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Aviation History
1919
1919 - 0985.PDF
JULY 24, 1919 isse SOME DEVELOPMENTS IN AIRCRAFT DESIGN AND APPLICATION DURING THE WAR By the Right Hon. LORD WEIR OF EASTWOOD, P.C., Honorary Fellow of the N.E. Coast Institution of Engineers and Shipbuilders. (Continued from page 959) PART II.—Construction and Application Wing Structure.—General development in wing construction of aeroplanes, during the War, has been more in the nature of refinement of detail, than of evolution of methods. With increased knowledge concerning the loads to which the structure is subjected during flight, has come the possi bility of more scientific proportioning of the structural members ; but beyond this the general arrangement of the wing has remained unchanged The structure percentage weight.has shown the effect of increased knowledge, and this percentage has now reached a very low value. • The effect of attention to detail has also been to give a general cleaning up and simplicity to the appearance. In external bracing, changes have taken place in some particulars. In 1914 the duplication of the main lift wires was considered to be very important, if not essential ; now. in the event of one wire getting shot away, the loads are taken through the incidence wires, which are those seen when looking at a wing from the side. At the commencement of the War, hard-drawn piano wire was used for the main lift and anti- fiying wires in many types ; when this was not used, stranded cable was employed. The use of these materials has been superseded by streamline wires. These consist of high tensile steel rods of a lenticular section rolled from bar. The ends are left circular, and threaded to form a simple means The splicing of spars was not found sufficient to overcome the timber shortage, and the need became evident lor some change in design by which small scantling timber could be used, even ior the spars o'. large aeroplanes. Further experimental work was put in hand, this time to determine the effect of laminating spars, i.e., glueing thin strips together, and from this built-up section constructing spars in the ordinary way. At the same time experiments were made on box spare, i.e., the spars instead of being made of the common "I" section were made in box iorm, the flanges and webs being formed of separate pieces of materials, glued and bradded together. Both the laminated and box spars were found to be very successful, and both types were immediately put into service. No trouble has been met with from their use. Indeed, it is probable that better quality spars are obtained by this means than by cutting from the solid, at any rate for large aeroplanes. The smaller the scantlings, the easier inspection becomes, and the more guarantee is there that first-class material only is being used. Splicing and the building up of spars have proved such successful innovations, that there is no doubt that these methods of spar construction, introduced purely as War measures to overcome the serious timber shortage, will remain as standard in the future. In M IIBX-«R "'" ^\ jWSf N^j^y. . ,,. . Fig. 8.—Braced N-girder fuselage. Fairey. of attachment to the fittings. These wires have proved very satisfactory, and initial troubles, due to crystallisation of the metal through vibration, having been overcome, this is now the commonest form of bracing. Hard-drawn piano wire has disappeared, but cable is still extensively used, par icularly in types where high performance is not essential. Tthe design of interplane struts has undergone certain changes, The earlier aeroplanes had struts of streamline section made from solid spruce, and occasionally steel tubes faired off to a steamline form by light fairings were used. With the growth in size of aeroplanes and the increasing scarcity of suitable wood, built up or laminated struts were used. Also, with the standardisation of steel tubes for aero plane work, and an enormously increased production of these, this form of strut became very popular. It is a most efficient construction, particularly for the landing chassis. Internal bracing wires have gone through much the same process as the main-plane bracing wires—at the commence ment of the War, piano wires were invariably used, but these have now almost universally been superseded by swaged rods, screwed at the ends for fitting purposes. During the War also, the scarcity of silver spruce occasioned the development of new methods of spar construction. These were, originally always in one length from the centre section to the wing tip and spindled from a solid section. The first effect of timber shortage was to introduce a system of splicing, and much experimental work was carried out to determine the best form of splice to be used. The result of this work was that a straight scarf joint sloped 1 in 9 was adopted as a standard. This scarf was glued, pegged and wrapped with fabric. Its efficiency was high when compared with the solid spar, and it was a simple job when considered from a production point of view. Fig. 8a.—Braced N-girder fuselage. Camel. Fuselage Construction.—Fuselage construction has under gone very few changes, during the period of the War, as regards methods. Larger aeroplanes have been built, and this has naturally meant greater attention to the construction, but the broad lines of design have remained fairly well the same. There are three principal types of construction adopted in present-day practice :— 1. The braced N-girder fuselage (Figs. 8 and 8A). 2. The three-ply covered fuselage (Fig. 9). 3. The monocoque fuselage (Fig. 10). The first of these consists of four longerons, or fore and aft rails, braced by a system of struts and diagonal wires in all four faces. Fabric covering completes the structure. This is the commonest type of design. The second system retains the longerons and the vertical and horizontal struts, but in place of bracing wires and fabric, thin three-ply is glued and bradded to the framework. The monocoque fuselage dispenses with the longerons and consists of a single sheet of three-ply or veneer moulded to shape on formers. As a general rule, three-ply frames are provided as bulk head bracing at intervals along the length. There are several varieties of this construction, but the essence of them all is the tubular construction of thin veneer, the formers or bulkheads merely being inserted to stabilise this skin. Of the three methods of construction, the first and third were known and adopted before the War, and modifications have been more in the nature of improvements in manu facture than in fundamental alterations in principle. The second method was, I believe, first used by the Germans in their early Albatross scout at the beginning of the War, 987
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