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Aviation History
1957
1957 - 1482.PDF
572 FLIGHT, 11 October 1957 ROLLS-ROYCE AVON 200 SERIES . . . The intricate casting which forms the compressor outlet casing. Certain features are annotated in the drawing on the right: A. top and bottom struts slotted aft of dividing wall; B, engine anti-icing air bleed; C, vent to atmosphere, front section of strut; D, seal vent; E, struts (other than upper and lower) slotted for- ward of dividing wall; F, air- frame anti-icing and cabin- pressure air. system. This device remains engaged when the engine isstationary and until a predetermined r.p.m. has been reached, whereupon the pawls disengage and the motor is stopped. Thetorque is transferred by splines to a large bolt housed inside the forward end of the compressor shaft. Drive is transmitted tothe inner face of the shaft itself by further splines at the rearward end of the bolt. A fabricated sheet-metal extension is bolted to the front of thefront bearing housing to form the true engine intake. In an open recess in its inner surface it carries a temperature-sensingelement which provides indications , for the inlet guide vane ram. The oil feed and scavenge pipes for the front bearing andstarter reduction gear are led through two passages cast for the purpose in two of the struts of the intake annulus. As described above, the compressor casing is divided into aforward section (magnesium) and an after part (aluminium alloy), both being split longitudinally into two halves. Each section isflange-bolted to its neighbour, the stator blades being held by tongues in grooves cast into the compressor ribs and machinedat the locating diameters. The blades are threaded into each half of the compressor shells before assembly. The forwardstages are in aluminium alloy, while the rearward stages, exposed to higher temperatures, are of steel. To prevent fretting at theroots under high loadings, the fourth and fifth and tenth to fifteenth stages are slipped into aluminium-alloy rings which arethemselves a tongue-and-groove fitting in the case. End stops are incorporated at the compressor joints to prevent circumferentialmovement of the stator blades. In the first four stator stages—that is, in the more sharply Hollow inlet-guide vane with its operating lever. tapering section of the compressor—the tips of the stator bladesare supported by being brazed in threes, fours or fives to rolled aluminium shrouds which also make a tip seal against flanges onthe compressor spacers. At the rear end of the compressor, the stator blades are unsupported at the tips, but are brazed togetherin groups at the roots. The bases of the seventh-stage stator blades are cut away so asto leave a rectangular gap between adjacent blade roots; and these provide a passage for the main compressor bleed designedto relieve blade root stresses at lower r.p.m. settings. Air thus escaping is collected in an annular manifold cast round the rearof the forward casing. At the lower engine-power settings, a ram operated by fuel pressure opens a gate valve mounted onthe upper starboard side of the manifold to bleed unwanted air to atmosphere. The manifold extends forward over the third,fourth and fifth stator stages. Fuel to open the bleed valve is supplied from the inlet-guide-vane operating system, so that thebleed opens when the vanes reach a certain setting. Two flanges, cast in the outer surface of the rear section ofthe compressor casing, provide attachments for the tubular front suspension and its swing links. Further contours are also cast inthe compressor surface to accommodate the various engine accessories which are mainly located underneath. Described earlier in this article as something of a triumph incasting, the compressor outlet casting forms an annular passage for air passing from compressor to combustion section and alsoprovides support for the centre bearing, wheelcase and ancillary drives. In addition it has integrally cast manifolds for anti-icingand pressurizing air bleeds, and provides the forward support for the flame-tubes and their burners.In the main air passage, eight struts carry the structure loads to the bearing housing. Each strut has two passages cast in it, andslots connecting the foremost of these with the main airflow effect the main bleed for anti-icing and aircraft services. Theair passes inwards through most of the struts and is collected in the manifolds cast inside the main casing; but through two strutsit flows outwards to the two bleed outlets. The use of the anti- icing system causes a certain reduction in thrust and it is thereforeonly applied when needed. An electrically operated gate valve is located at the relevant outlet to control the bleed. The top andbottom struts are slotted at the rearward passage to collect bleed air, while other air bled from the seals at the last stator stage isvented direct to atmosphere through the forward passages. The centre bearing housing is bolted into the compressor out-let casting and holds the central ball bearing which supports the shaft at this point. At the rear is attached a further aluminiumplate, carrying the bearings for the two spur wheels which transfer shaft rotation, via a helically splined bevel drive, to the twoaccessory shafts; the bearings of the latter are accommodated in the outlet and wheelcase castings. Primary and main fuel sup-plies for the burners are taken from the fuel-control unit under the engine direct to the distributer pipes which connect with eachburner. The cast aluminium snout of each burner is located by setscrews in the outlet casting; and the burners themselves havetheir supports in the casing at this point. Here the structure of the engine is carried by the steel casinginside the combustion section. The outer casing and the intermediate casing, colloquially known as the "milk churn," andmade in Fortiweld steel, are thus directly bolted to the inner and outer flanges of the compressor outlet casing and extend aft as faras the end of the flame-tube proper and the beginning of the nozzle section. At this point they are in turn bolted to the rearbearing housing structure and the Crown Max centricast nozzle- box assembly. A sheet steel shield is applied to the intermediatecasing to form a double wall inside the air casing through which high-pressure air is bled off to seal the turbine. Some of the structural loads are transferred from the "milkchurn" and rear bearing housing to the outer nozzle casing by hollow radial struts between the flame-tube nozzles. These strutsserve also as channels through which low pressure cooling and sealing air, which has passed through and round the main shaft,is transferred to the cooling manifold round the nozzle casing. It is thence vented direct to atmosphere.Each of the eight flame-tubes consists of four sections, the cast-aluminium snout (mounted in the compressor outlet casing),the flare, the tube and the nozzle, all in sheet Nimonic 75. About eight per cent of the compressor air passes directly into the snout
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