Some suggestions for designing ram air inflated single line kites.Ram air (soft) kites are those built by the Jalbert principle of using pressure captured from flow stagnation points to inflate the kite's internal spaces, and using these as the kite's structural elements.
In their pure form they use no other framing or battens and have the great virtues of easy packing and no rigid parts that can cause injury to users or innocent bystanders.
Some single line soft kites are built purely for efficiency and stability with no concessions to other aesthetic considerations except maybe for surface decoration. Examples are the Jalbert Parafoil and the Sutton Flowform. Increasingly though, soft kites are built as theme kites with some specific representational shape. For these, flying efficiency is not the prime requirement. For theme kites, that they look like what they are supposed to represent and that they fly reliably are the prime requirements rather than efficiency as measured by angle of flight or pull-for size. In fact, especially for larger theme type kites, NOT developing much pull is usually desirable.
Soft kites are all relatively new, the first flew in about 1953 and theme style soft kites are even more recent than this, not becoming common until the middle 1980's and after.
This has not been nearly long enough for there to be yet much understanding of how to make such kites fly satisfactorily. Designing such theme kites is still very much a matter of trial and error,- and more often error, unfortunately.
The following are some suggestions that can help..
The starting point for designing new kites is shape selection. The inviolable principle here, (and the only principle of kite designing that I'm absolutely certain of) is that: "Not all kites are created equal".
Shapes that are difficult to contrive using only ram air inflation:*It is difficult to make any shape with long thin appendages using ram air inflation as the sole structural element unless they can trail to rearward. Even when such appendages can be caused to inflate and hold their required position such kites will still be less reliable than they would otherwise be during launching and in unstable winds.
This makes most insect shapes difficult to do because they usually have feelers etc projecting forwards. An exception is some beetles that have rearward sloping feelers.
Another exception, or rather, a way around this problem is to design such kites to fly tail first so that the long thin things can trail to rear rather than project at the front. Jurgen Ebinghaus has made a beetle that fly's this way.
*Flat surfaces are also difficult to contrive on ram air inflated kites because any pressure difference across a flexible membrane will cause it to become convex, even if it has been cut flat. For example most of the flattish panels on car shaped soft kites (the sides, roof, and bonnet) tend to bulge out grotesquely.
There are some ways to control this bulging.
Firstly, any surface that lies in or near a plane at right angles to the wind direction will be a stagnation area for airflow and hence have near enough no pressure difference across it from outside to inside. This will therefore cause it to conform approximately to whatever shape it is cut to rather than bulging out- but such surfaces generally flap a lot so the effect won't be smooth. Unfortunately also, surfaces that are required to be flat can rarely be placed so as to meet this criteria.
Secondly, it is possible to rig multiple internal cords or very close spaced ribs inside the kite to hold a surface flat or even concave. The problem with this is that unless there are a huge number of such cords or ribs the surface will still either have an obvious quilted or ribbed appearance.
The third is to construct the areas that are required to appear flat so that they are recessed- either by tying them back with cords or ribs or by contriving some variation of a ram air inflated tubular perimeter and then to stretch a second fabric skin across this recessed area and vent it to atmosphere rather than to the ram air pressure so that the outer skin retains it's flat appearance. Because of the tiny margin over atmospheric that ram air inflation allows (except in ferocious winds), there is not enough compressive strength in any ram air structure to stretch such secondary skins really tight- but it is still a useful, though complex, solution.
Ram air inflated shapes that fail the "aesthetic" test:*Precision shapes can be difficult. The human eye immediately notes even minor asymmetries in common geometric forms. Circles, spheres, squares, straight lines etc have to be perfect or they leave most viewers with a sense of dissatisfaction, of "something not quite right".
An example of this response is the wheels on ram air car shaped kites- because there is a stagnation area along each "tire's" leading edge they always look distorted and wrong.
*Representational forms that we are accustomed to viewing from the top often look unsatisfactory when viewed from underneath. The much belaboured ram-air inflated car is a typical example of this. Car underbodies are unfamiliar except to road kill and mechanics but they are the most visible part of car kites.
Shapes that have stability problems:There are shapes that are more likely or less likely to be stable. However, because kite stability is so complex and poorly understood there aren't many rules about shape/stability that can be stated with confidence.
*From experience to date, long thin ram air inflated forms (when length is, say, more than 5 times diameter) tend to have serious, even terminal stability problems. However, I strongly suspect that some new insight, or a few thousand more hours of stuffing around, or getting lucky just once, might make a nonsense of this rule.
*Kites with reverse sweepback or that are wider across their leading edge than their trailing edge, should, according to theory tend to undercorrection. This is because when they become a bit misaligned with the wind, the leverage exerted by drag on the by now more downwind tip exceeds that of the more upwind tip, resisting correction. Without changing the overall shape there are two things that can be done to minimise this tendency. The first is, for obvious reasons, to make the 'wing' tips as thin and drag free as possible. The second is to contrive some sweepback in the tips, so that when viewed from above, the kite's widest point is somewhat back from it's nose. This increases tip drag on the side that finds itself more upwind when any disturbance occurs, automatically opposing and correcting the displacement. When parafoils or flowforms exhibit this form of undercorrection, a cure is to taper each cell so that the kite is wider at it's trailing edge than across their leading edge. For the Mk 1 Pilot (a parafoil) about 5% of taper in each panel (narrower at the front) was a complete fix. However, Parasleds are much wider at their leading edges and fly very well.
*Appendages projecting out laterally anywhere in a kite's "head" or "shoulder" area appear to benefit stability. Analogously to the use of a balancing pole by tight rope artists, there are good theoretical reasons why this should be so. Human form kites (including anthropomorphic examples such as teddy bears) can therefore be very good fliers.
*Kites that are tubular in their main form and bridled to mid range angles of attack tend to suffer terminally from a form of instability that probably derives from a von Karman flow effect (see "Even more reasons why kites (don't) fly"). A cure is to provide some interrupter that prevents flow attaching over the rear of the section- but except for fish that have continuous fins, this is not often graphically acceptable.
*Aerodynamically efficient forms tend to be less stable. This might seem paradoxical but is true because lift forces are the driving engine of instability, while drag forces generally assist stability. The higher the ratio of lift to drag (usually written as L/D, the main measure of aerodynamic efficiency for everything from aeroplanes to kites), the less inherently stable a kite will be. This doesn't mean that high performance kites cannot fly reliably. They can, but for such kites, the demands that stability makes on bridling and detail shape are such that there can be little freedom left to allow for aesthetic considerations. My Ray style kites are a good example. These are aerodynamically efficient but have been the very devil to develop. Starting in 1988, I have spent more time trying to understand the characteristics of Rays than the total for all the other soft kites I have designed and developed. Notwithstanding the occasional random success (balanced by some miserable failures), it wasn't until 2009 that I managed , finally , to design a version that looks realistic and doesn't require a high drag bucket style tail.
Yes But:While it is true that some shapes will inherently look better and fly more reliably than others, skill, luck, endless iterations and persistence beyond reason makes a difference. There are some shapes that should make ideal kites but endlessly resist and other seemingly less endowed candidates that fly straight off.
Rolf Zimmerman's Lobster is an example of how careful design and clever thinking can make possible soft kites in shapes that do not at first thought appear to be suitable. This is a very fine example of soft theme kite design. Marco Casada and Claudio Capelli's Cherub is another.
Because most soft theme kites are, almost by definition, not ideal shapes from a purely performance perspective they tend to be unstable- often violently so, and usually of the overcorrection type (figure-eighting progressing to looping). Further, it's almost never acceptable, for visual reasons, to have flares fins, or keels to prevent overcorrection, and tails (the best stabilisers of all) aren't always aesthetically satisfactory either. There are however, four other palliatives for overcorrection that are available for kites that are unable to utilise tails or extra lateral area for this purpose.
The most obvious is to attach drag devices. These can often be incorporated into the shape without causing visual offence or are already part of the shape that has been chosen anyway.
An example is octopus tentacles. The Octopus was an obvious choice for a theme kite. However, I found that the drag from tentacles alone was not quite sufficient to eliminate overcorrection. At first I added an extra drogue to the central tentacles. Although this was made in the form of a fish so as not to be visually offensive, it was still less than 100% satisfactory because it often snagged on things (like innocent bystanders) when launching and tended to cause undercorrection (flying off slowly to one side while descending) in some conditions. The eventual solution was to construct the suckers on the tentacles so as to form drag buckets- which solved the problem without offending visual requirements.
Another answer is to increase the kite's angle of attack by changing bridle lengths. This can reduce overcorrection by increasing drag relative to lift (lift forces drive instability, drag forces resist instability). The disadvantage of this approach is that it also usually increases the pull- which is undesirable for large kites. Unfortunately, theme type soft kites commonly start life as not at all naturally stable shapes, so bridling changes by themselves rarely if ever reduce overcorrection sufficiently (or undercorrection either if this is the problem) without unacceptable side effects such as more pull (as above) or poor light wind flying.
A third answer is to remove all rear bridles (or lengthen them until they have only an occasional restraining effect) and change the front bridles so that the kite has a strongly "nose up" aspect curving away to minimal angle of attack from say 1/3rd of the body's length. This effectively makes the front of the body fly as a kite, stabilised by the remainder of the body functioning as a tail. For this system to work there should be at least some minimum flexibility between the "head" and the "tail" but soft kites are generally naturally flexible enough to exceed this minimum.
A fourth answer is to increase the kite's aspect ratio (width relative to length) and making the furthest out parts (the wing tips if it has them) to have as much drag as possible. This works because the further out from a kite's centre of rotation that drag acts at, the better able it is to damp figure-eighting. Conversely, reducing aspect ratio reduces undercorrection if this is the problem. Fortunately, aspect ratio has a very strong effect on stability so changes that are within acceptable graphics restraints are often sufficient. A problem though is that each aspect ratio change requires a new prototype.
Considering carefully in advance what will likely be required to stabilise a new design of kite can avoid a lot of what are basically "selection" problems - that is caused by inherent characteristics of the shape initially chosen or by specifics of the details of that shape as it is built.
By far the most reliable predictive indicator of how suitable a particular shape will be is to judge from similar shape kites. The best example of this is how human form shaped kites of many different specific forms are proliferating. Although not automatically perfect fliers, once examples such as Martin Lester's Scuba Diver, Pierre Fabre's "Spaceman", No Limit's "Super Grover" and Jos Valcke's Clown show what is possible, variants such as our Teddy Bear become obvious. Human form shapes are suitable candidates for soft kites because their head and shoulders can be set up as the lifting surfaces while the body and especially the shoulders, arms and legs function as the "tail".
Inflation points for ram air kites.The basic principle for inflation is to create an entry point to internal spaces from, and only from some point where the airflow over the kite is stalled. Anywhere that airflow is stopped, by Bernoulli's theorem, the kinetic energy of the wind will be converted to static pressure, which by Jalbert's inspired discovery is sufficiently greater than the ambient to be useable as the structural element in inflatable kites. This is called ram air inflation to distinguish it from pump inflation which is required for kites that use pressurised bladders structurally.
The points at which airflow is stalled are called stagnation points. A classic example is the centre line of the leading edge of a cylinder placed at right angles to the wind direction. For most kite shapes, there will be many such stagnation points, at all of which exactly the same maximum pressure is available.
It makes no difference whether a stagnation point at the rear of the kite or at the front is chosen for a given wind speed, they all offer the same boost to the ambient atmospheric pressure.
However, like kite shapes, from a practical point of view, not all stagnation points are created equal. For example the leading edge of a fin or flare is a stagnation point and such have been used as inflation points for soft kites by using two layers of fabric for the fin and venting the gap between them to the kite's interior. The problem with this system is that the volume of pressurised air from such a slit is not great. The actual internal pressure developed in a kite is generally markedly less than stagnation pressure because kites leak (seams especially unless taped) and more so as kites get older. All leakage reduces the internal pressure but if there is a higher volume of replacement pressurised air bleeding in, the loss of internal pressure will be less. The stagnation points from which the highest volume of adequately pressurised air is available will be from locations where there is the greatest area of stalled airflow. Typically this will be in the middle of blunt noses or of relatively large blunt sections.
This is assuming that it is desirable to have internal spaces at maximum available inflation and this is generally true , because forward parts of the kite (stagnation areas or adjacent to) will otherwise appear to be buckled or malformed.
Less than 100% inflation is rarely acceptable for a kite's forward sections but can be advantageous further back by creating more drag to help stability. That this is so is shown by how much more stable kites such as my Octopus design become as they age. Fabric becomes porous and stitched seams stretch with use, causing some inflation loss- and improved stability. Unfortunately, the front outside edges of the head also indent severely. Too much inflation loss also causes tentacle tangling.
Internal pressure can be deliberately reduced by providing some adjustable vent from the kite that can then be used to let some air to continually escape.
For kites that require maximum inflation in some parts but benefit from reduced inflation in other parts to assist stability it is possible to make separate internal spaces and have them independently inflated and controlled. A useful technique when closing one space from another is to first sew in a cylindrical sleeve by one circumferential edge and close the opposite end with a fabric disc- this eliminates the visually annoying sucked in appearance that comes from just sewing a disc or plate in directly.
Because overcorrection tends to worsen with increasing wind speed, sometimes inflation points are deliberately placed where they will be correct when the kite is flying at a higher angle of attack, as is usual in lighter winds, but incorrect as the kites angle increases. This is so that some proportional inflation is automatically lost to assist stability as the wind gets stronger.
Open versus closed leading edges:Closed leading edges are favoured by most designers of single line ram air inflated theme kites because of appearance . Having big open holes at the front of a kite that's supposed to look like some critter quite spoils everything.
However, open leading edges (of the conventional single line parafoil style) can be inherently more stable because they cause more drag and because having a sharp (single layer of fabric projecting forward) upper edge, tend to prevent attached flow over a kite's upper surface, especially when the kite is flying at high angles of attack (which when this can be most destructive of stability). Attached flow causes instability because it increases lift, whereas, turbulent flow adds drag so improves stability.
The exception is the form of open leading edge commonly used for traction kites. These have quite narrow openings positioned below the centre line of the leading edge with carefully formed curved upper leading edge surfaces so are effectively closed leading edges from the perspective of single line kite stability.
Closed leading edges can be valved (a flap of fabric that acts as a non-return valve so as to retain internal pressure through brief periods of adverse or zero wind) or they can be open (inflating through a gauze area like used in my Octopus design for example). Valved systems are advantageous when the available inflation area is small (usually because larger would be unsightly) because every bit of available airflow can be captured and retained - for faster inflation and re-inflation after lulls. Valves do increase pressure stresses enormously when a kite impacts ground or water though- and are a major case of bursting.
If a design tends to overcorrection, and is to have a closed leading edge, then extra stabilising features such as keels, flares, a tail, more 'nose up' or increased aspect ratio will need to be used to compensate.
Peter Lynn, updated July '09.
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