretically 36 times what it will be in steady state flight once it stabilises at apex.  In practice it's not this extreme though, nearer 5 times in the present state of the art - because I use extra rigging that automatically de-powers the kite as pull increases.  Currently this is done by pulling the tips closer together as apparent wind speed increases, reducing both the kite's angle of attack and its effective area.  Perfection would be something around 2 times though- so there's work still to do.

Generally, Arc models with lower aspect ratio (that is less span more chord ) are well enough behaved to be useable single line kites with the rigging systems already developed, but those with the highest aspect ratios and potentially therefore the best performance, (F Arcs in particular) are still a challenge to rig and fly.  They tend to be TOO steady, so when disturbed a bit, move sideways a long way before recovering- and if tipped right over, they can dive straight for the ground from whatever height- at about 100km/hr- which would be really scary except that the line always breaks about then.                                                                                             F Arcs were designed by Chris Brent in '01, and are the highest aspect ratio Arc style kite we've ever produced.  They have, still, the best straight line performance we've ever achieved.  By current kite boarding standards though, their steering is too slow and they lack de-power.  By single line standards, their angle of flight is unmatched- near enough to vertical- but they need better stability (a combination of steadiness and recovery) and improved auto de-power to keep line pull within a narrower band irrespective of apparent wind speed.

It's worth trying to make them fully manageable because they have the potential to set a new world altitude record.                                                                                                                                                                                     There are three things that are really important for high flying: Size (the bigger the better because line drag is proportional to its diameter but its strength scales with diameter squared), angle of flight (that is kite L/D), and effective auto de-power (so that the line doesn't have to be oversize just to deal with occasional surges).  One hundred F Arc 16's flying in train (there are at least this many available, and I have already successfully tested a system for training them) would have an effective lifting area of more than 1000sq.m- and their flying angle is excellent.  De-power though?- not yet good enough.

But what is the current altitude record?  Rudolf Grund, kite designer at the Lindenburg Observatorium set the standards 95 years ago.  His box style kites had only moderate L/D, and weren't very big by modern soft kite standards, but he did have an excellent auto de-powering system and a technological edge over anything we can do now by his use of high tensile wire for flying line.  This is a lot superior to the Dyneema /Spectra we use now- so our kites will have to be much bigger to compensate.                                                                                      Various max. altitudes for kites are claimed, but to my view, the only indisputable high ones are from Lindenberg where heights of more than 7000m's were recorded multiple times*.

If enough kite fliers are interested in participating, these few hundred kites could be distributed now.  Shipping costs would have to be charged, and there'd probably best be a token price on each kite to discourage looters and tire kickers (NZ$10 and NZ$50 depending on size and age?), but they're mainly perfect new unused kites with fabric value alone in the hundreds of dollars.

This project is actually up and running already in a minor way but with just four participants so far,- so everywhere above where I've said "I" it should have been "we": Andreas Fischbacher(Germany), Orlando Ongkingco (Phillipines) and Johan Hallin (Sweden)- and there are others who have expressed interest.

We will share all the Slarc bridle and rigging systems developed so far (probably 10 or more across perhaps 20 sizes and models), and I'll write up some of the theories I've developed so that not everyone needs to start by re-inventing the wheel.  However, I'll be surprised and disappointed if some contributors don't ignore all this received wisdom and find better ways to make things work than I've never considered.

As soon as possible, we'll also publish a list by model, size, and condition (there are some ex-demonstrators as well) and their location (roughly equal numbers in Singapore, Holland and New Zealand as of now).

*I'm sure there'll be lots of discussion about this, but the over 9000m height often claimed for Lindenberg was from after a kite had broken free- which doesn't count.

Peter Lynn, Ashburton, 1 December '08