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Aviation History
1942
1942 - 1960.PDF
3U FLIGHT SEPTEMBER 17TH, 1942 PLASTICS AND AIRCRAFT is a product of skimmed milk. The process consists of heating the skimmed milk in a stirring pan and adding '"rennet" until a curd forms. It is then washed, pressed and partially dried, and the material is then formed into rods or sheets. It can be toughened and rendered proof against decomposition by soaking in a formaldehyde solu tion . As an example of the properties of plastics which are common in the plastics industry, the following are quoted: " Bakelite " (phenol-formaldehyde) i-35-2o 4,000-8,000 25,000-35,000 8,000-12,000 Specific gravity Tensile strength, lb., sq. in. Crushing stress, lb., sq. in. Shear stress, lb., sq. in. " Beetle " (urea* type) Specific gravity .. .. 1.49-1.63 Tensile strength, lb., sq. in. . . . . 8,000-10,500 Shear strength, lb., sq. in. . . 13,000-16,000 So far the subject of plastics has been considered from the point of view of the plastics industry, to give an idea of the various resins used and their origin. Now attention can be turned to how some of these plastics can be, and have been, made use of in the aircraft industry. Properties Required If one were asked what would be the properties of an ideal plastic material to "be used for the construction of aircraft, the first qualification would be that it must be comparatively cheap to produce ajatf'be of an unvarying and reliable quality throughout; non-inflammable (an essential feature), free from shrinkage and any liability to decay. Secondly, it would need at least the equivalent strength-to- weight ratio of, say, spruce or ash timber, with the added advantage that it could be welded as easily as mild steel, and could be formed by the use of a mould or press in addition. So far as is known no such material has ever yet made its appearance, but in the aircraft industry there are vari ous materials available of a,-synthetic nature, consisting of artificial resins and sjjtftable fillers, usually all termed "plastics," and familiar under their trade names to most people in the industry as Bakelite, Tufnol, Perspex, Plexi glass, Rhodoid, Micarta, etc., all of which have a very useful purpose ; but they are not structural materials. There is also an application used in the manufacture of airscrews known as the " Schwarts" method and the "Jablo" method. These have both been described in Flight in the past and can be dismissed lightly here by saying that they rely upon the impregnation of wood veneers by a plastic resin, and it is this process that appears to have beer/applied most successfully so far in aeronauti cal engineering not only from the point of view of airscrew production but, as will be seen later, of complete aircraft. Another important advancement is the use of plastic resins as an adhesive in place of glue in the manufacture of aircraft plywood. This has resulted in a much improved material. The use of plywood in the construction of aircraft in the i % i • . "'••• I 1 • ) i last war, and after wards for the light aircraft class, has been extensive in spite of its early dis advantages. Inflarri: mability was not its major disadvantage, since with fuel present in any air craft that is always the major fire hazard. The chief disad vantages of plywood as an aircraft material during and just after the last war seems to have been the character of the adhesives, mainly casein and albumen glues. Such adhesives would not stand the mould and fungi tests, and both encouraged parasitic growths. During the past twelve years, how ever, in the develop ment of the plastics industry resin ad hesives have come into use such as has been already de scribed of the phenol formaldehvde and urea formaldehyde group. As a result, the following advantages have been secured: -— (a) Complete waterjafoofness, the adhesive being in soluble until the wwfid decays. (b) High resistance to bacterial growth. (c) Boilproofness which facilitates bending and curving after steaming. Liquid Phenolics The liquid phenoiics are found to possess the same desir able qualities as the resin in film form, but the technique of bonding with a film is much more simple than with the liquid form. Whichever method is used, the process is similar, inasmuch as the assembled veneer sheets with the adhesive are subjected to simultaneous heat (300 deg. F.) and pressure (125 lb. to 250 lb. per square in.) during manu facture. The urea groups can be set at the much lower temperature of 170 deg. F. Thus it is possible by the aid of plastics to produce an idpfflf plywood for aircraft con struction, having superior' strength characteristics and enabling the craftsman to meet design requirements which are not even possible with normal wood or plywood. As will be seen later in a description of some of the pro cesses at present utilised, the so-called "plastic aircraft" so far have been made of moulded wood veneer or ply wood with resin adhesives and finish ; and a very good job Spar root designed by Dr. N. A. de Bruyne. It consisted of a " sand wich " of duralumin plates and synthetic plastic material, bolted together. &ORD6N hi ROUTE Cord material and "Gordon Aerolite " compared. The latter is twice as strong as the former.
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