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Aviation History
1941
1941 - 2165.PDF
SEPTEMBER I8TH, 1941. FLIGHT AIRSCREW BLADE MATERIALS An Evaluation of the Merits and Demerits of Wood, Compressed Wood, Steel, Magnesium and Duralumin By K. B. GILLMORE IN recent years there has been muchdiscussion as to what is the bestmaterial to use for the construc- tion of blades for variable-pitch air- screws. Improved wood of various forms, solid duralumin, solid mag- nesium and hollow steel have all been used with varying degrees of success, and there has been a good deal of con- troversy as to whether each of these materials has its own particular place, or whether there is one of them which is definitely superior to all the others. From the earliest days of airscrew- propelled aircraft, wood seemed to be a naturally suitable material for air- screws with integral-type blades, and enjoyed almost undisputed popularity for many years. Increase in the severity of operating conditions led to a tend- ency towards the use of harder woods and the introduction of various forms of reinforcing material and protection. Apart from isolated experiments, such was the situation until shortly after the termination of the last major war. From that time onwards, more attention began to be devoted to the use of various metals as alternative materials, and it is of interest to note that after the year 1920 metal-bladed airscrews were beginning to be used on many occasions when maximum efficiency was required, or particularly arduous operating conditions encoun- tered. In this connection one may cite such examples as the Schneider Trophy Competitions, the world's long-distance record, and numerous air races, etc. It became an increas- ingly regular thing for private owners to purchase a metal-bladed airscrew upon deciding to enter a machine for a racing event, and the superiority of performance which could be attained despite the, at that time, serious weight increase, was widely acknowledged. About 1925 the first British control- lable-pitch airscrew was constructed and flown, and this airscrew had forced duralumin blades. Meanwhile, in America the growth of airline operations and the use of air- craft on dirt and desert aerodromes had given considerable impetus to the development of the ground-adjustable duralumin-bladed airscrew. When, a few years later, the variable-pitch airscrew was first introduced, it brought with it the necessity for hav- ing detachable blades. At that time, this meant of necessity that the blades must be of metal because no means had then been found of holding the root of a detachable wooden blade into a hub, and accordingly these first THE author of this article is on the technical staff of the airscrew division of the De Havilland Aircraft Co., Ltd. His reasoning and conclusions will be read with interest by all who are con- cerned with the subject of blade materials, but will not, we believe, be shared by everyone. It must be ad- mitted, however, that he presents the case for duralumin blades in a con- vincing manner and with scrupulous fairness. V.P. airscrews used solid duralumin blades. During all this time, when the materials of normal production of fixed- pitch airscrews were wood or solid dural, and all adjustable and variable pitch airscrews were entirely dural, quite a lot of research had been done on the possibility of using hollow steel blades. This work met with a limited amount of success on the smaller sizes of blade, but on the larger sizes trouble was always experienced, either with the welding along the leading- and trailing-edges or else with panting of the sections, giving rise to fatigue cracks in the material. The hollow steel blade was, therefore, not con- sidered as a serious competitor of wood or duralumin at this time. Shortly afterwards the manufacture An early wooden four-blader. This particular one was fitted to a B.E. of '' improved '' wood became pos- sible, and it was found that by com- pressing the wood and impregnating it with a synthetic resin its strength could be raised sufficiently to make it possible to hold a detachable wooden blade in an airscrew hub by means of a sere wed-on sleeve. As the dural V.P. airscrews of this period were un- doubtedly extremely heavy, the intro- duction of improved wood was imme- diately seized upon as an opportunity for lightening the airscrew. Two main systems were used, though various minor alterations to both of them were tried out. The first system, introduced by Schwarz in Germany, consisted of using compressed and impregnated wood for the root of a blade only and attaching an outer portion of lighter wood by scarf-jointing the various planks from which the blade was made up. This system was also adopted by the Airscrew Company in this country. The second method was that devel- oped, also in Germany, by the Heine Company, and consisted of using thin veneers which were impregnated and compressed throughout their length, though the degree of compression and impregnation was varied from the root to the tip of the blade. This system of construction is used in this country by Jablo and Rotol. At this stage it is not without im- portance to add that, whilst both these systems of wooden blade construction originated in Germany, there is no evidence to show that they now enjoy any appreciable measure of popularity in that country, and almost all Ger- man aircraft are fitted with forged duralumin blades. Furthermore, the High efficiency was achieved in the Fairey airscrews designed for the Schneider Trophy seaplanes.
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