Note: The tables referred to in this article have been scanned from originals and may be viewed by clicking on the relevant hyperlink. As graphic files they may take a while to load. We recommend you close the new window each table will generate after viewing. IN RECENT YEARS the advances in aircraft performance have been, very striking. In 1939 speeds of the order of 350 m.p.h. were exceptional, but now they are almost commonplace. Such advances have been made possible by improvements in power unit output, and by aerodynamic refinements, and as a result the designers of aircraft structures have been faced with increasingly difficult problems. They must design their aircraft to withstand very heavy loads, and at the same time they have to bear in, mind the stringent aerodynamic requirements. For example, the thickness of wings must be a minimum and their surfaces should be as smooth as possible. Undoubtedly these requirements will become even more important, and consequently the difficulties of the structural designer will be more acute.
(Above) MODERN WING CONSTRUCTION - A wing panel with "Reduxed" stringers, immediately after removal from the press. They must design their aircraft to withstand very heavy loads, and at the same time they have to bear in, mind the stringent aerodynamic requirements. For example, the thickness of wings must be a minimum and their surfaces should be as smooth as possible. Undoubtedly these requirements will become even more important, and consequently the difficulties of the structural designer will be more acute. The advance from the Mosquito to the Hornet is a good example of how structural designs have developed to meet more exacting conditions. The Mosquito wing spars have wooden tension and compression booms, but this would have been impossible for the Hornet, because of the large cross-section of wood necessary for the tension booms. . As a result the tension booms were made of aluminium-alloy extrusions, while the remainder of the spars were of wood, i.e., the compression booms and the web. The, metal and wood were then welded together so as to form a final spar of remarkably low weight and high strength, and of small depth. One result of the success of this design, and of the testing carried out by the Royal Aircraft Establishment, Farnborough, was the approval given for Redux bonding by M.A.P A D.T.D Specification, No. 775, will shortly be issued to cover the process.
(Above) MODERN WING CONSTRUCTION - A wing panel with "Reduxed" stringers, immediately after removal from the press. They must design their aircraft to withstand very heavy loads, and at the same time they have to bear in, mind the stringent aerodynamic requirements. For example, the thickness of wings must be a minimum and their surfaces should be as smooth as possible. Undoubtedly these requirements will become even more important, and consequently the difficulties of the structural designer will be more acute. The advance from the Mosquito to the Hornet is a good example of how structural designs have developed to meet more exacting conditions. The Mosquito wing spars have wooden tension and compression booms, but this would have been impossible for the Hornet, because of the large cross-section of wood necessary for the tension booms. . As a result the tension booms were made of aluminium-alloy extrusions, while the remainder of the spars were of wood, i.e., the compression booms and the web. The, metal and wood were then welded together so as to form a final spar of remarkably low weight and high strength, and of small depth. One result of the success of this design, and of the testing carried out by the Royal Aircraft Establishment, Farnborough, was the approval given for Redux bonding by M.A.P A D.T.D Specification, No. 775, will shortly be issued to cover the process.
Application to All-metal Aircraft
However, it seems, likely that in the, next few years high performance aircraft will, in a majority of cases, be of all metal construction. The need for smooth, thin wings is greater than ever, and the hitherto normal riveted construction must necessarily come under very severe criticism. Most aircraft skins are literally covered with rivet heads, and consequently determined efforts are being made to diminish their aerodynamic effect, either by polishing operations or by finishing the surface with special paints. Other methods are being tried, including spot welding, but even this leaves a small mark where the electrode makes contact with the skin. This represents one general line of approach to the problem, but there is an alternative which is already, in use on one production aircraft. The de Havilland Dove has fuselage and wing stringers " Reduxed" to the skins. Unlike riveting and spot welding, this does not consist of a large number of small local attachments, but is a bond covering the whole area of contact between stiffener and skin. Hence it achieves two important objectives. It avoids numerous small stress concentrations, and leaves the skin perfectly "clean." Theoretically it seems quite wrong to make a hole in a skin as a first step to attaching a stiffener, and Redux now makes it possible to avoid this.
Above) STATIC LOAD TESTS - This test is being done on the D.H. Dove with the bonded stringers. When a similar wing of riveted construction is tested, the buckles run through the rivet- holes. Once again the Redux joints are considerably stronger than the others. Nevertheless, these are only small-scale tests and the following results, published by the courtesy of the Aircraft Division of the English Electric Co., Ltd., Preston, are of more direct interest to aircraft designers. Tests were made on flat-ended 16 s.w.g. and 18 s.w.g. panels, each 22 in. long, and of three different aluminium alloys. On the bonded panels rivets were fitted to the end of each stringer. The results are given in Table III. With regard to the results it will be seen that in every case the Redux-bonded panel failed at a higher load than the riveted panel. Further tests on Redux-bonded panels at + 60° C. and -40° C. and under repeated loading at normal temperature all behaved satisfactorily. With the high strength aluminium alloy materials in use at present, wing spars will undergo considerable strains before failure occurs and as a result the panels must be capable of carrying a load not only after the buckling of the skin, but after the initial buckling of the stringers themselves, i.e., after the skin and stringer combination has passed its maximum load. In consequence it is not sufficient to substitute bonding for riveting without first giving a careful consideration to the area of the bond between the stringer and the skin. However tests have shown that, given an adequate area of bond, the Redux will take a considerable load even after buckling.
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