Traction kites have gained popularity in recent times, along with the increase in popularity of extreme sports. There are now several kite-related sports, using the various designs in new and inventive ways. The main kite sports are kite-buggying, kite-surfing and mountain boarding. There are other fringe sports such as kite-snowboarding and Kiteship, but the general applications are the same.

This technical comparison gives an overall assessment of the various kinds of kites currently available and their applications.

Theory

Kites can often be thought of simply as wings or sails, as the aerodynamic principles are the same, the dynamics of wings and sails have been known and understood for a substantial period of time.

There are a number of factors which affect how kites perform; these factors are related to the size, shape, design of the kite and its simple ability to move through the air.

There are several sets of equations that can give an idea of how kites will perform, yet there are limitless variations of kite design and due to the empirical nature of some of the variables in the equations; a paper comparison of different kites is often not possible.

Aspect Ratio

Aspect ratio (AR) plays a large part in defining how a kite will perform; it is a measure of how long and slender a wing is from tip to tip. Typically, high aspect ratio wings have long wingspans, while low aspect ratio wings have either short wingspans or have thick wings:

Where:

AR = Aspect ratio

d = wingspan of the kite (m)

A = flat area of the kite (m²)

This formula is very useful, as it allows non-rectangular kites to have their aspect ratio calculated in a consistent manner. The importance of aspect ratio is that a wing with high aspect ratio wing has a lower drag and a slightly higher lift than a lower aspect ratio wing. Drag is important: as illustrated in a later section.

Lift

Lift is generated when an object moves through a static fluid, or the fluid moves past a static solid object. It is generated by the difference in velocity between a solid object and a fluid. Lift acts perpendicular to the motion of the fluid versus the object.

Where:

L = lift (Nsm^-2)

A = area of wing (m²⁾

ρ = density of fluid (Nm^-2)

v = Apparent wind speed (ms^-1)

Cl = coefficient of lift

The lift coefficient is a value used to model all of the complex dependencies of lift, such as variances in air density and air viscosity. For simple flow conditions and wing geometries the value for the coefficient of lift can be derived mathematically. However, in general, this variable is determined experimentally.

Drag

Drag acts in a perpendicular direction to the lift of a kite, in the opposite direction to the airflow over the sail; it governs the speed of a kite in flight. Drag is caused by the motion of the kite through the air (induced drag) and by the design of the kite itself (inefficiencies in the properties of the wing). Mathematically, the total drag seen on the wing is:

Where:

D = Drag (Nsm^-2)

A = area of wing facing wind (m²)

ρ = density of fluid (Nm^-2)

v = apparent wind speed (ms^-1)

Cd = Coefficient of Drag

Where:

Cd = Cd_s + Cd_i

Cd_s - coefficient of drag due to wing properties

The Cd_s is similar in definition to the coefficient of lift, but refers to the complex dependencies of drag on shape and inclination. Again the value for the coefficient is determined experimentally, due to its complex and thus empirical nature.

Cd_i - coefficient of drag induced

Where

AR = Aspect ratio of the kite

e = efficiency of the kite

Cl = Coefficient of lift

The efficiency factor is equal to 1.0 for an elliptic distribution and a value less than 1.0 for any other lift distribution.

Ram-Air

The ram-air design is the single biggest development in kite design during recent times. The ram-air design is a two sailed design, allowing it to be used in a variety of applications. Ram-air kites typically come with two varieties of sail support: Sparred or Bridled. The support affects the characteristics, usage and design of the kite. The ram-air style of kites use a two sailed aero-foil, the shape of which is maintained through air intake, via vents along the leading edge, at the front of the kite.

Sparred Ram-Air

This design provides minimal lift and little power, but these kites are very fast and manoeuvrable. Generally only used in a dual-line configuration. It was the original ram-air design and was developed in the 1970’s. The kite which holds the world kite speed record of 160mph is of this design (A Flexifoil Stacker).

Sail shape is maintained with a firm, though slightly flexible, spar. The spar runs the entire length of the kite at the leading edge, below the intake vent. This allows the angle of attack, presented by the sail, to alter throughout flight as air speed varies and the spar flexes.

Bridled Ram-Air

Although this style of design is easily categorised, it has variations and some sub-categories, each having its own characteristics and uses. The variations in design are dependant on the type of venting used to inflate the sail, through the ram-air effect (open intake, partial intake and valved intake).

The bridled ram-air design is similar in conception to the sparred ram-air, but instead of a solid spar the sail is supported by a complex bridle. This reduces the weight of the kite and makes assembly simple. The removal of the spar from the design also allows a quad-line control application to be used, as well as the simpler dual-line concept.

Open Intake

The open intake bridled kites are the original implementation of the ram-air and quad-line approaches to kite design. They have been refined and are now used to produce some of the highest performing and most versatile kite designs. These designs are typically low aspect ratio kites, giving them high efficiency, stability and controlled flying characteristics. They also produce the best power characteristics. They have medium speed in-flight and are easily controlled. Open intake kites are easily inflated, allowing easy launching and landing. As there are no solid parts to this design the kites pack away into small bags and do not require any special operating apparatus.

Due to the nature of the open intake these designs are not water re-launchable. If landed in water they will deflate, fill with water and sink rapidly. They can be used in water related sports due to their flying characteristics i.e. they fly high up in the wind-window and do not fatally over-fly.

A major drawback of this design is the use of 2 sails: when wet the sails stick to one another and degrade performance, water can also collect between the sails. Another disadvantage is that in gusty conditions kites of this design have a tendency to ‘luff’ or partially collapse mid-flight. In the next generation of kite designs this flaw will be worked out.

Closed/Valved Intake

One of the two main types of kite design used in water based kite sports (kite-surfing); this design uses the ram-air style of design, however, along the leading edge there are only a few (typically ~4) valved vents allowing air into the kite. The vents are simple non-return valves made from the same material as the kite. The usage of valves performs two functions:

Due to the small open area, water ingress is limited and so the kite will not sink if landed
Air cannot escape, so the kite will not deflate or sink if landed

The use of valves in place of open intake addresses many of the issues of the ram-air design: There is no problem with kite collapse, or luffing. Air is trapped between the sails permanently the only drawback of the kite getting wet is the extra weight caused by the water sticking to the outside of the kite.

The use of valves also creates its own problems; initial inflation of the kite is far harder and the leading edge of the kite has to be much thicker, in order to accommodate them. This design therefore has to have a higher aspect ratio, slowing the kite, lowering efficiency and power but raising the lift.

Partial Intake

The partial intake design is somewhat of a hybrid between fully open and valved foils. This design is very similar to the fully open intake type, however there are several cells at the end of the leading edge which are not open to allow air intake. The size of the intake is also significantly smaller. The overall effect of these variations on the open intake design is a sleeker kite, more stable kite with higher efficiency. This style of kite has found a home within buggy racing, due to its ultra efficiency and stable performance.

The aspect ratio of this design is very similar to that of the fully open intake style; the extra efficiency is gained from the smaller intake, through the lessening of drag caused by the intake itself.

The negative side to this design is similar to that of the valved design; they are harder to inflate, initially. This design also shares the advantage in being harder to deflate than an open intake style.

Single Sail Designs

The single sail designs have more classic seated roots, and operate on similar principles to that of older non-traction related designs such as stunt kites and the ancient Chinese fighting kites. The more recent developments in single sail design have seen it developed for the likes of kite-surfing, using more modern principles of aerodynamics.

The single biggest flaw in the single sail designs is fatal stalling. Once the kite ‘luffs’ or ‘stalls’, it simply falls out of the sky, recovery is almost impossible. This characteristic is endemic of all the single sail designs.

Inflatable

The new sport of kite surfing has been dominated so far by the inflatable kite. The backbone of the inflatable kite is the air pressure inflated tubes which provide the bony frame to support the sail, presenting a single surface wing profile. These tubes must be somewhat rigid to support the kite shape both span wise and cord wise. Unlike a ram air kite, very few lines are required to support the shape. Control lines are connected at both wing tips only (sometimes in 2 line and more commonly in a 4 line configuration). The air tube floats easily on the water and, without structural damage, will prevent any water from getting inside. The rigidity of the inflated tube however provides some performance limitations. The primary limitation comes from the fact that the front shape must be in a deep arch, generating a low projected aspect ratio. This design typically has a high drag associated, a necessity for re-launching brought about due to the aspect ratio.

Inflatable kites are comparatively slow due to their high drag. Together, slow speed and high drag enable the pilot to handle the kite easily. They do not need to move the kite in the air to generate power because the power is generated by the high drag characteristic. Some of the disadvantages of the design are as follows:

The tubes need to be pumped up by hand prior to launch
The kite body is heavy
Re-launch requires a skilful technique

Sparred

There is only really one model of kite that uses this style of design: the Peter Lynn C-Quad. The C-Quad was the result of the search for a kite that would fly like a traditional foil without filling with water. Whereas in the inflatable style of design the sail-support is provided by rigid, inflatable structures, the sparred design uses stiff but slightly flexible thin fibreglass rods (spars) to support the sail accompanied by a bridle to maintain its shape. As an added extra floatation tubes are available which attach to spar, to aid water re-launch. The profile of these kites is very low; the sails are completely flat pieces of material, only gaining shape when in flight.

Bridled

The NPW or NASA Power Wing is the only example of this style of design. The NPW was originally designed as a brake parachute for the space shuttle but it has since found a limited application in kite-buggying.

The design allows for very good down-wind performance, being simply a shaped sail, but has limited up wind capability. When compared to all of the other kite designs, this style produces by far the largest pull downwind. It produces little lift, and has a large usable wind range. This style of kite uses a simple bridle to maintain its shape and the action of the wind to support the sail.

Comparison

The design of a kite is only a component of what defines its performance characteristics. Variations in the design have a large bearing on how it will perform. Therefore the following comparison is only a generalisation, looking at some typical characteristics of the specified design categories. The usage of the particular design is not exclusive to any one application Since all kites have some form of ‘pull’, kites designed for one application can often be used in others (although the suitability of the kites’ characteristics define the main usage). In order to compare the different designs it is assumed that the kites are of similar area (m²); it is also worth noting that single sailed models are more powerful than with double sailed designs of similar area. When talking about a kites’ power one is usually referring to its lateral lift - namely its ability to ‘pull’ the pilot – which is produced by the kites’ horizontal movement.

Ram-Air kites

Below is a table showing a comparison of the ram-air kite designs, with 4 basic criteria.

	Sparred	Open intake	Closed intake	Partial intake
Lift	Negligible	Medium	Medium/High	Low
Power	Low	Medium	Low/Medium	High
Speed	High	Medium	Low	High
Agility	High	Medium	Low	Medium

Sparred

Although these are the original traction kites, the current use of this design in traction activities is minimal. The main use of this design is recreational, ‘static’ flying. These kites are often considered as beginner’s kites, good for those wishing to learn the basic control principles of powerful kites.

Open intake

This is a good all-round design that suits most applications and is a favourite among mountain boarders and beginners at buggying. Skilled surfers have also been known to use these kites.

Closed intake

The closed intake kites are the only major ram-air design used in water based activities. They are used in mountain boarding activities as well as kite surfing, but are not as high performance as the other kites used in these applications.

Partial intake

These kites are the ideal buggy ‘engine’ (a power source for the buggy). With low lift meaning the pilot does not get pulled out of the buggy and along with high power, speed and agility, these kites are the sports cars of kite design. Predominantly used as full race kites, high performance kites of this design are used throughout buggy racing. These kites are harder to fly, requiring a skilled pilot to safely control them. But they are ideally suited to buggy-racing, with no lift to pull the pilot upwards and out of the buggy and capable of high speeds with the agility to respond quickly.

Single Skin Kites

Below is a table showing a comparison of the single skin kite designs, with 4 basic criteria.

	Inflatable	Sparred	Bridled
Lift	Very High	Low	Low
Power	Medium	Medium/High	Medium/High
Speed	Low	Medium	Medium
Agility	Low/Medium	Medium/High	Low

Inflatable

The inflatable kites are pure surfing kites, with little other application, due to their enormous lift and slow speed.

Sparred

The sparred single sail kites are mainly used as buggy ‘engines’, the low lift makes them ideal candidates for this application.

Bridled

The bridled single skin design is again ideally suited to use as buggy ‘engines’, but this design has reasonably poor upwind performance, coupled with its low agility this design is mainly used as a high wind kite and as a budget ‘engine’. The low cost of this style of kite is due to its simple design.

Conclusion

Tools which can perform many jobs are often not the most suited to any one particular job, and this applies to kite design also. The most versatile kites are the closed ram-air kites, but they are out-performed by most of the other designs in each individual application. The open intake designs have slightly more limited applications, but perform well in most situations, as well as being the best mountain boarding kites available.

Progress in design is not limited to style, with modern technologies and materials more adventurous designs can be undertaken. Simply the application of new materials allows lighter, faster kites, broadening the designs possible. The use of modern supercomputering power is allowing advanced modelling and analysis of the complex situations, presented by kites, and chaotic airflow to take place.

One of most significant recent developments, in the application of kites, is the ‘Kiteship’ endeavour (using very large kites of a bridled single sail design) to race yachts in the International Americas Cup Class events. The ‘Kiteship’ application uses a replacement spinnaker design of kite, up to ~300m² (below right).

Another exciting development is the use of a NASA Parafoil as a descent control device for ‘lifeboats’ from The International Space Station. The ‘kite’ is ~700m² in size and is the largest of its design ever made (shown below). Prototypes have been successfully tested and the design payload is ~10,000kg.

It seems unlikely that there will be any major advances in kite design; the activity itself is possibly thousands of years old. There are many flaws in the current design strategies and these will inevitably be overcome with time. The flaws are generally design specific, and for Ram-Air some of them are as follows:

Open Intake designs are renowned for the wingtips folding over and impairing flight, they also are prone to luffing. (The new generation of Open Intake designs have a greatly reduced incidence of this occurring.)
Partial Intake designs over-fly the pilot, and can have a ‘twitchy’ flight characteristic, making them unpleasant to fly.
Valved Intake designs can be hard to inflate and have a very slight tendency to over-fly.

Some of the most common flaws in the single sail designs are as follows:

The fatal stalls, prevalent in all single sail designs are the only major flaw present in these designs.
The increased weight of the inflatable kites is their only flaw. This is being overcome through the use of new materials.
The Sparred designs are easily broken, no progress has been made to reduce this, new materials may lead to this flaw being worked out, but changes in design will have little affect on this.
The Bridled Single Sail kites are renowned for being hard to steer, with the introduction of the new NPW9 generation of kites; this has been remedied to a degree. The Bridled design itself seems to have reached its limit in terms of performance.

The future developments in kite design are as yet uncertain; the kite industry is in a period of change. Where once there was one large company controlling prices and competition (Flexifoil), as interest in the kite sports has increased and other companies have grown, the scope of the industry has changed. At one point in time there were many small ‘artists’ producing some ingenious kite designs, now the kite design companies are large enough to apply modern research technologies; such as wind tunnels and aerodynamicists. The future in design will most likely lie in the refinement of the current design styles, rather than the development of new ones.

Bibliography and References

Boyce, J. (2003) “The Flexifoil book of Power-Kiting” Navigator Guides

Moulton, R (1997) “Kites: a Practical Handbook for the Modern Kite-flyer” Special Interest Model Books

Van Der Horst, S. and Velthuizen, N., (1996) “Stunt Kites to Make and Fly”. Thoth

Van Der Horst, S. and Velthuizen, N., (1994) “Stunt Kites II: New Designs, Buggies and Boats”. Thoth

Cobrakites “Flexifoil History” at http://www.cobrakite.com/flxhstry.html

(Accessed 12th May 2003)

T. Benson “A Short Index of Aerodynamics Slides” at

http://www.grc.nasa.gov/WWW/K-12/airplane/short.html

(Accessed 17th May 2003)

Viokite “Designer memo for kite surfing” at

http://www.viokite.com/DevelopmentMemoEnglish.htm

(Accessed 17th May 2003)

S. Potter “Windthings – Kiting Advice Centre” at

http://www.windthings.co.uk/static/advice/default.aspx

(Accessed 10th May 2003)

http://www.flexifoil.com

(Accessed 11th May 2003)