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Written by: Albin Cheenath
Written on: October 1st, 2002
Tags: aerospace engineering, sports & recreation
Thumbnail by: Dembinski/SXC
About the Author
At the time of publication, Albin Cheenath was a junior majoring in Computer Engineering and Computer Science at the University of Southern California.
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Volume I Issue II > Engineering Kites Beyond Flight
Kites have existed for thousands of years, but even today, little is understood about them. While the aerodynamics of a kite are known in theory, in practice deformability makes its behavior highly unpredictable - yet, precise control of kites has rarely been a concern since kite flying has been relegated to the hobbyist's realm. The past decade has seen a revival of interest in kites due to new kite-based sports, novel commercial applications, and scientific uses of kites. As a platform for different activities, kites are attractive due to their low cost, portability, and easy maintenance. This renewed demand is pushing kite-makers to mass-produce and thus develop the science and engineering of kite production on a new level of sophistication. The role of engineering in kite making has led to several innovations and discoveries in aviation science and has shown that there is much to be grasped and many inroads to be made in this field.

Introduction

In 1826 an English schoolmaster named George Pocock patented an unusual invention: a carriage drawn by kites that could travel at about 20 miles per hour. Wherever it went, the carriage baffled and amazed people. A more significant perk resulted from the toll fee conventions of the time - since a carriage was charged according to the number of horses pulling it, Pocock paid nothing for his kite-drawn vehicle [1]. Such innovative kite usage contrasts with the conventional image one envisions of young children tugging at their little kites.
Kites are highly complex, belonging to the class of deformable flying structures that are more difficult to control than rigid flying structures, such as airplanes. The deformation of such objects alters the flying characteristics, thus leading to difficulties in maneuvering. Consequently, the way in which the flying object is controlled needs to be continually readjusted. Currently, one cannot predict the way in which the object will deform in response to continually changing factors in the environment, like wind and temperature. There is much to be learned about kites, and the work of engineers reveals the potential possibilities.
Dembinski/SXC
Figure 1: Although most commonly seen in recreational settings, kites also have relevant commercial applications in other fields.
Despite the prevalence of kites through the centuries and in many cultures, not much is understood about them. Even when kites were used for highly sensitive or important work, their ability to soar and cover long distances was the main focus; precision control was unimportant. Though human lives often depend on other deformable objects, such as sails and parachutes, such was not the case concerning kites, so proper research was rarely conducted. While most commonly seen in recreational settings, kites are capable of and have been used in much more influential endeavors, serving as the launching pad for several revolutionizing discoveries and technologies (Fig. 1).

A Short History Lesson on Kites

If it weren't for kites, Wilber and Oliver may never have invented the airplane - the brothers originally explored the idea of creating a flying machine by testing many kite designs. Their final airplane design was based on the box kite invented by an Australian scientist, Lawrence Hargrave [2]. Their belief that successful human flight could utilize the elements of kite design set them apart from the numerous would-be inventors. Benjamin Franklin's famous lightning experiment using a kite also exemplifies the way in which kites allowed scientists to conduct experiments at high altitudes.
Kites also played an important role during World Wars I and II, in which the French used them to quickly detect enemy movements, and the U.S. Navy implemented them for target practice. A Chinese general in the Han Dynasty used a kite to measure the distance into an enemy's fortress and built a tunnel that led him to a quick victory in battle; even the ancient Egyptians used kites to lift heavy loads when building structures [3]. Despite all this, kite usage has been limited in scope, and little scientific research has been done on them.
In recent years, increased interest in harnessing the power and simplicity of kites has turned the art of kite making into a science. Recreational kite flying is separating into new sports like kite-buggying, kite snowboarding, and kite sailing. Kites are also being developed for the purpose of capturing images in aerial photography at a lower cost.

How A Kite Flies

Though most people associate kites with the popular diamond-shape, they actually come in a variety of shapes and sizes depending on purpose. All kites share the aerodynamic principles concerning airfoils - objects that can unevenly vary air pressure around them by changing orientation and direction. Kites are heavier-than-air objects that overcome gravity by "catching" winds that force them upwards. As air tries to go around a kite, it travels faster above the kite and slower beneath the kite; this difference in pressure causes the kite to fly upwards since it is pushed above into the area of lower pressure. Swiss mathematician Daniel Bernoulli discovered that the pressure decreases as the velocity of a fluid (or air) increases - the Bernoulli effect helps explain the behavior of airplane wings, rudders and kites.
The forces of gravity, drag, thrust, and lift also affect the flight of a kite. Gravity is the Earth's force pulling the kite downward, while lift is the upward force due to greater air pressure beneath a kite. The force exerted by air pressure under a kite due to wind resistance is called thrust, and drag is the downward pull resulting from a combination of the kite's weight, shape, and angle to the ground. The lift must surpass the drag for a kite to fly, and the lift-to-drag ratio is determined by the angle of the kite to the ground - the greater the ratio, the higher and more easily the kite soars [4]. Drag can, however, be used to gain greater stability and control over a kite. The manner in which a kite flies greatly depends on the design choices made by the kite-maker; as kite flying has regained popularity in the past decade, a plethora of sophisticated kite designs have been introduced and revived.

Past Kite Innovations Still At Use in the Present

Just as recent interest in kite flying has triggered a renewed interest in kite-design-related engineering, a similar period occurred a century ago as humankind tried to take to the skies. Lawrence Hargrave, one of the first scientists to work on kite design during this period, invented the box kite and extended kite design into the ambit of engineering. While exploring the possibility of human flight, Hargrave experimented with the goal of a design that would be stable during flight. His subsequent research revealed that curved surfaces were far superior to flat surfaces for creating lift; the empty central portion of the box kite lends stability. This kite is useful in meteorological study, aircraft design, and reconnaissance, most currently as a stable platform for elevating payloads containing scientific equipment.
Another innovation in kite design was Francis Rogallo's flexi-wing kite created in 1948, which eventually became the basis of hang-glider design. Due to its likeness in shape to the Greek letter Delta, flexi-wing kites are more commonly known as delta kites. They are considered one of the easiest types to fly and, unlike others, soar with the wind rather than against it, allowing flight in very light winds.

Latest Kite Types and Their Designs

The merging of extreme sports, such as snowboarding and mountain climbing, into mainstream sports has been the result of sports enthusiasts pushing the limits of tradition. Consequently, a whole new genre of kites called power kites has been developed. These massive kites are used in sports like kite-buggying and kite surfing, in which the kite pulls a person at 20-30 miles per hour on land or water. Created by Domina Jalbert in the 1960s, these kites, also known as parafoils, are unique in that they have no frame and are completely made of fabric. A parafoil consists of multiple cells filled with air by the wind. The air-filled cells give the kite its structure and have enough lift to carry heavy payloads. Modern parachutes are, in fact, large parafoils.
Stunt kites are another type of kite becoming popular among kite enthusiasts. These kites can perform a variety of tricks and are generally 5-7 feet wide. Also known as sport kites, they generally have two to four separate lines that make them highly maneuverable [5]. Light and boomerang-shaped, stunt kites can travel at speeds up to 120 mph. Specialty stunt kites called ultralights are made of extremely lightweight materials, like graphite rods and polyester cloth, allowing for flight in nearly windless conditions.

Conclusion

The development of kites beyond the traditional diamond shape and paper-and-sticks construction is reflected in the necessity of sub-categories appropriate for the variety that exists. The designs of these kites range in traction properties, control, ease of use, and are flown in very different ways. Rather than simply discounting kites as children's toys, people have begun to recognize the multitude of benefits that kites offer, such as simplicity, inexpensiveness, and ease of set up, repair, and transport. These strengths coupled with their ability to soar into the sky with minimal aid make them very attractive to sports enthusiasts, scientists, and businesses alike.
However, serious research on kites has long been overdue - the development of new testing platforms and more standardized methods are being used to explore the possibilities of kite design [6].

References

    • [1] "History of Kites." The Drachen Foundation. Internet: http://www.drachen.o​rg/history-5.html, [10 Jan 2002].
    • [2] Joseph F. McKenna. "Mr. Hargrave's Box Kite." Tooling and Production 66.1 (2001):104.
    • [3] Shawn Carlson. "Using a Kite as An Experimental Platform". Scientific American, vol. 283(3), pp. 98-9 [2000]..
    • [4] Tom Benson. "Aerodynamics of Kites." Glenn Research Center. Internet: http://www.grc.nasa.​gov/WWW/K-12/airplan​e/kiteaero.html, [10 Jan 2002].
    • [5] K. Alexander and J. Stevenson. "Kite Equilibrium and Bridle Length." Aeronautical Journal, vol. 105(1051), pp. 535-41, 2001.
    • [6] K. Alexander and J. Stevenson. "A Test Rig for Kite Performance Measurement." Proceedings of the Institution of Mechanical Engineers. Part B, Journal of Engineering Manufacture 215.B4, pp. 595-8, 2001.
    • [7] B. B. Balsley, J.B. Williams, and G. W. Tyrrell. "Atmospheric Research Using Kites: Here We Go Again!" Bulletin of the American Meteorological Society, vol. 73, pp. 17-29, 1992.
    • [8] Damien du Toit. "Modern Kites." Your Online Introduction to Kites and Kite Flying. Internet: http://www.geocities​.com/Colosseum/4569/​modern.htm, [10 Jan 2002].
    • [9] "Kite Science: Aerodynamics." Aeolian Kites. Internet: http://www.win.tue.n​l/~pp/kites/fak/scie​nce/kite.aerodynamic​s.html.