by the multiplication of smaller kites into a cellular structure the results are very different. My own experiments with compound kites composed of triangular cells connected corner to corner have amply demonstrated the fact that the dimensions of such a kite may be increased to a very considerable extent without materially increasing the ratio of weight to supporting surface; and upon the tetrahedral plan (Fig. 16) the weight relatively to the wing-surface remains the same however large the compound kite may be. The indefinite expansion of the triangular construction is limited by the fact that dead weight in the form of empty framework is necessary in the central space between the sets of cells (see Fig. 6), so that the necessary increase of this space when the dimensions of the compound kite are materially increased-in order to preserve the stability of the kite in the air-adds still more dead weight to the larger structures. Upon the tetrahedral plan illustrated in Figs. 14, 15, 16, no necessity exists for empty frameworks in the central spaces, for the rably adapted for the support of aerosurfaces of any desired kind, size, or shape (aeroplanes or aerocurves, etc., large or small). In further illustration of the tetrahedral principle as applied to kite construction, I show in figure 17 a photograph of a kite which is not itself tetrahedral in form, but the framework of which is built up of tetrahedral cells. This kite, although very different in construction and appearance from the Aerodrome of Professor Langley, which FIG. 18-THE AERODROME KITE JUST RISING INTO THE AIR WHEN PULLED BY A HORSE mode of construction gives solidity I saw in successful flight over the Potowithout it.. Tetrahedral kites combine in a marked degree the qualities of strength, lightness, and steady flight; but further experiments are required before deciding that this form is the best for a kite, or that winged cells without horizontal aeroplanes constitute the best arrangement of aero-surfaces. The tetrahedral principle enables us to construct out of light materials solid frameworks of almost any desired form, and the resulting structures are admi mac a few years ago, has yet a suggestiveness of the Aerodrome about it, and it was indeed Professor Langley's apparatus that led me to the conception of this form. The wing surfaces consist of horizontal aeroplanes, with oblique steadying surfaces at the extremities. The body of the machine has the form of a boat, and the superstructure forming the support for the aeroplanes extends across the boat on either side at two points near the bow and stern. The aeroplane surfaces form substantially two pairs of wings, arranged dragon-fly fashion. FIG. 19-AERODROME KITE IN THE AIR The whole framework for the boat and wings is formed of tetrahedral cells having the form of the regular tetrahedron, with the exception of the diagonal bracing at the bottom of the superstructure; and the kite turns out to be strong, light, and a steady flyer. I have flown this kite in a calm by attaching the cord-in this case a Manila rope-to a galloping horse. Figure 18 shows a photograph of the kite just rising into the air, with the horse in the foreground, but the connecting rope does not show. Figure 19 is a photograph of the kite at its point of greatest elevation, but the horse does not appear in the picture. Upon releasing the rope the kite descended so gently that no damage was done to the apparatus by contact with the ground. Figure 20 shows a modified form of the same kite, in which, in addition to the central boat, there were two side floats, thus adapting the whole structure to float upon water without upsetting. An attempt which almost ended disastrously, was made to fly this kite in a good sailing breeze, but a squall struck it before it was let go. The kite went up, lifting the two men who held it off their feet. Of course they let go instantly, and the kite rose steadily in the air until the flying cord (a Manila rope 38 inch diameter) made an angle with the horizon of about 45° when the rope snapped under the strain. Tremendous oscillations of a pitching character ensued; but the kite was at such an elevation when the accident happened, that the oscillations had time to die down before the kite reached the ground, when it landed safely upon even keel in an adjoining field and was found to be quite uninjured by its rough experience. Kites of this type have a much greater lifting power than one would at first sight suppose. The natural assumption is that the winged superstructure alone supports the kite in the air, and that the boat body and floats represent mere dead-load and head resistance. But this is far from being the case. Boatshaped bodies having a V-shaped crosssection are themselves capable of flight and expose considerable surface to the wind. I have successfully flown a boat of this kind as a kite without any superstructure whatever, and although it did not fly well, it certainly supported itself in the air, thus demonstrating the fact. that the boat surface is an element of support in compound structures like those shown in figures 17 and 20. Of course the use of a tetrahedral cell is not limited to the construction of a framework for kites and flying - machines. It is applicable to any kind of structure whatever in which it is desirable to combine the qualities of strength and lightness. Just as we can build houses of all kinds out of bricks, so we can build structures of all sorts out of tetrahedral frames, and the structures can be so formed as to possess the same qualities of strength and lightness which are characteristic of the individual cells. I have already built a house, a framework for a giant wind-break, three or four boats, as well as several forms of kites, out of these elements. It is not my object in this communication to describe the experiments that have been made in my Nova Scotia laboratory, but simply to bring to your attention the importance of the tetrahedral principle in kite construction. APPENDIX Copyright, 1903, by the National Geographic Magazine Through the courtesy of Dr Bell the NATIONAL GEOGRAPHIC MAGAZINE is able to present as an appendix to this article a series of some seventy illustrations of experimental forms of kites and structures used by Dr Bell. The illustrations were selected by the editor from several hundred pictures in Dr Bell's notebooks. The pictures were taken and developed by Mr David George McCurdy, the photographer of his laboratory, with the exception of Plate III, which was taken by Mr F. Tracy Hubbard. The notes explaining the illustrations were written by Dr Bell by request. |