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Aviation History
1919
1919 - 0145.PDF
JANUARY 30, 1919 gives improved efficiency, thereby opening up future pos sibilities of extraordinary value. II. Progress in Heavier-thaw-Air and Lighter-than-Air Machines: 1914—1918. —The development of rigid airships lias been even more rapid than that of aeroplanes. In 1914, the average endurance of a German rigid at cruising speed was under one day,*and the maximum full speed about 50 m.p.h. In 1918 (German L. 70 class, 2,195,000 cub. ft. capacity), the endurance at 45 m.p.h. has risen to 177.5 hours (7.4 days) and the maximum full speed to 77 m.p.h. The ceiling has correspondingly increased from 6,000 ft. to 23,000 ft. The British R. 38 class (2,720,000 cub. ft. capacity) lias an estimated cruising endurance at 45 m.p.h. of 211 hours (8.8 days), 34 hours greater than the German L. 70 class. It is a matter of some difficulty to make a fair and at the same time simple comparison between two types of transport possessing widely different qualities. The figures for rigids are comparatively simple, as they are few in number and all of approximately the same class, compared to the many differ ent classes of H/A machine which have been developed for various purposes. The Avro has been taken as the best all- round machine actually in nse in August, 1914 ; large mach ines such as the Sikorski, the Caproni, and the Graham White live-seater, were then only in the experimental stage, and, besides, did not possess the all-round efficiency of the Avro. The D.H. 10a has likewise been taken as the best all-round machine in August, 1918. Although the two-engined Handley-Page and Caproni have greater endurance and weight- carrying capacity, their all-round efficiency appears inferior to the D.H. 10a. The Handley-Page V. gives promise of having a slightly better performance than the D.H.ioa, but this machine is still in the experimental stage, and reliable performance figures are not available. Tables 1 and 2 give the progress in L/A and H/A since 1914. Table 3 shows the difference in performance between cor responding L/A and H/A craft of 1918. TABLE I. L/A August, 1914 Average figure for German August, 1918. Naval Zepp. German L.70. Progress. Maximum speed at Per cent. 10,000 ft. .. 50 m.p.h. 77.6 m.p.h. 55 Endurance at 45 m.p.h. 20 hours 177.5 hours 787.5 ^ , ,., (7-4 days) iotal lift .. .. 30 tons 66.64 tons I22 Disposable lift .. .. 8.5 tons 38.84 tons 357 Efficiency ratio .. 27.3 per 58.3 per 113.5 cent. cent. Static ceiling .. 6,000 ft. 21,000 ft 250 Indicated h.p .. .. 800 2,100 162.5 TABLE 2. H/A August, 1914. August. 1918. Avro. D.H.ioa. Progress. Speed at 10,000 ft. 70 m.p.h. 125 m.p.h 78.3 Endurance .. • • 4 hours 10 hours 150 Total weight loade<j .. .737 tons 4.02 tons 455 Useful load .. ... 268 tons 1.45 tons 445 Efficiency ratio .. .. 36.6 per 36.1 per 0.55 cent. cent. (Decrease) Ceiling 14,000 ft. 22,500 ft. 35.7 Indicated h.p. .. .. 80 810 913 per cent. August, 1914. August, 1918. Avro. Handley-Page. Progress. Per cent. Speed at 10,000 ft. .. 70 m.p.h. 85 m.p.h. - 21.4 Endurance .. 4 hours 12.5 hours 212.5 Total weight loaded .. .737 tons 5.97 tons 524 Useful load .. .. .268 tons 2.17 tons 709 Efficiency ratio .. .. 36.3 per 36.4 per 0.28 cent. cent. Ceiling .. .. 14,000 ft. 15,000 ft. 7.1 Indicated h.p. .. .. 80 720 800 TABLE 3. COMPARISON OF EXISTING H/A AND L/A MACHINES H/A L/A Percentage August, 1918. D.H.ioa. German L.70. Superiority. Per cent. Speed at 10,000 ft. 125 m.p.h. 77.6 m.p.h. H/A 61.1 Cruising endurance 14 hours 177.5 hours L/A 1167 Total lift (- weight loaded) .. .. 4.02 tons 66.64 tons VA '557 Disposal lift (- use ful load) .. 1.45 tons 38.84 tons L/A 2580 Efficiency ratio .. 36.1 per 58.3 per L/A 61.5 cent. cent. Ceiling .. .. 10,000 ft. 21,000 ft. L/A 10.5 Indicated h.p. .. 810 2,100 L/A 159 Hi HT It will be seen that at the present time the largest rigids in commission have over 10 times the total lift of the cor responding H/A, and that the disposable lift is about 25 times greater. The proportion of useful lift compared to gross lift is much higher in airships than in aeroplanes. An approxi mate figure for an aeroplane of average engine power is one-third, while in a rigid the useful life available for fuel, crew, passengers, freight, etc., is well over one-half. In the case of L.70 the figure is 58.3 per cent., although this ship is the most heavily-engined and fastest airship yet built. It cannot be too strongly emphasised that many of the advantages apparently possessed by H/A at the present time result from their relatively small lift. Thus, an aeroplane of 60 tons total lift, if found to be possible at all, would certainly be very much less convenient to land and handle on the ground than aeroplanes of existing sizes, and would require overall dimensions about twice those of the largest existing machines. III. Technical Advantages in the Design of Airships.—Im provements in design, materials, machinery, etc., may be expected to produce great advances both in H/A and in L/A. It may be assumed also that most of the difficulties now experienced, such as the landing of large H/A and the handling of large L/A upon the ground, will be overcome by various means to a similar extent, and that the general utility of both types will enormously increase. Increase in Size Unfavourable to Aeroplanes.—It is important to note that in H/A there is no automatic improvement in efficiency resulting from greater dimensions. In L/A, on the other hand, such an automatic improvement takes place to a very marked degree. The reason for this difference is as follows :—In similar H/A machines of different dimenions, the total lift, air resistance, and b.h.p.—other things being equal—all vary nearly as the plane areas, i.e„ as the square of the linear dimensions. weight of machinery It follows that the ratio total lift much with size of machine. The ratio • does not vary weight of structure total lift tends to increase with size of machine. Unit area of wing surface can only exert a definite lifting effect, and as the machine increases in size it is necessary to increase pro portionally the weight of each preceding unit area of wing surface to make the machine proportionally strong. A point is finally reached where every additional unit of wing surface results in as much increased weight as its lifting effect, so that any further increase in size will involve a definite falling off in the total lifting capacity of the machine. A further small increase in size can be effected by increasing the number of planes; but, owing to the inefficiency of middle plane surfaces, due to the blanketing effect of the top and bottom planes, the resulting gain in endurance will not be large, and it will only be effected at the price of a loss in efficiency. As the relation of size to performance is one of the govern ing factors in future development, it is considered from another point of view : In all similar structures the strength of the given structure is inversely proportional to its linear dimensions. This general property of structure applies both to airships and aeroplanes, but gives a very different numerical value in the two cases. In aeroplanes, if a given wing area in a small aeroplane weights * lbs. for a span of y feet, then to double the span and still maintain the structure proportionately strong the weight of the second structure will be 2 x + x)~ 4* ; and if the width (chord) of the wing structure be cor respondingly doubled, the weight of the struture will equal 8 x. Now the lift of the structure is directly proportional to the area, and if the span and chord are doubled, the area, and likewise the lift, will be four times as great, whereas the weight of the structure will be eight times as great to be correspondingly strong. This unfortunate property of structure applies equally to airships ; but the airship depends for its lift on volume (y*), while the aeroplane depends for its lift on area (y2). To give a concrete airship example, a 10,000,000 cub. ft. capacity rigid has five times the lift of the present 2,000,000 cub. ft. capacity rigid, but the length of the former is only 1.7 times greater, and therefore the weight of the structure only five times greater (1.7)*—»-«•• the weight of the structure is directly proportional to the total lift. This theoretical property of structure may, to a certain extent, be modified by the material used. Thus, in aero planes, there is a certain size of machine which can be built most economically of wood. Any machine of smaller or larger size entails a certain proportion of uneconomical weight, due to the inherent qualities of the material. This question of material applies equally to rigids ; thus, for ships of 2,000,000 to 4,000,000 cub. ft. capacity, duralumin is the H5
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