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Aviation History
1952
1952 - 1442.PDF
626 FLIGHT, 23 May 1952 PLASTICS IN AIRCRAFT Recent Progress in the Development of Major Components and Structures REGARDED from an historical point of view, the entry of plastics into aircraft is generally presumed to have taken place in the 1930s, when acrylic resins in thick sheet form were first employed for windscreens and, later, for canopies, gun turret glazing, etc. In actual fact there was a prior, and a surprising, application—the use of trans parent cellulose-acetate sheet in place of fabric to cover the wooden structure of wings and fuselage and so to produce an "invisible" aircraft. This was achieved both by the Germans and the French about the year 1917, and served some small use for artillery spotting. While it was not really invisible below 2,000ft such an aircraft was very difficult to see above this height and was definitely a very difficult target to hit. With later, more powerful, aircraft, such structures were useless. The poly-methyl methacrylates (the British variety is known as "Perspex") are now general for canopies and also for radomes, and while the simple methacrylate structure was the earliest form, modifications have been employed to prevent cracking at fixing- points. Thus glass-fibre reinforcement is introduced to reduce chances of failure at bolt holes, while success has been achieved in the bonding of a rubber interlayer to metal on the one side and to methacrylate on the other. While this resin has been widely used for radomes, the increase in aircraft speeds has demanded a much greater strength /weight ratio and high erosion resistance. The last few years, therefore, have seen the development of glass fabric reinforced with polyester resins capable of being hardened at very low pressures. These structures may be used alone or, for the shorter energy wave lengths, may be sandwiched with lightweight cores such as expanded rubber, expanded P.V.C., or honeycomb structures made from P.V.C. or paper impregnated with phenolic resin. Radomes of all types are produced by several of the plastics fabricating companies and, indeed, by a number of the aircraft constructors themselves. Turning now to the subject of plastics as adhesives, it may first be recalled that during World War I, when almost all air craft were of wood, glue was an essential part of the structure, and all the glues were of natural origin—the "gelatine" type, from hides, hoofs, bones, etc., and the casein type, from skimmed milk. The synthetic adhesives made from phenol and urea formalde hyde have, over the past 10-15 years, almost wholly replaced the older types of glue in most of the modern applications of wooden adhesion on a large industrial scale, and came into prominence during the late war for aircraft structure bonding. The most out standing example of this was the Mosquito, which proved so valuable in service during the latter half of the war. Nevertheless, since the metal aeroplane had long existed and was obviously to be the transport aircraft of the future, the atten tion of certain scientists was drawn to the problem of bonding metal-to-metal with synthetic resin adhesives, the prime considera tion being the production of a strong bond with the hope that it would replace costly riveting and, for the first time, introduce a really smooth, and thus efficient, surface. An outstanding worker in this field is Dr. N. A. de Bruyne of Aero Research, Ltd., who introduced the "Redux" process in 1941. This consists in using polyvinyl formvar as the adhesive, employing phenol-formaldehyde as a flux to induce the formvar to "wet" the metal. The first all-metal aircraft in which this method was employed was the de Havilland Dove, but prior to this, a prototype panel was made and bonded with Redux, and was subjected to 330,000 loading reversals to induce skin wrinkling. The panel was found to be intact after this severe test. The success of the Redux process has reached its apotheosis in the construction of the de Havilland Comet. The synthetic- resin adhesive is employed in the assembly of the whole fuselage, all wing skin panels, tailplane, control surfaces, etc. In general, the advantages of adhesion compared with riveting are as follows : reduction in weight up to 20 per cent due not only to the elimination of rivets but also to the use of thinner metal sheets; smooth surfaces without the use of expensive flush-riveting; reduction in production costs which approach 50 per cent, and in increased static and fatigue strengths*. The next logical step in development has led to the considera tion of plastics as aircraft structures in themselves. Some idea of the rate of progress in this field can be gauged by remembering that at a lunch during the early stages of the last war, a famous aircraft builder told the plastics industry that plastics were neither strong enough nor stiff enough to be incorporated as aircraft structures. This was so obviously true that the plastics industry accepted the fact; it was true even for the then reinforced plastics made by incorporating strong fabrics and wood veneers. Today, the situation is so much more hopeful that it induced Dr. Megson of the Chemistry Department, Royal Aircraft Establishment, to state : "The days are gone when plastics are used simply for the production of small items in which mechanical strength is of secondary importance, and we are beginning to realize the poten tialities of the materials for structural purposes." The new situation has arisen more specifically because of the development of two very different types of plastic materials during the last 10 years or so. The polyester resins, which were first developed in the U.S.A. about 1941, resemble closely the alkyd resins used for the past 20 years in paints and enamels. They differ considerably from the normal thermosetting resins in hardening at very low pressures and at room temperatures, and when employed to impregnate glass fabric or fibre and built up to form laminated sheet, result in structures of excellent strength and stiffness and of great toughness. The widest development of such glass fibre resin laminates has taken place in the U.S.A. where radomes, boats, fishing rods, washing-machine components, and even small air craft and experimental motor-cars, have been fabricated. In Great Britain similar lines have been followed, although on a smaller scale. Several boat-building companies are using the material; radomes are fabricated and full-size experimental omnibus roofs have been so constructed. In the field of aircraft construction, the Comet is fitted with three resin-bonded glass- fibre panels in the undercarriage flap as part of the aerial system, whilst air ducts of the same material are also employed. The Ambassador, too, is fitted with large laminates of resin-bonded glass-fibre. The second new constructional material of special note is that used in the construction of the Delta wing first shown at the S.B.A.C. Display at Farnborough last year, and recently exhibited in the Earls Court section of the British Industries Fair. The basic material is asbestos, and is bonded with phenolic resin to form a felt-like sheet. This aircraft construction is specifically British in conception, having been developed by J. E. Gordon and his co workers at the Royal Aircraft Establishment. The phenolic- resin reinforced asbestos felt has been developed and produced by Turner Bros, Asbestos Co., Ltd., under the name of "Durestos." In his paper before the Anglo-American Aeronautical Confer ence in September, 1951, Mr. Gordon indicated the reasons for choosing this type of material in preference to others for the con struction. Since one must use reinforced plastics to obtain the necessary strength and stiffness and since the cellulose types are known to shrink or swell with change of weather, the choice lay between glass-fibre polyester resin and asbestos phenolic resin structures. Because both glass-fibre and polyester resin are com paratively costly, because reinforced asbestos is specifically twice as stiff as resin-bonded glass-fibre, and because of the compara tively short "pot- life" of polyester resin once the catalyst has been added, it was considered that the work at the R.A.E. should be concentrated on the felt type of reinforced structure. It is less strong and less tough than the polyester/glass-fibre structure, but it is stifjer, cheaper, and permits greater ease of moulding. The weights and general performance of such laminates 'com pare well with metal, and cost is considerably less. There are also advantages of surface smoothness, heat and sound insulation and radio and radar wave transparency at low frequencies. In general terms, it may be concluded that both the new types of material have wide fields of application, each perhaps more applicable than the other for specific purposes. %* © N. A. de Bruyne, "Plastics Progress 1951," p. 147.
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