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MODELING
Kites. Tips for a modeller
Directory / Radio control equipment Who among you hasn't flown a kite? But does everyone know what they are? When did they appear? ...For the first time a kite flew into the sky 25 centuries ago. At that time, no one could explain why a kite takes off and what forces act on it in flight. At first, snakes were launched for fun, entertainment. In the countries of the East, for example, kite fights were held. Two kites were launched into the sky, having previously been smeared with glue and sprinkled with crushed glass on the twine holding them on a leash. The winner was the one who was the first to manage to saw through the enemy's twine. Later, kites began to be used for scientific purposes. In his experiments on atmospheric electricity, the American physicist Benjamin Franklin used very large kites. The lifting force of some of them was so great that the scientist could hardly keep them on a leash. Kites helped Franklin prove the electrical origin of lightning, establish the existence of two positive and negative charges - and test the idea of a lightning rod, And at the end of the last century and the beginning of this century, snakes were widely used for meteorological research. With their help, scientists raised instruments to a height of more than 1000 m and measured wind speed, air temperature and humidity, atmospheric pressure ... In our time, interest in kites has not been lost. The creative thought of the inventors of many countries gives rise to more and more new designs of kites: diskoplanes, flywheels, etc. Today we will talk about twenty-three snakes. In the selection there are simple, non-labor-intensive models, there are also more complicated ones. There are no two identical among them: all kites differ from each other either in their flight qualities, or in design, or in manufacturing technology. Any snake from this selection can be made in a pioneer camp or in the yard. Especially for beginner modellers, we have selected four designs. We talk about them in more detail (they are combined in the figure). So kites... Why does a kite fly? A simplified drawing will help us answer this question (Fig. 1). Let the line AB represent the cut of a flat kite. Suppose that our imaginary kite flies from right to left at an angle A to the horizon or the oncoming wind. Consider what forces act on the model in flight.
On takeoff, a dense mass of air impedes the movement of the kite, in other words, puts some pressure on it. Let's denote this pressure as F1. Now let's build the so-called parallelogram of forces and decompose the force F1 into two components - F2 and F3. The F2 force pushes the kite away from us, which means that as it rises, it reduces its initial horizontal speed. Therefore, it is the force of resistance. The other force (F3) pulls the kite up, so let's call it lifting. So, we have determined that two forces act on the kite: the drag force F2 and the lift force F3. Lifting the model into the air (towing it by the rail), we kind of artificially increase the pressure force on the surface of the kite, that is, the force F1. And the faster we scatter, the more this force increases. But the force F1, as you already know, is decomposed into two components: F2 and F3. The weight of the model is constant, and the rail prevents the action of the force F2. This means that the lifting force increases - the kite takes off. It is known that wind speed increases with altitude. That is why, when launching a kite, they try to raise it to such a height where the wind could support the model at one point. In flight, the kite is always at a certain angle to the direction of the wind. Let's try to determine this angle.
Take a rectangular sheet of cardboard (Fig. 2). Attach it exactly in the center to the O-O axis. Let us assume that the sheet rotates about an axis without friction and that in any position it is in a state of equilibrium. Suppose the wind blows with constant force perpendicular to the plane of the sheet. Naturally, in this case, he will not be able to rotate the sheet around the O-O axis, since his action is distributed evenly over the entire sheet. Now let's try to set the sheet at some angle to the wind. We will see how the air flow will immediately return it to its original position, that is, put it under direct angle to the direction of the wind. From this experience it follows: half of the sheet, tilted towards the wind, experiences more pressure than the one that is on the opposite side. Therefore, in order for the plane of the sheet to remain in an inclined position, it is necessary to raise the axis of rotation O-O. The smaller the angle of inclination of the sheet, the higher you need to move the axis. This is how the center of pressure is determined. And the wind force that maintains the plane in an inclined position is the lifting force applied at the center of pressure. But the angle of the kite does not remain constant: after all, the wind never blows at the same speed. That is why, if we tied a string to a kite at one point, for example, at the point where the center of pressure and the center of gravity coincide, it would simply begin to tumble in the air. As you understand, the position of the center of pressure depends on the angle a, and with a gusty wind, this point is constantly shifting. Therefore, in order to make the model more stable, a bridle of two or three or more strings is tied to it. Let's do one more experiment.
Take a stick AB (Fig. 3a). Let it also symbolize the section of a flat kite. We hang it by a thread in the center so that it takes a horizontal position. Then we attach a small weight P not far from its center of gravity, imitating the center of pressure. The wand will immediately lose balance and take an almost vertical position. And now let's try to hang this stick (Fig. 3b) on two threads and tie the same weight to it again: the stick will maintain balance in any position of the weight. This example clearly demonstrates the importance of the bridle, which allows you to freely move the center of pressure without disturbing your balance. Simple calculation Why a kite takes off, we figured it out. Now let's try to calculate its lifting force. The lifting force of a kite is determined by the formula: Fз=K*S*V*N*cos(a), where K=0,096 (coefficient), S - bearing surface (m2), V - wind speed (m/s), N is the normal pressure coefficient (see table) and a - angle of inclination. Example. Initial data: S=0,5 m2; V=6 m/s, a=45°. We find in the table the coefficient of normal pressure: N=4,87 kg/m2. We substitute the values in the formula, we get: Fз=0,096*0,5*6*4,87*0,707=1 кг. The calculation showed that this kite will rise up only if its weight does not exceed 1 kg. The flying qualities of a kite largely depend on the ratio of its weight to the bearing surface: the smaller the ratio of these values, the better the model flies.
What to make snakes Use lightweight and durable materials to build models. Remember: the lighter the kite, the easier it is to fly, the better it will fly. Glue the frame from thin, even shingles - pine, linden or bamboo. Sheathe small models with thin paper (preferably colored), foil or, in extreme cases, newspaper, and larger snakes with cloth, plastic or lavsan film, or even thin cardboard. Separate knots and parts are interconnected with threads, thin wire, glue. Be sure to lubricate the threads wound on the part with glue. For bridles and lifelines, pick up a thin, strong thread. Simple snakes These are paper models for beginners. Some can be done in an hour or two, while others in just a few minutes. Such kites fly well and do not require complex control. So first... paper birds The experience of many researchers has shown that the curved surface of the kite has more lift and stability than the same size, but flat. The simplest snakes of the American engineer Raymond Ninney are surprisingly similar to small birds. They fly well, showing excellent stability in flight. There are several of them in Figure 1 (see a, b, c). In just two or three minutes, the inventor cuts out a rectangle (4:5 aspect ratio) from thick paper or thin cardboard, veneer, foil and bends a bird out of it. Then he attaches a bridle to the body in one or two places - and the kite is ready. In this way, you can make models of any size - it all depends on the strength of the material.
The next design (Fig. 2a) was developed by the American inventor Daniel Karian. Doesn't it look a bit like Ninney's birds? Please note that this snake is stiffened by a frame assembled from pine or spruce sticks and wings closed in a semicircle. For sheathing the frame, the author suggests using fabric: silk, twill, thin linen. Those who wish can experiment with a two- or three-wing design. The inventor believes that if several geometrically similar wings are attached to a long rod, a very funny kite will be obtained (Fig. 2b). Both Raymond Ninney's birds and Daniel Karyan's snakes will fly even in large rooms and corridors, but with one condition: the person launching them must move at a constant speed. Flat snakes... At first, all kites were equipped with bast tails. But... Once a Canadian meteorologist Eddie, who was a lot of kite lover, noticed that the inhabitants of a Malay village were flying tailless kites of an irregular quadrangular shape. Observations helped the meteorologist construct his kite, which you see in Figure 3. This quadrilateral with pairs of equal sides resembles a parallelogram. Such a figure is obtained when two triangles are added with their bases, of which one, ABD, is equilateral, and the other, DIA, is isosceles, with AB:SD as 4:5. Side AB is tied at the ends with a slightly smaller metal string. Therefore, it is slightly curved. The bridle is attached at points O and D, and the fabric (sheathing) is stretched in the upper part, where it forms two small folds. Under the influence of the wind, the kite bends and takes the form of a blunt wedge. In flight, its leading edges, as it were, throw off the oncoming air flow in both directions, so the kite is stable.
Forty years later, the Englishman G. Irwin improved Eddie's design (Fig. 4). It is known that the separation of the air flow behind the leading edge leads to the formation of a swirling region above an obtuse-angled kite. As a result, stability is violated in gusty winds. Irwin did it simply - he cut out two triangular windows in the casing, and the oncoming stream began to rush into these windows. The position of the kite in flight has stabilized. The model shown in Figure 5 was proposed by the Frenchman A. Milie. It consists of a wooden lath AB, pulled together by a string into an arc (the chord AB is 9/10 of the length of the lath). At points O and O1, two identical strips SD and EF are attached to the rail (AO1=OB=0,2*AB). Like the AB rail, the planks are also pulled together by a string into an arc and form an equilateral hexagon in plan. The ends of all the rails are fastened with another string passing through the vertices of the hexagon. The kite you see in Figure 6 is well known in Korea. Its quadrangular frame, glued from bamboo sticks, is covered with fabric. If the size of the two sides is taken equal to 800, and the other two - 700, then the diameter of the hole in the middle should be 300 mm. Look at figure 7. This model, similar to a bird of prey, was invented by the American Sandy Langa. The inventor first tried to test on it the principles of flight, borrowed from nature. The fuselage and tail assembly Lang made from a single wooden slat. At one end, he split it, and inserted the round slats of the supporting wings into the holes of the wooden sleeve. I tied the split part of the tail, the ends of the wings and the nose with a thick fishing line - a very flexible design turned out. And the wing slats were also sprung with rubber shock absorbers. The Langa snake is sensitive to the slightest gusts of wind. In flight, he, like a butterfly, flaps his wings, thereby changing the magnitude of the lifting force, and the drag force, and stability. ...and box Figure 8 shows one of the options for a box kite. It is stable in flight, because its carrier planes are oriented towards the oncoming flow at an optimal angle of attack (the lift generated on them is greater). In addition, its cross section can be not only square, but also rhombic. For a rhombic, the ratio between the vertical and horizontal diagonals is 2:3. The depth of the box is 0,7 times the length of the larger side of the kite. The framework consists of four longitudinal and four spacer rails of rectangular section. The figure shows how the spacer is connected to the longitudinal rail. But the Russian inventor Ivan Konin proposed the design of a box kite, somewhat reminiscent of an airplane. It has two wings (Fig. 9). Thanks to them, the kite rises faster, maintains stability in flight and does not tip over in case of sudden lateral gusts of wind. More difficult snakes Both in design, and in the use of materials, and in the time of manufacture, these aircraft differ from previous ones. They are more modern and sophisticated. But, probably, the more pleasant it will be for experienced modellers to tinker with them: to understand the scheme, to understand the principle of flight, to catch some features. Reactive Many of you have probably observed that if a river overflows widely, the speed of its flow becomes much slower. And vice versa: in a narrow place, the flow velocity increases sharply. In air, as in water, this physical law also operates. Try directing the airflow into the wide end of the conical tube (tapering diffuser) and you will see how the air velocity changes: it will be higher at the exit than at the entrance. In order to obtain jet thrust in practice (namely, this is how the change in the flow velocity in the pipe can be regarded), one condition is required: to fix the diffuser on a large plate. When a flat kite is in the air, a zone of high pressure is created below it, and a zone of low pressure is created above it. Under the influence of the pressure difference, the air flow breaks into the diffuser and passes through the pipe. But the diffuser is conical, so the speed of the outgoing stream will be greater than the incoming one (think of a river). So, the diffuser works like a jet engine. In figure 1 (see page 6) you see the kite of the Englishman Frederick Benson, in the design of which the diffuser effect is used. The inventor claims that the jet thrust not only increases the rate of ascent of the kite, but also gives it additional stability in flight.
The jet kite is arranged quite simply. Two rectangular crossbars are fastened crosswise in the center and tied at the edges with a strong thread. A diffuser bent from thick paper or foil is installed on this frame. Sheathing is ordinary: paper, fabric ... According to the WUA principle It is known that air-cushion vehicles (AHP) rise due to the pressure difference: the pressure under the bottom is always greater than the top. And the stability of the apparatus is created by a special device that evenly distributes the gas flow around the entire perimeter. The American engineer Franklin Bell proved that devices similar to WUAs can fly in the air. Fantasy? No. The kite model is a witness to this (fig. 3 on page 7).
Smooth bottom and sides, a small keel, smooth hull contours - a complex design. But on the other hand, the oncoming air flow flows around the body without disruptions and turbulence and easily lifts the kites. It is easy to see that these aerodynamic advantages are effective not only in climbing. The curved sides of the hull stabilize the position of the kite in the air at high altitude. And the last. Take a closer look: isn't it true that in the longitudinal section the model is somewhat reminiscent of a high-speed motor boat? Taking off... parachute It is generally accepted that a parachute descends only down. A parachute cannot lift a person up, even in an updraft. But a group of Polish engineers tried to refute this opinion. They proved that, under certain conditions, a parachute can rise up. Recall the game familiar from childhood. If you blow on a small parachute - a dandelion seed - from below, it will rise up. Of course, comparing a dandelion and a modern parachute can only be conditional - Polish inventors create a vertically ascending stream of air with powerful fans. But even the usual wind cannot be discounted, says the American Jack Carmen and offers a toy - a parachute kite (Fig. 4). The air current hits the slightly tilted canopy of the parachute and lifts it up. Structurally, the model is no different from the well-known children's parachutes. But there are also differences. For example, to stabilize the flight, a tail is attached to the kite-parachute, and a telescopic tube is fixed in the center under the dome. It serves both as a rigid frame and as a regulator of the position of the center of gravity of the model. In flight drive The device will acquire good stability in flight if you give it the shape of a disk. One of the options for a flying disk is shown in Figure 2. The model is very similar to two low cones stacked together. But the cones will not fly well, according to the inventor Wilbur Bodel from Switzerland, so he complements the design with a keel, as well as a small weight that shifts the center of gravity down (thus increasing the stability of the device), and a hole in the bottom of the skin. But what is this hole for? At altitude, the wind blows stronger than near the ground. And this means that not only its speed changes, but also the pressure. Is it possible to use pressure drops to create additional jet thrust? It turns out you can. With a strong gust of wind, the inner cavity of the kite is filled with a slightly larger amount of air. This means that excess pressure is created inside the snake. When the gust weakens, the pressure from the outside drops and air from the inside rushes out through the hole in the skin. There is, though weak, but a jet stream. It is she who creates additional lifting force. A characteristic feature of this kite is that it can be launched at night. To do this, instead of a weight, Bodel installs a miniature flashlight with a reflector, a light bulb and a 1,5 V battery. In the "Side View" figure, it can be seen that the kite frame is assembled from many rails rigidly fastened together. Pay attention to the characteristic knots connecting the slats to the outer rim, hub and keel. But the diskette of the French engineer Jean Bortier already has three keels. It takes off well, maneuvers smoothly in the air, even in high winds, and hangs motionless on a leash in weak ones. We will tell you in more detail how to make it (see the figure on page 10). Like many other kites, its frame is made of thin wooden slats, fastened with a wire rim and covered with thin paper. So, everything is in order. Prepare four even slats with a section of 3x3 mm for the frame, put them together as shown in the "Top View" picture, glue in the center, tie with threads and coat with glue. Along the perimeter of the frame, bend a rim of steel wire with a diameter of 0,4-0,5 mm and tie it with threads with glue to the ends of the rails (see Fig.). Connect the ends of the rim together and wrap with threads with glue. It is most convenient to dock them in front, in the area of \uXNUMXb\uXNUMXbthe central rail "a". If you do not have a suitable wire, then make a rim from a thick thread. Don't forget to glue it to the rails. Cover the disc and keels with tissue paper or newsprint. Glue the casing to the disk from below - this will significantly reduce the resistance of the model. But you can put paper on top. True, then the skin will have to be glued to all the rails and the rim, otherwise a strong gust of wind will tear it off. Install three keels on the lower surface of the disk (you can get by with one or two, but then the size of the keels will have to be increased) - Keel rims are easiest to make from thin bamboo or pine slats - these materials bend easily, and you can get smooth contours. If you want to make a large kite, then do not forget to strengthen its frame with two or three more slats. Tie a bridle to the finished snake - three short threads. They hold the model at the required angle of attack. Cut the central thread of the bridle in half and tie its ends with a rubber compensator ring. This ring, stretching with strong gusts of wind and unexpected jerks, removes part of the load from the frame. Tie a handrail to the bridle. For a small snake, harsh threads (cord line) are suitable. Test the finished model. As we have already said, the disk kite can be launched even in light winds. And if it's not there at all, try launching the model while towing behind you on the run. Be prepared for any surprises. If the kite suddenly flies in loops or starts to drop sharply, release the rail from your hands without delay - the model will not break when it hits the ground. Pick up the kite and examine it closely; correct distortions; if necessary, reduce the angle of attack (increase the length of the center line) and fly the kite again. If it cannot be adjusted, then the plane of the disk is irreparably skewed. Try attaching a tail to the model from a strip of paper, or a bundle of threads a meter and a half long, or from a lump of paper on a thread. Instead of a frame... air Many inventors do not use slats and paper to make their models, but ... air.
Look at Figure 5. This is an inflatable kite by Canadian inventor Paul Russell (see page 7). In the picture, it only looks complicated from the outside. Very simple actually: two sheets of airtight material were all Russell needed to make the model. Longitudinal and transverse seams-solders divide the internal volume into several interconnected inflatable cavities. Seams give the entire structure the necessary bulk strength. And further. The inflated body has no sharp protruding edges. And this means that there will be no turbulence on the surface of the inflatable kite, and therefore the model will be stable in flight. But to make such a kite is not easy - certain conditions are required in the work. The model of the Finnish engineer S. Ketola (see drawing on page 11) is much easier to manufacture. Seems like it could be easier? I took two pieces of plastic wrap, welded them around the edges and in the middle with a hot iron or soldering iron - and the kite is ready. But how many of you know how to weld the film so that the seams are sealed? We warn beginner modellers in advance: this operation is not easy. Before you start making a kite, try welding a few seams on some plastic bag and test them for leaks. Use an iron with a temperature control. Do not forget to degrease the polyethylene blanks before welding. According to the dimensions indicated in the figure, open two blanks from the film. Put them together and, stepping back from the edge by 10-15 mm, slowly draw the edge of a hot iron or soldering iron around the entire perimeter of the workpieces. In three places of the resulting seam: on the sides - at the bottom and at the top anywhere - leave small holes. Through them you will pump up the snakes. Then weld the blanks diagonally. And so that you are calm about the tightness of the seams, melt the edges of the blanks on the fire of candles. Do this in the fixture shown in the picture. To attach the bridles and tail, burn six holes in the seams with a diameter of 1-2 mm. Do this with a very cold nail or the tip of a candle flame. Inflate the finished model and weld the holes in the outer seam with a candle or, folding the edges of the skin in half, fasten them with paper clips, after moistening the holes with water or lubricating with technical oil. When you learn how to make small inflatable kites, try to make and run a large model - a meter or two meters. Are you strong enough to keep her? helicopter kite Here is a model (Fig. 7, p. 8). But what? "Helicopter", some of us will probably think when they see the rotors. "A kite," others will say, noticing the model's bridle and handrail.
Both are right, according to the author of the invention, American Al Whitekhest. The model successfully combines the properties of a helicopter and a kite. This is easy to verify if you follow how it takes off. The oncoming air flow hits the plane of the kite (in this case, the rotor), a lifting force arises, and the model rises. So it could be if the rotor stood still. But after all, it rotates, which means that lifting force also arises on its blades. Therefore, in flight, the kite receives an additional impulse of energy, pushing the model up. As you can see, the advantages over other types of kites are obvious. And this helicopter kite was made in Brazil by R. Fugast (fig. on page 10). In our opinion, the Brazilian model is the most interesting of the subclass of helicopter-type aircraft. This kite has three rotors: two carriers and one tail. The main rotors, rotating in different directions, create lift, and the tail rotor stabilizes the position of the model during takeoff and keeps it at a height. The design of the kite is extremely simple. The frame is assembled from two longitudinal, glued at an angle, and two transverse rails. The slats are glued together and reinforced with threads with glue for greater rigidity. Carrier rotors are installed on the transverse rail, tail rotors on the longitudinal rail. To ensure that all rotors rotate easily, they are mounted on wire axles. The manufacture of rotors is the most responsible operation. It is necessary to glue the parts carefully, without rushing. The lifting force of the kite depends on how well you make the rotor. We offer you two options for rotors, but there may be more. Try to design a rotor yourself. Test it in action. In the meantime, let's talk about those shown in the figure. First option. This rotor is most suitable for large models. A kite with four, six or eight blades takes off well and keeps well at a height. The rotor is made like this. Glue two pine or bamboo slats crosswise and sheathe them with whatman paper or lime (birch) veneer. In the center of the rotor on both sides, stick a washer made of thin plywood, veneer or celluloid and drill a through hole for the axle. Second option. This rotor resembles a children's spinner. It is good for a small light kite. Such a rotor is assembled from thin bamboo slats (section 3x3 - in the center and 1,5x1,5 mm - at the ends), tissue paper or newsprint, two washers (veneer, celluloid) and a strong thread. Glue the slats together, as shown in the figure, and pull their ends with threads to the base of the blades. Snake or spinner? Watching the flight of an artillery shell, Gustav Magnus discovered a strange phenomenon: with a side wind, the shell deviated from the target up or down. There was an assumption that aerodynamic forces are involved here. But what? Neither Magnus himself nor other physicists could explain this, and perhaps that is why the Magnus effect did not find practical application for a long time. Footballers were the first to use it, although they did not know about the existence of this effect. Probably, every boy knows what a “dry leaf” is, and has heard a lot about the masters of this blow: Salnikov, Lobanovsky and others. Today, the physics of the Magnus effect is explained simply (for more on this, see "Young Technician", 1977, No. 7). Now there is even a whole independent subclass of kites, the principle of flight of which is based on the Magnus effect. One of them is in front of you (Figure 6 on page 8). Its author is American inventor Joy Edwards. This kite is somewhat reminiscent of a spinner. In flight, the body of the kite, like the artillery shell that the German physicist observed, rotates around its axis. At the same time, the wings-blades convert the wind pressure into lift, and maintain the stability of the kite due to the symmetrical streamlined hull and round keel. The kite is designed like this. The central rod of a rectangular section, a round keel and wings-blades form a sufficiently strong body that rotates on two axes fixed on the ends of the rod. The lugs and bridle connect the body with the handrail. It should be emphasized that kites of this type are an almost untouched area of inventive creativity. Now try to make a model that was invented by the American S. Albertson (fig. on page 11). The principle of operation of the Magnus snake (as the author calls his model) is clearly visible from the figure.
Semi-cylinders, mounted on rails and closed at the ends with disks, rotate around their axes under the pressure of the oncoming air flow. If you hook a bridle on these axles and tie them to the handrail, then the device will easily take off. The kite consists of a frame with axes, two half-cylinders, four half-disks and a bridle. The frame is assembled from four longitudinal and two transverse rails (pine, bamboo). Start with him. Glue the rails together, and tightly wrap the joints with threads with glue. Bend the ends of the central longitudinal rails on a soldering iron, as shown in the figure, glue and tie with threads. Then fasten the wire axles to them (the mount is the same as that of the kite-helicopter). For the same axes, tie the bridles. Bend half-cylinders from whatman paper and glue them to the longitudinal rails of the frame. Lastly, install the keels on the frame. (Each of them is made up of two half-discs.) Glue them to the cross rails from the inside so that the rails are on the outside. So, you have built and tested the Magnus kites in flight. What's next? Try experimenting with this aircraft. For example, increase the size of the half-cylinders and the body of the kite. Or make a flying garland of several kites (see fig.). Authors: V.Zavorotov, A.Viktorchik
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