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Aviation History
1919
1919 - 0683.PDF
MAY 22, IQIQ \f * <^ggfrl describe methods of utilising these materials in an efficient and practical way. The structure of an aeroplane may be divided into two main portions : (a) the vital members, such as spars, struts, longerons and lift wires, and (b) the members giving form or bulk, such as ribs, ailerons, elevators, webs so that they meet together, thus forming the section into tubular units. Each web is formed of two pieces joined together at the neutral line. This is necessary in view of the limited width of strip available. The free ends of the flange are turned inwards so that the maximum stress is not H • H B E H E B S3 Figs. 8 and 9.— Photographs of Rudge spars showing respec tively how the spar is tapered towards the tip and how it is reinforced at the root E B E E E E B a a a E a • a B BB B a • a a • a B a a a E E B • E &c. The latter members are not stressed to the maximum limit in the same way as the vital members, and present no great difficulty in competing with wood. Thus, metal ailerons have been made and tested which were 40 per cent, lighter and 60 per cent, stronger than the wooden ailerons they were designed to replace. Improvements in design are to be looked for only in increasing the cheapness of manu facture. With regard to the vital members the problem is far different, and here all our ingenuity and knowledge is required. Metal designs are required for three main types of machine : (a) small machines up to 3,000 lb. ; (b) intermediate machines from 10,000 to 15,000 lb. ; and (c) large machines of from 30,000 to 200,000 lb. The largest members in a machine are the wing spars, and these present the greatest difficulties in construction— the next largest members are the struts. As it is impossible to deal with the whole vast problem of metal construction in one paper it is proposed to limit the description to these members for the above types. Rudge Spars. A section of the Rudge spar, developed from designs cour teously placed at the firm's disposal by Lieut. Commander H. Wylie, and suitable for the Avro machine, is shown in Fig. 1. In this spar an effective strength of 80 tons per square inch in the strut is utilised. To meet the weight requirements the thickness of the flange strip is reduced to 1 per cent, of its width, while the thickness of the web strip is reduced to i per cent, of its width. To prevent local failure the strips have been formed into longitudinal corrugations, the radius of each corrugation being from 30 to 100 times the thickness of the metal, a smaller radius being used when the highest compressive stresses are to be withstood. These corrugations are so effective that the Rudge spar shown has developed a compressive stress of 83 tons per square inch in the flange, while the webs have remained intact under a shear stress of about 30 tons per square inch. When tested at the Royal Aircraft Establishment it was found that this spar is 10 per cent, lighter and 5 per cent, stronger than the wooden spar it was designed to replace. Fig. 2 shows a modification of the Rudge spar for inter mediate type machines. The first step is to deepen the central corrugations in the developed at the free edge. This considerably increases the strength of the flange. The best disposition of the free ends of the strip constitutes one of the problems of metal construction. The solution lies in placing the free ends in position where they will be stressed as little as possible. Failing this they must be locally supported or strengthened if the full strength of the metal is to be developed without local failure. The Huns solved this difficulty very well with their Zeppelin bracing. They stamped the diagonal bracing members to the section shown in Fig. 3. Thus, the free ends AB were on the neutral line and were not stressed when the member was in compression as a strut. Fig. 4 shows a modification of the Rudge spar for a larger machine of the intermediate type or a small machine of the large type. With this type of spar the metal must be increased in thickness to retain sufficient stability. These derived sections are obviously weak about the waist and support must be given by cross frames, or formers, which suitably occur at each rib. With the addition of these frames the section is made stiff in all directions even when the strip composing it is of extreme thinness relative to its width. The flanges are supported in the middle by extending the web into contact with the inner surface. Fig. 5 shows a modification of Fig. 1. The depth of the spar may be readily altered by varying the depth of the central corrugations of the webs, the minimum depth being obtained where the corrugations meet in the centre. In the event of the corrugations not meeting, as in Fig. 5, it is necessary to join the webs together at intervals by a cross tubular member riveted to each. The above designs are adapted to be riveted together either by hand or by machine. Dunlop Spars. Several designs have been adopted to utilise spot or strip welding—this dispenses with the difficult, and at present somewhat expensive, process of riveting. One such design, known as the Dunlop single spar, is shown in Fig. 6. Three coils of strip of the required width are mounted on the rolling machine. After passing through the banks of rolls each strip emerges of the required shape and passes .68? .
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