SLARCS to NOW

Arcs as single line kites.

When the first Arcs were being developed in 1998/99, we noticed straight away that they would fly as single line kites. 

This was in direct contrast to the LEI (leading edge inflatable) style of sled form traction kites which aren't stable on a single line, (except when they're flown upside down), undoubtedly because their CL is too close to their CG. 

Compared to LEI's, Arcs don't have such a high proportion of their weight at the leading edge, and their CG position is (just) far enough back, to make stable single line flying possible.

The Arc style kites that I've been developing specifically for single line flying are, naturally, called Slarcs.

As single line kites, Slarcs have advantages and disadvantages.

Their major advantage is excellent lift/drag ratio (L/D).  Slarcs fly (sometimes!) at higher line angles than any other single line kites I have ever had or seen- which apart from being intrinsically pleasing, is also an excellent characteristic for very high altitude flying.

Their main disadvantage is that, by their intrinsic form, they tend to undercorrect.

There are three contributing causes to this:

The first is that Slarcs have a relatively short distance between their CL and CG; their pendulum effect is weak.  This is inherent in their arch form as Andreas may provide a diagram showing.

The second is that they have a lot of lateral area relative to their lift area-the ratio is 50% when the kite's form is a half circle.  By itself, this is not too much- there are many very successful single line kites with ratios of 100%, and more- but is significant when added to other characteristics of Slarcs that also promote undercorrection.

The third is that they have relatively high AR (aspect ratio) for single line kites- that is; they have quite substantial drag elements (the wing tips) well outboard of the kite's centre line.  These act to damp out dynamic instabilities and any unwanted changes of direction that may initiate- but also slow down desirable corrections.

The combination of these three effects is that if Slarcs become misaligned with the wind direction for any reason (like when there's a wind shift) they can take quite a long time to get back straight, and during this time, may traverse off to one side or the other- which is not desirable.

And there's another inherent problem:

The main advantage that Slarcs have over other forms of single line kites also makes for an extra problem: Their efficiency, by L/D, manifests as close to 90 degree line angle, but also as speed through the air.  The rule here is that kites accelerate to the true wind speed times their L/D.  A Slarc with L/D of, say, 6, in true wind of, say, 20km/hr could achieve a theoretical (infinite line length, zero line drag) maximum of 120km/hr, at which speed it will have (theoretically again), 36 times the line pull that it has steady state at 20km/hr. 

This is a significant danger to the kite itself, its lines, and to people,- and to national infrastructure.  

It's also a problem for stability, by the third paragraph from "Single LIne Stability":  "-- while the lift (and drag) forces that drive dynamic instabilities increase with the square of wind speed, the weight force (from which the kite derives its upward seeking tendency) is constant.  At some wind speed therefore, the pendulum effect will be overwhelmed by aerodynamic forces and the kite will crash- if it doesn't break first."

At 20km/hr, the lift from a Slarc is likely to be about 5 times its weight, at 120km/hr it'll be 180 times (if there's no de-power system).  Any puny influence that the kite's weight can have in keeping it pointed upwards becomes increasingly inadequate when forces of this magnitude are pushing it around.

Therefore, it's essential to have effective automatic de-power systems- line pull should increase at a much slower rate than by the square of the wind speed.

De-power is also important for high altitude flying because it's maximum pull that determines line selection.  If this maximum pull can be reduced relative to the average pull, the flying line can be of smaller diameter and the kite will then be working against less weight and drag.

High performance single line kites are also particularly prone to another annoyingly anti social habit.  When a single line kite launches in wind that's barely enough for it to sustain steady state flight in, it will climb fast initially because of apparent wind, overshoot the apex it could fly steadily at, stall, and fall back.  This is not usually a terminal problem, because it won't fall back right to the ground and so, won't overfly so much the second time up, and will soon settle to steady state flying.  The exceptions are kites that are bridled at a high angle of attack (they don't re-start reliably after falling back the first time) and very high performance kites.  If a kite goes up past 90degrees on the first surge (and Slarcs will do this), when they run out of apparent wind and fall back, they do so at exactly wind speed- so have no relative flow at all to get them flying again- until they get back to a line angle of 45 degrees or even less.  By zero degrees line angle-that is horizontal- they will of course have true wind speed again, and will definitely re-ignite if the wind is above their minimum, but if a kite's up 5000m it's not entirely desirable to have it going through this cycle as it will end in tears -sooner rather than later. 

There are palliatives and solutions for this.

A palliative is low stall speed.  Kites that will fly in very light wind are much less likely to exhibit this bad behaviour.  Slarcs that are 'forward bridled' (by wedging or centre bridles for example), have a lower stall speed and will therefore be much less inclined to this behaviour than those set for power.  Power control systems that automatically decrease a Slarc's angle of attack as apparent wind speed increases therefore also need to have a feature by which at very low wind speeds the angle of attack is reduced to the optimum for light wind flying.  This can be accomplished without active (servo style) control, by using differential spring rates for example.

A solution would be a speed sensitive air brake- that prevented overflying (would also be a useful de-power system in that overpull is caused by overspeed).

Until the last year or two, I stuck to the basic Arc layout of an arch shaped kite with lines only to the tips (there were some exceptions to this, like an Arc from 5 years or so ago that transitioned to a fully bridled foil when the bar was pulled in).

Up to early '08, most development was towards getting as much de-power as possible, by using ridiculous amounts of 'wedging'.  ("Wedging" in Arc parlance is when the leading edge is concave- kite stretched out.)  Wedging brings twin advantages; more de-power and better light wind flying, and I did get some excellent single line flying out of highly wedged Slarcs in steady winds.  Unfortunately, highly wedged arcs are incapable of flying except when 100% inflated- they collapse in a heap if they lose even a bit.  For Slarcs with extreme wedging in marginal wind, even 30 seconds spent drifting back caused their tips to come together.  From this position they usually didn't recover- and if ever they did, it was not until they had drifted almost to the ground, after which they would take off like a rocket, usually horizontally, sweep everything else from the sky then break their line.

A solution to this is forced inflation- and this was tried (by Pete) for conventional Arcs about 3 years ago.  Using battery powered ducted fan as supplied for model aircraft, this was completely successful in overcoming all problems with underinflated flying and significantly boosted performance as well.  They also, incidentally, allowed for remote controlled inflation, which would be a great party trick while kite surfing.  Used in conjunction with conventional valves, these fans had only to provide a small boost during normal flying, so battery life could be many hours.  Batteries and electricals are a reliability concern around salt water though, so developing this to a commercial level is going to be neither quick nor cheap- which is why this project lounges in the unfinished business file at present.

Another solution may be to create separate inflated chambers in the Slarc- one of which would be along the leading edge and given priority inflation.  Highly wedged Arcs collapse because their leading edges don't have sufficient compressive strength to hold the tips apart until they're fully inflated, so this would help by directing available inflation pressure to the leading edge first rather than last as for conventional rib and skin form ram air inflated kite which inflate their trailing edge first. . 

For Arcs, at various times, we did try priority inflation to centre span cells by fitting restrictive valves in ribs at approximately 30% and 70% span (inflation inlets are at centre span because this is where the highest stagnation pressure can be captured).  This definitely did improve underinflated flying, but I expect that splitting the inflated cavities chordwise rather than spanwise would be even better.

I was just about to try this, procrastinating because it's more difficult to avoid creating flow discontinuities with spanwise cavities than it is with conventional rib and skin chordwise cavities, when I left for another kite trip, March '08 I think.  With lots of pondering time but no kite making facilities, I then started to think about extra centre span bridles and pulling the tips together instead.

Spanwise priority inflated cavities will still be worth trying at some time though.

Centre Bridles and Horseshoeing.

Adding a few centre bridles (just back from the kite's leading edge and about one for every 3 cells) makes almost any Arc fly better as a single line kite than purpose built Slarcs ever did.  The primary reason why this works so well is that it shifts the kite's centre of pressure forward -which makes for more de-power and better light wind flying- as for extreme wedging- but it does these things without any penalty to partially inflated flying.  In fact, centre span bridles greatly enhance underinflated flying, making it superior even to unwedged Arcs, and, as a consequence, make launching much easier also.  Maybe Andreas will make one of his excellent drawings to show this, my drawings aren't half as good.

Since this development (adding centre span bridles), work has been mainly in exploring every likely form of these bridles, especially towards the goals of getting more de-power and better light wind flying, but also to reduce the natural undercorrection tendency (explained above) that Slarcs have.

At about the same time as I started working with extra bridles, I also found that when a Slarc's wingtips are pulled together into a horseshoe form- or even right together into a closed circle- they pull a lot less AND correct quicker.  That they'll have less pull in this form is obvious enough- their projected or lifting area is smaller.  The reduced tendency to undercorrect (better recovery) comes from the reduction in aspect ratio that horseshoeing causes and from the reduced lift force relative to a relatively unchanged corrective pendulum moment.   The down sides of horseshoeing are poorer light wind flying (less projected area for the same weight of kite) and lower L/D (lift reduced, drag not reduced, may even be increased a bit because of the 'biplane effect' by which wings in close proximity interfere with each other).

For the last 6 months or so, we've therefore been looking for bridles that make Slarcs horseshoe more as apparent wind speed increases.  Such systems allow a Slarc to take full arch form (maximum projected area) in low apparent winds for best possible light wind flying (providing the centre of pressure position is adequately far forward also), but then, as apparent wind builds, the increasing amount of horseshoeing counteracts increasing undercorrection,- and provides line saving depower.

The Phantom chart published by Andreas is the best we have for this so far.  It's adaptable to any Arcs that have a spanwise braid near their leading edge (all Arcs except the original series).

It isn't optimised- need to be tweaked a bit here and there to get good performance on each new application- but will then work great, especially in very strong winds.

In very light winds, it has a minor problem:  If the Slarc stalls, the bungee can prevent it's getting going again.  This happens because any gust that arrives will tend to just stretch the bungee rather than acting to increase the apparent wind over the Slarc.  We've thought of possible solutions to this but as yet haven't tried any.  Basically we're looking for an approach that doesn't use letting the Slarc out by a metre or so in order to pull the tips together as wind speed increases- or, at least doesn't let it out by as much.

The F Arc bridle chart published is not the best general F Arc bridle (it's non de-powering) but is the best single wind speed bridle I have found so far.  In 'steady mid range winds' (which are in to the same category as 'the foreseeable future' and 'goat proof fences' by my experience), F Arcs with this bridle fly vertically- within any reasonable margin of measurement error- just before they fall back, re-engage and become Shiva, destroyer of the world.

This F Arc bridle has been a parallel development, while trying various horseshoeing systems, because we've also been looking for the best possible performance by L/D, to give us a benchmark by the assumption that eventually a system will evolve that will use this optimised-for-L/D bridle in light winds but with some de-power system and some anti undercorrection system coming in as apparent wind speed increases. 

The Phantom style bungee bridle is of course adaptable to F Arcs, as is the F Arc bridle to other models.

Peter Lynn, Ashburton, December 31 '08