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Aviation History
1920
1920 - 1292.PDF
DECEMBER 23, 1920 CAMBRIDGE UNIVERSITY AERONAUTICAL SOCIETY (By C. I. R. AN attempt has been made in the following paper to set down some of the various principles and facts which have directed the development of rigid airships. As this paper deals mainly with airships, whose uses for war are not very extensive, and cannot here be discussed, it is proposed to refer only to aircraft designed and used for peaceful purposes. 1. Reasons for the Use of Aircraft.—The decisive reason is"to save time in transit of goods or persons. Taking every- thing into consideration, we cannot expect that air transportwill secure adequate public support unless the time taken is at least halved, on the average journey, as compared withcompeting systems of transport. Adopting this basis, we see that the necessary average speed over the ground wouldvary from 90 or even 100 m.p.h., where competing with fast railways, to 10 miles an hour or ever, less in some out-of-the-way parts of the world. Where the highest speeds are required, aeroplanes alone can compete with existing formsof transport, but such cases are surprisingly few, and only occur over a small portion of the earth's surface. Wheremoderate speeds and endurances of not more than 1,000 miles are required, either H/A craft or airships may be employed.Where moderate speeds are adequate and endurances of over 1,000 miles are required, airships alone can be used. 2. Airship Speeds.—In the case of airships, we have thecondition that the most economical speed is commonly below that which is commerciallydesirable : endurance and economy must therefore be sacrificed in order to givesufficient regularity of running. Airships (OFFICIAL ORGAN, "FLIGHT") -[ Rigid Airships* CAMPBELL, O.B.E., M.I.N.A., F.R.Ae.S. weight of these items to total lift is a minimum at a value of total lift which is not very definite, but which we will suppose to be about 80 tons. Below this total lift the ratio increases at a rate which is accentuated as the smallest sizes are reached Above 80 tons the increase is slow at first, but over a range of total lift from, say, 150 tons to over 300 tons is sufficiently rapid to neutralise the steady fall in the proportion of total lift absorbed by machinery weights, with the result that over this range the total empty weight is nearly a fixed proportion of the total lift, i.e., about 40 per cent, in airships having normal speeds of 65 to 70 m.p.h. This does not mean that there is no gain in efficiency to be obtained by increasing size above 150 tons. On the con- trary the steady reduction in horse-power per ton of total lift involves a reduction in the proportion of lift which, for a given journey at given speed, must be allotted to petrol and oil, and enables proportionately more cargo to be carried. Mention has already been made of the heavy cost of increasing airship speeds. This is shown by the curves in Fig. 2. The loss of carrying capacity is, of course, due rather to the increased weight of petrol than to increased weight of machinery. 4. Types of Airships for Commercial Work,—As previously stated, airships increase in efficiency with increased size. Small airships may be used for a variety of purposes, com- Cent +0 / / / / / ffe/ation between Per *\ tcorryincc7PS. Continuous Running J/oeecf for on Airship of s^ooo.-ooo Cubic Feet Capacity for a Journey of 3,000 through tht Air. 1 Sea Miles Fl X. 6 .2. \ 4O 45 SO SS Running Speed in Knots. 70 will show to greatest advantage where used over journeyson which their quality of long endurance is called into play. For journeys of, say, 1,500 to 3,000 miles, it veryseldom happens that existing means of transport average 25 miles an hour. In most cases airships averaging 50 m.p.h.speed over the ground would fulfil the requirement of halving the time. Taking the weather conditions into consideration, it isconcluded that on long voyages the necessary saving in time can be effected by airships having normal speeds of 65 to70 miles an hour through the air. For short voyages higher speeds are feasible when desirable, but, generally speaking,the reduction in commercial value of an airship which accom- panies increase in speed is so great that the lowest speedadequate to secure traffic will usually be adopted. 3. Airship Performance.—Unlike H/A craft, airship per-formance is greatly affected by size, increase in total lift giving improved performance up to values of the former, almostcertainly above 10,000,000 cubic ft. gas capacity (about 300 tons total lift). The reasons for this are as follows :—Roughly speaking, the horse-power per ton necessary for a given speed varies as the two-thirds power of the total lift.This law is fairly correct for airships of, say, 25 tons total lift and above. If we start from the smallest airships and consider theeffect of increase in size, we find that the weight of machinery per ton of total lift for a given speed decreases at first some-what quickly, later more slowly, but it never ceases to decrease. The other fixed weights—hull, fabric, equipment—do notbear any simple relation to total lift. The ratio of the * Extracts from a Paper read before the Cambridge Univtnity Aero-nautical Society OD November 34, 1930. mercial and otherwise, but the more important transport work will almost certainly be done by rigid airships of the largest size. 5. Rigid Airship Types.—A semi-rigid airship relies for the maintenance of its transverse form mainly upon gas pressure in the envelope. Its longitudinal form is main- tained by a rigid, or nearly rigid, longitudinal keel at the bottom of the envelope. A rigid airship relies entirely upon a rigid framework for the maintenance of its external form. Clearly, between the semi- rigid and rigid types many intermediate types are possible. The true rigid type is itself capable of many variations. Inventors have usually given excessive attention to the provision of strength longitudinally to resist vertical forces, and have proposed such devices as the use of very strong keels in the lower parts of the hull; or forming the sides of the hull into deep braced girders ; or even fitting vertical struts and diagonal wiring along the middle line of the ship so as to form, with the top and bottom longitudinals, a braced girder the full depth of the ship. In considering these it must be remembered that the hull has to resist the action of very considerable lateral forces when turning. The effect of such forces will be combined with that of vertical forces due to weight and buoyancy. It is apparent, therefore, that the resultant shear forces and bending moments may at times be in a plane more nearly horizontal than vertical, and that the best hull structure will be one suited to withstand such forces and moments acting in any plane through the axis, with the greatest and least strengths in the vertical and horizontal planes respectively. A hull structure having the general characteristics of a tube naturally presents itself as the most scientifically correct, and such a structure is accord- ingly found in all rigid airships. ; 1294
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