April 2008

Newsletter, April '08

More boring kite sailing stuff.

In 1987 at a kite event in Lincoln City (USA), John Waters (inventor of the spinsock and general purpose kite pioneer) talked up the prospects for kite traction, especially kite sailing.  Flexifoils, Peter Powell's, Delta style sports kites, and some re-rigged ram air sky diving 'chutes were already showing the way- so I didn't take much convincing.  This was an epiphanic event for me; it did strike me that up until then I'd been preparing for this moment. 

By five or ten years ago I'd abandoned hope that commercial shipping could usefully be kite powered or kite assisted- except in some yet to be found minor niche- and except as a bullshit green scam rather similar to bio fuel ethanol from corn (which uses much more fuel to make than you get back).  However, by then it was clearer than ever that kites could advantageously drive yachts- including big ocean going ones.  

There have been times of despair and periods of hope.

Right now, I believe it's about to happen; no ifs or buts.

A week ago, I wasn't so convinced.

Some background.

At least since George Pocock (1820's, father of kite traction) two main 'inconveniences' have stood in the way of practical kite sailing.

One is that kites, whenever they get the opportunity, momentarily accelerate to a speed through the air equal to the wind speed multiplied by their efficiency.  For a reasonably efficient kite (say L/D 6) this will be 6 times wind speed, at which velocity their pull will be 36 times that which a sail of the same size attached to a boat would experience in the same wind.  Unless controlled in some way, this is lethal - breaks kites or lines or boats or those sailing them- or all of the above. 

The other is that kite sailing is only possible if the kite is flying- obviously true- but when the wind is light, sails don't fall off their masts but kites do fall out of the sky.  For kite sailing to be practical, kites have to fly in very light winds, and be easy to launch and retrieve.

A solution to both of these problems, and of a lot of other less serious ones, is the dual mode kite.

Dual mode kites are traction kites that have a secondary mode as a stable flying single line kite.  In the single line (default) mode the kite auto zeniths and flies stably in all winds without control inputs and with, ideally, 90% or more reduction in pull.

This solves launching because the kite can be launched from the boat's deck, then let out until it's at traction kite altitude.  It also eliminates the need for winches able to control multiple lines.  Winches capable of doing this might seem to be simple enough, but they're not.*  If just one line (out of 4 or 5 with some systems) ever gets a "wrap"- that is, catches around it's spool (almost impossible to prevent when line tensions are sometimes negative)- the kite usually commences  to loop uncontrollably with max. pull.  I've built some sophisticated winch systems, but have never had one that is safe. 

However, the main advantages the dual mode approach provides is immediate de-power (when the power lines are released) and the convenience of not requiring the kite to be retrieved and re-launched when pausing for a period.

I started working seriously on dual mode kites about 5 years ago, initial direction coming from the natural auto zenithing tendency that Arc style kite have.  The Slarc series of kites resulted from this- but their single line/de-power mode (about 1/4 pull) also, unfortunately, is at max efficiency.  They therefore tend to fly too fast (generating higher than desirable pull until they zenith) and often overfly.  They are delightful single line kites though- flying at a higher angle than any other kite I've had or seen- but, because of their high efficiency, not at all docile.

Another style I've tried is NASA/pilot hybrids. (See attached photos)  These work very well except that their max. efficiency is limited to about L/D 3- but they just can't compete with conventional yachts using sails of L/D 10 and more.  There may well be some applications for this approach though, because the crossover point for NASA's versus high performance Arc, LEI or bridled 'foil style kites is at much higher wind speed than I expected- up to about 15km/hr, which covers a lot of sailing. 

But 3 weeks ago I heard about an LEI (leading edge inflatable) dual mode kite that works.  I haven't seen it but believe it's a standard C form LEI kite surfing kite modified and rigged so that when the tension is taken on a 5th (centre leading edge) line, the leading edge tube buckles back at its centre to about 45 degrees dihedral,- and in this form, the kite flies stably single line and with very little pull.  This seems plausible.  I can think of quite a few ways that it could be achieved- and even if nothing better turns up, such a development would enable kite sailing.  A disadvantage would be that quite a bit of line length change would be required to change mode, but this is a minor inconvenience relative to the advantages.

Then, in the last few days I have found that if an Arc's wing tips are tied together to make a horseshoe form and an extra bridle (or two) is taken up to the centre leading edge area, they fly reliably as single line kites, with very little pull- and no overflying.   

I've now tried it on three different Arcs style kites (two Slarcs and an 8m Venom), and all worked immediately.  For now I've only been able to get smooth transitions from single line to power by using 5 lines, but I expect 3 line versions will be achievable. 

Just to check, I will also try horseshoeing a standard C form LEI- but I expect it's more forward c of g position will militate against satisfactory single line flying.  Perhaps some special design form (like the Naish Sigma's for e.g.) will fix this, but no matter, the approach described earlier can be expected to work for LEI's even if horseshoeing doesn't.

I therefore expect that kite sailing will take off sooner rather than later now.

Dual mode kites will also have implications for kite surfing and land based traction kiting.  Kite pull in single line mode can be so light that up to any normal kite surfing size, I expect fliers will be able to launch kites from their hands and let out on the front line(s) until the bar engages, retrieving the same way.

Damn, thoughts of backing off and relaxing a bit seem to be receding; looks like it's now safe to get back in the water!

Peter Lynn, Ashburton, March 25 '08

*To prevent line wraps in multi line winches, the challenge is to maintain line tension in the gap (usually a metre or more) between the nose pulleys and the winch drums.  This can be achieved by driving the nose pulleys faster than the winch drum when line is being let out and slower than the winch drum when pulling in.  This is conventionally done mechanically or electrically, but here's another way:  I tried enclosing the lines between the drums and the nose pulleys in loose fitting tubes with rubber seals at the winch end.  By pumping water into these tubes by way of a tee fitting adjacent to the seals, constant tension was applied to the lines for minor energy and complexity cost.  With 3.5mm lines, 3 bar water pressure is sufficient to constantly "extrude" the line at about 4m/min.  Water flow was a bare dribble. An inelegant photo- but an elegant solution- to a problem that we don't have now seems!