SPRING is coming, and with it will come the moderate winds that are good for flying kites. Kite making has recently undergone a small revolution, at least from the amateur's viewpoint, through the introduction of new materials and tools that facilitate the construction of durable kites. Nevertheless, the making and flying of kites are still highly experimental activities. There are no simple definitive equations by which one can predetermine the lift and stability of a kite; the fluid dynamics of an airstream passing a kite is just too complicated for solution unless one gets into fairly sophisticated aerodynamics.

And one cannot treat a kite like an airplane, fabricating a small mock-up for a wind-tunnel test and then determining whether the kite will lift and remain stable. About the only wind tunnel the amateur has is the natural wind itself. Thus regardless of whether you are building a basic design or inventing your own there is only one way to tell if the kite is satisfactory: fly it. The late C. L. Stong wrote an excellent introduction to kites for this department in April, 1969. Recently several good kite books have been published describing not only kite designs but also the new kiting materials. Here I shall discuss the aerodynamics, design and flying of kites and describe a few good designs that are not difficult to build. For much valuable information I am indebted to Peter Pruden of Cleveland and to Judy Neuger, who owns the Kite Kompany, Inc. (33 West Orange Street, Chagrin Falls, Ohio 44022). Kites and kite materials can be bought from that store and many others. A list can be made from the advertisements in Kite Lines, the quarterly magazine of the American Kitefliers Association, to which one can subscribe for $6 per year (7106 Campfield Road, Baltimore. Md. 21207). It is great for both beginners and experienced kite makers and fliers.

Kites are no fun unless you can get them up in the air and then keep them stable or at least controllable. Lift comes from two features of the passing airstream. One feature, the impact and deflection of the airstream on a flat surface flying at an angle to the horizontal, pushes the kite upward. The magnitude of the pushing depends on the wind speed and the cross-sectional area the kite presents to the wind. Increasing either of them increases the lift. With too much wind deflected, however, the kite becomes difficult to handle and may even break up. The greater the wind speed is, the smaller the angle between the kite face and the horizontal should be. Part of the kite's adjustment is automatic, and part must come from proper adjustment of the bridle attaching the kite to the kite string.

Another component of the kite's lift is similar to the lift on a classic airfoil. The airflow around an airfoil can be considered as having two parts, one circulating around the airfoil and the other nonrotational. The actual airflow, at least in a simple model, is a superposition of the two, with the result that the airspeed is greater above the airfoil than below it. Because the air pressure is less the greater the airspeed is, there is more pressure below the foil than above it. This difference in air pressure lifts the foil. A kite gains lift in much the same way, although the application of the simple airflow models may not be very straightforward.

The instability of a kite is divided into three types: roll, pitch and yaw. Roll is a longitudinal rotation, pitch is a lateral rotation and yaw is a rotation around an axis perpendicular to the kite face. Several features can be built into a kite's design to stabilize its flight. They include proper bridling and frame balancing, tails, a bowed or boxed shape. fins on either the face or the back and vents.

The frame of the kite must be carefully balanced to have a symmetrical distribution of weight from left to right as the kite faces the airstream. If one side is heavier than the other, the kite almost certainly will fly erratically.

The bridle is the combination of lines running from the kite to the kite string. Sometimes (but rarely) it is a single string (the kite string itself), in which case the definition is academic. Usually a bridle consists of two or more strings attached to strategic points on the kite and tied in a common knot or to a small ring on the kite line. The tasks of the bridle are to distribute the stress of the air pressure as much as possible, to provide stability and (under some circumstances) to enable the kite flier to maneuver the kite.

The combined pull of the bridle lines provides a force that counters the combination of the other two forces on the kite: its own weight, which acts downward, and the force due to the passage of air, which acts upward and to the rear for a nicely riding kite. Another function of the bridle is to keep the kite at a proper angle of attack: the angle between the kite face and the wind direction. Once you get your kite into the upper winds you may find that its angle of attack is wrong. The only solution is to bring the kite back to the ground in order to adjust the bridle. Some bridles have a spring or a strong rubber band on the lower line so that as the wind speed increases, the angle of attack is automatically adjusted because the wind pressure stretches the spring or the rubber band.

The tail provides lateral stability and is needed on most flat kites. It should not be too long or too short; the correct length can be determined only by experiment, By adding extra drag (as essentially an extension of the kite's spine) the tail should respond to the lateral motion of the kite by pulling in the opposite direction and thereby damping out the lateral oscillations. The tail can be made of a strip of cloth, a length of paper or plastic or a cup or several cups designed to catch and funnel the wind.

Stong's article in this department described the use of cups, called drogues. that were first employed in the 19th century to stabilize kites. If the cup is tapered, with the narrow neck to the rear, it funnels the wind down through a narrower opening and so forces the wind to speed up. The outgoing airstream, moving faster than the surrounding one, helps to maintain the orientation of the drogue. Some kites have side tassels or two tails on outriggers instead of a single tail to provide longitudinal stability.

A great deal of stability can be built into the shape of the kite. Some kites present to the wind a face that has either a sharp angle or a blunt one: it is termed the dihedral angle. The Eddy bowed kite, first introduced by William A. Eddy in 1897, is a classic example of an otherwise flat kite that has been bowed backward to present a curved face to the wind. Stability against gusts is provided by the bowing. When the kite is turned by a sudden gust on one side, the other side rotates to present more surface area to the wind and thereby experiences a greater force, which prevents the kite from turning further. The Eddy kite is so stable against gusts that it needs no tail. The extent of the bowing is usually adjustable, so that more bowing can be created for greater wind speeds.

Other kite designs employ fins on the face or the back to keep the kite stable against gusts. Still others have vents to allow air to stream through unimpeded. Such vents have several functions. They reduce drag on the kite and therefore the force on your end of the kite line. They can also help to eliminate vortexes shed by air passing the sides of the kite.

You may have noticed the seemingly paradoxical venting of most parachutes. Aircraft parachutes have a single hole in the center. Parachutes for slowing down race cars, dragsters and landing aircraft sometimes have so much venting area that the parachute resembles two crossed strips of bandage. With an unvented parachute vortexes are shed alternately on opposite sides of the parachute as the air passes the edges. The air pressure in the vortexes is lower than the ambient air pressure, so that the parachute finds itself with different and periodically changing air pressures on opposite sides. The parachute begins to swing. The swinging builds in amplitude just as the amplitude of swinging for a child is built up by periodic pushes on the swing.

The swinging could be disastrous for the load at the other end of the parachute. Air is vented through the parachute to break up the vortexes and thus reduce the swinging. Venting on some kites serves the same purpose. The vent for a kite can be a simple rectangle or circle or it can be subtler, as in the venting achieved through the cellular structure of a box kite.

You should be somewhat careful about choosing a site for flying your kite. An upward deflection of the wind on the windward side of a hill would be helpful, but the turbulence on the other side would not. Hence it is better to stand at the foot of a hill on the windward side rather than at the top of the hill. Relatively warm ground, as in a parking lot, may provide rising thermals of warm air to contribute lift to your kite. Once the kite is airborne it can ride the stronger winds higher up.

Never fly a kite near a crowd or over motorists, who may be distracted by the sight. Flying a kite in the air lanes near airports is illegal. Never fly a kite with wire as the kite line; even with a string never fly a kite near power lines or during a thunderstorm. (Contrary to the impression many people have, Benjamin Franklin conducted his famous experiment not during a thunderstorm but as a thunderstorm was approaching. Even so, he was lucky not to have been electrocuted.) In anything but gentle breezes wear heavy-duty gloves so that you can grab the line without risking a bad string burn. Finally, be wary of kite eating trees: they have been known to gobble up hours of work in a minute.

To launch a kite hold it about 20 degrees forward of the vertical and in approximately the correct flying attitude, release it as a gust of wind passes and slowly reel out line. Keep the line taut to maintain lift. If the breeze is light, have a friend stand with the kite about 150 feet away from you while you hold the line taut. When a gust comes, your friend releases the kite upward and you pull the line hand over hand. By tugging and bringing the kite forward into the light breeze you give the kite lift.

If the kite flaps and fails to rise, the bridle point is probably too high and should be lowered. If the kite goes crazy and continuously loops, the bridle point is too low and should be raised. When a flat or a bowed kite climbs to the zenith, it no longer presents any cross section to the wind and therefore loses its lift. As it falls (partly upside down) it presents a negative angle of attack, but it can correct itself after it has fallen a sufficient distance. If the kite is high in the air, this type of looping may be fun to watch; if the kite is low, it may crash at the bottom of the loop.

To bring the kite down you probably will reel in the kite line, but with a stronger breeze reeling in may be hard work or impossible. An alternative is to walk down the kite. Secure the end of the line to a stationary object or have a friend hold it as you walk toward the kite and pull the line down hand overhand. Once the kite is lowered into the lighter breezes near the ground reeling is easier.

To build a kite you first must decide on the materials for the two basic parts of the kite, the framework and the covering. In the Orient split bamboo rods traditionally formed the framework. They were light and strong and could be bent without too much trouble. In the U.S. pine sticks are usually employed. Recently fiberglass rods and hollow tubes of aluminum have been offered for the purpose. They are much stronger but are not as easy to work with and are inexpensive. (Cutting fiberglass might even require wearing a mask, since breathing the glass particles is dangerous.) The fiberglass and aluminum rods must be connected with some type of slightly larger plastic tubing. Unless you are already an experienced kite maker, I suggest you build with ordinary round wood dowels 36 or 48 inches long and 3/16 or 1/4 inch in diameter. Rectangular cross-sectional strips with dimensions of about 1/8 by 3/8 inch will also serve. Spruce appears to be the best type of wood, although most other kinds of wood will do. Dowels can be bought inexpensively in any hardware or lumber store. Before buying a dowel carefully inspect it for straightness and straight grain.

The traditional material for covering kites has been either cloth or strong lightweight paper. Cloth is strong and durable but has to be stitched. Paper tears more easily but has the advantage of requiring only common white glue or rubber cement to be fastened around the kite's framework. Recently the thin plastic film Mylar has become popular. It is strong, relatively cheap, light in weight and can be attached with adhesive tape or reinforced tape. You can buy Mylar from kite stores and some hardware stores. Other modern materials are Zephyrlite and Stabilkote, which can either be sewn or taped in place. Both are light and strong and have good resistance to tearing.

To me the best material available is Tyvek, a Du Pont synthetic fabric. Depending on what grade of weight you buy. it feels like either cloth or paper, but it is almost impossible to tear by hand. It has all the advantages of paper in that it is fairly cheap, can be fastened in a few minutes with rubber cement and can be colored with common paints and markers, and its high tear strength makes it superior to paper. Du Pont is selling Tyvek only in large lots, but a smaller amount (27 feet by three feet) can be bought from the L. G. Striegel Mfg. Co. (1223 Arcade Avenue, Louisville, Ky. 40215). Other covering materials include sailcloth (you may be able to obtain remnants from local sailmakers) and nylon rip-stop. Some household materials are also suitable: wrapping paper, brown paper bags and plastic trash bags (the kind used to line the inside of garbage cans).

The kite line can be ordinary cotton string, but nylon string is lighter and stronger. One limitation on the maximum height a kite can reach is the weight of the string it must support, and so a string with a high ratio of strength to weight is best. The breaking point of the string should be high enough to spare you from losing the kite the first time out. Wyatt Brummitt, writing in Kite Lines, has a rule of thumb on what strength you should buy: the breaking point (expressed in pounds) should be at least three times the total front cross-sectional area of the kite (expressed in square feet). The spool on which the kite line is wrapped can be anything from a wood rod, which is what I use, to more convenient spools such as fishing reels. Some kiters fly their kites from the end of a fishing rod, which presents a rather curious sight to passers-by.

Other equipment can make kite building much easier. If your kites are made with wood dowels, the dowels can be glued together after they have been lashed with string. White glue has been favored, but recently kiters have been buying hot-glue guns (about $6 in most department and hardware stores). White glue can take up to an hour to dry, whereas glue from the gun requires only about two minutes. The guns therefore speed up the construction and make it possible to do quick repair work on broken frames. If you use a glue gun, however, you had better keep a bowl of water nearby because eventually you will get hot glue on your skin. Quickly submerging the skin in water is the only way to avoid a bad burn. Two other convenient pieces of equipment are a razor saw (from a hardware or hobby store) for cutting wood dowels and an eyeleteer and eyelets (from a hardware or sewing-supply store) for making reinforced holes in materials.

The bridle can end in a small loop to which the kite line is fastened, but if your kite rotates in midair, the bridle will become twisted. To avoid the problem fasten the bridle to a small metal or plastic ring with a lark's-head knot and then attach a swivel clip (of the type used on fishing lines) between the ring and the kite line. Such a clip can be bought in a sporting-goods store.

Now let us build a kite or two. Here I shall describe several basic kite designs. There are certainly many more that would be interesting to build because of their different aerodynamics. Perhaps I shall return to some of the other designs in a future article in this space.

All the illustrations of the designs I shall discuss are drawn to the same scale in inches; the scale appears on the left side of the illustration below. To measure the length of any one stick or the side of a kite, mark the length in the illustration off on the side of a sheet of paper and then lay the edge of the paper down on the scale. If you want to make a kite that is larger or smaller than my plan, first measure the dimensions of my kite and then scale all of them up or down uniformly. Bear in mind a hazard in making large kites: you have to be able to get the kite through the door.

The first design is a flat diamond kite with a single spar (horizontal stick) lying perpendicular to a single spine (vertical stick) about a quarter of the spine's length from the top of the spine. Cut the wood dowels to size with a razor saw, keeping the cut edges smooth. Mark the intersection points for the crossing of the two sticks, taking care that the point is exactly halfway along the spar. With a touch from a hot-glue gun fasten the two sticks together. After a few minutes you can lash them in place with strong string (fishing line, for example), applying more glue if you like. Alternatively you can first apply glue and then cover the joint with masking tape for added support. Be careful about keeping the sticks accurately perpendicular to each other.

Before assembling the sticks cut narrow notches in the ends with a hacksaw blade. At one end tie the end of a string and run the string through the notch and then around through the other notches in turn until the string outlines the basic diamond of the kite. Tie off the other end of the string. Check the kite for balance by visual inspection and by inserting a needle through the spar-spine joint and seeing if the frame of the kite hangs straight from the needle. If it is obviously unbalanced, you will have to adjust the sticks and the string.

Lay the frame down on whatever material you plan to use for a kite covering. Cut the material in the shape of the kite, leaving extra width on the sides that can be folded over the outlining string to lie on the material inside the diamond. If the material is paper or Tyvek, fasten the flap down with rubber cement. If you are using cloth, stitch the flap down. With Mylar, tape it down. Run a two-legged bridle to the spine. Make a tail of six-inch lengths of cloth tied to a narrow piece of cloth that is about five times longer than the spine.

An Eddy kite is made in much the same way, with the exceptions that no tail is needed and that the spar is bowed backward after the covering is put on the frame. Place one end of the spar on the floor and bend it so that the covering is on the outside of the curve. Run a line from one end of the spar to the other to keep the spar bowed. The bridle lines are run through small holes pierced in the covering and are attached to the spine at the points indicated in the left-hand part of the top illustration on the opposite page. The bowed shape provides the dihedral angle needed for the kite's stability. This kite performs well in light to moderate winds.

The first step in making a third kind of kite, the Nagasaki Hata, is to connect the spar and spine at right angles after making certain of their balance. Lash the joint well. Bow the spar downward and fasten it in place by attaching it to the covering. Attach the kite covering to the spine and add tassels on the spar ends. The tassels can be ribbon of the kind used for a Christmas package. Be sure to balance the kite by appropriately trimming the tassels. Properly balanced, they aid in stabilizing the kite as a tail does. The bowed spar is pushed backward by the wind, producing a curved face similar to that of the Eddy kite, except that with this design the extent of the bowing is determined by the strength of the wind.

A fourth kind of kite, the double-lozenge kite, is similar to the Eddy kite except that a smaller diamond is below the main one. Both diamonds are built on the same spine and are bowed backward. The smaller diamond acts as a rudder to stabilize the kite, which is flown in light to moderate winds.

Many of the Oriental kites are rectangular. The Musha kite is easy to build and has an interesting aerodynamic feature: the trailing edge (below the lower spar) helps to stabilize the kite much as a tail does. This kite can be flown in light to gentle winds.

Bibliography

KITES. Wyatt Brummitt. Golden Press, 1971.

KITE CRAFT. Lee S. Newman and Jay H. Newman. Crown Publishers, 1974.

THE PENGUIN BOOK OF KITES. David Pelham. Penguin Books, 1976.

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