Fig. 177A
Fig. 177B
Figs. 177A and B.—Details of Twin Cellular Box Kite
[Fig. 177A] shows another simple form of box kite. It is made from four strips of straight grained wood (preferably spruce), 2 ft. 6 in. by ⅜ in. by ½ in. Obtain also four other pieces, each 1 ft. 7½ in. long, but ¹/₁₆ in. wider and thicker than the foregoing, and halve their ends to a depth of ⅛ in. by ¼ in., in order that when the false end A ([Fig. 177B]) is tightly bound on, these cross sticks will firmly grip the long pieces edgewise; the sides of the cell are indicated by dotted lines. It is advisable to make the cross-pieces a trifle long, to ensure their straining the kite to its correct form.
CHAPTER XVIII
Building a Model Airship
It has not for obvious reasons been possible, in the design here presented, to rigidly adhere to the lines of the prototype, as in the adaptation of the design to rubber-driven model form several modifications have necessarily been introduced. It has been the aim of the writer to bring the model within the constructional capabilities of the amateur; indeed, it is hardly possible to have simplified the construction further. Now, the success of a dirigible, whether full size or model, depends primarily on the observance of the fact that an airship is lighter than air, and thus, unlike the aeroplane, does not rely on speed to obtain lift. Secondarily, the lifting power of hydrogen must be remembered, and although this varies according to temperature and the purity of it, it may be taken as a general rule that hydrogen will lift 80 lb. per 1,000 cubic feet. It is a good rule to adopt a lower figure, say 70 lb. lift per 1,000 cubic feet, to allow for discrepancies. In the design here submitted, aluminium (or what is equally as good, magnalium) tube forms the framework of the body or envelope. The general arrangement will be apparent from Figs. [178] and [179], which show the model in plan and side elevation respectively. The framework is of hexagonal cross section, the longitudinal members of which terminate at each end in a brass cap, to which they are riveted with soft brass pins. It will be necessary to anneal the tubes before bending them to impart the conical shape to each end of the frame, and this can best be effected in a weak spirit flame, care being taken to keep them on the move in it, to obviate fusing them. Where it is necessary in the construction to rivet the tubes, solder should be run over the pin to take up any play. [Fig. 180] shows the method of securing the longitudinal members to the brass end-caps. The tubes are first flattened out, as at B, and then riveted to the caps. This figure also shows the method of adjusting the angle on the rudder. A piece of brass tube is soldered to the end-cap; and it should be of such a bore that the No. 18 gauge wire of which the rudder is constructed makes a bare fit through it. Two similar pieces of tube, ⅛ in. long, are soldered to the rudder, to maintain the position of it. Thirteen hexagonal cross members will be required, and each is formed from ³/₁₆ in. aluminium tube. In order that they may not become out of truth, they are cross-braced with No. 35 s.w.g. piano wire, the ends of each wire being made off in a small hole drilled through the tube. [Fig. 180] will make the detail clear; each cross member is riveted to the longitudinal, and the latter is flattened out at those points where the cross member is attached.
Fig. 178.—Side Elevation
