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Aviation History
1926
1926 - 0405.PDF
JUNE 17, 1926 stability. In the same way that the frames and the plating co-operated structurally, the fins and the hull co-operated aerodynamically. Having determined that the short compact shape of hull could be satisfactorily used, one problem was out of the way. The short fineness ratio made it possible to build not only a large airship of the metal-clad type, but an extremely small one. Mr. Upson stated that the demonstration airship which they were building first was to be one of only 200,000 cubic feet, or the same size as the fabric iion**igids being operated by the United States Army at the present time. The new airship would, however, not only be a rigid but a metal- clad rigid. Coming to the structural part of the design, Mr. Upson said that at first it was considered by many almost impossible to calculate the stresses arising in a metal-clad airship. In the Zeppelin type of airship there was a very high degree of redundancy. Offhand, one would say that nothing could be more redundant than a metal-clad airship. Strangely enough, however, the approach to the extreme in that respect had been in effect a simplification. There were two ways to simplify a structure from the stress analysis point of view ; one was to reduce the number of elements to the minimum •which made a statically determinate structure out of it. The other was to multiply them so much that in effect they produced a uniform material. In other words, if the number of parts was increased to such an extent that the material to a large extent closed in on itself and became perfectly uniform, so that the stresses could be carried in any direction, then one could go back to first principles and consider in "what direction that stress was going to be carried. It was no child's play to work out the stresses over the whole of the metal-clad airship hull. The undertaking was a very arduous and difficult one, but it was susceptible to very close analysis. The stresses in the actual airship were first worked out by purely analytical methods. Then to be thoroughly on the safe side they repeated the same kind of experiments as had been made for aerodynamic purposes. The lecturer then described and illustrated how tests were made on what is known as a water-model, which is, fundamentally, simply a scale model made out of some kind of material, everything in proper proportion, and which is then suspended upside down and filled with water. The weight of the water is a volume factor in the same way as is the lift of the gas, and so the water-model enabled tests to be made on the structure of the airship as a whole. Mr. Upson pointed out that in water-model tests it is usual to choose the size of the water- model as one-thirtieth of the linear dimensions of the full- sized airship. In order to simulate over-loading in the same way that an aeroplane wing is tested to destruction by sand loading, they chose a different size of water-model, which was approximately one-fourteenth instead of one-thirtieth. By using this larger water-model and keeping the material the same that would be used on the full-sized airship, they got the equivalent of over-loading the airship approximately five times, which was severe enough to give a good big load factor. At the same time, it accomplished another incidental purpose of practically eliminating any effect of the stiffness of the material, which was almost nothing on the full- sized ship. The principal result of the exhaustive water-model tests, which covered a period of about five months, during which the model was inflated most of the time, was that they could not tell any difference between calculated results and the results of the water-model tests. At one time it was thought they had found a place where the two methods did not check. A series of small wrinkles were discovered which indicated excessive shear stress at one portion of the model. The calculated stress analysis had not indicated these stresses, and it was thought at first that somehow the calculations had gone wrong. It was later found, however, that the cause was that in the stress analysis they had not taken the stresses at a sufficient number of different points. As already indicated, the airship was designed as a rigid structure at all times, that is, it was entirely non-deformable, which was one of the main essentials for a metal-clad hull, and it would be far more rigid than the rigid airships of the present day. That still left two problems to be taken care of. One of them was the stiffening of the surface locally against wind pressures and tendencies to vibrate. The other was to utilise the strength of the plating to as great an extent as possible in unusual weather conditions, such as those which destroyed the " Shenandoah." The manner in which these two problems have been solved was indicated by the lecturer by means of diagrams, without which it is almost impossible to convey an idea of how this result is achieved. It may be 351 said, however, that by a suitable choice of curves, etc., as the speed increases and more local support for the plating is required, the natural flow of the air as induced by the speed itself is such as to re-arrange automatically the pressure dis- tribution in the desired manner. The Discussion. Commander Burney thought there were three fundamental points which had been dealt with by the lecturer. First ; the^was the differen^m fineness,.ratio developed primarily to reduce the area of the outer cover. The results obtained in America were borne out by the work done in this country which had also shown that there was no difficulty in getting down to a very much smaller fineness ratio. He rather agreed with the lecturer when he said that there was very little difference between a fineness ratio of 3 and a ratio up to 4£. Where he did rather disagree with the lecturer was that there were other considerations to be taken into account when dealing with large airships. He quite agreed that on small ships it was feasible to use a small fineness ratio, but in very much larger ships, say of the 5,000,000 cub. feet type, there were other practical considerations, and it was these considerations which led the British experts to choose a 5j to 1 fineness ratio. He did not think it fair to compare a small metal-clad airship of small fineness ratio with a large Zeppelin type airship of much greater fineness ratio, because obviously there was a very considerable inherent gain in structural efficiency in going to a small length/diameter ratio. The second point to which Commander Burney wished to refer dealt with the metal skin. It would seem that the determining feature of the ship must be the maximum local compressive stress. Therefore it would seem that unless the envelope layers were made up in variations of thicknesses to take such aerodynamic stresses as one got at the tail and the fins, the determining factor would be the local compressive stress in every case. If one considered it upon that basis it would seem that to get the highest efficiency one would begin to crowd the outer cover into such forms and such spaces throughout the ship that one would eventually come back to the type of structure which was termed the Zeppelin type, leaving the cover as a bare cover for reduction of resistance. It would, he thought, appear from a cursory consideration that the metal-clad type of vessel would be heavier in struc- tural weight than a vessel built under normal conditions. At the same time he would not for a moment say that the extra weight might not be justified by increased safety and increased capacity to resist fire and so forth. The third point raised by Commander Burney was that in a type of airship in which one was dependent upon the whole outer surface and on pres- sure within that outer cover for security it was very difficult to meet conditions such as one had to meet in the normal type of a deflated gas-bag. He would be interested to know how that condition was to be met. Mr. Upson said he was afraid to put off answering Com- mander Bumey until others had spoken, as he might by then have forgotten some of the queries. He had not expected anyone to draw any unwarranted conclusions as regards com- parisons between fineness ratios. He merely intended to bring out certain rather striking facts to show that the short fineness ratio was feasible at least from the standpoint of resistance and stability. The matter of accommodating the fineness ratio to the structure was a different kind of problem. The small and medium sized airships practically demanded a rather low fineness ratio. It was true that from a structural standpoint one could afford, and should have, rather greater fineness ratios for the larger sizes of ships. The bigger the airships became the more serious became the effect of dia- meter. No matter what the details of the design might be, the diameter had a fundamental effect on the weight, particu- larly with large sizes. He thought that with airships round 3,000,000 or 4,000,000 cub. feet, one would have to consider the weight of the skin primarily as a structural part of the ship. Anything beyond that would make desirable an increase in fineness ratio. As regards the point raised by Commander Burney dealing with the subject of weight as influenced by the metal-clad construction, at first they were fully prepared, and con- sidered it would be necessary, to make considerable sacrifices in that direction in order to obtain the other more obvious advantages of the metal construction. The idea at first was that, considering the rather radical nature of the design, they wished to build the first ship about as small as it could be made, and have a satisfactory demonstration with a ship that could be put to some useful purpose. The first guess (Mr. Upson said it was not much more than a guess) at that size was 1,600,000 cub. ft. The design of that ship was worked out to a point where they began to get a fairly good
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