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Aviation History
1946
1946 - 0354.PDF
F LtG \o\ FEBRUARY 21ST, T046 The nozzle junction box with skirt, diagonalstays, and combustion chamber seatings SERIES II COBLIN 1 ween inlet and delivery approxi- mate to 3.5 and 150 deg. C.respectively. At 10,500 r.p.m., the gyroscopic couples are about hall those for a comparable pis-ton engine/airscrew installation. The impeller is a one-piece,heat-treated light-alloy forging, anodised and polished. It is thirty-one inches in diameter, has seventeen vanes and reaches a maximum tip speed of 1,430ft.per second. On the back ol the impeller anumber of concentric labyrinth grooves (the number has beenincreased in later Goblins) match up with similar grooves on thesealing plate at the back of the compressor unit.The gas turbine which pro- vides driving power for the com-pressor and auxiliary drives is of the single-stage axial-flow type. It has seventy-seven stator blades and eighty-threerotor blades; numbers having no common denominator to avoid resonance. Tip diameter is about 27 in. and theblades, tapering slightly from root to tip, vary between 1.3 and 1.1 inches in width. Under operating conditionsthe centrifugal stress at the blades is nine tons per sq. in. and the maximum gas bending stress is 12.5 tons per sq.in. The constructional material, Nimonic 80, is an alloy with very high nickel content. Thecreep limit in the ferritic steel turbine disc is 0.1 per cent., but this has notin fact been approached on compon- ents checked after the 300 hours'running period. The moving blades are attached tothe turbine disc by the "fir tree" method, that is, evenly spaced ser-rated slots are broached in the peri- phery of the turbine disc and the rootsoi the blades bearing corresponding serrations are slid axially into theslots. The blades are held in position by peaning at the roots on each side.On the upstream side the peaning is a little heavier to resist rearward thrust,blades and the slots in the turbine disc appear 011 page 189. Bolted to the turbine and the impeller, the large-diametercentre shaft is machined from a steel forging, and these three components form the main rotating assembly whichis carried in two ball bearings. Any thrust loading is taken by the front bearing which is located on a stub or pivotshaft between the air intake ducts in front of the impeller. This stub shaft is shrunk into and bolted on to the impeller The fixed nozzle ring assembly showingthe diaphragm for radial location. Compressor iaby-r'nth sealing rings. Illustrations of the COMBUSTION CHAMBER FRONT SWIRLVANES, CONNECTING EL8OW- and all the bolts for the attachment of both stub and mainshafts pass through tubular dowels which take all shear loading, the studs being in tension. Drives for theauxiliaries are taken from the stub shaft through bevel gears. The rear bearing is fitted in a special sliding housing andis designed to take only journal loading. It is air cooled, carries a heat insulating plate, and runs at a temperatureof about 130 deg. C. The cooling-air circuit and the lubri- cation is described later. Turbine and impeller are first balanced separately andthen as a unit. The limits permitted are respectively 5 and 7.75 grm. inches, and subsequent measurements of vibra-tion for the whole power unit have shown that the vertical oscillation is less thar ± 0.001 inch Improved Combustion Chambers Sixteen combustion chambers are fitted to the Goblinseries units, and on the Goblin II these are of " flower pot" design as illustrated on this page. They comprise a flametube and an outer casing—each of which is made in two basic sections—and a burner. With the primary part ofthe flame tube, the fuel burner forms an annular space through which passes a metered quantity of air, sufficientfor primary combustion. The metering annulus is, in fact, located in the outer dome, between the " Snout" and innerdome, and approximately one quarter of the air to the combustion chamber passes through it. The air for primarycombustion also passes through swirl vanes which promote a back vortex round the flame. As a result, the inrush ofair helps to promote flame stability instead of blowing the flame out. In addition, a small quantity of air whichpasses round the outside of the swirl vanes through holes in the inner dome and flaredcover plate, produces enough outer turbulence to mix thefuel and air. The remaining bulk of the intake air is admitted to the flame tube in stages as it passes round between tube and outer casing. Its pur- pose is dilution and cooling of combustion gases which, from an initial 2,000 deg. C. near the burner, enter the turbine blades at not more than 790 deg. C. Combustion is completed within one-third The Goblin II " flower pot " type com-bustion chamber. The metering annu- lus, formed by the burner tube andouter dome, is above the swirl vanes. Ill NOZZLEil VANES ill .
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