Making Ram Air Inflated Single Line Kites:Principles and Practical Suggestions.
Design considerations are covered more fully in "Some suggestions for designing ram air inflated single line kites" and stability is covered separately in "Why kites (don't) fly" and etc...
- Shape selection:
Some shapes are not suitable for ram air inflated construction; insects with long thin feelers projecting forward for example. Thin flat appendages, like butterfly wings, are also difficult; they require a large number of bridles and have an almost unstoppable inclination to overfly and cross over the body. Realistic bird shapes are difficult for the same reason. On the other hand , human form shapes can be very successful; the chest area provides the lift while the arms provide stabilising drag. Necks have to be thick enough to prevent the head falling off to one side or the other however which can look ungainly. As explained in "Why kites (don't) fly", aspect ratio (width relative to length) has a strong effect on undercorrection/overcorrection. If a kite tends to overcorrect (figure eight's progressing to looping), this can be reduced by making the kite wider relative to it's length. If it tends to undercorrect (fly's slowly off to one side and descend), make it longer relative to its width. Choosing a shape that's likely to be stable right from the start is the single biggest determinant of success for new designs- after persistence beyond all reason that is!.
- Camber versus reflex:
Over the last 20 years, soft kite have tended towards using reflexive rather than cambered forms. Reflexive or "nose up" profiles have significant stability advantages (not least being their resistance to luffing), but there are occasions when highly cambered forms are useful- see "The main reason why single line kites (sometimes) don't fly". There is an instability mode at high angle of attack (generally in light winds) that can be addressed by using considerable camber- to kick the rear section up at lower wind speeds, reducing the excessive angle of attack that causes the problem. Forward sections still need to be 'nose up' enough to resist luffing at low angles of attack (higher winds) though. Diagonal thru cords are an excellent way to control and adjust reflex/camber- particularly because no re-sewing or skin re-shaping is required for a large range of shape changing.
- Open or Closed leading edge:
Because they have rounded form rather than a sharp edge at the upper leading edge, closed leading edges (by using valved inlets at stagnation points) can cause airflow to remain attached over a kite's upper surface rather than become turbulent. This increases the kite's lift relative to it's drag and therefore tends to make it less stable. Closed leading edge forms can be successful, but it's useful to be aware that extra drag then has to be provided elsewhere.
- CAD (computer assisted design) programs:
These are now VERY good for generating three dimensional shapes from two dimensional pieces- but they're slow. In the time it takes to generate the shape and print out the panels ready for cutting using a typical CAD systems (even for experts), I would expect to not only have designed and made physical patterns for an equivalent kite, but to have built and test flown it in prototype form as well.
The general kite viewing public like conventional colour schemes. These can be boring to kite makers who live with them every day, but pastel combinations generally don't work for the wider audience, while bright colours that are universally regarded as "going" together; rainbow sequences, colour progressions etc, do. If your aim is to make a splash and get noticed at kite events, then go for less 'art' (of the minimalist school), and more 'Disney'. Apropos: Eyes should generally be larger than life, and use colours that are different and brighter than anything else in the kite. Eyes may clash with the main body colours.
- Pilot kite use:
Pilots make kite designing much easier. New designs don't need to be nearly as highly developed before they can be flown in public if they're first used with a Pilot. I notice though, that kites which will initially only fly under a Pilot, often will fly alone after a year or two's refinement. It seems that the characteristics that make kites behave well in a wide range of conditions when flown with a Pilot are the same characteristics that enable solo single line flying.
- Sewing machines:
Yes, it is possible to do accurate work on a domestic machine or an out-dated industrial- provided they're capable of sewing through double cording that is- but why would you want to? Very good new industrial machines with auto thread snipping and precise stitch by stitch speed control and reliable sewing through 6mm and more, cost less than a single maxi kite and at least halve your sewing time while improving the result. Go on- treat yourself!
- Sewing Thread:
Although for seam strength, fine thread (80 to 100 din) and very close spacing is optimal, the usual failure mode for seams on kites is when a thread gets snagged on something, breaks and then gradually unstitches. In practice, 40din at 3.5mm spacing is best. Thread tension should generally be the lightest tension that doesn't loop- more than this will shorten seams and can have deleterious effects on flying and appearance.
- Cording versus ribs:
Except for high performance kites (Pilots and traction kites for example), and some special effects, cording wins every time: Cords are lighter (saving up to 25% by weight). Cords are less expensive (ribs are typically 30% of fabric used). Cords enable shape changing without re-sewing (by length adjustment and adding diagonals ). And, corded kites have slightly more drag (from chordwise bumps), which makes them generally more stable than their ribbed equivalent. Also, cords enable the enclosed air mass to react independently of the kite in rotation- whereas if it's trapped between ribs it can't. This can be beneficial for very large soft kites that tend to undercorrection.
- Lateral symmetry:
Although some kite styles are less sensitive than others, lateral asymmetry is VERY important, a major cause of flying problems. Generally, low performance kites and kites with tails won't be overly sensitive (eg maxi octopus's). High performance kites (eg Rays) and kites without tails (eg Pilots) will be.
Soft kites don't scale: As they are made larger, their weight increases approximately with area, but the mass of air inside increases with the cube, not the square. This mass slows responses and causes very large soft kites to undercorrect. It's a problem that has to be addressed at the design stage (usually be decreasing the kite's aspect ratio as it's made larger): Once a kite is built, nothing much can be done to fix air mass induced undercorrection except by adding weight to move it's centre of gravity rearward.
Most kite makers use far more bridles than are necessary (I used to): It's almost never useful to have any bridles to the rear half of a kite- or even the rear two-thirds. This is because in low wind speeds, when higher angle of attack can assist flying, the kite's weight will cause this anyway, but in higher wind speeds, lower angle of attack allows the kite to de-power (pull less) which is highly desirable. This is best accomplished by having bridles only to the leading edge area.
- Bridle stretch:
This can be a major (and puzzling) cause of instability: When one wing happens to momentarily move faster than the other, it generates more pull. If it's bridles then stretch significantly, the kite can 'self steer', often amplifying the initial movement into a destructive dive or loop.
High tenacity nylon ripstops are excellent for soft kites. Coatings that combine polyurethane and silicon are especially effective; they double the fabric's tear resistance and increase useful kite life from perhaps three hundred to as much as a thousand hours. Not all colours are equally fade resistance though, and this varies from one manufacturer to another. Flouro colours are never satisfactory for high UV countries. Polyester fabrics, although having higher nominal strength and better inherent UV resistance are not nearly as tear resistant as nylons.
Fabric warp and weft are not always at right angles. In critical places, this can be more than enough to cause lateral asymmetry- unless paired panels are placed as mirror images.
Cording (and bridle) size should be appropriate to the loads they will take; neither over-designed nor under-designed Spectra/Dyneema is best because it's lighter and stretches less. But, most important of all is that cording should be hard not soft. Soft line when passed under a sewing machine pressure foot shrinks by as much as 10%. This gives kites a 'hungry dog' appearance, which can occasionally be a deliberate feature (for segmented critters like Trilobites for example), but generally makes a kite unattractive. Cording is critical for lateral symmetry: Cord from the same roll (and that has been equally pre-stretched) should be used for each opposite side pair of cords. For sewing on cording, file a "V" on the underside of your sewing machine's pressure foot, about 1mm to 1.5mm deep. This centralises the cording for high speed stitching - works even for 4mm and larger line. Diamond files (less than $20) are the best because pressure foot steel is usually very hard. Cording may not change direction unless there is another attached cord sewn out from the corner. When bridles or diagonal thru cord are to be attached at a cord cross point, the cords should be attached sufficiently well to each other (by knotting or extensive back tacking for example) so that the bridle or thru cord loads can't break this connection. Bridles should be tied to cording crosses in such a way that they can not strip out along one cord or the other. Even when there appears to be a unidirectional load at an attachment point, the cording should be able to resist loads from every direction, because bridles and rigging lines will occasionally get snagged or pulled from unexpected directions. Thru cords (and bridles) connecting from a kite's flying line to any Pilot kite should usually be stronger than any others on the kite.
- Stress Points:
Bridle points and the attachment point for the Pilot kite require extra reinforcement. Also, intersections of appendages with the main body require special reinforcement: Anywhere there's an angle between sections that is significantly less than 180degrees will be liable to damage. Under arms and between legs for example (both of which have angles of less than 90degrees). Appropriate cording is a complete answer: Sew two cords at approximately 90degrees to each other and crossing at the maximum stress point.
When sewing two panels of the same cut length together, it's quite difficult not to have some fabric left over on one or the other by the end. Tubular tails are illustrative of this- frequently they'll have a twist in them because of progressive miss-registration Experienced sewers can cause either the lower or the upper piece to stretch or shrink a bit as they sew, but the result is often just that they then register correctly at the ends- but will still rarely register perfectly at every intermediate point. Sometimes this is critical; parafoils for example, which won't fly straight unless every seam is correctly registered at every point. For any seam that is thought to be critical, taping or pinning can be used but is far too slow. Making registration marks at 500mm intervals and lining up each pair of marks as you sew is completely satisfactory- and much quicker. For large kites, 10 or even 20mm of miss-registration doesn't make a difference to flying, but at panel intersections, even 3mm will be noticed- an aesthetic concern only , but one I'm rather obsessive about. Using pleats or folds in seams to get things to line up is never acceptable.
- Fabric orientation:
For leading edge and flare panels, either the warp or weft should generally be oriented so as to be exactly parallel to open leading edges and forward facing flare edges. In other places, fabric orientation can have major effects on appearance and flying and bias stretch can often be used to advantage. In parts of kites where lateral symmetry is most important (towards the nose and at wing tips), mirror image panels (to counter out of square warp/weft) should be used - but they make cutting less efficient, and aren't required everywhere.
- Fabric waste:
It's well worth spending a few minutes to save a square metre. In some places where warp/weft orientation is not critical, templates can be swung a few degrees to enables a panel to be cut from an available piece- and this is never a problem if such panels are placed as mirror pairs.
- Sand and water holes:
Every closed space needs at least one sand/water hole. They should be big enough to let out lumps, and should be at the lowest points when the kite is flying at it's usual angle.
- Finishing the job:
When making kites for other people, what may seem like a trivial mistake or easily corrected miss-bridle or flying problem to you, can completely destroy the value of the kite for them.