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Aviation History
1919
1919 - 0413.PDF
MARCH 37, 1919 under the influence of the rigging tension ; and secondly, the pressure which is necessary to prevent the bow of the envelope being blown in, due to the excess of external pressure caused round the bow of the envelope by the motion of the airship forward. It is found that the excess of pressure which takes place at the bow of the ship extends for only a short distance aft. By reinforcing this area it is possible to fly with an envelope pressure considerably lower than the external pressure at the bow of the ship. This stiffening of the bow becomes a matter of greater importance as the speeds increase. There is always the possibility in " bumpy " weather, when the height of the ship Yaries rapidly, that the pilot may let his pressure fall momentarily too low. The bow of the envelope then blows in and forms a curious concave cup shape in which it remains till the speed is reduced or the pressure is raised. No damage will probably be done so long as the reinforcement of the bow is not of such a nature that it will break and puncture the envelope. It may very probably be necessary to provide a separate small compartment at the bow and kept at a higher pressure than that of the rest of the envelope. This would allow high speeds to be attained without a corresponding increase in envelope tension. Planes In order to stabilise the motion of the ship it is necessary to provide the envelope at the after end with planes. These are provided at their after edges with rudders capable of steering the ship either vertically or horizontally. The construction of these planes somewhat resembles that of the wing of an aeroplane. Fabric is stretched over a framework and is doped in order to render it taut. The surfaces are, however, practically flat and the loading is considerably less than that provided for in an aeroplane. The planes are supported from the surface of the envelope by g^y wires attached to suitable points on the envelope, and are prevented from pushing their inner edges into the envelope by skids or wooden bearers. The importance of the rigidity of this resistance to inward thrust is very often under-estimated. A small inward movement of the foot of the plane allows the plane as a whole to sit over a serious angle. This lack of rigidity in the planes has a serious influence on the stability of the ship. The maximum intensity of air pressure occurs towards the forward edge of the planes, and it is therefore desirable that the leading edge should be short. The long narrow plane, increasing in width as it goes aft, is therefore adopted in preference to that of large aspect ratio which, although aerodynamically more effective, would be much more difficult to hold with adequate rigidity. The planes of a rigid ship are attached rigidly to the hull framework. In the latest German ship these planes are made some 6 ft. thick at the root, and faired off into the rudder and tapered to the outer edge. They are, therefore, almost totally self-supporting and require no guy wires. Fabrics The fabrics used in airships are of three main types :— 1. Gastight fabric, such as that used for gas bags of a rigid ship. 2. Outer cover fabric, of which the principal function is to form a rain and weather proof outer cover to the ship, both as a fairing to reduce her resistance and to protect her internal bags from variations of temperature, due to radiant heat, and from deterioration caused by sunlight. 3. The envelope of a non-rigid ship requires a combination of both properties. Gastight Fabric.—The lightest method of rendering a fabric gastight is the application of goldbeater's skin. This material is a membrane which, although water will easily pass through it, has a very pronounced ability to resist the passage of hydrogen. The skins are obtained from the messentry of a cow, each animal contributing a piece which averages about 8 in. by 20 in. These skins are stuck to the fabric by means ol glue or rubber solution, and are varnished to protect them against moisture and damage. The gastightness of a non-rigid envelope is obtained by lubber proofing only. The same fabric has to fill the functions of outer cover, as stated above, and also has to withstand considerable stress produced by the internal pressure of the gas and by the tension of the riggings attached to it. In order to obtain the necessary strength—and more particularly strength to resist tearing—a number of plys of cotton fabric are stuck together with layers of rubber solution between them. Fabric for small size ship can be given the required strength by two plys of cotton, the inner one being diagonal. Stronger fabric is usually made of three plays, the middle of the three being diagonal. The diagonal ply is formed by cutting strips of fabric and sticking them to the other ply so that their threads run at 45* to the threads of the main ply and to the length of the built up strip of fabric. These diagonal threads have a very pronounced effect in dis tributing a stress evenly over threads of the main ply. The rubber is made into a thick solution and spread by a kind of scraping knife on to the layers of fabric before they are stuck together. The rubber on the outer surface usually contains aluminium powder as this forms a surface that reflects much of the radiant heat, thus preventing rapid temperature change, and is also opaque to the light which would injure the fabric inside. It is usually found that the outer layer of proofing has perished so badly as to be easily noticeable before the strength of the fabric has become appreciably reduced by weathering. Outer Cover Fabric.—Outer cover fabrics generally re semble the fabric used on aeroplane wings. It is necessary that after the outer cover is placed on the ship a certain degree of contraction should take place in order that the cover may be well tensioned to resist any tendency to flap. It is found that a contracting cellulose acetate dope is gen erally most satisfactory but the extent of the contraction must be considerably less than that customary on aeroplanes. If the contraction is excessive it is liable to bring a serious crushing strain on the iramework of the ship. Considerable difficulty has been experienced in obtaining a satisfactory dope for the outer cover, but its function in resisting fight and heat and in providing a waterproof cover ing, are so closely analogous to those fulfilled by the latest aeroplane dopes that it is probable aeroplane practice will be adopted, the only modification being a considerable reduc tion in the amount of contraction allowed. The outer cover of a rigid airship constitutes a very serious problem because the unsupported areas of the fabric art large, and it is of the utmost importance that no part of the fabric should flap or even tremble to a small extent. Such action would very rapidly increase and in the prolonged flights which these ships have to make, very serious results might follow any small flapping which was allowed. Engine Requirements Our very extensive experience of airship flying, extending over nearly 3,000,000 miles during the war, has shown that by far the most fruitful cause of failure is connected with the engines. This is the case although a large proportion of the small difficulties which occur in aero engines are of a type which can be made good in an airship, but not in an aeroplane The length of time for which an airship's engine is running continuously is very considerably greater than that of an aeroplane. The requirements to be expected from a good airship engine therefore differ from those of an aeroplane engine in several important respects :— 1. The engine must be suitable for running for very long periods without breakdown. 2. All gear on the engine must be arranged so that small defects can be made good in the air, the engine, if necessary, being stopped for a short period. 3. The fuel and oil economy, more particularly at reduced powers, are of far greater importance to an airship than is the initial weight of the machinery. Although these differences between the requirements of airships and aeroplanes exist at the present time, they will be very considerably reduced as soon as the aeroplane develops into a machine of longer range and capable of flying with a smaller proportion of its power. The airship engine require ments of to-day are very largely the requirements which the aeroplane will call for to-morrow. Useful Carrying Capacity The carrying capacity of an airship is perhaps the feature of greatest importance both from a Service and commercial point of view. The weight, which is available for bombs, passengers, merchandise, or fuel, depends upon the volume of gas contained by the ship and upon the weight of the ship's structure and all necessary parts. The volume of gas will increase as the cube of the linear dimensions of the ship, and it will be readily understood that the weight of the ship will not increase as such a high power. This indicates that as the size of the ship increases the proportion of her gross lift which is available for lifting capacity will also increase. The non-rigid ship having no hull structure will for the same size have a considerably greater proportion of available lift. It may be assumed that it is at the present time practicable to design both a rigid ship and a non-rigid ship which will be able to carry as useful load a weight equal to that of the ship, i.e., 50 per cent, of the gross lift of the ship will be avail able for useful purposes. The size of a non-rigid which will 413
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