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Aviation History
1956
1956 - 0242.PDF
240 Hollow-steel blade de Hayilland airscrews, a distinctive feature of the Bristol Britannia, predominate in this fine winter's morning study of B.O.A.C.'s first two Mk 100s outside the Corporation's maintenance base at London Airport. ENTERPRISE IN AIRSCREWS . . . for November 2nd to 23rd, 1944) but a brief recapitulation will provide a useful preface to a description of the new D.H. airscrew. The key unit is the constant speed controller, or governor, mounted on the engine. A gear pump in the unit takes oil from the engine and delivers it to an hydraulic valve, the setting of which is determined by centrifugal fly-weights whose position is governed by engine speed. This valve distributes oil via a rotating transfer bearing within the hollow engine shaft to the blade pitch-change mechanism in the hub. This mechanism (Fig. 1) selects coarse or fine pitch (Fig. 2), so either increasing or decreasing the airscrew's resistance to rotation. Coarse pitch results when oil is delivered to the front of the piston (Fig. 2), and vice versa. The fore-and-aft movement of the piston is converted into rotation of the blades by rollers which run in the slots of two concentric cam rings, one fixed and the other—gear-meshed to the blade root—free to rotate. Thus any change in r.p.m. triggers off a sequence of events which instantly restores the selected value. This value is of course determined by the position of the pilot's r.p.m. control lever, which alters the datum setting of the flyweights. Blades are, in both senses of the word, the most absorbing part of the airscrew. Many and varied are the plan-forms chosen—rounded tips, square tips, tapered chords, parallel- sided chords, and variations too many to recount. The plan- form eventually chosen is, needless to say, a delicate com- promise between available diameter (the fundamental design parameter) and weight. A discussion of all the factors which eventually decide plan-form and section is outside the scope of this article, but a brief commentary is pertinent. As powers increased during the 1939-45 war there was a steady tendency towards increasing the number of blades to provide the required solidity for absorption. Four-bladers began to re- place three-bladers on high-performance fighters; even a five- blader saw service, and six-blade counter-rotaters were not uncommon. A further tendency, as higher powers had to be dealt with for the same diameter and r.p.m., has been for the predominant blade area to move towards the tips, where it can do more useful work. This trend has been most notice- able since the war, especially on turbine installations, where take-off is the critical condition. Since cruising r.p.m. of a turbine are a much higher proportion of take-off r.p.m. than is the case with a piston engine, take-off r.p.m. have to be kept comparatively low too in order to keep cruising tip-speed, and therefore noise, at an acceptable level. Hence broader blades to restore take-off performance. Hollow steel blades are considered by de Havillands essen- tial for big airscrews required to handle 4,000 h.p. and above; and, indeed, without them further progress would be barred. D.H. were the pioneers outside the U.S.A. of this type of blade, having foreseen soon after the war that, without a heavy weight penalty, Duralumin blades would be unable to provide the necessary stiffness for the big blades needed by the very powerful new turbines. The engineering and manu- facture of steel blades (described by our sister journal Aircraft Production, July to August, 1951) is a matter of con- siderable ingenuity and skill: examples in operation on a British aircraft are the 16-ft units of the Bristol Britannia. At the same time, however, the design and development of solid Duralumin-bladed airscrews has not lagged; in fact, progress in design and metallurgy, and especially in vibration research, has led to their use for powers that only a few years ago could not have been handled by light-alloy blades. (An example is the new de Havilland airscrew for the Rolls- Royce Tyne). Vibration research is undoubtedly the key to a successful blade, and into it is put the major design and development effort. By artificially applying vibration to the blades, and to the hub also, inherent vibration and its unpleasant effects in operation are eliminated in the laboratory. The word "laboratory", with its associations of scholarly research in an environment of monastic quiet and seclusion, is perhaps a misnomer for the de Havilland vibration department at Hat- field, where electro-magnetic exciters pound all day long at blades, hubs, spiders and—such is the scope of D.H. vibration analysis nowadays—a wide variety of aircraft components from industry at large. We recently illustrated some of this work (Flight for November 11th, 1955) and observed that the controlled application of high-frequency vibration stresses occupies the greater part of an airscrew's design and develop- ment period, in which the paramount quest is for longer life, lower weight, and smoother running. So much for the airscrew—and its background—as we know it today—a remarkable synthesis of design and engineer- ing ingenuity. For nearly two decades the airscrew's charac-
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