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Aviation History
1920
1920 - 0526.PDF
MAY 13, 1920 Side view of R 24 at mooring-mast two conflicting causes will be a minimum. This point appearsto be reached with hulls of a length /diameter ratio of about 6. Although the size of the hull is large compared with the sizeof the various appendages, wires, struts, etc., which are con- sidered under the general head of appendage resistance,nevertheless, this resistance is, for the early parallel-sided rigids, in the order of 40 per cent, of the total combinedresistance of the airship, and for modern rigids in the order of 30 per cent. It will, therefore, be seen that very carefulattention must be given to reducing, by every possible means, the parasitic resistance as well as to improving the streamlineform of the hull. The form of the hull of a rigid airship must be a compromise.The smaller the length/diameter ratio, or the more nearly spherical the form, the lighter will be the structure. On theother hand, after a certain point, i.e., wherever the ratio length/diameter is less than about 6, the higher will be thehead resistance and the greater will be the weight of the machinery necessary to drive the airship through the airat a given speed. From these two considerations the following axiom can bedeveloped :—The longer the distance a rigid of a given size is required to fly, i.e., the greater the percentage of its dis-posable lift needed for fuel, the greater will be the economy in obtaining a true streamline form with low head resistanceeven at the expense of a certain amount of lift. On the other hand, if an airship is only required to fly a short distance,the greater weight-carrying capacity more than offsets the greater fuel consumption of the short ship of large diameter. Size The effect of size on performance is of fundamental im- portance, and has a marked .bearing on all future develop- ment. In airships of similar form, but of different size, the total lift varies as the (linear dimensions)", being proportional to the total volume of the gas contained in the gasbags. This should be clearly distinguished from the lift of an aero- plane which is proportional to the area of the wing surfaces and therefore varies as the (linear dimensions)*, while with airships, as already stated, the lift varies as the (lineardimensions)8. Generally speaking, with increase in size, the weight ofsuch items as fins and planes increases more or less propor- tionately to the volume. The hull structure fabric and cars,.however, can be confidently predicted to increase in weight with increase in size at a rate distinctly less than the (lineardimensions)3. The remaining item, however, the propelling installation, shows the most marked percentage saving inweight in proportion to increase in size. This is a matter of great importance as, in a rigid of 2,000,000 cub. ft. capacityand maximum speed 80 m.p.h., the weight of the propelling installation is as much as one-third of the total fixed weightof the ship. The head resistance of an airship, and consequently theb.h.p. necessary to drive it through the air at a given speed, is roughly proportional to the (linear dimensions)2. Ittherefore follows for a given speed that with increase in size the ratio weight of machinery/gross lift only increases as the(linear dimensions)"1, in other words, inversely as the linear dimensions. Besides this very material saving in percentageweight of the machinery installation with increase in size, a corresponding economy in petrol consumption is attained,which is of far greater importance, as in the large modern rigids the petrol capacity is well over five times as great asthe total machinery weights. This definite improvement in performance with increase in size does not allow for a stillfurther probable improvement, which can be predicted with the greatest confidence due to general refinements in designand more skilful use of materials. This- gain in performance with increase in size is shownpractically in the accompanying table by comparing the range of a 10,000,000 cub. ft. capacity rigid with a similar shipof 2,000,000 cub. ft. capacity. In both cases a disposable lift of 60 per cent, and a maximum speed of 75 m.p.h. havebeen arbitrarily assumed. Even though the gain in range at a constant speed of the 10,000,000 cub. ft. ship is verymarked in the example, a still further gain would be obtained in practice by the larger ship because it would actually possessa larger percentage of disposable lift than the small ship. R 34 moored at Mineola on the three-wire system at night. Side view 526
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