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Aviation History
1944
1944 - 1341.PDF
JUNE 29TH, 1944 FLIGHT <")7 W- Wooden ades A Preview of Airscrew Retfmremen&yor Post-war Civil Air Transport By L. G. FAIJWURST/A.M.l.Mech.E. THERE is little doubt that ijrscrews as a avians of aircraft propulsion will rerrmjii for/ a ^mside. period after the war, particula?r^fc*«|^Bi|W«^5e' of aircraft envisaged for the civil air transjMf field Of the major requirements which civil airline operators , will demand, the maximum possible pay-load will hold a prominent place. This requirement at once emphasises the necessity for the utmost care in reducing the weight of aircraft, engine and airscrew components to the mini mum, commensurate with safety and reliability. This article is intended to deal solely with the possibilities of weight-saving on the airscrew, mainly by the *fse of com-C -'ed wooden blades against their .rnative metal counterpart. First, in case the important part which wooden blades have played dur ing the present war and the rate of progress achieved in their use on mili tary aircraft are not fully appreciated, a few remarks should be of interest. Aircraft of such divergent types as the heavy,bomber and the high-speed altitude fighter have been operated most successfully during the present war with airscrews fitted with com pressed wooden blades. The performance has l^en proved equal in all cases to the metal counterpart, j^th the out standing advantage of a big saving in weight. Development has proceeded at a tremendous rate, and the outstanding ability of the wooden blade to absorb vibrations from engine harmonics has considerably enhanced its reputation, particularly since the vibration problems are a constant source of aggravation with ever- increasing engine power output. The ease with which wooden blades can be repaired after damage in combat or forced landing has been shown to be of great value under wartime conditions with their problems of material supply. Further, the fact that THE author of this article is chief engineer of Rotol, Ltd., and thus has had wide experience of metal airscrew blades as well as'ofthe different wooden blades, such as the " Weybridge," Jablo and Hydulignum. For use on post-war com mercial aircraft he strongly advocates wooden blades, chiefly because of the great saving in weight. rboden blades fracture readily in a crash landing makes possible a higher percentage of salvage of airscrew hub an^r engine reduction gear mechanism than is the case with the metal blade, due to the shock loads imposed by the latter on the hub and engine mechanism. Compressed-wood Blade Construction Before dealing with the main question of the weight - saving which can be contributed by the airscrew, tlu- reader will no doubt appreciate a few notes on the con struction of the compressed wooden blade. The blade is produced from profiled blocks, each block being built up by gluing together several rectangular-shaped boards approximately o.Sooin. thick. These boards are made by compres sing in heated presses a pack ot veneers, interspersed between each veneer being an agglutinant which, can be either in the form of a resin coated on eaci veneer or an impregnated pajip^rilm. In each case the application of heat and pressure consolidates the pack ol veneers into a rectangular board of constant thickness. The arrangement of the veneers and the method of pressing is such that one end of the board has a density of 1.30, while the remainder (the greater length) has a density of 0.90. The blade aerofoil portion is machined from the 0.90 density portion and the root from the 1.3 portion, thereby ensuring that the root shall have adequate strength to withstand the stresses imposed by the centrifugal and bending loads from the blades. During manufacture extreme care is taken to ensure a moisture content at which the blade will remain stable, so as to prevent any change in blade balance when exposed to thfl<^arying atmospheric conditions encoun- Stages in the making of a Hydulignum wooden blade. f" From right to left: A pack of loose venders ; board con solidated by hot-pressing ; board trimmed for side pressing ; finished side pressing ; finished assembly in glued block. FINAL STAGES. From right to left: Finished assembly butt-ended and threaded; metal adaptor mounted ; blade rough-shaped ; finished white-shaped ; blade finished and covered.
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