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Aviation History
1926
1926 - 0022.PDF
with hand-operated altitude control of the extra diffuser air type and is bolted to a cast aluminium induction T-piece, which is attached by studs and nuts to a broad facing on the underside of the magneto and pump housing on the rear cover. The throttle and magneto advance and retard are inter-connected by a suitable arrangement of levers and links. The altitude control is independent except that it is closed automatically if the throttle is closed. The air intake JANUARY 14, 1926 to the carburettor is an exhaust jacketed steel elbow,induction pipes run from the T-piece parallel to cylinders, and are fitted into it with airtight Thethe- expansionjoints, and are provided with bosses to take primer jets. Mounting.—The engine is mounted frcm screwed extensions on the ends of the four crankcase bolts at each corner of the crankcase. A standard connection for a tachometer is arranged on the port side above the magneto. EXPERIMENTAL STRESS ANALYSIS. THE paper on the above subject, read before the Royal Aeronautical Society on January 7 by Prof. A. J. Sutton Pippard, M.B.E., D.Sc, F.R.Ae.S., was "in many ways a most remarkable one, but unfortunately it was of such a technical character as to preclude any possibility of making a useful resume of it for the benefit of readers of FLIGHT, and we can only advise those interested in the subject to obtain the forthcoming issue of the Royal Aeronautical Society's Journal, in which the paper will be published in full. Prof. Pippard is, of course, an authority on matters pertain- ing to stress calculations, and it may be recollected that he is the joint author with Mr. J. L. Pritchard of the book " Aero- plane Structures." which has become the recognised text-book on the subject of aeroplane stress calculations in this country. In his paper Prof. Pippard referred to the report of the Accidents. Investigation Sub-Committee of the Aeronautical Research Committee, and quoted from this report the follow- ing passage : " The existing methods of calculation at present in use for determining the scantlings of structural members of an airship are insufficiently accurate for the purpose, and more exact methods should be developed." The lecturer then briefly recalled the formation of the Airship Stressing Panel, under the Chairmanship of Mr. R. V. Southwell, and the published report of the Aeronautical Research Committee (R. & M. 800) containing certain results of the work of the Airship Stressing Panel. He pointed out that these results were only obtained by the introduction into the analysis of various simplifying assumptions, the validity of which required checking. The Panel recommended that experiments should be initiated on models representative of the structure of a rigid airship, as a check on the theoretical results of their report. At the end of 1923, Prof. Pippard, who was then in a position to undertake experimental work, suggested to the Aeronautical Research Committee that he should carry out a research into the properties of redundantly braced frameworks. This course was agreed to, and the experimental work was begun almost immediately, and has proceeded continuously up to the present date at Cardiff University College. It was with the results of these experiments that Prof. Pippard's paper dealt. Prof. Pippard then referred briefly to a series of investiga- tions commenced by Mr. Southwell, inspired by the problems of the torsional stresses in an aeroplane fuselage, and gave an outline of this work. The lecturer then showed slides illustrating the model used by him in his experiments, which was a framework three bays in length, each bay being 30 in. long. In section the framework was hexagonal, the side of the hexagon being 25 in. As the theoretical analysis had been based on the assumption that the structure was pin-jointed throughout, an attempt was made to design a suitable joint of this type. It may be imagined that to design a frictionless joint in which there should be no fixing factors would have been well-nigh impossible, and ultimately a suggestion made by Mr. Southwell that the members of the structure should be attached to the joints by short dowels or pins was adopted. Preliminary tests on these joints indicated that the dowels might be relied upon to serve as a satisfactory means of attachment without introducing any serious fixing effect. Accurate means for measuring the stress in all the members of the structure were essential, and for this purpose none of the standard types of extensometers were suitable, and conse- quently a special instrument had to be designed, and this was done in collaboration with the Cambridge Instrument Co., Ltd. The result was a micrometer microscope which gave the high degree of accuracy required. A series of experiments on the stresses in various members in this structure under various loads was then described, but space does not permit of referring to these in detail, and all that is possible here is to give the general conclusions arrived at by Prof. Pippard, which were as follows :— " 1. When a tubular framework with redundant bracing is provided with efficient bracing in the plane of the applied load system, the stresses in the members tend quickly to become independent of the arrangement of the load system. " 2. The provision of additional bracing in other planes parallel to that of loading produces a much quicker equalisation of stresses. " 3. Unless efficient bracing is provided in the plane of loading to act as an initial distributor of the external load,, even if the tube exhibits a high degree of redundancy in other planes, the process of equalisation is a very slow one, and the stresses even at a distance from the plane of loading would be dependent to a considerable extent upon the arrangement of the load system. " 4. If in the design of such structure (e.g., the hull of a rigid airship) formula1 are used which determine the stresses in the members in terms only of the resultant actions at the section considered, it is important that effective bracing should be provided in the plane of the load system. Unless this is done there is a very serious liability of error." Effect of Non-operating Members In conclusion the lecturer dealt with the case when certain members of a framework are capable of resisting compression only up to a definite value, such as slender struts in which the load will increase as the external load is increased until its magnitude reaches the Euler critical value, after which it will bow. and the load remain constant at the critical value. In the case of an initially tensioned wire, the member behaves as a strut until the induced compression just balances the initial tension. It then ceases to function and is no longer an integral part of the framework. " A framework containing such members," the lecturer said, " presents difficult problems, since all members are operative under small external loads and the structure may exhibit a high degree of redundancy : under large external loads it may reduce to a simply stiff frame, while inter- mediate loads may produce any degree of redundancy between these extremes." For practical stress analysis, it was generally sufficient to take account of the two extreme cases, but in the design of rigid airship hulls the work involved was prohibitive. Approxi- mate methods had been given by the Airship Stressing Panel in R. & M. 800, but while this analysis was legitimate for normal flight conditions of loading, some or all of the counter- bracing panel wires resisting compression might beccme inoperative under exceptional loads. It was essential that the stresses in the frame under the extreme loading con- ditions should be known. A method had been suggested by Mr. Southwell by which such stresses could be simply deduced from a knowledge of the stresses in the various members of the framework in its normal fully redundant state. To apply this method it. was necessary to calculate the loads in all members of the framework under the extreme conditions, assuming it to be fully redundant and all mtmbeis capable of resisting compression of any required amount. In order to obtain such conditions, external forces were imagined, to be applied to the joints connected by the members, of such magnitude as to reduce the hypothetical stresses in them to the actual values. An example was given in which reversed loads were superposed on the panel, and it was pe>inted out that if this superposition were made for each panel in a frame- work and the complete structure analysed under such super- imposed loads, an exact solution of the problem would be obtained, but the work would be heavy. It was therefore suggested that each pane] should be treated as if it were an independent frame. If this were done the work was very easy, but it necessitated an assumption that required check to determine the degree of approximation involved. During his experiments Professor Pippard made a check of this point, and the check showed that the general agreement was good. It was therefore concluded that the method described was sufficiently reliable to give a good indication of the stresses in the extreme condition when redundant members had ceased to operate. Finally Professor Pippard referred to some experimental work now in progress, in which the longitudinal members of the model structure were con- tinuous instead of being pin-jointed at the transverse frames. 22
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