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Aviation History
1943
1943 - 1907.PDF
JULY 29.TH, 1943 FLIGHT 121 PLYWOOD AND PLASTICS — II PRESSURE ,CONNECTION floors, seats, tanks and other units. Aero- dynamic efficiency, largely dependent upon external form and finish, is achieved with marked success with this technique, which un- doubtedly will be used extensively in the post- war development of civil aviation. The manufacturing processes involved have already been reviewed at intervals in past issues of Flight, so that only a brief description need by given here. In moulding aircraft and component parts in plywood the fundamental principle is the use of heat and fluid pressure applied simultaneously. During the moulding process the wood veneers become impregnated with synthetic resin, which fluxes under the application of heat and is forced by pressure into the wood fibres. The operation is per- formed with the plywood in contact with a suitable mould. When, after the curing period, the resin has set, a plywood unit of the .required shape is obtained. In most cases fluid pressure is obtained by means of an inflated or deflated rubber bag which is used either as the male or female member of the mould. Pressure supplied by this means develops in a direction everywhere normal to the surface of the work, irrespective of curvature. For certain complex forms veneer sheet is not always practic- able. In such cases narrow strips are sometimes used, cut to tapered, trapezoidal or triangular form where necessary to facilitate the forming of the final shape. Use of Hot- and Cold-setting Adhesives More recently veneers have been impregnated prior to "laying-on" the mould, a practice associated with the manufacture of laminated plastic materials. In either case the veneers are bonded by the hot process with a thermo- setting adhesive. This, it is claimed, gives improved dur- ability and has greater moisture resistance. In contrast to •frthis, however, the Langley Aircraft /Corporation, in the construction of. their twin-engined plywood aircraft by the Vidal process, selected Vinyl thermo-plastic resins for the bonding agents as being the most satisfactory over the required temperature range of 40 deg. C. to + 160 deg. C. Due ' to the fact that thermo-plastic resins can be reworked it would appear that their use will facilitate repairs and • more convenient servicing. In general, the use of hot-setting adhesives is advocated for panels and the cold-setting type for the produc- tion of heavier frames or final assem- blies. This is recommended because of the danger of damaging the outer wood by over-heating when the hot- setting type is used, the cure time being proportional to the thickness of the members. The necessary steam heat and pressure for moulding the - plywood are obtained by using auto- claves, which are relatively easy and cheap to construct. < In America such autoclaves are some 8ft. in diameter and up to 35ft. in length. It is understood that larger equipment is in course of construc- tion, so that limitations of the process which at present confine its applica- tion to complete light aircraft and trainer types may be removed. The use of moulded plywood offers TONS/so. IN 10 15 20 ULTIMATE TENSILE ULTIMATE COMPRCSSIVe 0 10 YOUNGS LB/SQ.lNS.2O MODULUS 3O 1-4 DENSITY WOOD VENEER MATERIAL I 0* DENSITY WOOD VENEEB MATERIAL I FABRIC BASE PHENOLIC LAMINATED MATERIAL •H GENERAL PURPOSE PHENOLIC Fig. 7. Comparison showing the superior characteristics of wood veneers with otherplastic materials. MOULD Fig. 8. Completely enclosedforms may be moulded by the internal moulding pro-cess shown above. RESTRAINING SHELL PRESSURt. BAG PRESSURE ADMISSION VALVE — - — -•= PLYWOOD RUBBER Fig. 9. Moulded plywood construction. Top: The inflated bag method, positive pressure. Bottom : The deflated bag method, vacuum pressure. all the advantages of moulded plastics, and many of the difficulties which accompany the use of the latter material disappear. Its superior qualities in comparison with other plastic materials are grouped in the chart, Fig. 7. The weight of plywood is even lower than that of plastics, and the vibration-damping properties of this material are well known. The application and future development of this modern technique to aircraft engineering is indeed promis- ing, and for that reason will be approached later from the design point of view. In this country Merron, Ltd., and Renn's Shaped Ply, Ltd., are engaged in this class of work and employ both the external mould and the internal rubber bag method. By the internal method it is possible to produce completely enclosed forms, such as the cylindrical or conical units, shown in Fig. 8, as the bag is collapsible and can be with- drawn after releasing internal pressure. Among the successful processes operating in America are Duramold, Vidal and the Timm Aero- mold process. Moulding the support- ing structure integrally with the ply- wood skin is a characteristic feature in certain of these processes, in which the following are among the advan- tages claimed: Fire resistance (sirtce the thickness of the plywood skin will resist the effects of heat longer than a thin light-alloy skin), freedom from pulsation; external rivet heads and overlapping joints, combined with a superior finish which improves aero- dynamic efficiency. Typical examples of the process are illustrated in Fig. 9. It is conceivable that a high-speed air- craft of this construction could be quite a good deal faster than a similar aircraft of metal construction. From what has been written it will be noticed that apart from the plastic element the use of wood veneer pre- dominates throughout in these latest applications of wood to aircraft en- gineering, and it becomes equally essential to study the characteristics of such' material. The production of veneer offers several distinct advantages to the manufacturer in addition to the en- gineer. For example, thin planks and veneers can be seasoned much more rapidly and economically than ordin- ary timber ; sliced or rotary-cut veneer gives greater yield per log, as there PRESSURE ADMISSION VALVE AILERON INSIDE VENTED BAG
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