SOME YEARS AGO TWO BOYS OFFERED to mow my lawn if I would help them to build and fly a kite. I agreed. The resulting kite performed so nicely that we extended the contract at the rate of one kite for one mowing and spent a memorable summer discovering the joys of designing, building and flying kites. Although one of the boys ultimately became an aeronautical engineer and the other a biochemist, their enthusiasm for kites lives on, as does mine.

What is a good kite? The answer depends on your tastes and interests. To the skillful enthusiasts of the Orient, where the sport of kite-flying predates recorded history, kites are of three kinds. The first category consists of fighting kites. A good one is so agile and sturdy that it can knock down all competitors in controlled aerial combat. The second category consists of acrobatic kites that can be made to dive, loop, perform figure eights and so on. Designs in the third category appeal primarily to the eye and ear. These kites range in form from simple diamond shapes to figures resembling birds, animals and mythological creatures including dragons up to 100 feet long. The kites are usually gaily colored and may carry bamboo pipes and other instruments that emit musical sounds.

The kites we built belong to still another category: those that reach the highest possible altitude for a given length of string, or, as aeronautical engineers would put it, kites of maximum lift-to-drag ratio. We had fun trying novel designs and contriving jobs for the kites to do, such as measuring the temperature of the air at altitudes up to a few thousand feet and making aerial photographs. We obtained much of the information we needed from the files of the U.S. Weather Bureau, which relied heavily on kites to carry meteorological instruments aloft before the advent of radiosondes, rockets and satellites. At the turn of the century the bureau operated 17 kite stations across the country and set a number of records. For example, on May 5, 1910, the uppermost unit in a train of 10 Weather Bureau kites carried a load of meteorological instruments to an altitude of 24,000 feet. Collectively the kites in the train had a sail area of 683 square feet and exerted a pull of more than 400 pounds on the nine-mile flying "string" of piano wire. The wire was paid out and drawn in by an electric winch.

Our efforts were much less ambitious. Our first kite was a simple diamond formed by a pair of crossed sticks covered with paper. The spline, or vertical stick, was three feet long and the spar, or horizontal stick, was two feet long. The middle of the spar crossed the spline a foot from the top. We fastened the sticks together at the cross with a drop of glue and a binding of string. A saw kerf about a sixteenth of an inch deep was made in the ends of each stick before assembly [see illustration at left].

When the glue had dried, we stretched a string through these kerfs and glued the paper covering to the strings. The kite was then fitted with a bridle, which consisted of a loop of twine tied to the top and bottom of the spline on the papered side of the kite and another loop across the spar. The length of the bridle was adjusted so that the string became taut when the loops were pulled a foot from the paper at a point directly above the cross. The flying string was tied temporarily to the bridle at this point. The point at which the string is attached to the bridle determines the angle at which the air strikes the kite: the angle of attack. Kites, like the mainsail of a sailboat, perform best at angles of attack ranging from about 20 to 25 degrees, greater than the angle at which an airplane wing meets the wind because of the pronounced curvature of the paper. The exact angle of attack at which a kite performs best depends on the strength of the wind and must be determined experimentally. In general the angle must be decreased as the speed of the wind increases. The change is accomplished by shifting the string toward the top of the kite.

Diamond kites are inherently unstable and must be fitted with a tail to hold them upright. Part of the stabilizing force is provided by the weight of the tail, part by friction between the tail and the moving air and part by turbulence generated in the airstream by the tail. The last two forces are known as drag. If the kite is to fly, the downward forces must be exceeded by the lifting force that is developed by the flow of air under and over the paper covering. The tail provides stability but at the cost of lift. The amount of stability required increases with the speed of the wind. A kite that flies nicely with a short tail during a light breeze will spin out of control in a stiff wind. Yet a kite with a long tail that flies well in a stiff wind may not fly in a light breeze.

One solution to this problem is to provide the kite with a tail that consists of a series of wind cones [above]. The cones can be made by removing the bottoms from paper containers, such as paper cups or ice cream cartons. The force developed by $2 air flowing through the cones increases with the speed of the wind and so provides optimum stability through a fairly broad range of wind speeds. Kites with light tails of this kind fly substantially higher than those with tails made of strips of cloth or bundles of paper tied at intervals along a string.

The shape of the diamond kite can be easily modified. For example, three sticks of equal length can be crossed to form a hexagon. A circular form can be made by bending bamboo into a hoop. Combinations of straight sticks and bamboo can be formed into figures of birds. All these shapes fly about equally well when they are equipped with an appropriate stabilizing tail. In the Orient comp]ex designs are often made by stacking a series of kites closely behind one another. The kites are tied together by strings that extend from the corners of the first kite to the comparable corners of succeeding kites. The bridle is attached to the first kite of the series. Succeeding members of the series function both as kites and as the stabilizing tail. To launch a kite of any kind grasp the flying string in one hand and the bottom of the kite in the other. Incline the kite at an angle of about 25 degrees into the wind, with the paper side facing the wind, and let it go. As the kite rises pay out flying string at a rate that allows the kite to continue its ascent. If two people do the flying, let one hold the kite while the other pays out about 50 feet of string. When the string has been pulled snug, the inclined kite is tossed into the air.

The materials for kite making are readily available. Our sticks came mostly from a local lumberyard. We selected clear, straight-grained spruce 3/4 inch thick, six inches wide and six feet long and had the yard saw it into strips from 3/16 inch to 1/4 inch in width. We also made kite frames of wooden dowel stock 1/4 inch in diameter. Dowel stock is heavier than spruce, bends rather easily and is more difficult to assemble than flat sticks. On the other hand, it is available ready-made in most hardware stores.

We used nylon flying string. The breaking strength of nylon is high in proportion to its weight and thickness, and it develops less drag in the wind than other strings we tested, particularly the fuzzy cotton string known as butcher twine. Do not use piano wire for the flying string or any other material that conducts electricity. Kite-fliers have been electrocuted by electrically conducting "string" that made accidental contact with high-voltage power lines. Never fly a kite with a string that is wet or attempt to duplicate Benjamin Franklin's experiment of drawing "the electric fire" from a thunderstorm. Franklin was lucky. A European scientist who followed his instructions for performing the experiment was killed by a bolt of lightning that traveled down the wet string.

Kites up to four feet in diameter can be covered with paper, even newspaper. Light, closely woven silk is better and stronger and develops less drag, but it is expensive. The strongest and aerodynamically most efficient material is Mylar, a plastic sheet that is available from mail-order firms that cater to farmers. The sheets are used to protect grain and hay from the weather.

We next made and flew several Malay kites. Essentially they are diamond kites that achieve stabilization through their shape and therefore need no tail. They fly much higher than kites with tails and are more maneuverable. Stability is achieved by bending the spar back ward with a bowstring [see Figure 3]. Stability increases with the depth of the bow. In the case of a spar two feet long the bow is typically made about four inches deep. When the kite faces squarely into the wind, the force of the wind acts equally on the left and right sides of the surface. When a puff of wind turns the kite so that one side faces more squarely into the wind than the other, the forces are unbalanced. The side facing into the wind then experiences greater force, which rotates the kite to restore the balance. Airplane wings are similarly joined to make a shallow V, known as the dihedral angle, that provides lateral stability. Again, stability is gained at some cost in lift. Malay kites of minimum bow fly higher than those of maximum bow but tend to dart back and forth, to loop and spin. Kites of maximum bow fly steadily but lower. The paper covering of Malay kites should be slack, like the sails of a boat, so that the wind can bend the surface into a uniform curve. A Malay kite has a single bridle string that is tied to the spline.

By fitting a Malay kite with two or more flying strings and a crossed bridle the flight pattern can be controlled from the ground. Two bridle strings of equal length are tied to the top and bottom of the spline and a third string is tied to the ends of the spar. The vertical strings are separated to the left and right and tied symmetrically to the horizontal string [above right]. The flying strings are attached to the points at which the bridles cross. During flight the kite will drift sideways, toward the string that is pulled most, and at a speed that increases with the amount of pull on the string. At the limit of the lateral drift the kite will loop, dive and reverse its lateral direction. The maneuver will twist the flying strings together. They can be unwound by pulling the slack string, which will cause the kite to perform the same maneuver in reverse. The art of making the kite perform other acrobatic stunts can be learned by practice.

For lifting a load such as a thermometer, a barometer or a camera we prefer the French war kite, a triangular box kite that has a pair of triangular wings [above left]. The framework consists of five sticks. In a design of modest size four of the sticks may be three feet long and the fifth one 14 inches long. Two of the long sticks are used for parallel splines, spaced 12 inches apart. The spar crosses the splines a foot from the top. The short stick crosses a foot below the spar and parallel to it. All of the crosses are cemented and bound with string. The completed frame is covered with paper, as in the case of a hexagon kite. The paper is then slit diagonally in the square center space bounded by the splines, the spar and the short stick. The resulting flaps of paper are folded over the adjacent wooden strips and cemented in place. The structure is now a hexagon kite with a square hole in the middle.

The triangular box section is made by equipping the edges of two paper strips with cover strings. Drive four nails partway into a bench top to mark a rectangle a foot wide and three feet long. Stretch a cover string snugly around the four nails and tie the ends together. Cut two strips of paper 14 inches wide and 26 inches long. Slide one of the strips under the rectangular loop of cover string and center it with respect to the nails. Fold an inch of the paper over the string at the sides and cement it in place. Cut the loop of string at the ends midway between the nails. The sides of the strip are now reinforced with strings, the ends extending from the four corners of the strip.

Slit an inch of the paper, close to the string, at the four corners and bend the edge up into one-inch flaps. Prepare the second strip in the same way. To assemble the strips to the body of the kite, first make four small holes in the paper of the body at the outer edge of the splines, where they cross the spar and the short stick. Thread the strings of the paper strips through the holes and tie them securely to the splines. Cement the one-inch flaps of the strips to the body covering. Finally, slip the remaining spline into the bands of paper, center it and cement it to the paper strips. Centering is easy if the paper strips have been marked with a right-angled crease across the middle. The kite can be suspended upside down by strings attached to the ends of the center spline while the cement dries.

Just before the kite is launched the spar is bowed and fastened with a bowstring, as in the case of a Malay kite. The bridle is attached to the center spline of the triangular section. When a French war kite of these proportions is flown in a wind of 15 miles per hour, it will lift a payload of about two ounces per square foot of sail area and will exert a pull on the flying string of about a pound per square foot.

The amount of lift a kite develops in relation to the force of drag that tends to pull it downwind-the lift-to-drag ratio-increases within limits as the width of the kite is increased in relation to its length, a proportion known as the aspect ratio. The aspect ratio of airplane wings is about seven to one; they are long and narrow. The French war kite can be regarded as a triangular box kite with triangular wings of comparatively low aspect ratio. The aspect ratio can be increased by using a spar that is longer than the splines. This improves the lift-to-drag ratio. The modification, when carried to the extreme, results in instability and also in structural weakness unless the spar is made in the form of a truss. We had great fun constructing kites of various proportions and observing their performance.

To achieve maximum altitude we would launch a kite and pay out as much flying string as it would support. When a pronounced sag developed in the flying string, we would attach a second kite and let it rise until the flying string sagged again. A third kite was then attached, and so on. The number of kites that can be flown as a train is limited only by the strength of the flying string. Occasionally the top kite of the train is caught in an updraft and lifted into a wind that blows in a direction that differs from the wind at the surface. The entire train is then carried skyward. On one occasion the upper most kite in one of our trains reached an altitude of more than a mile. The string spiraled downward so that the top kite was directly above our heads. It remained there for about an hour.

We fitted kites with various payloads that could be dropped by parachute or otherwise manipulated. For example, a parachute can be attached to the kite by a simple wire hook that has an extension to serve as a trigger for releasing the load. The trigger can be operated from the ground by placing a paper cone, called a messenger, around the kite string and letting the wind carry it up to the kite, where it strikes the trigger and releases the hook.

It is also possible to release loads and operate camera shutters by means of a fuse. To prepare the fuse soak ordinary cotton twine in a solution made by dissolving as much saltpeter (potassium nitrate) as possible in a cup of water. Let the string dry. When the string is lighted, it will burn without flame at about three inches per minute.

To make a photograph by this technique tie the shutter release of a camera in its closed position with a short length of fuse. Stretch a rubber band over the shutter release from the opposite side, so that it exerts an opposing pull on the release. Fasten the free end of the rubber band to a convenient point on the body of the camera.

Cut a length of fuse for the desired time interval. If you want the shutter to click in 10 minutes, make the fuse about 32 inches long. Thread two inches of this length through the fuse that ties the shutter release and secure it with a knot. The shutter will operate 10 minutes after you light the free end of the 32-inch fuse. The camera can be kept pointed in any direction by fitting it with a wind vane.

The angle at which the kite meets the wind can be increased or decreased during flight by inserting in the bridle a loop of string held closed by a triggered hook or a fuse. When the loop is released, it opens, increasing the length of the bridle and so altering the angle of attack. The modern kite-flier could perform these and other operations by radio control. Versatile radio receivers that weigh less than an ounce and are capable of obeying a number of commands can be built with transistors.

The approximate height of a kite above the ground can be determined by multiplying the length of the flying string by the trigonometric sine of the angle made between the kite, the end of the string and the point directly below the kite. We marked the string with dabs of colored paint at 100-foot intervals: red, yellow, blue, green and black through the first 500 feet, then red-red, red-yellow, red-blue and so on for the next 500 feet. Thereafter we used combinations of three colors, four colors and so on. To measure the angle we fastened a protractor and a plumb bob to a stick with a pair of sights on it [see illustration below]. Multiply the calculated height by .96 to correct for the error caused by sag in the flying string. For example, how high is a kite that flies at an angle of 30 degrees on a 500-foot string? The sine of 30 degrees is .5, and 500 times .5 times .96 is 240 feet.

The aerodynamic features of kites invite innovation. We made several designs, one of which gave us great satisfaction until we learned that it had been invented independently several times. It occurred to us that the lift of a kite might be improved by borrowing a trick from the designers of racing sloops. These boats have both a jib and a mainsail. Aerodynamically the jib makes only a small direct contribution to the forward motion of the boat, but by directing a jet of air at the proper angle behind the mainsail the jib increases the effectiveness of the mainsail about threefold, according to the German yacht designer Manfred Curry The "wing slat" of high-lift airplane wings is another application of the jib principle. Why not fit a kite with a pair of jibs? We decided to do so.

The frame of our jibbed kite resembles that of a Malay kite, consisting of a crossed spline and spar. A strut extends vertically from the cross to leeward and is braced at the outer end by a pair of bowstrings attached to the spline and spar. The sails are made of thin, closely woven nylon. A triangular keel sail resembling a pennant is tied between the strut and the bottom end of the spline [see illustration left]. The triangular mainsail is tied to the ends of the spar and the bottom end of the spline, and it loops over the keel. Two corners of each jib are tied to the ends of the spar and the top of the spline. The third corner of each jib is tied loosely to the cross of the frame. The jib sheets are adjusted so that air flows over the lower surfaces of the jibs and passes over the leading edge of the mainsail. As in the case of a sailboat, the optimum adjustment of the jibs varies with the strength of the wind and must be altered experimentally for maximum lift. The kite appears to develop about twice the lift of a French war kite at wind velocities up to about 12 miles per hour. We had been planning to make a series of wind-tunnel tests of the jib kite, but our memorable summer ended before the tunnel was ready, and my collaborators returned to school.

A note of caution: Always wear leather gloves when you hold a kite string, particularly if you use nylon. A running string can burn and cut your hand. Never fly a kite above the heads of a crowd or on more than 100 feet of string within a mile of an airport.

Bibliography

AERODYNAMICS OF SAILS AND RACING TACTICS. Manfred Curry. Charles Scribners Sons, 1924.

YOUR BOOK OF KITES. Clive Hart. Faber and Faber, 1964.

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