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Aviation History
1942
1942 - 2014.PDF
342 FLIGHT SEPTEMBER 24TH, 1942 PLASTICS <ua4 CRAFT Part II of an Article on Their CompositioiyManufacmre, and Application W. NICHOLS/A.R.Ae.S. IN Part I of this article were described the varioils resins, fillers and materials likely to be met'with in trie plastic^ group as applied to aircraft engineering. Of jhese tj most common are '' Acrylic '' and '' Venyl'' Vthcjifio- plastic) and '' Phenol'' (thermo-setting). From what has already been written it will be realised that so far there is not a material available to the aircraft engineer by which a fuselage or wing could be moulded without the use of wood or metal. But with the aid of plastic resins the ideal is being approached, as will be seen latef'. There is no plastic with which one could construct air craft on the girder fuselage principle unless it be " Gordon- Aerolite," referred to earlier. Unfortunately this material has never been used outside of the laboratory. Its manu facture requires the use of linen as a reinforcement, and the manufacture of linen requires flax. In this war the demand for linen in other directions is already heavy, and the increased cultivation of flax is impossible at the moment, so this material also cannot be utilised for the present, but here again by the use of plastic resins and plywood a sound structural material is obtainable^ Plastics Specifications The list of approved plastics or plasticised materials to D.T.D. specifications is a short one at present, but the majority are given hereunder and show that the aircraft engineer has not by any means a large choice of plastic materials whereby strategically important materials could be economised. D.T.D. 216A.—Synthetic resin bonded fabric mouldings for magneto gear wheels. D.T.D. 315— Pigmented cellulose-acetate sheets. - D.T.D. 320.—L.T. varnished insulating tubing. D.T.D. 339A.—Transparent synthetic sheets and mould ings. D.T.D. 451 to 454 inc.—Synthetic resin (Phenolic) mould ing materials and mouldings (various shock resist ance) To this may r>e added D.T.D. 370.—Laminated compressed wood. D.T.D. 427.—Plywood—lightly stressed parts, and 5V3.—Birch plywood impregnated and resin bonded - materials. The list is not complete,'.but these are the materials already in common use. The purpose here is to deal with the manufacture and fabrication of the more important plastic materials used in the construction of aircraft and components, and to give some idea of what can be accomplished by the use in com bination of plastic resins and plywood, wood veneer, fabric, etc. From the list of specifications just given probably the most commonly used is D.T.D. 339A, known in this country under the trade name of "Perspex" ; a similar material used in America is known there under the original .German name "Plexiglass." This transparent material is one of the best so far obtained from the plastics industry and has been used extensively for windscreens, cabin hoods, windows, and landing light covers. Perspex or Plexiglass is even more transparent than glass and retains this superior transparency in spite of the effects of sun and weather in extremes of temperature and humidity such as aircraft operating with the Services are now subjected to. The impact strength is superior to that of glass, though this plastic material is lighter in weight. As a thermo plastic it adapts itself to being conveniently formed to two- or three-dimensional sections, an essential feature if good aerodynamical shape is to be obtained. Mounting this material either as a windscreen, hood, or cover presents 'no difficulty if certoifi rules are observed, and bracings or supports whichjffight become necessary with glass can be dispenseduj*mto some extent and vision thereby improved. JPgjcspex can be obtained in sizes to suit nearly all require ments and in various thicknesses, as follows: Thickness Tolerance -. • 1 IO Jin. •.,m. in. ..,"., in. and over 1,000 Nowadays sheets can also be obtained in other thick " i nesses to assist production, such as A in. in lieu of gin. 1. in lieu of in lieu of ,in. in. 1 'i gjin. i f j;iB* Tensile strength from the specification is given as not less than 2.75 or more than 4.25 tons per sq. inch. Manufacture Considerable care is necessary. Starting with a flat " cast " sheet protected from dust or foreign matter which might later cause distortion of vision, the sheet is hung vertically in a hot-air oven. Small areas may be im mersed in hot oil or glycerine. At a temperature of from 220 deg. F. to 250 F. the sheet becomes pliable, but can be handled by operatives wearing gloves. Above this temperature the sheet becomes very soft, A Wadkin router makes a good tool for cutting and trimming the edges of synthetic transparent sheets, also Plasti-ply to be referred to later.
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