Kites have been put to a variety of uses since they were invented; they have lifted military observers, been used as a vehicle for scientific experiments, and have of course been used as toys by many generations. As a tool, they have been neglected for some time, but currently a renaissance is under way, and they still have a part to play in the further understanding of our world. Kites were used by weather bureaux until the 1930s, reaching altitudes of almost 32,000 feet (Eden, 1989). (In keeping with international aviation convention, altitudes are given in imperial measurements, whilst to further confuse matters, horizontal distances and weight are given metric values). After a long absence, kites are once again being used in meteorological research, lifting instruments into the atmosphere, and holding them in one location for weeks at a time. In one recent concept proving trial, kites reached an altitude of 11,400 feet, carrying 32 kilograms of instrumentation. Future attempts will be based upon the design knowledge gained from this flight, and altitudes in excess of 60,000 feet are predicted (Tyrell, 1992).
Kites have been objects of interest and fascination to people throughout the world for at least 2000 years. There is evidence to suggest that kites were being flown in China as long ago as 200 BC, when a general in the Han dynasty is recorded as having used a kite as an instrument of war, by using it as a method of determining the correct distance to dig a tunnel to enter a palace and end a siege (Hart, 1967). Other uses for kites in Asia included a novel way of fishing (also practised in New Zealand), scaring birds from crops, as a way of lifting construction materials to the tops of buildings, and as a toy. In some Asian countries the kite had considerable religious significance. In Korea, newly born children had kites flown and released for them, taking away any bad luck they had been born with. It appears that kites spread throughout the rest of Asia from China (Hart, 1967), and by the fifteenth century had also reached Europe. Drawings, as well as detailed instructions on how to make simple kites, exist in several manuscripts dating from 1405 AD (Pelham, 1976). Curiously, the best descriptions from this period are in books dealing with magic. Kites served not only as toys, but also as tools in building construction and wars, in scientific experiments and as lifesaving devices. At the end of the last century they were a prime tool in gaining the information and experience that led to the development of the aeroplane.
In eighteenth century Europe the kite showed its usefulness as a scientific instrument. In 1749 a Scottish meteorologist named Alexander Wilson used kites to lift thermometers to a height of 3000 feet to measure temperature variations at altitude (Pelham, 1976). Three years later, Benjamin Franklin used a kite to demonstrate that lightning was similar to the static electricity that scientists were experimenting with at the time. By flying a kite in an electrical storm he was able to observe sparks coming from a key he had suspended from the flying line. Until that time there had been no direct evidence that lightning was an electric current travelling from the ground to the storm cloud. Franklin's experiments led to the development of the lightning conductor, and placed him so high in the regard of the French people (who called him the modern Prometheus) that he was able to travel to France during the American war of independence and obtain financial and military aid for the Americans (Hart, 1967).
George Cayley experimented with kites between 1799 and 1809 in the quest to develop a heavier-than-air flying machine capable of carrying a passenger. He was the first person to describe scientifically the problems that would have to be overcome before man would fly in such a machine. He made a number of observations based upon his practical experiments in the field. Based upon his perceptions of the limitations of the flying devices he had made, he was able to express the conceptualizations that form the basis for the science of aeronautics.
The whole problem is confined within these limits, viz. To make a surface support a given weight by the application of power to the resistance of the air. (Quoted in Gibbs-Smith, 1962, p 217)
From kites, Cayley moved to gliders that incorporated two kite-shaped wings. His experiments culminated, in 1853, in a full sized glider that supported the weight of one of his servants on a flight that lasted perhaps 40 seconds (after the flight the coachman promptly resigned). Cayley had identified the separate properties of lift, thrust and drag, and made a number of prophetic suggestions about what he called "aerial navigation" (Gibbs-Smith, 1962).
By 1826, George Pocock had patented a four stringed kite used for pulling carriages. The four strings allowed the kite to be controlled so that the carriage it pulled behaved much like a sailboat, and could even tack into the wind. Using kites rather than horses, the carriage apparently was capable of reaching speeds of 30 km/h (Pelham, 1976).
Through the latter half of the nineteenth century a number of people experimented with kites as a lifesaving device. It was anticipated that some of these would be carried on the ship, and flown onto the lee shore if the ship was wrecked. Once the kite had been retrieved by people on the shore, heavier lines could be drawn out by the kite's flying line, until a cable strong enough to carry the weight of an adult could be passed from the ship to the shore, and a rescue of the crew and passengers effected by a breeches buoy, basically a seat suspended from a pulley that could be pulled along a supporting line. Whilst the idea is good, there is no recorded rescue using these methods.
In 1833, a British meteorologist, E. D. Archibold, started using kites to lift anemometers to measure wind speed at various altitudes. Meteorological observatories around the world used kites to lift instruments thousands of feet into the air. This gave a great deal of information about the atmosphere, and vastly improved the weather forecasting of the time. Kites were to continue in this role until the mid 1930s, when aircraft and radiosonde balloons finally eclipsed them (Balsley, Williams, Tyrell and Balsley, 1992). In 1887, Archibold was the first person to take an aerial photograph using a kite to lift a camera, an application that is still practised today. Kites have been used as a cheap alternative method of obtaining aerial photographs of archeological sites, reefs, and the remains of shipwrecks. A photograph taken from even a comparatively short distance above ground level can show details not readily apparent from the ground (Pelham, 1976).
By the late nineteenth century the kite was being seen as a serious scientific instrument. Kites were seen as a good starting point in the development of powered, heavier-than-air flying machines. Potential aeroplane builders were tackling the problems of powered flight in a more disciplined manner, making small steps forward, and discovering their new craft as they went along. Rather than trying to create a flying machine in one step, the more careful experimenters worked on one problem at a time, solving each small part of the puzzle and then, and only then, bringing all the parts together in an attempt to build a working aircraft. There were still many people who did not believe that it was possible for a heavier-than-air machine to fly, amongst them being Professor Simon Newcomb (a well-respected astronomer and, in 1903, the only American since Benjamin Franklin to be made an Associate of the Institute of France) and Rear Admiral George Melville, chief engineer for the United States Navy. Even after the Wright brothers had flown, there were many people saying that powered flight was impossible. In hindsight, it seems strange that such a world changing event was virtually ignored until several years after the fact.
Throughout Europe and America, experiments were being undertaken to determine the best sort of design for powered aircraft. The Wright brothers were by no means the only people trying to build an aeroplane; they were simply the first to achieve powered and controllable flight. There were still many fanciful designs being touted as the solution to the problem of powered flight (human powered, flapping wings built onto a bicycle being but one example), but in the main the advocates of powered flight were making gradual but definite progress towards overcoming the intricacies of lift, thrust, drag, stability, and control. Lawrence Hargrave experimented near Sydney in the 1890s with a number of kite designs. He finally settled upon what he called a cellular, or box, kite. He was looking for a stable lifting surface, to which he could add an engine. His experiments led to the development of the cambered aerofoil, a feature that generated much more lift than a flat surface. Put simply, a curved wing surface causes the air traveling over the upper surface to move faster, and travel further, than the air passing along the lower surface of the wing. The air passing over the top surface has a slightly reduced pressure. The greater pressure below the wing exerts an upwards force, called lift, and within certain limits increases with airspeed.
Some of Hargrave's later "gliding kites" were so efficient that they flew at angles of up to 110 degrees. The kite could well be further into the wind than its tether point, which caused a problem if the wind dropped. Hargrave solved this problem by flying this style of kite from a trapeze-like structure, which prevented the kite from reaching the ground if the wind did ease at all.
Most kites fly at angle of less than 50 degrees above the horizon. Efficient kites normally fly at angles of up to 70 degrees. The reason Hargrave's gliding kites flew so well was due to the shape of the wing he developed. It was shaped very much like the cross section of a modern aeroplane wing, with a slight reflex curve at the trailing edge, tilting the nose of the kite up slightly, and producing enough just enough drag to stop the kite falling forwards into the wind.
The cambered aerofoil is the basis of the shape of all aircraft wings today. In his quest for a light power source he also invented the rotary engine. Unfortunately, he was unable to overcome the weight problems that beset so many of the early aviation pioneers, and his dream of a full sized, powered box kite never came to pass. In the course of his research he presented 23 papers and exhibitions to the Royal Society of New South Wales on aviation related topics. Hargrave was more interested in solving the problem of powered flight than in being the person to do it. He did not patent any of his inventions, preferring their benefits to be available to all researchers into the development of the aeroplane. (Grainger, 1978, Walker, 1984, Note 1). In Australia, his achievements are acknowledged by a statue at Stanwell Park, New South Wales, where he carried out his experiments. His likeness, and some of his designs may be seen on one side of our twenty dollar note, and the engineering and science library at Monash University is named after him.
The Wright brothers eventually overcame these weight problems and flew an aeroplane of their own design in 1903. This was the climax of several years of experimentation using kites and gliders. The wing warping system they used to control their aeroplane had been developed by flying their smaller versions as kites, and twisting the wings with four lines from the ground. Because of the extensive flight testing of their designs as kites, and their use of wind tunnels to test ideas about wings and propellers, they were able to collect a great deal of information about the stability of their designs, as well as the amount of lift the glider developed for a given wind. This gave them invaluable information about the necessary size and curvature for the wings of their 1903 "flyer" (Kelly, 1944, Wright, 1953).
Alexander Graham Bell was also trying to invent the first powered aeroplane. He knew of the work being undertaken by Hargrave, and he also experimented with kites to determine the most suitable lifting surface. He finally settled on a cellular kite made of regular tetrahedrons (the shape is best known now as that of the tetra pack that Sunny Boys and similar icy poles come in, a "pyramid" with four triangular sides). Later, he and a number of like minded people set up an association with the express purpose of developing an aeroplane. This association included Tom Selfridge, who later was the first person to die in an aeroplane, when he and Wilbur Wright crashed whilst conducting trials for the United Stated Army, and Glenn Curtis, who later became one of the first aircraft manufacturers in America. Curtis later had to defend himself in court against the Wright brothers, who claimed an exclusive patent on the aeroplane. (Hart, 1967).
Kites again attracted attention in the 1950s and 1960s when Francis Rogallo developed a completely flexible kite, with no rigid supporting spars. Instead of spars, this kite uses the wind itself to hold it open and maintain its shape. Rogallo was an aeronautical engineer working for NASA. He was searching for a controllable recovery system for spacecraft. This kite was developed with the assistance of wind tunnel testing, and is an indication of how far kites have come since they were simply a child's toy. The "Rogallo wing", rather than being used just as a kite, has been put to numerous uses by the American military, and is the basis for hang gliders, and through them, for many of the ultralight aircraft designs being flown today.
Another recent and widely used kite is the parafoil. It too has no spars, sticks, or other form of rigid bracing. It is a true aerofoil, using the force of the wind to inflate the kite and maintain its shape. Of all kite designs, this style gives the greatest amount of lift known to date. This design has been developed into steerable parachutes, and is the basis of the paraglider, a non rigid hang glider. With the development of the parafoil, the evolution of the kite has undergone a new twist. Rather than aeroplanes imitating the shape and form of kites, as happened at the end of the last and the beginning of this century, kites are now starting to imitate the form of the aeroplane (Schimmelpfennig, 1988; Pelham, 1976).
Throughout this and the previous century, kites have been regarded as a tool, as a way of discovering more about the laws of aerodynamics, as a way of testing new ideas safely. It is because of the work of people like Hargrave, Bell, and the Wrights that we now have aircraft that can fly around the world. Their work formed the basis of the body of aeronautical knowledge we now have. They used kites to test their ideas because they saw their kites as scale models of the aeroplanes they were trying to build. The meteorologists who seized upon Hargrave's box kite were trying to find out more about our atmosphere, and they needed a reliable tool that could get their instruments into the air. Kites could, and did, lift their instruments to heights in excess of 30,000 feet, giving the meteorologist much more information to use in the development of forecasting. Kites were a tool that was used to help humans expand their view of the world.
Kites are currently undergoing a resurgence of interest, due mainly to the development of controllable two line kites. At the forefront of this new interest is a group of people using kites to provide traction for small vehicles, on both land and water. The techniques used borrow heavily from sailing and hang gliding, and the kites being used are far in advance of those used by George Pocock in the nineteenth century. It is anticipated that within a few years people will be attempting sea voyages between continents, propelled by kites, and a voyage between continental America and the Hawaiian islands has already been made (Note 2). While the techniques are borrowed from sailing and flying, much of the technology comes from the aerospace industry. The flying lines are a good example of this. High performance, dual control kites require flying lines which have minimal stretch, and which have the narrowest profile possible. They also need to have great strength. Minimal stretch, or elasticity, is needed so that the kite responds instantly to the control inputs from the flier. A thin line presents less surface area to the wind and won't create as much drag,so it won't bow in the wind as much as a thicker line, again allowing the flier to have more precise control over the kite. In search of such lines, we have turned to materials and technologies developed by the aerospace industry. Kevlar and Spectra are now the two most common materials used in flying lines for high performance kites. Kevlar is commonly used in the construction of composite aeroplane skins, and is also used in bulletproof vests. It stretches about three percent during flight. A disadvantage of kevlar is that it readily degrades in ultra violet light. Spectra, which is the best line currently available, is the material used to make the tethers used by astronauts during space walks. It has less than one percent stretch, is thinner than kevlar for the same strength, and is not affected by ultra violet radiation.
Without access to these sorts of materials it would not be possible to build and fly the sorts of kites commonly seen in the sky today. A technology will develop to fulfil a perceived need, but only if appropriate materials and supporting technologies are available.