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Aviation History
1920
1920 - 0527.PDF
MAY 13, 1920 Length .;.• '' "... Diameter Gross lift (at 68 lbs./ 1,000 cub. ft.) Disposable lift (60 per cent, gross lift) Crew, ballast, food, etc. (15 per cent, gross lift) Dischargeable lift (45 per cent, gross lift) .. Maximum speed 70 m.p.h. (normal full speed)— * Maximum range .. fHorse-power developed .. 45 m.p.h. (comparison , speed)— * Maximum range Horse-power :, developed PERFORMANCE TABLERigid of 2,000,000 cub. ft. Capacity.645 ft. 79•• 5 ft. 60 • 7 tons 36-4 tons 9 • 1 tons 27-3 tons78 m.p.h. [67-9 hours 4,125 naut. miles Rigid of 10,000,000 cub. ft. Capacity. 1,100 ft. 135-5 ft- 303-6 tons 182-2 tons 45-5 tons 136-6 tons78 m.p.h. ("115-5 hoursJ 6,950 naut. I miles [_4,75° stat.miles |^8,ooostat. miles 1,700 5,000 f2i8 hours f 364 hours. 8,530 naut. I 14,200 naut. miles "I miles 9,820 stat. miles ^16,4000 stat. miles JO 1,580 If the question of streamline form and size of ship is studiedfrom the point of view of carrying with the utmost economy unit weight unit distance at unit speed, it will be found thatthe greatest economy is effected if a ship is designed with the most perfect streamline form available and of such size thatthe normal range of the ship is a little over twice the distance between the two termini between which it is proposed to fly,allowance being made when estimating the normal range for the necessary safety margin of fuel. It can, therefore, be stated definitely that for Service re-quirements, where high speed and high performance are of the greatest importance, the most perfect streamlineform available is required, and that the size of the ship will vary according to the performance required. For com-mercial airships the same also applies, and the best possible streamline form should be used whatever the variation indistance between the termini, the factor to be varied being the size and not the form of the ship. These axioms are, of course, only general, and may bemodified to a certain extent: for example, when the housing sheds are not of sufficient size and the ships have to be modifiedfrom the ideal streamline form to get the best performance out of, for example, a low shed. In general, the cramping effectof too small housing sheds has been very acutely felt, and has tended towards the production of rigids with a large amountof parallel portion. Unfortunately, the question of streamline form is one ofconsiderable difficulty, and although many experiments have been carried out in wind tunnels, owing to the very large scaledifference between the actual ship and the model, the data obtained is at present of a vague and somewhat unsatisfactorynature. Recent results, however, are most encouraging. Up to a short time ago the R 33 streamline form, a copyof the German L 33 form, was considered about as efficient as could be obtained and far in advance of the previous parallel-sided ships built by the Germans and by this country. Certain changes in streamline form have, however, been instituted inR 38 design, whereby it has been found possible to increase the lift by about 6 tons more than if the ship had been of the33 streamline form with only a very small increase in weight of structure and without, it is hoped, appreciably increasingthe head resistance. If this proves to be the case, the advant- age is obvious. There is now every reason to believe tnat astreamline form of appreciably lower length/diameter ratio can be developed with an even lower head resistance thanthat of the R 33 streamline form, this ratio being reduced from 8 to 6, or even lower. . , ,, ' 11t is possible at the present time to construct ships of the R 33 streamline form with an efficiency ratio, i.e., disposaDie lift/gross lift, of some 60 per cent, and a maximum speed of * * Fuel and oil consumption has been taken on the basis of .53 lb. P« brake horse-power hour. , f xrakohnrse- t Thrust horee-power has been taken as equal to 70 per cent, of brake horse power of full speed, and 60 per cent, at 45 m.p.h. . over 75 m.p.h. for ships of under 2,500,000 cubic ft. capacity.If the experimental model results are justified and a streamline form is obtained of equally low or lower head resistance with alength/diameter ratio of 6 or under, it is quite reasonable to suppose that the ratio disposable lift/gross lift can be increasedto nearly 70 per cent, without loss of speed or range. These theories appear to be confirmed by fact in the case of theGerman commercial airship the " Bodensee," which appears to be of far more dumpy outline than the 33 streamline form,and as far as her weight-carrying capacity, range and speed are concerned, having regard to her very small size, appearsto have the highest performance of any rigid yet built. Speed. The speed for any given airship is proportional to the(horse-power) J. The machinery weights are proportional to the horse-power and the speed is, consequently, proportionalto the (machinery weights) \. A point, therefore, is rapidly reached at which it is no longer practical to increase the speedof the ship, owing to the large reduction in disposable lift per unit increase in speed. A reasonably high speed, in the orderof 80 m.p.h., is, however, essential because the whole jus- tification for the use of airships is, in the case of Servicerequirements, rapid and extended reconnaissance and patrol duties, and for commercial purposes reliable and fast transportover long distances. Naturally, it is of the greatest importance to obtain astreamline form of particularly low head resistance when speeds of over 40 m.p.h. are required. A reasonable amountof speed is also an economy, taking into consideration adverse winds, since, in practice, a ship with a large reserve of speedcan be flown between any two points at a given speed with a smaller safety allowance of spare petrol than a low speed ship,when the given speed is nearly her maximum speed. Up to a point the smaller safety margin of petrol necessarilycounterbalances the dead weight of the extra machinery. Very much greater reliability is also achieved in the event ofone or more engines breaking down. There are several important speeds which can with ad-vantage be defined :— (1) The speed below which it is impracticable to fly, owingto the necessity for jettisoning ballast. This speed varies according to weather conditions. In the mornings andevenings, owing to superheating effects, this speed must necessarily be in the order of at least 45 m.p.h. under normalconditions. For a few hours before dawn, when the ship is probably more or less in equilibrium, it can naturally be flownat a very much lower speed. With increase in size this speed tends to increase approximately at the (dimensions) £ asexplained under the heading of dynamic lift. (2) Theoretical economic speed depends directly on thespeed and direction of the wind. This is, in practice, usually determined by the Scott construction as shown. With afavouring wind in the same direction as the path of the ship DETERMINATION OF ECONOMICAL, SPEED SCOTT Mr-THOD. Economical speed. Scott construction for wind inany direction 527
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