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Aviation History
1937
1937 - 0321.PDF
FEBRUARY 4, 1937. FLIGHT. 121 PLASTICS in AIRCRAFT CONSTRUCTION Present Position in the Use of a Promising New Material Bruyne''s R.Ae.S. Lecture Dr. N. A. de PART I. THAT synthetic resins are a good deal nearer to being suitable materials for aircraft construction than had been generally supposed was revealed in the ex tremely fascinating lecture given by Dr. Norman A. de Bruyne, Fellow of Trinity College, Cambridge and director of Aero Research, Ltd., Duxford, to the Royal Aeronautical Society last Thursday. Shortcomings of plain resins can evidently be met to a very great extent by reinforcing the material, and the stiffness and impervious- ness to damp, petrol, acids and so forth, as well as its fire proof qualities, make it attractive from many points of view. Dr. de Bruyne had written a remarkably informative paper on the subject under the title " Plastic Materials for Aircraft Construction." This, together with the 77 refer ences to other works, can be studied by anyone who wishes to possess all the information available. To save time (the printed paper was very long), Dr. de Bruyne gave a short talk, illustrated by slides, for the benefit of those who had not previously made a study of plastic materials and their properties. The following notes attempt to com bine both without claiming to follow either closely. The lecturer pointed out that an account of a subject in an early stage of development must contain many first thoughts and guesses, and the author must be prepared to take the risk of having to eat his words at a later date. However, he thought it was better to have blundered than never to have thought at all [a point of view which is all too rare among research people.—ED.]. Vfft.f* *T Comparative response of steel and Aerolite forced vibration. This characteristic of Aerolite should be useful in aircraft con struction for damping vibration. RESONANCE CURVED fo* STELI AND AEROLITE The study of the mechanical behaviour of synthetic resins was largely the study of matter in an amorphous state. In contrast to that of single crystals, our knowledge of poly- crystalline matter was meagre. Practically all we had we owed to Professor L. Prandtl of Gottingen (a name more familiar in connection with concepts such as the boundary layer and induced drag). The property of resins which attracted the aircraft designer was the possibility of moulding them into intricate shapes. Most resins are "thermoplastic," i.e., they become soft when the temperature exceeds a certain value. Some kinds of synthetic resins have the valuable * characteristic of being "thermosetting," i.e., when once moulded they set to permanently infusible products. Thus all resins could be divided into these two classes. Thermoplastic resins were not of great interest for struc tural purposes, as the - necessitated uncomfortably high mould ing temperatures, but two of them, cellulose acetate and methyl methacrylate, were widely used for windscreens and fairings. Thermosetting synthetic resins are essentially con densation products formed by linkages between molecules with the elimination of water, while thermoplastic synthetic resins are polymerisation products formed by the simple process of joining up similar molecules as pearls strung on a thread. Thermosetting Resins The two main groups of thermosetting resins are the phenol formaldehyde group and the urea formaldehyde group. All synthetic resins are weak in tension, and urea formaldehvde resins have an affinity for cellulose which results in a brittle structure and prevents the use of cellulose, which is otherwise the best reinforcement on a strength-to-weight basis. The urea formaldehydes, by the addition of suitable retarders and cata- clysts, can be made into excellent cements, which, unlike casein cements, are proof against attacks of mould and fungus. Dr. de Bruyne explained that for convenience he would refer to phenol formaldehyde resins by the name of Bakelite. although this was a trade name; it had, however, the merit of doing honour to Dr. Baekeland. He did not think it was too much to hope that control of the molecules in thermo setting resins, at present somewhat uncertain, might one day be attained, thus giving us a product, without reinforcement, having the strength of cotton or silk. When synthetic resins were tested in compression they showed a similarity in hysteresis to that of stone, cement and UuTVMATE STRENGTH 2S,i30L6jfsQ|i STRESS STRAIN CURVE fO" CORP AEROLITE IN TENSION —1— oil 5TRO 04 O A O-t OS I'O >2 FREQUENCY /m. Up to 6,000 lb. per sq. in. hysteresis is present, but no "elastic after effect" ; above this stress the strain rises gradually on applying the load and does not reach a final steady value until a few minutes after the application of the load.
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