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Aviation History
1955
1955 - 1078.PDF
190 Gas flow in the Oryx. Established figures at the NOr.l rating: A, intakes at 288 deg K and 14.7 Ib/sq in abs; B, turbine outlet at 855 deg K and 25.4 Ib/sq in abs; C, auxiliary compressor delivery at 346 deg K and 25.5 Ib/sq in abs; D, final delivery to rotor at 678 deg K and 23.5 Ib/sq in abs. D NAPIER ORYX... • —a prerequisite to the maintenance of the fine clearances neces-sary in so small an engine. Considerations of mechanical stiffness were also principally responsible for the attachment of shroudsaround the first three rows of stator blades. Like the compressor casing itself, these shrouds and the stator-retaining rings aredivided into top and bottom halves. The materials used in the compressor are Hidurax 1/12A aluminium-bronze (supplied byHigh Duty Alloys) for the rotor blades, which are secured by fir-tree roots to discs of S.62 steel for stages one to 11, and S.82for the final stage. Magnesium-zirconium RZ5 is again used for the casing. There are six main bearings in the Oryx (all by Hoffman),disposed as shown in the drawing on pages 192-193. The com- pressor is carried by a front roller bearing and a rear ball bearingmounted in the main support-plate casting. A muff coupling shaft connects the compressor shaft to the turbine shaft and thebearings on these shafts share a common drainage sump. In like manner, the bearings at either end of the shaft connecting theauxiliary compressor to the turbine are also housed in a common box-like structure. A simple turbojet, with two or three bearings, can have tiedshafts with compressor and turbine loads roughly balanced. The Oryx, however, has sliding couplings and the loads on the threemain shafts have to be individually taken out by the appropriate pair of bearings.In all their new engines the Napier company are taking great care to keep bearing loadings as low as possible. In the Oryx, forexample, the load on the compressor rear bearing is the algebraic sum of the aerodynamic end loads, comprising pressures on therotor blades, discs and end sealing disc, and it has therefore been necessary to keep the air pressure on the compressor back-plate as low as possible. There is, therefore, extensive labyrinth sealing, and an air-leak, discharged overboard, is permitted throughthe rear labyrinth seal. In order to accommodate thermal ex- pansion of the cast support-plate, this seal is mounted on swinginglinks so that it remains centred at all temperatures. Bearing-cooling is provided by oil jets. Air is tapped from themain compressor in line with the sixth rotor stage, passed through a felt filter under the compressor, and then ducted to small pres-surizing chambers (at 2 to 31b/sq in gauge) on one side of each bearing (excluding the main-compressor inlet bearing) to keepback the oil. Any excess leakage of air through the seals which cannot be accommodated by the scavenge pumps passes acrossthe bearings and eventually into the shaft, which is breathed to atmosphere. The leakage air from the main compressor-outlet Three more Oryx details. From the left: components of the auxiliary com- pressor, with the half-casing and inter- stage stator bidding removed above and the rotor below; a two-piece combustion chamber with flame tube and up- stream-injection burner; and the col- lector, showing internal arrangement and port engine-mounting trunnion. FLIGHT bearing and the turbine-inlet bear-ing (similarly for the turbine-outlet and auxiliary - compressor - outletbearings) flows first into a large chamber surrounding the couplingshaft before entering the shaft. The oil carried by the air into the largechamber lubricates the couplings. The turbine section always remainssubstantially in line with the com- pressor and the drive is accordinglytransmitted through an ordinary muff coupling. The auxiliary com-pressor, however, being bolted on the back of the hot mixing box, cansuffer greater misalignment as a result of thermal distortion, and it is- .-...- " '•""" accordingly driven through gear- ' * "'• ' type couplings centred in Ferro- bestos ring bearings. The latter are employed to keep thecouplings central during the run-down, when the lack of drive- torque results in slight back-lash having to be accommodated. All bearings are scavenged and will function in any of the un-usual flight attitudes which may be assumed by a helicopter. The slight pressurization of the interior chambers results in anair/oil mist being forced into the interior shafting, where it is centrifuged as a spinning column, the air passing out at theauxiliary-compressor end of the shaft in a dry state. Air lost from the labyrinth at the delivery end of the auxiliary compressorpasses into the canister inside the mixing chamber and reduces the metal temperatures between the oil mist and the hot canware. Asa result, silicon rubbers are rendered unnecessary, ordinary black rubber seals being employed and the air finally being spilt froma downward-ejecting vent. The inside of the support-plate is left open to the atmosphere.Surrounding the muff coupling shaft is an annular chamber communicating with five holes in each diffuser passage tothe five combustion chambers. Although the air entering these holes has to perform an abrupt 90-deg turn, and thus loses itsdynamic head, the static pressure is high and two feed-pipes trans- mit pressure from the annular chamber to the main balance pistonimmediately forward of the turbine section. The. pressure on this piston is matched with the various turbine load conditions toreduce the bearing end loads. The high-pressure air is then taken out through holes in the shaft distance-pieces and finallyflows over the back of the first-stage turbine discs and the front and back of the second-stage discs. A proportion passes througha restrictor labyrinth (which matches the considerable pressure drop across the first-stage nozzle) and escapes over the front ofthe first-stage disc. To return to the description of the major components, most ofwhich have now already been mentioned in tracing the flows of air and oil, the main compressor support-plate—which, as in allNapier gas turbines, can be considered the structural foundation of the engine—is cast in D.T.D. 708 light alloy. At its forwardend, it incorporates two rows of straightener blades and then divides into five separate diffuser sections, one for each of thecombustion chambers. The casting also incorporates the annulus through which is extracted the turbine pressurizing and coolingair, and a trunnion mounting attachment is provided on each side (everything forward of these attachments is cantilevered.Following typical Napier practice, the turbine section is held in an external assembly of brazed steel tubes forming a rigid frame,bolted to the back of the support-plate and forming the back- bone of the engine. The combustion system, which was designed in conjunctionwith Joseph Lucas, Ltd., comprises five tubular chambers. Outer casings are fabricated in S.84 and the flame tubes in D.T.D. 703(Nimonic 75 sheet). Spherical joints are included in the outer casings 4to allow for slight misalignment on assembly, eachchamber being articulated so that it does not fight the rigidly controlling tubular frame. As in the Eland, upstream-injectionburners are fitted, and each burner can be removed without removing any other component. Ignition is effected by a high-energy, high-voltage system of theR.A.E. type, manufactured by B.T.H. or Rotax. Igniter plugs are provided in two of the chambers, interconnectors carrying the
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