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Aviation History
1949
1949 - 0256.PDF
FLIGHT Engine mark and aircraft are as follows: WS.51.L vertical shaft)—Westland Sikorsky 51. LE.21.HM (vertical shaft)—early Bris- tol 171. LE.22.HM horizontal shaft)—Fairey Gyrodyne. LE.23.HM vertical shaft)—Bristol 171. The WS.51.L replaces the American Pratt and Whitney Wasp, which has opposite rotation. To alter the direction of rotation of the rotor blades would be a major undertaking; the power plant is, therefore, fitted with reverse gearing as well as its 0.8:1 redaction gear. The lubrication system has been made suitable for the installation with the minimum of modification, and the rear cover remains near standard. A small production order has been placed for Leonides WS.51.Ls For Bristol 171s two horizontally operating units have been specially developed, namely, the 21. HM and 23. HM, known respectively as the long and short engines. The 21.HM, which has an earlier type of rear cover and accessory arrangement, is now being superseded by the 23.HM, which is 8in shorter due to an improved rear cover with integral sump, redesigned pumps and rearranged accessories. Certain important aircraft accessories—generator, vacuum pump, etc.—have, in fact, been removed to the rotor gear box above, where they are driven from the tail rotor shaft. Here, therefore, they remain operative for a forced landing in the event of an engine failure. Completing the list of helicopter units is the LE.22.HM, produced for the Gyrodyne. This more nearly resembles Power for Helicopters ... the standard Leonides 501 series in being mounted in the conventional vertical manner. No reduction gear is fitted, and accessories have been modified. The standard Leonides engines, 501/1 and 501/2, were awarded type-test certificates in December, 1945, and January, 1947, respectively. The WS.51.L has completed its test and confirmation of type approval is awaited. The LE.23.HM has not yet been type-tested. It is, however, this latest LE.23.HM (short unit) for the Bristol 171 with which we are principally concerned in the subsequent detailed description. The basic Leonides is structurally unaltered so far as the power section is concerned, although increased output has been ob- tained by increasing the r.p.m. and boost. Latest figures for maximum (5 min) S.L. power are 550 h.p. at 3,200 r.p.m. and + 7 1b boost. Other data are tabulated. To summarize power-plant features briefly, the main and tail rotor are driven through a "vertical" shaft extending between engine crankshaft and rotor gear box. Interposed are a large centrifugal clutch and a flexible shear coupling. To the clutch are attached the blades of the cooling fan. This circulates air down- wards over the cylinder fins, which have close-fitting baffles and are surrounded by a metal cowling. A separate oil cooler is provided. Starting with the front or top of the t-ngine, it may be seen that the reduction gear housing has been replaced by a tim- ing case cover plate, through which pro trudes the standard splined crankshaft. The splines which would normally carry the main driving pinion of the reduction Cowled Leonides horizontally installed in the power compartment of a bristol 171. Cooling air enters through a forward-facing grilled aperture in the roof of the engine compartment. gear engage with an internal component in a short steelcoupling sleeve. This sleeve is bronze-cone-mounted at each end and carries a flange to which is bolted the clutchback-plate. Borg and Beck have played an important part in the development of the clutch, which is of the single-plate centrifugal type. Engagement is brought about by eight- bob weights mounted on Torrington needle bearings j dis- engagement is controlled by four springs adjustably ten- sioned with the aid of their retaining nuts. The main clutch driving shaft is splined to the driving plate and again to the upper steel coupling sleeve. At its lower end it extends into the hollow crankshaft and is carried in a needle bear- ing, itself located in an "egg cup" sealing plug. The upper end of the main shaft revolves, when the clutch is disengaged, in a large ball race. A provisional engagement speed of 960 r.p.m. has been settled, and the rotors are completely free at 820 r.p.m. A grease nipple is provided on the clutch driving plate which feeds through into the hollow drive shaft so that it can be packed with grease to lubricate the needle rollers at the lower end, from the inside, and also the main bearing at the upper end. A number of Angus seals prevent leak Hie drive from crankshaft through clutch, disengaged and engaged (below left), to rotor drive shaft, shown in simplified form. Below right) Diagrammatic layout of the fuel system. COLD START FUELINJECTION NOZZLE
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