Popular Science Monthly/Volume 64/December 1903/The Tetrahedral Kites of Dr Alexander Graham Bell
|THE TETRAHEDRAL KITES OF DR. ALEXANDER GRAHAM BELL.|
By GILBERT H. GROSVENOR,
EDITOR OF THE NATIONAL GEOGRAPHIC MAGAZINE.
I HAVE been asked by the editor of The Popular Science Monthly to write an article for that journal describing the tetrahedral kites of Dr. Alexander Graham Bell. I am glad to comply with his request, especially as I have had the good fortune for several summers past to watch the marvelous kites which Dr. Bell has been building in his laboratory at beautiful Baddeck, Nova Scotia. In this brief article there is not space to describe all the experiments that have been made, and I shall endeavor to explain, therefore, only the more important principles that I have seen evolved.
Dr. Bell began building kites in 1899. He was led to experiment with them because of his interest in the flying machine problem and his belief that a successful kite will also make a successful flying machine. A kite that will support a man and an engine in a ten-mile breeze will probably also support the man and engine when driven by a motor at the rate of ten miles an hour. This proposition has not been actually proved, but there can be little doubt that it makes no difference whether the kite is supported by the motion of the air against it or by its own motion against the air.
In a calm a kite rises when it is pulled by a man or horse, because of its motion through the air; there is no reason to believe that it would not also rise when urged through the air by propellers. A kite then can be changed to a flying machine by hanging a motor and propellers to it and dropping the string which attaches the kite to the ground.The first kites that Dr. Bell built for his experiments were of the Hargrave box type, which had been the standard kite since its invention by Mr. Laurence Hargrave, of Australia, in 1892. Small Hargrave box kites flew very well, but their flying ability became poorer as their size was increased; a gigantic Hargrave with two cells as big as a small room would not sustain itself in the air, and experiments showed that only a hurricane could make it fly. To obtain much lifting power with box kites it was necessary to send up a number of them hitched on one line. But Dr. Bell's object was great lifting power in one kite and not in a team of kites. He realized that he was thwarted at the very outset by an old law, which was recently formulated by Dr. Simon Newcomb and which has made many believe that the flying machine is impossible without the discovery of a new metal or a new force. This law is that the weight of kites or machines built on exactly the same model increases as the cube, when all the dimensions are increased alike, while the supporting or wing surface increases as the square.
A Hargrave box kite two meters on a side weighs eight times as much as one that is one meter on a side, but it has only four times as much sustaining or wing surface; the weight is tripled, while the wing surface is doubled; hence as the size of a box kite is increased a point soon comes when the weight is so great that the wing or supporting surface will not lift the weight.
Dr. Bell then set to work to see if he could not outwit this law by devising a new form of kite which he could enlarge indefinitely without the weight increasing faster than the wing surface. He saw that if he could get a large kite by combining many small kites instead of by increasing the dimensions of his model the weight would not increase faster than the wing surface. He decided, therefore, to combine many small cells into one large kite instead of using two large cells each as big as a barn door. The Hargrave box cell however did not lend itself to combination. Two box cells fly well, but when a number of them are tied together they do not act with the same harmony. A box cell is structurally weak in all directions and requires a great deal of bracing to keep it from being twisted in a strong breeze; this bracing adds to the weight and makes head resistance to the wind; the more cells combined together, the more bracing required proportionally. Furthermore, the cells must be grouped in two sets at a distance from each other, and as the sets tend to pull apart, the framework connecting the two sets has to be very strong and heavy. As a result the experiments showed that neither
Fig. 1. Kite built of Twelve Triangular Cells. It is formed of two triangular kites, one inside the other.
Fig. 2. Giant Kite built of Triangular Cells. The superstructure consists of seventy kites, like the one in Fig. 1, tied together at the corners and arranged in two sets of thirty-five kites each. Each of these kites was tested individually before being combined and found to fly well by itself. There are a total of 840 triangular cells in the giant kite. The total length of the kite is 29.5 feet. The picture shows the kite rising into the air.
The Method of Building up Kites with Tetrahedral Cells. The four-celled kite B weighs four times as much as one cell and has four times as much wing-surface; the sixteen-celled kite C has sixteen times as much weight and sixteen times as much wing-surface; and the sixty-four celled kite D has sixty-four times as much weight and sixty-four times as much
the efficiency nor the size of a kite could be increased by using many small Hargrave box cells instead of two large box cells.
The problem was then to invent a new cell, one that could be used in combination. Circular cells, polygonal cells of six, eight and
wing-surface. The ratio of weight to surface, therefore, is the same for the larger kites as for the smaller. In the middle of the kites there is an empty space, octahedral in form, which seems to have the same function as the space between the two cells of the Hargrave box kite. The tetrahedral kites that have the largest central spaces preserve their equilibrium best in the air.
twelve sides, and cells of various other shapes were devised, tried and thrown away.
Finally the triangular cell was hit upon. It immediately proved an immense advance over the rectangular Hargrave, being stronger in construction, lighter in weight and offering less head resistance to the wind.
Diagram 2 shows a drawing of a kite built of two triangular cells. The triangular cell needs bracing in one direction only, on its flat surfaces; in a transverse direction it is self-braced, so that internal bracing, which causes head resistance, is unnecessary.
By tying a number of kites built of triangular cells corner to corner, as shown in Fig. 1, Dr. Bell was able to construct a giant kite, Fig. 2, in which the ratio of weight to wing surface is not much more than that of the smaller kites of which it is composed. Combinations of
triangular kites, however, must be arranged in two sets with a powerful connecting framework as shown in Fig. 2. The larger the two sets, the farther apart must they be, and, therefore, the connecting frame becomes exceedingly stout and heavy. This connecting framework is of course dead weight; it is a very serious handicap and soon limits the size of kites that can be built of triangular cells.
By his invention of the triangular cell Dr. Bell was able to build larger kites than he had been able to build before. The old limit of size was stretched considerably, but a limit remained none the less.
The principal improvements of the triangular cell, greater lightness and strength, are due to the cell being self-braced in a transverse direction, from side to side. Longitudinally, fore and aft, it is, however, very weak, like the box cell. Dr. Bell reasoned that a cell could be made self-bracing in every direction by making it triangular in all directions or tetrahedral in form.
Accordingly a number of regular tetrahedral cells, Diagram 3, were built in the laboratory. The experiments made with these cells have given startling results:
First.—A tetrahedral cell has astonishing strength even when composed of very light wooden sticks. As Dr. Bell has expressed it: "It is not simply braced in two directions in space like a triangle, but in three directions like a solid. If I may coin a word, it possesses 'three-dimensional' strength; not 'two-dimensional' strength like a triangle, or 'one-dimensional' strength like a rod. It is the skeleton of a solid, not of a surface or a line."
Second.—A large kite constructed of tetrahedral cells is as solid as a small one, for it is likewise self-braced in all directions.
Third.—A kite built of tetrahedral cells is an almost perfect flier; it is steady in squalls, a good 'lifter' andalmost directly overhead. Tetrahedral cells when combined do not interfere with each other in the least or hurt each other's flying ability as box or triangular cells do when combined.
Fourth.—By the use of the tetrahedral cell it is possible to build kites unlimited in size and in which, however gigantic the kite, the ratio of supporting surface to weight remains the same as in a small kite.
The successive doubling in size of the kite shown in Fig. 3 may be carried on indefinitely without the weight increasing faster than the wing surface. The cells all act in harmony; no part of a kite built of tetrahedral cells has to be strengthened to counterbalance an opposing force or a weakness in some other part of the kite; no weight is thrown away.
By his invention of the regular tetrahedral winged cell, Dr. Bell thus got around the old law which said you can build kites up to a
certain size, but no greater. The adherents of that law have always admitted that the law might be circumvented if a kite could be combined of many small models, but they have denied or at least doubted that a working combination of small models effective enough to carry a man, and to be called a flying machine, could be made. With his tetrahedral cell Dr. Bell has, however, been able to build kites of tremendous power, strong enough to carry up several men. One of his first kites lifted two men off their feet in a squall, and they only saved themselves from an undesirable ascent by instantly dropping the rope. Later this same kite (Fig. 4) snapped its rope, a three-eighth-inch new manila rope, as quickly as a thread. Kites much more powerful than this one have since been built and prove beyond a question that a practical, efficient and powerful method of combination of small forces has been discovered.
Dr. Bell has been building during the past summer thousands of tetrahedral cells varying in size from 25 cm. to 1 meter. Some of them are covered with light red silk weighing about 40 grams to the square meter and others with nainsook, very fine cotton, about as light as the silk. Some of the earlier cells were covered with cheesecloth, but the cheesecloth weighed so much—over 100 grams to the square meter—that Dr. Bell has discarded it. The framework of the cells is usually of black spruce, which is light and strong.
To make a tetrahedral cell, take six sticks of equal length and place three of them on a table so as to make an equilateral triangle. Erect one of the three remaining sticks at each corner of the triangle and bring the tops together. It is the old-fashioned puzzle of making four triangles with six matches. Then cover any two sides and you have a tetrahedral winged cell.
A number of cells outlined against the sky look like a flock of birds; for instance look at Fig. 18; the wings of a cell are also like a bird's wings in that they are not rigid like a board; the silk covering yields somewhat to the pressure of the wind as the feathers of a bird's wing.
Hundreds of tetrahedral cells are now being made in which the frame consists of hollow aluminum tubing. The aluminum weighs very little more than the spruce wood hitherto employed and gives much greater strength to the frame.
Using these cells just as a mason uses bricks to build houses of many forms, he has been constructing kites of every shape that a fertile brain could devise. Steadiness in the air and lifting power have been the main object in all. Some of his combinations are gigantic, exceeding twenty-five feet in length and twelve and fifteen feet in height and width, but in spite of their strength all are so light that his trained assistants send the giant kites up into the air as easily as the little fellows.
The kite shown in Fig. 5 is tetrahedral in form and built of sixteen tetrahedral cells. This was the first tetrahedral kite constructed by Dr. Bell. It is a wonderful flier, darting up from the ground with a shrill whistle and climbing to extraordinary heights. It is a pretty sight to see the operator bring the kite in after the experiment is over.
The kite flies steadily without a turn or quiver as the line is reeled in and finally alights on his hand as gently as a bird. Figs. 6 and 7 show a sixty-four-celled kite made of four kites like the preceding. The kite is two meters on a side. The most remarkable feature of this kite, aside from its perfect equilibrium and steadiness in squalls, is its ability to fly almost directly overhead. Even in the lightest breeze I have rarely seen it flying at an angle of less than eighty degrees. The kite is admirably adapted for meteorological observations at great heights, as it can carry considerable weight with the greatest ease. Fig. 8 shows a kite of the same size but with sixteen cells instead of
Fig. 9. Sixty-four Celled White Flier resting on its Keel.
Fig. 10. White Flier. (Front View.)
sixty-four, the cells being four times as large. The kite is not as successful as the preceding one. Dr. Bell's experiments have convinced him that the small cells are better; when the wind varies in strength as in a squall, the shifting of pressure on a small cell is less than the shifting on a large cell; hence the resultant shifting of pressure on a kite built of small cells is considerably less than on a kite built of large cells. Fig. 8 shows the method of attaching five pounds, a piece of lead in this case; the kite is not disturbed by the weight. The kite
shown in Figs. 9, 10 and 11 is also tetrahedral in form and built of tetrahedral cells. It is twice as large as the red flier, being four meters on a side. Fig. 9 gives a side view and Fig. 10 a front view of the kite as it rests on its keel. The average pull of this kite in light winds is 80 pounds; in a heavy wind it exceeds 150 pounds.
The strength of the kites made of tetrahedral cells is something remarkable. I have seen one of these kites towed on a tetrahedral float for more than a mile on the bay at a speed of eleven or twelve knots without breaking, though one end was dragging one foot under water all the time. As I saw the kite pulled along I expected to see it shattered to pieces, but beyond a few broken sticks it was as well and strong at the end of the journey as when it started.
The big tetrahedral kites, twelve feet and more on a side, look like awkward things to travel with or to store away, but they may be packed as handily and in as small compass as blankets or rugs. Each kite is made in collapsible sections, which open and then fold up, as shown in Fig. 12. Half a dozen large kites can in this way be carried in a trunk from place to place and put together in a few minutes.
The more recent experiments made have been to obtain a giant manlifting kite, or flying machine, that will rise from the surface of a lake. Any one who has ever watched a heavy bird rise from the ground has doubtless noticed that it runs along the ground for a few feet before it rises—the bird must acquire some momentum before its wings can lift its heavy body into the air. The natives of certain parts of the Andes understand this fact very well and by means of it catch the great Andean vultures. A small space is shut in with a high fence and left open at the top. Then a lamb or piece of carrion is placed on the ground inside. Presently a vulture sees the bait and swoops down upon it; but when once he has lighted on the ground inside he can not get out for he has no running space in which to acquire the momentum that is necessary before his wings can lift him. In the same way the first difficulty of all flying machines is to acquire the first momentum that will lift the machine into the air. To overcome this difficulty the flying machine inventor usually shoots his machine from a high platform which makes it necessary for the machine to rise immediately. But if the flying machine can not start in a natural way the chances are its method of working is not right and it is doomed to failure. And even if a machine could fly perfectly after it had been started how could it get up again if it came down for food or fuel at some point where there was no platform and starting apparatus? In a word the solution to the whole flying machine problem if to get a machine that will start of itself without being shot off as if from the mouth of a cannon. The successful machine in rising will probably have to imitate the start of a large and heavy bird—that is glide along the ground or surface of a lake for some distance with constantly increasing speed until it rises of its own momentum.
A little kite, such as that shown in Fig. 5, darts up from the hand if there is the least breath stirring. The larger kite, shown in Figs. 6 and 7, is equally nimble, but in a faint breeze, to raise the large White Flier, shown in Figs. 9 and 10 and which is more than twelve feet on a side, the operator has to run a few yards towing the kite behind him.
Kites larger than the White Flier Dr. Bell sends skyward by tying the rope to the collar of a fast horse and then sending the steed galloping down the field. Of course, when a good wind blows all these kites soar upward as easily as the little fellow.
But to raise the giant kite Mabel II., shown in Fig. 15, Dr. Bell found a more serious problem. It would be difficult for a man or horse to pull the great frame so steadily as to keep her from being dashed against the ground and smashed before she could rise.
The kite has power enough to lift several men, but how was Dr. Bell to get her up into the air? If he could raise Mabel II. naturally, like one of the smaller kites, he could be pretty sure that she would go up when a motor, with propellers, was suspended to her. A pull or a push would be identical in its effect. In a word, if Dr. Bell could get this great man-lifting kite into the air by towing, as he did the smaller kites, lie would succeed in obtaining a successful form for a flying machine.
There are two ways in which Mabel II. might be towed—on wheels along a track or on floats on the surface of a lake. Dr. Bell preferred to try the second method first, as it is simpler and easier.
With tetrahedral frames he built three long boats and covered them with oilcloth to make them watertight. The boats possess great strength, and yet, because of their tetrahedral structure, are so light as not to overweight the kite. The three boats were then ranged parallel to one another and the whole structure placed upon and securely fastened to them.
Fig. 15 shows Mabel II., just before she was launched. This figure and Figs. 16 and 17 give an excellent idea of the construction of the kite. Across the floats extend two bridges, built of tetrahedral cells. Resting on the bridges are four large kites, like the one shown in Fig. 8. The spaces between the four kites are filled with smaller tetrahedral cells. In all there are 272 cells in the structure.
Fig, 18 shows the kite floating merrily on the water waiting to be put to the test. With her tiers of red wings above and white wings below she was a beautiful sight. But would she fly? A small model of Mabel II., shown in Fig. 13 had flown beautifully on land. The flying weight of this model was greater than the flying weight of Mabel II., and Dr. Bell had therefore every reason to believe that Mabel II. would also fly if he could raise her.
When everything was ready Mabel II. was towed out to the center of the bay and her flying line cast aboard the steamer which Dr. Bell had engaged for the experiment. The flying line was made fast to a cleat on deck and the steamer started ahead at full speed, twelve or thirteen knots an hour.But Mabel II. was working under two bad handicaps—first, a
Fig. 15. Mabel II. before Launching.
Fig. 16. End View of Mabel II.
heavy downpour had begun some minutes before the start and had thoroughly drenched the kite, making her so heavy that every one but Dr. Bell urged that the experiment be postponed (when Mabel II. was
weighed after the experiment it was found that the rain water and leakage in the boats had increased her weight by sixty-four pounds); second, the operator on the deck of the steamer had given Mabel II. too short a Hue, so that she was blanketed by the big hull of the steamer and therefore received but a small fraction of the wind of motion.
In spite of these two serious disadvantages, however, as the steamer gathered headway, the great kite first trembled for a few moments, and then rose gracefully from the water and flew steadily the full length of her line.
Fig. 19 shows the kite as she rose from the water after being lowed a short distance. The rain was pouring down in such torrents at the time that my other pictures were not successful.
The experiment was thus a success, and showed conclusively that Dr. Bell has obtained a man-lifting kite, or flying machine, that will rise of itself. If a pull will make the kite rise, there is no reason to doubt that an equally powerful push, such as propellers would give, would be equally successful in causing the kite to ascend.
Though the tests have proved that Mabel II. can easily carry a man and engine, no actual ascents have been made this summer. When ascensions are made the man will sit in the center of the open space between the two bridges (see Fig. 16).
One of the beauties of Dr. Bell's models is that in every one there is a large roomy space in the center where the operator and his passengers can sit. This position is much safer and more comfortable than sitting in a chair suspended some yards below the machine. As the ultimate machine will probably be of tougher material than wood and silk, in time of war the operator and the motor would be protected as well as hidden, instead of being a splendid target for every shot from below.
Kites that are tetrahedral in form, as the red and white fliers shown in Figs. 6 and 9 and those used to form the superstructure of Mabel II., have perfect equilibrium, but because of their small resultant area of horizontal or sustaining surface, their lifting power, though considerable, is not as great as Dr. Bell is satisfied to obtain. His latest combinations have, therefore, been made in the hope of obtaining greater horizontal surface, and thus greater lifting power. In Figs. 21 and 22 is shown a new form of kite, Victor I., which is undoubtedly the most wonderful kite ever devised and put together.
This great H-shaped kite rose from the hand, without running, in a breeze so light that a flag on a pole fifty yards away hung limp and motionless. It glided up and up until it was flying six or seven hundred yards high, steady as a table top. The breeze at that elevation was perhaps five or six miles, though on the ground the movement of the air was so light as to be imperceptible even on the grass or trees. In a breeze of fifteen miles it flew as steadily as before, but nearer the perpendicular and with a tremendous pull.
A glance at the photographs will readily explain what makes the kite such a remarkable flier. The cells of the two wings are reversed, the keels of the cells pointing up instead of down, and the tips pointing down instead of up, while above each tier of cells stretches a wide aeroplane. This wide expanse of sustaining surface helps the winged cells tremendously and at the same time does not interfere with their working. Victor I. is three meters long, three meters wide and one meter deep and weighs only twelve pounds. The flying weight is only three hundred and fifty grams to the square meter of horizontal surface. A smaller kite of similar model has been constructed whose flying weight is about two hundred grams. The wonderful lightness of this model will be better understood when we realize that it carries twenty-five square feet of supporting or horizontal wing surface to one pound of weight, while a wild duck
has only one half of one square foot of wing surface to one pound of weight. The model almost rivals a mosquito in lightness—one pound of mosquitoes represents an area of wing surface of forty-nine square feet.
Dr. Bell is now making a wind boat on this model, and it would not be surprising if this new wind boat should eclipse even the redoubtable Mabel II.
The framework of this latest model is also strong enough to support a man, and yet its flying weight is, as I have said, only 200 grams to the square meter of supporting surface. When we consider that the flying weight of other machines in which the greatest lightness has been striven for is nearly one hundred times as great as in this kite, we realize the tremendous advance made by Dr. Bell in at least one direction—a marvelous combination of lightness and strength.
In not one of the successful kites of Dr. Bell has the flying weight exceeded 500 grams to the square meter of supporting surface, whereas in various other machines the ratio exceeds 10,000 grams to the square meter.
Dr. Bell has thus constructed one form of successful flying machine, Mabel II. Another form, which may be even more successful and of which Victor I. is a model, is nearly completed. To obtain the form of a flying machine has been the principal problem to solve; the matter of a motor is comparatively simple.
The next step is to place a motor on Mabel II., or an enlarged Victor I., with a propeller extending from each side of the kite like an aerial paddle wheel. Strong and light motors are in the market and to be had easily. Then, as the operator sits inside with spinning propellers he can drive the kite up and down the surface of the bay testing how to control and steer her. Later, with the propellers going faster, he can send the kite skimming along a few yards above the surface and continue the experiments at this small height above the water without danger to life.
Finally, by still further increasing the speed of his propellers he can shoot upward and leisurely proceed wherever he may desire. Great speed is not Dr. Bell's object. Ten or fifteen miles an hour is enough to start with.
Dr. Bell has now reached the point where the flying machine is no longer problematical. It is simply a question of time necessary to put things together. Whether the first flying machine carrying a man is built by him at his laboratory in Beinn Bhreagh is probably immaterial to him, but the chances are that if some one else does not build a successful machine within the next yea,r or two he will have a flying machine of his own by that time.
- This article and the illustrations are protected by copyright. The copyright of the first three diagrams and the first four pictures is in the name of the National Geographic Society and the remaining pictures in the name of Gilbert H. Grosvenor.
- 'The Tetrahedral Principle in Kite Structure.' By Dr. Alexander Graham Bell. National Geographic Magazine, June, 1903.
- Figures 1-8, 10, 12, 13, 14, 15 and 20 are from photographs by Mr. David George McCurdy, the photographer of Dr. Bell'? laboratory; the photographs shown in Figures 9 and 11 and those of Mabel II. and Victor I. were taken by the author.