This is of critical importance. Lift to drag is a measure of the efficiency of the kite. It is easy to measure - in a horizontal wind, the lift is is up and the drag is across. You can resolve the ratio, simply by taking the tan of the angle of the kite-line. Note that this is much more important than the pull of the kite. It is trivial to make a kite that pulls harder, simply by making it bigger, but it is not so easy to make a kite more efficient.
For ease of reference, here's a few angles and their L/D's:
|45 degrees||L/D = 1|
|63 degrees||L/D = 2|
|72 degrees||L/D = 3|
|76 degrees||L/D = 4|
|78.7 degrees||L/D = 5|
|80.5 degrees||L/D = 6|
|81.9 degrees||L/D = 7|
|82.9 degrees||L/D = 8|
|83.7 degrees||L/D = 9|
|84.3 degrees||L/D = 10|
Getting up to 80 degrees requires a pretty efficient kite. Those last 5 degrees to get up to 85 degrees is exceedingly difficult.
It is also rather difficult to measure. Kites that fly high tend to overfly. It is difficult to measure with an accuracy of 1/10 of a degree when the kite is osscilating in a 10 degree area and is easily capable of overflying past 90...
Note that there are only a few degrees of difference between a very efficient kite and an ordinary one. If there is any appreciable sag in the lines (due more to aerodynamic drag than weight), then that advantage is lost. For best performance, use the thinnest line you can manage.
The lift to drag of Chevron kites is very good in comparison to other kites. Unfortunately, all kites fall short of the performance of a good sail on a boat, even without the kiteline.
This is a measure of how much lift the kite gives in relation to it's size. On many of the earlier prototypes, the coefficient of lift of the kite was smaller than that of it's contemporaries. Although many fliers rate this as significant, I don't feel the same way. If the lift provided by the kite is small, it is a simple matter to just make a bigger kite. It is better to have a large, efficient kite than a small, inefficient one with a large ceofficient of lift. This approach only fails when the kite becomes too large to handle.
In the first 7.7m Chevron, I stumbled across a bridle setting which did, in fact significantly increase the coefficient of lift, without a large drag penalty. This is exploited in the reefing system to maximise the bottom end of the power-band, to provide as much pull as possible in the lightest conditions.
Note that the coefficient of lift varies across the canopy - the kite produces more lift in the centre to enhance low-wind grunt, while producing less at the tips, to vigerously address tip-drag issues.
The kite must have sufficient structural strength to be able to fly without collapse. Large cells are stronger than small ones. Fat plans are stronger than spindly shapes. The kite must be easy to launch and easy to recover from a crash.
This is an area where the Chevron is has a clear advantage. It is strongly disinclined to collapse, and when it does collapse, it is remarkably easy to recover. This makes the kite particularly tollerant of novice fliers and allows the experienced flier to get away with buggying in more difficult conditions.
It is difficult to explain how to recover the kite from a collapse. New fliers will simply shake the kite about, hoping that the canopy will sort it'sself out, but with experience you will pick up the technique, it is a matter of pulling the tip with the problem towards you, then letting the line slack to let the pressure off, to give the canopy an opportunity to fall back into shape.
With experience, you will also find that you are less inclined to collapse the kite in the first place. Although the kite pulls like a truck, it also has sensitivity - after all, it's only air that's powering it. You will begin to recognise the feel in the lines as the pressure comes off just prior to a stall, you will begin to appreciate when the kite is about to fly outside the window, and take evasive action (just a push on the lines to take the power out and slow it down) to keep it just where you want it.
One of the rare joys of buggying is to do a long, downwind tack on a smooth surface. The wind may be blowing 40mph and you're hurtling along, but then suddenly you find the spot where you're travelling at windspeed - the noise dissapears, you notice that the sun is out, and the kite hangs stalled, on the verge of collapse in the sky, bowling along at 40mph with only the sound of the wheels to be heard. Light wind performance is important even on the windiest days.
You need to be able to turn the kite, or it will crash into the ground. Moreover, the control input reqired must be available from the confides of the buggy. I consider that the ability to turn on a wingtip is a reasonable requirement
With my cross-bridle, a 7m span kite can be made to turn on a wingtip but uses an arms-length of movement to do it. The 6m span of the 7.7m^2 kite is much more responsive, giving the full control required with movements that are more convenient in the buggy.
If you look closely at the bridle, you will notice that unlike the Peel bridle, which uses a 2-teir system, the Chevron has a 3-stage cross-bridle. This allows me to increase the steering sensitivity whilst also reducing the bridle-drag. The only down-side to this design is the increased component count, with it's associated labour cost.
Note however that the kite must be flying in order to turn, the kite is not steered according to the length of the lines, but the balance of the pressures on the lines. If it's not pulling against you, it won't steer properly, so in the buggy, with the larger kites, steering the kite is as much to do with steering the buggy to control the pressure as moving the hands.
There are two issues to consider with inflation
The vent on the Chevron is marginally larger than usual for this reason. I would have expected to have paid a drag penalty for this luxury, but my fears seem to have been unfounded.
As the air meets the leading edge of the kite, some of the air must go up and over the canopy, some must go down and under. Right in between, there is a very small point where the air is "unsure" which way to go, and flows very slowly. This is known as the stagnation point. Bernoulli tells us that the highest pressure will be at the point where the air is moving the slowest. By placing a vent at the stagnation point, we can ensure that the the canopy is inflated to this high pressure - a pressure higher than the pressure at any other place on the canopy.
Note that as the angle of attack to the apparent wind changes, then the stagnation point moves. The design of the vent must be such that the stagnation point always coincides with the vent.
A stall is a light wind manoever. With the kite moving upwards push forward, and kite kite will stop. With a little sensitivity on the part of the pilot, the kite will hang there, without collapsing.
The Icarex fabric, and good structural integrity due to the longer, fatter middle cells and the reduced bending moment from the smaller tips mean that the Chevron does this particularly well. On light-wind days at beach festivals, I'll spend much time stalling the kite and hovering, inches above the water. It looks impressive to hang a huge, deflated kite just an inch or two above the wavetops, but infact, it is easy to do - the kite doesn't do anything nasty in a hurry.
Be warned, however, that if the waves are moving away from you (eg: you are in the water and the kite is towards the shore) and a wave should break on the kite, the sudden force of the water on the bridles that are directly effected will almost certainly break them (and with average luck, it will tear some rib-seams too). Of course, if you do drop the kite in the surf, you will invariably end up with a challenging bridle-tangle (and the looks of people thinking "We knew that would happen").
When going slowly, upwind, with the kite at the edge of the wind, the kite sees an apparent wind approximately equal to the real wind. When the kite flies across the middle of the window, it speeds up considerably and sees a much higher apparent wind, so it pulls much harder. The ratio between the pull at these two points is the MMR. In an ideal kite, they would be the same.
Without the reefing system, the Chevron had a moderate MMR. In particular, the power comes on smoothly, rather than being inclined to rip you from the ground.
The reefing system is designed to reduce the MMR as much as possible. It takes as it's base two different bridle settings. A high-power setting that gives the 7.7m^2 kite a degree of lift approaching that normally available from a 10m^2 kite. The low setting has been shown to give a 10m^2 kite the lift expected from a 5m^2 kite. The difference between these two bridle settings is not a simple matter of changing the angle of attack, or pulling down a brake at the trailing edge, but rather, changing the effective profile of the ribs, changing the tune of the canopy, to produce entirely different performance.
The reefing system self-adjusts so that when the pull is excessive, the kite will deform towards the low-power bridle. Conversely, when the pull is light, the kite flies entirely on the high-power bridle.
It is very easy for the buggier to change the direction of the apparent wind, so that it comes from above, rather than below the centreline (just try turning towards the kite). This is usualy the precursor to the kite falling out of the sky in a heap, but if it is possible for the kite to survive this abuse and to continue to fly (or simply not to fall apart for long enough for the pilot to recover), then you donh't have to get out and walk.
The Chevron does much better at resisting luffs that I would ever have hoped. The best way to see this is simply to fly the kite to the top of the window, let both hands up as far as you can reach and pull down, hard. The kite will fly over your head and stall. Provided that no-one is pointing a video camera at the kite, it should just hang there, remain inflated and drift back into the window. Note that while it hangs, a very small amount of control is still available, even though the lines hang loose - pushing a line up means that your hand supports more of the line-weight and shifts the ballance of the kite. I have a suspicion that the kite is still producing lift as it floats back, and hence that it catches the wind again, higher than would be anticipated, but this may merely be wishful thinking.
The result of this is that you can get away with things that you would expect to trash the kite. When you have no choice, but to dart the buggy out of the way of some obstacle, you steer your feet, keep your eyes on the thing you're trying to avoid and you feel the lines go limp, because you had to steer into the lines. When the emergency is over, you steer the buggy back upwind and glance at the kite - it's still there, and powering up, ready to do business.
A noteable occurrance of the luff recovery occurred at Berrow. I was in the tandem, using buggy-hooks, when I messed up and found myself about to cross lines with someone doing the opposite tack. I threw out the anchors and came to a stop as quickly as possible. I stopped hard enough, with the assistance of the additional buggy in the buggy to power the kite way outside the window - something of the order of 45 degrees too far. I couldn't immediately recover. First I had to get the buggy to a stop, unhook the buggy hooks, then run *round* the person that I'd crossed, before I could run upwind to try to recover the kite. To everyone's astonishment (including mine!), the canopy was just stalled and drifting, waiting for me to catch up with it. The kite recovered without difficulty, I jumped back in the buggy and carried on...
For Dave Culp Speedsailing, I was challenged with building a Chevron specifically for use on water. Whilst I would have no hesitation in taking a regular Chevron for kitesailing, for this project I wanted to make a kite that was specifically designed for the purpose. The main features were:
Testing was, admittedly, on a very light wind day, but the evidence was convincing - the kite felt like it was made of lead. You give it a big heave to launch, it jumps in the air, stops and falls back to earth.
One of the things that was on my mind when working on the bladders was the idea of Helium inflation. Since the air inside a 7.7m Chevron has more mass than all the other components added together, it is worth considering replacing it with something lighter, such as Helium. The aim is not to build a kite that floats, but merely to reduce the weight below that currently possible. The weight of the bladders was sufficient that this was not attempted, the weight of a Helium inflated kite would be little different from one with neither Helium nor bladders.
I may return to the Helium issue again, but only with very large kites, perhaps 20m^2 and above.
When I said that the efficiency of the kite is Lift/Drag, I lied. It is (Lift - Weight)/drag. Any weight scraped off the kite means that less energy is wasted on flying the kite and more is available to the buggier.
Without a reefing system, Chevrons weigh somewhere around 120g/m^2. The bungees for reefing system adds something around 30g each to the total. For a sense of scale, compare this with photocopier paper, which weighs around 80g/m^2. This is the sort of wing loading which would usually be associated with ultralight sparred kites, yet because the strain is distributed through the bridle, there is no upper wind limit imposed on the kite. Note however that there may be an upper wind limit for the pilot!
Note however that this is the weight of the kite - the amount of weight that needs to be lifted in order to keep flying. Also of consideration is the mass of the kite, which is considerably higher because it includes the air trapped inside. Air has a mass of around 1.2kg/m^2. To take some easy figures, lets say that a 5m^2 kite has a weight of 600g. For easy calculation, let's say that the average thickness of the kite is 20% of it's average chord. That's 20cm, times 5m^2, gives 1m^2. 20% is a little high, but you can see that the air inside clearly has more mass than the kite that surrounds it. This extra mass slows down the accelleration of the kite and also gives it more momentum to overfly the window. I'm looking at the idea of Helium inflation, not to ensure that the kite can fly in low wind (it already outflys just about anything else in a lull), but rather to decrease the mass of the heaviest component to increase the acceleration response. Unfortunately, I don't see a technology that will allow me to build a Helium-tight canopy with:
It doesn't matter how efficient the kite is, if you can't afford to buy it in the first place. Most traction kites are only available through retail channels. The retailer generally doubles the price. I sell them direct. This means that I can undercut the price of comparable kites by a massive amount, to reduce the price to the customer. I generaly manage to sell my Icarex and Spectra kites for less than the retail price of similar sized Nylon and Polyester kites.
Currently, my prices are:
Try as you might, the kite will be crashed into things. It will be overloaded. The kite should survive. Damage should be repairable.
There are different ways to break a Chevron:
This patch also helps to avoid the wear on the lower skin that can occur due to the bridle rubbing the hole where it punctures the lower skin. This sort of wear is of very little significance - it makes no difference to the performance of the kite, and the construction is such that it is almost undetectable in use, however, it can look ugly on the ground and is *percieved* to be a problem, so I use the reinforcing patches to reduce it to a minimum.
Most bridle failures are due to some damage or tangle. Note however that if anchored securely, in a strong wind, the kite will blow it's self apart - overstrain on the complete bridle will find the weakest (or most damaged) link. This will fail, and the strain that broke that line will be transferred to the adjoining bridles. This happens extreemly quickly. You can expect the entire bridle to fail simultaneously.
Festival organisers do like to take a perfectly good field and stick spikes all over them to catch kites on... If you rip the skin, open up the seam on the trailing edge, and repair by using patches of white rip-stop repair tape on the inside. For a large repair, many small (say 2 inch) pieces is easier and stronger than one large piece. For a lasting repair, sew the patch in place. Note that white tape shows up least on all colours. Then simply roll the trailing edge and sew in place.
If you slam the kite into the ground hard enough, the air has no-where to escape and may burst through the kite with explosive force. Let go of the kite before it hits the ground. This type of accident can cause extensive damage, but for some reason that completely escapes me, I have never actually done it myself - I have little advice to give.
As the kite is used (or repaired), it may loose it's tuning. It should be possible to adjust the settings. The factory settings should be available, so that the kite can be adjusted to it's original setting.
As I learn about tuning kites, I generaly share my discoveries on rec.kites. I am open to discussing tuning problems with any customer. All stopper knots are figure-of-eight knots, which are easier to undo (and considerably less likely to fail) than the more common overhand knot. All bridle points are marked, assisting re-tying. I have no hesitation in passing on new and improved bridle configurations to existing fliers.
Aspect ratio is a measure of the wide the shape of the kite is. For a rectangular plan, the aspect ratio would simply be the width divided by the chord. Thus a square kite has an aspect ratio of 1. A 10' flexifoil with a 2' chord has an aspect ratio of 5. For more complex shapes, a more comprehensive formula is used, namely (span^2)/area.
Higher aspect ratio generally gives better aerodynamic performance, principaly becase the tips are so much smaller. On the other hand, a thin soft kite has less structural integrity. I strive to find the right compromise between these two issues.
On larger kites, the total width of the kite becomes an over-riding issue, on my current design, I don't build any wider than 7m, so that the kite is capable of steering sufficiently within a reasonable arm movement, thus the aspect ratios of the larger kites is smaller.
Most of the development work that I do is alone, judging to my own standards. However, whenever I get chance, I try to compare my work to the other kites around. The main competition is:
Others. There are many other good (and not so good) kites out there, but the ones above are the only ones that I take seriously as contenders for the top slot.
At the end of the day, I buggy not to win races, but to have fun. The ease of flying and the power available mean that I have more fun with Chevrons than with any other kite. The main features which contribute to this are:
Provided that you don't use anything silly, (such as cotton, which will rot), the strength of the thread is not a performance limiting consideration. The seam does not break by failure of the thread, but rather by failure of the fabric. To this end, I choose:
Unfortunately, much of this is guesswork. The forces involved in a crash are explosive and difficult to recreate for testing.
There are three primary types of bridle design to choose from:
After studying Andy Hawken's work, I have moved away from Peter Lynn's model of a 2-stage bridle (the primary bridle supporting the rib and the cross-bridle for steering) to a three stage bridle, where there are lines joining the primary bridle to the secondary bridle. This has several advantages:
It is common on soft kites for the grain of the fabric to allign precisely with the ribs. I'm not happy with this idea, because it means that all the perforations punched in the fabric hit the same thread. This must surely be an effective method for removing the maximum amount of strength from the fabric. The grain of my fabric is schewed a few degrees from this alignment. This means that each stitch on a rib perforates a different thread on the skin, helping to spread the load.
There may be some concern that such cross-grain construction may lead to stretch problems, but the stretch in Icarex is very low on the diagonal and at the small angles concerned, it is indistingishable.
When the distance between the ribs is "large" in comparison to the height of the ribs, the shape of the skin bears little more than a passing resemblance to the shape of the rib, due to billow. Conversely, too many ribs lead to excess weight, possible inflation problems (the jury is still considering their verdict on this issue), and unnecessary fabric and labour costs.
Since the ribs on a Chevron vary in shape considerably (being thinner at the tips than in the middle), it follows that the tip-ribs should be closer together. Indeed, the spacing of the middle ribs is about 50% wider than the tip ribs. On early Chevrons (Spring/Summer 1996), this was done by using 3 cells per bridle at the tips (as opposed to 2 cells per bridle in the middle), but I was unhappy with the transition from 3 cells per bridle to 2 cells. The spacing has now been re-calculated. The tip cells are the same width as before and the middle cells are also the width size as before, but there is no sharp change from one width to the other. The cells change size gradualy (each one is just over 3% different from the cell next to it). The effect is very pleasing - the change is inperceptible but still significant. There are still the same number of cells (28), so there is no drag penalty from extra lines.
According to the designers of other soft 2-line kites, the kite should be curved when viewed from ahead, in order for the kite to turn properly. I am beginning to work away from this mindset. Looking through the development of the Chevron, the kite has gradualy got flatter. The very first kite went through three cross-bridles, each one giving a flatter shape. Later, the middle of the kite took on a more gentle curve and the tips took on a radius centered on the tow-points.
On the latest models, the tips have now been flattened out, and only the middle of the kite retains a very gentle curve. Even this may go, in time.
I maintain that the reason that the kite turns is due to the distribution of the pressure of the lines to the canopy. The shape of canopy has very little influence on turning performance. A flat canopy does, however put extra compressional pressures on the kite, which concentrate on a position about 1/3rd from the tips. I'll be watching for problems here.
Finding the ideal gauze is difficult. The ideal gauze has the following properties: