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Aviation History
1946
1946 - 0818.PDF
422 FLIGHT APRIL 25TH, 1946 METRO-VIC K CAS TURBINE area of the outlet orifice progressively reduces the thrust efficiency of the fan stream. If, by having a variable out- let, the area of the orifice can be reduced proportionally to the rise in flight speed, for a given mass flow the stream velocity will be increased and thus the efficiency of pro- pulsion may be extended to higher speeds, with a conse- quent delay in thiust drop from the augmenter. Structurally, the build-up of the main elements can well be seen in the cut-away drawing, but we may with advan- tage briefly survey the salient points of the structural com- position. The casing of the compressor is an RR50 light- alloy casting in four parts, top and bottom, front and rear, the former bolted to- gether at horizon- tal meeting faces, and the latter bolted to- gether circumferentially in the transverse plane of the sixth rotor stage. Dovetail-section axial slots are machined in the bore of the casing, and the stator blades are held in these, spaced by distance pieces and locked by peening. Bolted to the front face of the com- pressor casing is the intake shroud which, by means of '' spokes '' across the intake an- nulus, supports a coned diaphragm in which the front thrust bearing is housed, blades are designed to '' screw a forward reaction, and as the turbine blades "windmill" in the gas stream and thus have a rearward reaction, the opposed thrust components of the respective blades tend to compensate each other and the thrust loading on the bearing is so reduced. The compressor drum is a single-piece forging in the outer surface of which are machined axial, parallel ser- rated slots for anchorage of the rotor blades, these also being spaced by distance pieces and locked by peening. Detail ofburner showing airflowthrough shroud tube to effectcooling. FUEL Again, since the compressor air into the unit, they have Augmenter fan assembly showingrelation of contra-rotating fans and their turbine. The build-up of fansand turbine can be seen n the cut- away drawing. Compressor half casing -showing stator blade stages andjoint between front and rear sections. A coned diaphragm, bolted internally to the drum at the third stage station, is spigoted to run in the front thrust bearing, and in- ternal splines in the bore of the spigot transmit the drive through a splined quill shaft for power- ing the auxiliaries. The starter motor is housed within the " spinner" and en- gages the forward end of the quill shaft through a plain dogged coupling. At the rear end of the compressor drum is bolted a conic extension, this having bolted to its rear end an inter- nally-splined sleeve which runs in the rear journal bearing and houses the splined shaft of the overhung turbine wheel. The journal bearing just mentioned is housed within the rear end of another cone extension member (this one being co-axial with, and enclosing, the internal extension cone) which, at its forward end, is bolted to the rear face of the compressor casing and is vented at this point to form the compressor outlet branch. Lubrication and Cooling Surrounding these two co-axial cone extension members is the annular combustion chamber, the form of which has already been described. Suffice to say it is fabricated in commercial quality, high-temperature sheet steel developed by Firth-Vickers before the war and known as Immaculate 5. Jointing faces are formed by machined flanges spot- welded to the main fabric, units being attached by through- bolting adjacent flanged faces. At the rear end of the combustion chamber, the stator blade stages for the turbine are anchored in peripheral rings bolted together, and the inner ends of the first row of blades (the nozzle ring) are supported on a cone diaphragm bolted at its forward end to the outer extension member, which houses the turbine bearing. Each of the twin discs of the fan turbine is separately supported on a pair of roller and ball-thrust bearings, both sets of bearings running on a common shaft, which is~ carried in a six-point, triangulated star-bracing of tubular members which also serve to support the fan ducting and jet tail cone or bullet. A glance at the cut-away drawing will serve to make clear the arrangement. The method whereby the fan blades are carried off the crown of the turbine blades has already been described. Before passing on to the starting and running control systems it may well be advantageous to review the extremely clever cooling and lubrication methods. Going back to the compressor end of the unit, radial holes tbrougfc the compressor drum, or rotor, at the fourth stage supply air under /fth-stage pressure to the interior of the drum, whence it flows along inside the cone extension and is admitted to the turbine bearing and, in so doing, energises a "scent spray" in order to provide an air/oil mist to lubricate the bearing. The mist is then evacuated back along the cavity between the inner and outer cone exten-
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