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Aviation History
1920
1920 - 0303.PDF
MARCH ir, 1920 MODEL AEROPLANES NOTE.—All communications should be addressed to the Model Editor. The Design of Model Airships As a number of enquiries have been received lately regarding the possibilities of making models of airships, the following notes are given :— The fundamental principle of lighter-than-air craft is the lifting power of hydrogen, which, for model purposes, may be taken as 70 lbs. per 1,000 cub. ft. Coal gas lifts only about 38 lbs. per 1,000 cub. ft. Little success can be ex- pected with small models of airships, owing to the difficulty in working to fine weights. The following calculation (which relates to an airship 3 ft. long and 2J ins. diameter) shows the extremely fine limits which must be worked to :— 22Capacity of airship = _ total weight lifted will be— <5 5x V x " 1 1728 and the - x 7 5 70 1726 = -028 1b. rs cejcu/oted from these po/nfa from which it will be apparent that goldbeater's skin is byfar the most suitable :— Goldbeater's skin .. .. 120 sq. ft. = r 1b. Varnished silk .. .. .. 80 sq. ft. = 1 lb. Prepared rubber .. .. 40 sq. ft. = 1 lb. Varnished cotton . . . . 25 sq. It. = 1 lb.The seams or joints necessary in the material materially increase these figures, and at least 30 per cent, should beallowed for them ; in the case of goldbeater's skin, which cannot be obtained in large pieces, at least 60 per cent,should be allowed. ShapeThe accompanying illustration shows the various forms (side elevational) imparted to model airships. The usualmethod of setting out a section of, say, A is to divide the length of the model into a number of parts and calculatethe circumference at those points, by multiplying the diameter by 3f. If the envelope is to be made in two portions only, the corresponding dimen-sion of each piece will be half the circumference.Thus, if the length has been divided into 12 parts,a line representing the length of the model isdrawn and sub-divided into 12 parts, and halfthe circumference at those points is plotted equi-distant on each side of Setting Out facets1 no n_t ow'nfctp notched Shapes Enfelopes the centre line. It is extremely doubtful whether a model of the foregoing dimensions could be built to so small a weight. Now, if we double the length, and let the same diameter remain, we obviously have double the weight, and also double the lifting capacity = -058 lb.—still too fine from a practical point of view. But if we double the diameter, and let the length remain, we have four times the lifting capacity for only twice the weight, as the following calculation proves :— 22 7 Axlxl 6 X 1 I 1 1728 -7?-1000 = .n 2lb This point should be kept well in mind when designing model airships, and the diameter should, therefore, be kept as large as is commensurate with a proportionate length and breadth, and the diameter should be varied to obtain more lift—not the length. It will clearly be understood that a model airship has in reality a higher lift per 1,000 cub. ft. than full-size craft, as owing to the greater quantity of material to be forced out to form the latter it must be necessarily inflated to a greater pressure per square inch, with a corresponding reduction in the lift; whereas with models a much lower pressure will serve to impart the intended shape to the envelope. The materials available for model airship work, with their superficial area per lb., are given in the subjoined table, Setting Out Facets It will, of course, beapparent that the length of the curve of each facetis actually longer than the length of the hemisphere.For example, if the length of the hemisphere is12 ins., the length of the facet will be greater than12 ins., being equal in length to one-fourth ofthe circumference of the hemisphere. Therefore,the centre line of the facet must not be marked offin lengths equal to the lengths of sections taken ; but in lengths equal to |- of thecircumference of a hemisphere sub-divided into the given sections. First draw a tangent to a quarter -circle whoseradius is equal to the hemisphere to be constructed, and vertical lines are then erected (at an equal distance apart)which just touch the curve. The lengths of the arc cut by these lines are transferred by the dividers to the centre lineof the facet in their proper order. In the accompanying drawing, the length LK on thecentre line of the facet is equal to the L' K' on the curve, KJ equals K' J', and so on. If there are to be 24 facets,the width of the facet is equal to Jj-th of the circumference of the hemispherical cap at the corresponding cap. Thus,if the circumference at K' is 24 ins., the width of the line MN will be 1 in.—£ in. on each side of the centre line. Thecircumference at each point is calculated and then divided by the number of facets (the greater the number of the latter,the more accurate the finished shape). In a similar manner, the remainder of the widths are marked off and the outlinedrawn in with a French curve—not forgetting to allow for the seam. It is a good plan to use a dummy facet in card-board for marking out. The overlap for the seam should be notched, to allow the fabric to form to its shape; joints shouldbe made with gold size. Only one overlap is necessary for each facet. p-tf bt t 303
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