The Rolls-Royce Derwent Mk. I was a jet propulsion gas turbine consisting of a centrifugal compressor connected by a shaft to a single-stage axial-flow turbine wheel. Energy was supplied by burning a paraffin-type liquid fuel in ten combustion chambers, which received the necessary air for supporting combustion from the compressor. Engine accessories were mounted on a wheel case at the engine front and were driven through suitable gearing from the compressor shaft forward end. The complete engine was installed in a streamlined cowling having a large air intake at the front and a projecting jet pipe terminating in a propelling nozzle at the rear.

Working Cycle (Figs. 1 and 2). Air at atmosphere pressure or above (due to ram effect caused by the aircraft forward motion) entered the compressor by intakes at each end of the compressor casing and gained kinetic energy as it passed through the double-sided impeller. The air's kinetic energy was converted partially to pressure energy in the diffuser, from which the compressed air traveled to the combustion chambers. Fuel was injected into the combustion chambers, where it was ignited, for starting purposes only, by igniter plugs situated in two combustion chambers. Burning took place in flame tubes, mounted concentrically within the combustion chambers, and was completed before the gas entered the nozzle guide vane ring mounted at the combustion chamber backs. Balance pipes were fitted between adjacent combustion chambers to equalize pressure and to allow the flames to ignite adjoining chambers when starting. The gases then passed through the nozzle guide vanes and were directed against the turbine disc blades, causing them to rotate. The turbine disc, connected to the compressor impeller by a shaft, provided power for driving the compressor. After the gases left the turbine blades they passed through the exhaust cone and jet pipe to the propelling nozzle. Gases left the propelling nozzle with great velocity produced the reaction and so imposed an axial thrust upon the power unit and consequently upon the aircraft. The thrust increased as the nozzle velocity was increased, (i.e. as the amount of gas passing through the engine was increased). Both these factors depend on the engine rotation rate, which depended upon the quantity of fuel being burnt.

The main shaft was carried in three plain bearings, one in front of the compressor impeller, one at the shaft centre by a flexible coupling, and the third positioned immediately in front of the turbine disc. The centre bearing had a plain journal with Michell thrust bearings on each side to take the shaft end loads.

Fig. 4. Fuel System

Fuel System. Fuel from an electrically-driven low-pressure pump in the aircraft fuel tank passed through a low-pressure filter and entered the high-pressure (H.P.) fuel pump, which was engine driven. This H.P. fuel pump incorporated an overspeed governor control that limited the maximum engine speed at all altitudes by controlling the maximum fuel flow to the burners under varying atmospheric pressure conditions. From the H.P. pump, fuel flowed to the throttle valve, which provided the pilot's manual engine speed control. Two connections were taken from the throttle valve. One was from the inlet union passes fuel through a barostat that bypassed it back to the aircraft tank. The other connection from the outlet side of the throttle valve passed to the combined H.P, cock and accumulator and dump valve (A.D.V.) unit and thence through the fuel manifold to the burners. Altitude increase with its corresponding atmospheric pressure decrease caused an engine rpm increase and, consequently, if a constant engine speed was to be maintained the fuel supply to the burners had to be reduced.

The barostat acted as an altitude-sensitive relief-valve and decreased the fuel pressure to the burners as height was gained and, conversely, increased the burner fuel pressure when descending. The barostat functioned automatically to obviate the dead movement of the pilot's throttle control in a corresponding manner to the automatic boost control used on normal supercharged aero-engines, and thereby enabled full throttle lever travel to be used at all altitudes.

The H.P. cock was provided for stopping the engine and was arranged to cut off the fuel supply to the burners completely, as the throttle valve was set, when shut, to enable the engine to idle. Since a certain pressure was required to produce an ignitable spray of fuel from the burners, a spring-loaded accumulator was arranged to ensure that when starting the engine, no fuel reached the burners until a suitable pressure has built up in the fuel system. A dump valve operated when the engine was being stopped to drain the fuel manifold and relieve the burner pressure before combustion actually ceased, preventing the formation of carbon on the burners and the accumulation of liquid fuel in the combustion chambers, which would ignite when the engine was next started. The accumulator and dump valve were made in one unit with the H.P. cock; this unit was the A.D.V.

Fig. 5. Accessory Locations

Accessories. The H.P. fuel pump, oil pumps, rpm indicator generator, and accessory gearbox drive were mounted on the wheelcase at the compressor casing front and were driven through various gear trains originating from a pinion on the impeller shaft. An electric starter motor was also mounted on the wheelcase and drove the impeller shaft pinion through a centrifugally-disengaging ratchet. Air from the compressor delivery side entered a manifold that pressurized the aircraft cabin. A jacket formed round the exhaust cone heated air circulating through the jacket; this warm air was used for cabin and gun heating systems. At the combustion chamber assembly rear a perforated fire extinguisher ring was mounted, and was connected to the airframe extinguisher fluid pipe system.

Fig. 6. Lubrication System

Lubrication System. Oil from a tank on the wheelcase port side was fed by a gear-type pressure pump, the upper of a triple unit, to a series of internal wheelcase passages, which provided lubricating oil to bearings of the various drive shafts, while one passage terminated in a union at the top right-hand wheelcase side. A pipe from this union led pressure oil to the centre and rear main bearings. The front main bearing was fed by oil from a further passage in the wheelcase. Scavenge oil from the centre and rear bearings was collected by a pipe leading to the lower scavenge pump and drain oil from the front bearing and wheelcase was led to the upper scavenge pump. The scavenge oil was discharged from the twin pumps through a common outlet to a thermostatically controlled oil cooler mounted on the starboard side of the wheelcase, and from the cooler the oil was returned to the oil tank. An inverted flying device and a gauze delivery filter were positioned in the base of the wheelcase.

Cooling Air System. The centre and rear bearings and the turbine disc front face were cooled by a separate air stream provided by a small centrifugal fan mounted immediately forward of the centre main bearing. Air was admitted through short stub pipes fitted to the centre bearing casing front end and passed through the fan to the rear bearing housing inside. The air passed over and around the bearing and radially across the turbine disc front face, and forward through the casing and curved outlet pipes to the cooling air manifold, and finally away to atmosphere outside the engine cowling.

Instruments. An rpm indicator, jet pyrometer, oil pressure and oil inlet temperature gauges were installed in the cockpit to enable the pilot to ensure that the Operating Limitations were not exceeded. A burner pressure gauge also provided a fuel system cross check.

Controls. With the exception of the starting switches and low-pressure fuel cock, there were only two engine controls, the H.P. cock, which was used for stopping the engine, and the throttle. Thus the only control used during flight was the throttle control, under the action of which the aircraft behavior was similar to propeller-driven aircraft.

A brief engine description along with an outline of how it worked have been covered. This section amplifies the earlier material and provides information on, and illustrations of the various engine components and explanations of their purpose. The fuel and lubrication systems were dealt with in a similar manner in later sections.

The double-entry centrifugal compressor functioned similarly to superchargers used on reciprocating engines. Air was admitted at the front and rear, through curved annular air intakes, to the eyes of the double-sided impeller from which it passes through a diffuser ring to ten outlets to the compressor delivery pipes. The compressor casing formed the engine backbone, as it was on this casing that lattice supports for the wheelcase and the centre and rear bearing housings were bolted.

Fig. 7. Air Intakes

Air Intakes (Fig. 7). Ram air from within the engine nacelle passed through the lattice supports to the front and rear intakes; metal gauge screens were fitted over these lattice supports to prevent small objects being drawn into the engine. The intakes were light alloy spinnings that were fitted to the compressor inlets to lead the air as smoothly as possible to the impeller intake vanes. The front air intake outer section was riveted to a flange ring bolted to the compressor casing eye, trapping the inner section between the lattice support and front bearing housing. The rear intake outer section was riveted to an air sealing baffle plate; the inner section was clamped between the rear lattice and the centre bearing casing.

Fig. 8. Impeller and Diffuser

Casing (Fig. 8). From the air intakes the air passes to the impeller, which was housed, together with the diffuser assembly, in the compressor casing. This casing was built up from two light-alloy castings, circular in shape, that were bolted together around their circumference. The front casting exterior was ribbed radially and had a circular facing upon which the lattice support was spigotted and bolted. Internally, it was shaped to conform closely to the shape of the impeller and towards the periphery provided a seating for the diffuser assembly. The rear casting followed the same design to the outer diameter of the diffuser seating, but from this point curved backwards and ran into the ten connections to the combustion chambers. In the rear casting interior, ten wedge-shaped blocks, held in position by bolts passing through both halves of the casing, helped to smooth the flow of air to these combustion chamber outlets. Bosses on the exterior accommodated the ten burners. The compressor casing front half had two integral bosses that were drilled and tapped to receive two eyebolts, which provided the means for slinging the engine.

The impeller, which imparted kinetic energy to the air, rotated in the compressor casing centre and was driven by the turbine. The impeller was a single light-alloy forging with 29 radial vanes on each side. At the inlet eye these vanes were curved toward the direction of rotation to minimize the shock to the air entering the compressor. The front impeller shaft flange was secured to the impeller by eight studs. The nuts, which were locked with split pins, seated in counterbores in the flange. Set between these counterbores was a series of holes, drilled and tapped to accommodate balancing dowels. These plugs were available in varying lengths to give the requisite degree of balance and were locked by centre punching the shaft at two opposite points on each plug's outside diameter. Forward of the flange radius was a parallel section which ran in a labyrinth sealing gland; the front of this the shaft was stepped down in diameter to form the journal for the front bearing. The shaft was bored to take an aluminium oil sealing plug, the rear end of which was flanged to rest against a shoulder provided in the bore of the shaft. The plug's forward end had a circumferential groove holding a rubber gland ring that formed an oil seal. Forward of the oil seal plug, the impeller shaft was internally serrated to receive the externally serrated wheelcase driving shaft. Lateral sealing plug movement was prevented by its front end bearing against the driving shaft and its rear end bearing against the impeller.

The flanged rear impeller shaft was also provided with balancing dowels and was secured to the impeller in a manner similar to the front impeller shaft. Behind the flange radius the shaft was parallel, but at approximately mid-length it's diameter increased to form a shoulder for the cooling fan assembly. Immediately to the rear of this shoulder the shaft diameter decreased and was splined and threaded in succession, the splines providing the drive for the cooling fan and the threaded portion receiving the securing ring nut. Behind this threaded portion the shaft diameter again slightly reduced to take the labyrinth gland, and then still further reduced to provide a seating for the centre ball-bearing inner race. The shaft bore was parallel for the forward two-thirds of its length and was then reduced in diameter and serrated to receive the male portion of the centre coupling. A stepped steel bush was pressed into the shaft bore to bear against the front end of the serrations; this bush located a clamping bolt for the coupling.

The diffuser partially transformed the air's kinetic energy as it left the impeller to pressure energy; the diffuser was fitted in the compressor casing around the impeller periphery, and consisted of a series of vanes, arranged to coincide with the air's path as it left the impeller and then curved to reduce the air speed with as little loss as possible. These thin light-alloy vanes were riveted to light-alloy side plates which were pressed into shallow grooves in the halves of the compressor casing.

Fig. 9. Combustion Assembly

Combustion System. From the compressor, air passed to combustion chambers via the compressor delivery pipes. The combustion chambers consisted of air casings in which were mounted flame tubes where the air and fuel combustion occurred. Air entered through colanders at the flame tube fronts and through secondary and tertiary air holes, at the flame tube forward and mid-length positions. Fuel was injected through burners into the combustion chambers. The gas travels from the flame tubes to the discharge ducts and thence to the turbine nozzle guide vanes. To provide a pressure balance and to ensure the simultaneous light-up of fuel in all combustion chambers, inter-connectors were fitted between each flame tube. Bosses were provided in Nos. 3 and 10 air casings to house the igniter plugs; these plugs were fed with high-tension current to give a spark for starting purposes.

The compressor delivery pipes (Fig. 9) were short connections between the compressor casing outlets and the air casings. Each was built up from thin steel sheet with an oval-shaped flange at the front end to mate with the delivery passage opening. A circular flange at the rear end was bolted to a corresponding flange on the air casing using a Walkerite joint. A union welded in the middle of each pipe's inner side provide a connection to the cabin pressurizing manifold.

The air casings that formed the combustion chamber outer casings were built up by welding from sheet steel. They were bolted to the compressor delivery pipes at the front and, with their axes inclining to the centre, were held in the discharge duct assembly at the rear. Each casing ran parallel for approximately half its length and then tapered gradually; the tapered end was trapped by an expansion joint in the discharge duct tapered orifice. To maintain a pressure balance between the air casings, and to allow the flames to travel to all combustion chambers when the engine was started, inter-connectors were fitted at their forward ends. These inter-connectors comprised two short flanged tubes located at their unflanged ends in holes in the flame tubes, and held together at their flanged ends by male and female coupling attachments on the air casings, the captive ring nut of the female coupling on one air casing screening over the threaded male connection on the adjacent air casing. Three bosses were welded at equal intervals around each casing's front end for screwed locating dowels that held the flame tubes in position. Externally screwed bosses were also welded in No. 3 and 10 casings to hold the igniter plug bodies. Air Casings Nos. 3 to 9 had additional bosses to which were connected pipes for draining the air casings of any liquid fuel remaining when the engine has stopped, and so preventing the fuel igniting when the engine was re-started; any fuel remaining in casing Nos. 10, 1 and 2 at the engine top, flows back to the turbine. The fuel in 3 and 9 drained away from bosses on the lowest points of the casings through pipes running to the nearest points on the adjacent casings, 4 and 8. This fuel, together with that remaining in 4 and 8, drained off through piping connected in bosses situated in the compressor delivery pipes of 4 and 8, and from there to connections at the lowest points of 5 and 7, where it was joined by any fuel remaining in these casings. From these connections at 5 and 7, the fuel runs down through pipes which connect with the drain from 6 and thence to the combustion chamber drain valve bolted to the flange of No. 6 casing.

Fig. 10. Air Casing Expansion Joint

The flame tube assembly consisted of the flame tube with a colander welded to its forward end. The colander was built up from a dome-shaped perforated outer casing, a concave circular inner plate welded at its edges to the outer casing inside, and a small housing containing eight tangential swirl vanes that was welded to the inner and outer casing apexes. The swirl vanes were designed to impart a turbulent motion to air entering the flame tube and thereby assist the complete fuel combustion. A bush welded in the swirl vane assembly centre housed the burner discharge end. The flame tube was built up from heat-resisting steel; its diameter was constant for approximately half its length and then decreased toward the rear end. The flame tube forward end was located by three dowels passing through bushes welded to the tube, while the rear was supported by small pips pressed out of the tube, which rested against a stepped ring on the air casing inside (Fig. 10). The flame tube assembly was drilled with a series of holes that admitted air in metered quantities for combustion. Primary air passed to the flame tube interior through perforations in the colander assembly inner and outer casings. Combustion was assisted by secondary air entering through two rows of equally-sized holes in the flame tube forward part; this secondary air supply also insulated the tube from heat as it approached its maximum diameter. Four rows of tertiary air holes were provided approximately half-way along the flame tube. Holes in the first row were half the diameter of the remaining three rows; air entering through these holes completes combustion and further insulated the flame tube wall. Excess air that did not find its way into the flame tubes passed along the exterior for cooling purposes, and passed into the discharge duct through the annular space between the flame tube and the air casing.

Fig. 11. Discharge Duct Assembly

Discharge Duct Assembly (Fig. 11). Gases from the combustion chambers passed into the discharge duct assembly, which was bolted to the intermediate casing at its centre and to the nozzle guide vane assembly and turbine shroud ring at its periphery. The assembly consisted of a thick supporting ring, hollowed out at the back to leave only a front facing, ten curved discharge ducts welded around it with an insulating sheet trapped between the ducts and the ring, and a circular flange welded on the outside of the ducts to which the nozzle guide vane housing and the shroud ring were secured by bolts passing through both outer flanges. The ducts changed in section from circular at their forward ends, where a stepped ring was welded on the inside of each duct to provide the female part of the air casing expansion joint, to segmental at the nozzle guide vanes. The inner rib of the support ring carried a flange to which the intermediate casing rear was secured by studs, and the outer rib was stepped and flanged to bolt onto the nozzle guide vane assembly inside ring. Air from the cooling system, flowing out at the rear of the intermediate casing, was deflected by the turbine disc dished surface into the annular space provided by the hollow support ring. Ten holes drilled in the support ring front face allowed air to pass into cooling air outlet pipes bolted to the ring face at each hole.

Fig. 12. Nozzle Guide Vanes and Turbine

Turbine. Gases leaving the discharge duct assembly passed between nozzle guide vanes and were deflected so that they flowed into the turbine blade curves as smoothly as possible. In their passage through the turbine blading; the gases changed their direction, and by so doing, lost some of their kinetic energy in driving the disc and hence the compressor impeller. The gases then passed from the turbine blades into the exhaust cone and finally away down the jet pipe. The turbine disc was bolted to the turbine shaft, which ran in the rear main bearing; the turbine shaft front end was supported by the centre coupling.

The nozzle guide vane assembly (Fig. 12) was mounted at the discharge duct assembly rear and consisted of inner and outer rings, which were "L" and "T" section respectively; these located the guide vanes. The inner ring was held between a clamping ring and the front of a gas-sealing labyrinth, the whole being bolted to the discharge duct assembly support ring. The outer ring was clamped between the discharge duct assembly outside flange and the turbine shroud ring. The 48 nozzle guide vanes were had curved aerofoil sections and were fixed by buttresses integral with the blades fitting into slots incorporated in the inner and outer guide vane rings. Endways vane location in the inner guide vane ring was effected by clamping them between the clamping ring and the gas-sealing labyrinth front face. The gas-sealing labyrinth was a ring with concentric grooves machined on its rear face that mated with a complementary labyrinth on the turbine disc front face.

Fig. 13. Turbine Blade Root

A turbine disc and 54 blades comprised the turbine wheel, which was bolted to a flange on the turbine shaft rear by six bolts secured by split-pinned nuts. The blades, with their curved aerofoil sections thickening toward the root, were fitted around the disc periphery in fir-tree shaped serrated grooves. The blades were held in place by roll peening a tongue of metal, which was left during manufacture, into a groove in the disc rear face (Fig. 13). The turbine disc front face was roughly dish-shaped and was cooled by cooling system air flow from in its path to the cooling air outlet pipes. Around the front face edge, at the blade roots, a series of concentric grooves was machined to mate with the corresponding grooves of the discharge ducts gas seal and formed a labyrinth; this labyrinth prevented hot gas from blowing down the disc front face.

The turbine shaft rear end was flanged to take the turbine disc. A series of holes was drilled and tapped in the flange for balancing dowel pegs, similar to those used on the impeller shafts. Forward of the flange the shaft was reduced in diameter in three steps, the middle step forming the journal for the rear bearing, the other two steps rotating in oil seal labyrinths. Forward of the front labyrinth step, the shaft was again slightly reduced in diameter and at its forward end six splines were took the female half of the centre coupling's internal splines. A circumferential groove and shoulder were machined on the splined portion to locate a coupling securing ring which locked the centre coupling rear portion. A shallow slot between two splines received a small spring-loaded plunger in the coupling which, when released, located in a corresponding hole in the securing ring.

Forward of the splines the shaft diameter was again reduced to form a plain section that was carried in a bush in the centre coupling. The shaft was bored parallel throughout its length and at its forward end accommodated a splined ball coupling shank. The coupling was a press fit and was secured in the shaft by a peg that passed through both shaft and coupling along a diameter. The truncated spherical coupling forward end had three wide splines machined in it, to allow its assembly to the corresponding female spherical seating in the impeller shaft coupling.

The front main bearing (Fig. 14), which supported the front impeller shaft forward end, was a plain white-metal bearing in a circular dished housing. The bearing rear face housing was secured to the forward lattice casting front face by a series of studs, while the housing front face carried the wheelcase spigoted over a shoulder on the housing. The housing centre was bored and faced at each end of the bore. The front facing and the bore located the front bearing, while the rear facing provided main bearing, the front end of the turbine shaft being supported in the centre coupling.

The centre main bearing (Fig. 15) was a deep-grooved thrust ball-bearing. The inner race seated in a shoulder cut in the rear impeller shaft; a tubular spacer clamped between the bearing and the coupling front forced the bearing inner race against the shoulder. The outer race seated in a steel bush held in a conical housing and was secured in position by a retaining plate bolted into the housing rear. The housing had an internally grooved cylindrical extension on the front to provide the labyrinth oil seal, and an outer flange secured by studs, with a shim interposed, to the centre bearing casing. The centre bearing casing was a truncated conical casting, held by studs through its outer flanges to the rear intake lattice at the front and to the rear bearing casing at the rear. A circular web inside the casing forward part acted as a support to the casing and an air seal for the cooling fan casing. Another circular web inside the casing rear was bored and faced to take the bearing housing that carried a labyrinth oil seal in the front. The housing rear spigoted onto the casing and was secured by studs. A circular rib cast on the casing rear face midway between the bearing and casing outside was faced to carry a truncated conical casting that had the rear oil seal labyrinth secured to its smaller diameter end. A circular flange inside this casting fitted around the periphery of the centre coupling and served to locate the coupling rear portion when the turbine shaft was withdrawn. A drilled thickening of the bearing housing web had a union connection at its top; oil from the pump passes through this drilling and corresponding drillings in the bearing housing to two jets spraying oil directly on to the bearing front face.

Centre Coupling. The rear impeller shaft end and the turbine shaft were joined by the centre coupling. This was a double coupling comprising an outer flanged coupling and an inner ball spline joint, so designed to provide sufficient flexibility to allow for a certain amount of misalignment in the three main bearings and to transmit the very high thrust developed by the engine. Drive was taken by the flanged coupling, the female component of which was fixed onto the turbine shaft and had teeth that mated with complementary teeth on the male member connected to the impeller shaft. The ball spline joint was made by a splined ball, fitted into the end of the turbine shaft, seating in a correspondingly-splined cap that was screwed to the male flanged coupling face. The necessary flexibility was ensured by the ball spline joint combined with the clearance allowed between the mating teeth on the flanged coupling.

The outer coupling female component was a short hollow shaft widening out to a flange at the front end; machined around the inside of this flange were the teeth that coupled it to the main component. At the flanged end the parallel portion was bored to take a steel bush, which carried the plain journal on the turbine shaft end; behind this, six internal splines mated with external splines on the turbine shaft. A circumferential groove and a series of short splines were machined on the coupling outside to take a securing ring, and housed in one spline was a small spring-loaded dowel that, when the coupling slid onto the turbine shaft, located in a shallow oval depression between two splines on the shaft. The securing ring, which retained the unit on the shaft, was machined with two internally-splined lips, one at the front, which seated in the groove on the coupling, and a deeper one at the rear, which seated in a similar groove on the turbine shaft. The assembly was interlocked by a spring-loaded dowel that, when released, located in a coinciding hole in the securing ring.

In addition to the flanged coupling female half, the turbine shaft carries the ball spline joint male half. This was a truncated spherical shape machined with three wide splines. Integral with the ball was a parallel shank that was pressed into the turbine shaft bore and retained by a peg passing through shaft and shank across a diameter.

The flanged coupling male component was complementary in shape to the female member. Teeth, which mated with those on the female coupling, were machined around the periphery of the flange, and the parallel portion carried fine external splines that correspond with serrations in the rear impeller shaft bore. The coupling was secured in the shaft by a hollow clamping bolt, the underside of the bolt head bearing against a stepped steel bush. The bush was inserted in the shaft bore and in its turn bore against the front end of the serrations. A washer was placed under the bolt head, and the bolt was prevented from slipping forward by a circlip resting in a groove cut in the bush. Two slots machined in the underside of the bolt-head provide seatings for two ears on the end of the coupling, the whole assembly being locked by a nut and tab-washer threaded over the end of the bolt. The ball spline joint serrated cap seating was bolted to the male flanged coupling rear face and retained by a locking plate; the three screws used for this purpose also act as locating pins for the female coupling. This coupling was designed so that on assembly, correct alignment was ensured; the female coupling would only slide onto the turbine shaft in such a position that the spring-loaded dowel on the coupling coincided with the oval depression on the shaft; the male coupling was located in the impeller shaft by a master spline, and locked in position by the ears on the coupling seating in the slots in the bolt-head; the locating pins on the male coupling rested in holes on the female coupling face and positioned the components when coupling took place.

The rear bearing (Fig. 16), which supported the turbine shaft rear end, was a plain white metal bearing bolted between two labyrinth glands and carried in a housing supported by webs integrally cast in the rear bearing casing rear. The bearing housing and bearing block were drilled to provide the air inlet to, and oil return from, the rear bearing. The aluminium alloy casing was tapering in section towards the rear, having a flange on its forward face that was bolted to a corresponding flange machined on the centre bearing casing rear. Two inspection covers were provided, one on each side of the housing near the centre coupling, to enable the coupling to be unlocked without unnecessary dismantling of the engine when removing the turbine shaft. The housing rear face was studded and accommodated the discharge duct assembly centre flange and the cooling air deflector casting.

Fig. 17. Bearing Cooling System

Bearing Cooling System (Fig. 17). Cooling air was directed on to the centre and rear bearings and to the turbine disc front face by a small fan unit mounted just aft of the compressor inside the centre bearing casing. This air was drawn through short stub pipes in the centre bearing front casing and delivered from the cooling fan diffuser to the centre bearing, through the rear bearing housing to the rear bearings, and radially across the turbine disc's forward face where it was deflected to twin air manifolds through exit pipes attached to the discharge duct assembly. The cooling fan impeller of the was similar to a small single-sided supercharger impeller and had sixteen radial vanes that were curved forward in the direction of rotation at the eye. The impeller centre was bored and machined with splines that mated with corresponding splines on the rear impeller shaft; the impeller bearing front bore against a beveled spacer that bore against a flange on the shaft. This spacer was fitted to ensure that correct clearance was obtained between the impeller blades and diffuser. The impeller was secured against the flange by a ring nut that was locked by plain and tab-washer.

Clamped between the rear air intake lattice and centre bearing casing front flange was a circular light-alloy casting that formed, together with a diffuser, the cooling fan casing. Additional support was given to this casing by an internal rib in the centre bearing casing that spigoted over a seating machined on the fan casing rear face; a rubber ring recessed into the seating ensuring an air-tight seal. The bore front part of this casting was lipped outwards to form the cooling fan intake eye. Deflection of the air to this cooling fan intake eye was ensured by an air baffle plate, which was a thin, circular aluminium sheet trapped at its periphery between a shoulder machined on the fan casing and a complementary recess in the rear intake lattice just below the clamping bolt. Midway between centre and periphery, the baffle plate was riveted to the outer section of the rear intake. The rear face of the cooling fan casing was machined to conform closely with the shape of the front face of the impeller.

The casing curved portion outside was machined to form the seating for the diffuser, which was secured to the casing by a series of studs. The diffuser comprised a series of vanes integral with a disc, which formed the fan casing rear part. The diffuser disc forward face was recessed to house the impeller. Its centre was bored and grooved to form a labyrinth seal over the ring nut, which secured the fan impeller to the rear impeller shaft.

A circular lipped casting attached to rear bearing casing rear face deflected the air stream across the turbine disc front face and into the annular space in the discharge duct assembly, from which the air was led to two curved manifolds fitted around the exhaust cone. These manifolds discharged air to atmosphere through two air discharge pipes.

The wheelcase was an aluminium casting bolted to the engine front. It provided mountings for the various engine accessories, and contained the necessary shafts and gears for the starter motor, rpm indicator, auxiliary gearbox, oil and fuel pumps. The front rectangular section was faced to receive the housings for the accessory gearbox drive and the main drive; a cover plate over the latter gave access to an auxiliary driving pinion used for test purposes only. Also on the front face and just above the drives, attachments oil tank breather fittings were provided. The fuel pump, throttle bracket, and a boss housing an oil supply union to the centre and rear bearings, were all mounted on the starboard face, while the port face had inspection holes for the rpm indicator generator and oil pump drives It also provided a boss to which was bolted a small casting housing the oil pressure relief valve. Mountings were provided on the upper face for the starter motor and rpm. indicator generator on the right and left respectively, and just to the right of the starter mounting a boss was bored to receive the pressure transmitter adapter. The fuel filter and the oil pump unit were both mounted on the lower face, and between these two a housing was provided in the wheelcase sump for the scavenge oil filter. On the circular flange to the left of the rectangular section, was a small cover plate which when unbolted, allows the extraction of an oil transfer bobbin. Internally, the wheelcase casting was webbed and bored to provide housings for the bearings and oil galleries for lubrication purposes.

Wheelcase drive assemblies (Fig. 20). The main drive, which carried the main driving pinion and starter mechanism, was an impeller shaft continuation that was coupled by a splined coupling piece. Drive was transmitted from the starter motor on the top of the wheelcase through a bevel on a vertical spindle at right angles to the ratchet gear. Running parallel with the main drive was the auxiliary drive, having a spur gear driven by the main driving pinion, and driven from a pinion on the auxiliary shaft, through bevels on vertical spindles, were the rpm indicator generator on the top of the wheelcase and the oil pump unit on the bottom of the wheelcase. The fuel pump was driven from a bevel pinion on the vertical oil pump shaft through a bevel integral with a horizontal spindle.

The main drive shaft assembly (Fig. 21), which was coupled to the impeller shaft, carried the starter ratchet and ratchet gear through which the drive from the starter motor was taken. Integral with the shaft at its rear end was the main driving pinion that transmitted the drive to the auxiliary shaft spur gear. This pinion was had internal serrations to mate with corresponding serrations on the short coupling piece, the other end of which splined into the front impeller shaft. Just in front of the pinion, the shaft ran in a ball bearing housed in a flanged collar that was inserted in a web in the casting and retained by three screws passing through holes in a cover plate and the flanged collar. The bearing inner race was forced against a small shoulder on the shaft by a tubular spacer placed between the bearing and the starter ratchet. The starter mechanism comprised an annular channel section carrying three spring-loaded pawls; these were located by pivot pins held in the carrier, which trapped the spring ends. This assembly fitted into a hollow ratchet gear whose teeth meshed at right-angles with those on the starter motor spindle gear . At low speeds the pawls free-wheeled around a series of ratchets machined inside the ratchet gear, but the pawl springs were so loaded that as soon as the engine reached a certain speed, the pawls flew outwards under centrifugal force and disengaged. Forward of the pawl carrier, the main drive shaft ran in a roller bearing housed in the ratchet gear shank. A flanged collar placed between the bearing and the carrier forced the latter against the tubular distance piece; the bearing being located by an auxiliary drive pinion used for test purposes only, which was splined onto the shaft forward end. The whole assembly was secured by a nut and tab-washer threaded over the shaft end. The ratchet gear shank rotated in two ball bearings, the outer races of which seated in two shoulders machined in an aluminium housing inserted in the wheelcase front. The rear bearing inner race was clamped against a small shoulder in the gear shank by a tubular spacer, and the front bearing inner race was held against the spacer by a circlip carried in a groove cut in the shank. The wheelcase front was faced to provide a mounting for the aluminium housing flange, which, together with a cover plate, was bolted to the wheelcase. To ensure correct alignment and gear wheel meshing, a shim was incorporated between two joint washers on the mounting face.

Starter drive assembly (Fig. 22). The starter motor drove the ratchet gear via a quill shaft and bevel pinion, which was borne in two ball bearings carried in a flanged bush that was spigoted and bolted into a web in the casting above the main drive. The outer ball bearing races seated in two shoulders cut in the bush bore, and the inner races were separated by a spacer. The pinion and bearing assemblies were secured by a nut and tab-washer threaded over the pinion shank upper end. The quill shaft splined into the hollow pinion and carried internal splines on its other end to provide the starter motor connection.

The auxiliary drive assembly drove the rpm indicator generator, oil pumps and fuel pumps through right-angled bevels, and carried the accessory gearbox coupling at its front end. A spur gear driven by the main driving pinion was bolted to a hollow shaft that rotated in a plain bush, which was pressed into a web in the casting bored parallel to the main drive. At its rear end the accessory drive shaft splined into the hollow gear shaft and at its front end, which was cup-shaped, it rotated in an oil seal pressed into an aluminium housing spigoted into the wheelcase front. Splined onto the shaft immediately behind the oil seal, and bearing against the shoulder formed by the cup-shaped portion, was a bevel pinion that ran in a ball bearing. The bearing inner race was seated in a groove cut in the pinion shank and the outer race fitted into a shoulder formed in the aluminium housing. To ensure that the bearing pinion and inner race were pressed home against the cup, a flanged bush was inserted in an annular recess formed between the pinion rear face and the shaft. The bearing outer race was secured by a circlip recessed into a groove in the aluminium housing. The cup-shaped shaft end had internal gear teeth to drive a short accessory gearbox coupling piece, which was retained by a bronze retaining ring secured by a circlip recessed into a groove in the shaft (Fig. 23). The shaft was hollow throughout its length in order to provide an oil breather for venting the wheelcase.

The rpm indicator generator drive (Fig. 24) was taken from the bevel pinion on the auxiliary drive, through a bevel splined onto a vertical spindle, the two being secured by a nut and tab-washer threaded over the lower end of the spindle. The spindle upper end rotated in an oil seal pressed into a bell-shaped aluminium housing. The housing was bolted into the wheelcase top and had an annular boss that spigoted into a cavity bored in a casting web; this boss was machined with a circumferential groove for lubrication. Two shoulders were machined on the spindle; the lower one rested on a boss on the housing interior with a shim interposed, and the upper one located the oil seal. Two pins in the spindle top face engaged with holes in a leather disc coupling, from which the drive to the generator was transmitted through similar pins in a flange on the generator shaft.

The oil pump drive was taken from the bevel pinion on the auxiliary drive shaft, through a bevel integral with a vertical spindle. Just below the bevel, the spindle rotates in a plain bush pressed into a web; its lower end was splined into the oil pump driving gear. The fuel pump drive was taken from the oil pump driving gear, through a bevel integral with a horizontal spindle splined into a connection on the fuel pump. The spindle was carried in two plain bushes pressed into a web bore near the wheel case base at right angles to the main drive.

Fig. 6. Lubrication System	Fig. 25. Oil Tank

Lubrication System. (Fig. 6) The light alloy oil tank (Fig. 25) was roughly rectangular in shape and had a 2 gal 6 pint oil capacity and a 7-pints air capacity. It was secured by two straps tightened by turnbuckles on two brackets bolted to the wheelcase port side; tank distortion was prevented by internal stiffeners. A curved filler spout, having a handwheel lock quick-release cap, was bolted to the rear by a flange joint. Housed in a casting bolted to the tank bottom, was a suction filter to which was attached a long rod that passed up the tank centre inside a tube; this formed the negative-G valve, and was spring-loaded against a cap screwed into an opening in the tank top. The filter was removed from the top by drawing it up through the valve tube, which avoided spilling oil when the filter was removed for cleaning. A boss at the tank base housed a union, from which oil returning from the cooler passed up through a tube to a ramp that de-aerated the oil before spraying it down into the tank body. Two vents allowed the air to pass through piping to a breather connection in the wheelcase; one was at the tank top for normal flying, and one was at the bottom for inverted flying.

Negative-G Valve. When the engine was in the normal flying position, oil flowed from the main tank body to the suction filter through an annulus formed by the valve tube lower end. A valve seat at the tank base had its valve held off its seating by a gravity-loaded lever operating on the valve tube. Under negative G conditions, however, the lever weight forced the valve tube down to close the valve. The valve tube end was welded to a bush, which slid on the inside of a short sleeve inserted in the opening at the tank top, and as the tube was forced downwards (or upwards if the aircraft was upside-down) the bush uncovered a series of holes in the sleeve through which the oil was drawn into the tube. It was then supplied to the filter from inside the tube.

A cylindrical coarse gauze suction filter on a brass housing was contained in an aluminium casting bolted to the oil tank base. The filter was held to a conical seating by a spring-loaded rod locked by the screw cap in the tank top. Oil flowed into the filter through evenly spaced holes drilled in the top and filtered outwards into the casing. From the filter outlet the oil flowed to the pressure pump inlet through a short pipe fixed with two rubber hose connections and jubilee clips. The oil tank was drained from an opening at the filter casing bottom, which was fitted with a drain plug, held in position by a locking plate that was in turn locked by a stud and spring washer.

The triple pump unit comprised three gear-type pumps. The pressure pump, which delivered oil from the tank to the engine, was the lowest pump; the centre pump scavenged oil returning from the centre and rear bearings, and the top pump scavenged oil returning from the wheelcase gears and front bearing. All three pumps had a common shaft driven from the auxiliary shaft through bevel gears at 0.25 engine speed. The maximum delivery of the pressure pump was 225 gph. The triple pump was built up from three aluminium castings with steel plates interposed, held together by three bolts, and the whole assembly was held to the wheelcase bottom by seven studs. The pump bushes and thrust washer were lubricated by the scavenge oil.

Fig. 26. Wheelcase Oil Passages

Wheelcase Lubrication. (Fig. 26) The pressure pump delivered oil to a passage drilled in a wheelcase casting vertical web. Smaller galleries branching off at right angles to this main passage fed oil to the auxiliary drive shaft, and plain bearings for the oil and fuel pump drive shaft. Three small holes drilled in the bearings allowed the oil to reach the auxiliary and oil pump shafts, while the fuel pump shaft received oil from a gallery between its two plain bearings. Towards the wheelcase top just above the main and auxiliary drives, the vertical passage ran into a horizontal passage having a non-adjustable relief valve at its port end, and the outlet to the main rotor shaft bearings at its starboard end. Immediately over the main driving pinion a small hole was drilled in the gallery, allowing the oil to be sprayed through a jet directly on to the gears. A relief valve, which controlled the oil pressure at 30 – 40 psi, was a spring-loaded disc valve housed in a small casting bolted to a wheelcase boss at the horizontal passage end. At its opposite end, the horizontal passage ran into a short drilling in the wheelcase at right angles to the passage, and from this drilling oil flowed down to the front bearing through a transfer bobbin, while a certain amount of oil was bled up to the pressure transmitter through an external banjo connection on the wheelcase front. Oil was supplied to the front bearing through the transfer bobbin, one end of which was housed in the drilling at the horizontal passage end and the other end was held in a boss on the front bearing casing. A small cover plate bolted to the wheelcase face gave access to the bobbin for removal. The oil was fed through the bobbin and down small drillings in the bearing casing and bearing housing to the front plain bearing. After lubricating the bearing, the oil drained into a space between the bearing rear and the labyrinth, and returned to the wheelcase bottom, through another passage drilled in the housing just below the bearing.

The oil pressure transmitter comprised two compartments separated by a diaphragm; one contained compass fluid and the other contained oil. A semi-spherical compartment containing the compass fluid was bolted around its circular flange to a corresponding flange on the side of a cylindrical compartment containing the oil, and the whole unit was held on a bracket on the wheelcase by four bolts. The oil pressure deflected the diaphragm, transmitting the pressure to the compass fluid, which flowed up a small-bore pipe to the pressure indicator in the aircraft. Oil was bled from the banjo connection on the wheelcase through a small-bore external pipe to the transmitter base. It flowed through the transmitter and was returned to the main oil feed through a second small-bore pipe. In this way the oil was kept circulating to prevent freezing in the transmitter at altitude.

Bearing Lubrication. From the wheelcase drilling, the main oil feed traveled through another drilling to a banjo union on the wheelcase outside, where it was joined by the oil returning from the pressure transmitter. It was then led by an external pipe to a thermometer pocket on the compressor casing top and then to the centre and rear bearings. The thermometer pocket was a tubular brass casing attached by set screws and a locking plate to the compressor casing top, into which oil piping was brazed at both ends. Screwed into the pocket and completely immersed in moving oil was an electric resistance type of thermometer bulb that was connected to a gauge in the cockpit. The centre bearing received its oil via an external pipe and a banjo union on the centre bearing casing top. Here it was fed through small holes in the banjo bolt shank and a restrictor jet, and down through a drilling in the centre bearing casing rear to the bearing housing. A groove on the bearing housing exterior allowed the oil to run into two holes from which it was sprayed through small jets to the ball bearing front. Oil escaping from the bearing ran down through two similar bearing housing drillings and a passage in the bearing casing to a connection on the casing bottom, where it was joined by oil returning from the rear bearing. The rear bearing oil supply was provided via a banjo union from which a pipe carried oil to the rear plain bearing. The oil was supplied through drillings in the rear bearing casing and the bearing shell, entering the bearing through three holes. Similar drillings below the bearing allow the oil to drain to a further union on the bottom of the casing, whence it was carried by an external pipe to join oil returning from the centre bearing.

Scavenge system. Return oil from the wheelcase gears and bearings and the front plain bearing drained down to the wheelcase sump, in which was housed a gauze suction filter retained in position by a nut and locking plate in the filter cap centre. The oil was drawn into the filter, flowed out of the top and entered the upper scavenge pump through an oilway drilled in the wheelcase casting. Return oil from the centre and rear bearings was brought by an external pipe to the lower scavenge pump. The scavenge pumps delivery sides were connected by an internal duct, and from this duct an external pipe fixed with rubber hose connections and jubilee clips led the oil up to a Serck oil cooler incorporating a relief valve.

Fig. 27. Oil Cooler

The oil cooler (Fig. 27) matrix was housed in a cylindrical brass container held with straps and trunnion bolts to two brackets bolted to the compressor casing starboard side. Spigoted on to the cooler top was a truncated conical sheet metal duct, which mated with a corresponding duct in the nacelle, through which air was passed to the cooler matrix. A casting bolted on the oil cooler side housed a spring-loaded relief valve at its base and was provided with two oil cooler connections; the upper connection was open to the matrix and the lower to a jacket surrounding the matrix. When the oil temperature was low enough to produce a pressure difference across the matrix exceeding 15 psi, the relief valve opened to allow the oil to bypass the matrix by flowing through the jacket, thus preventing bursting of the matrix. The jacket was provided to prevent complete freezing of the matrix under adverse conditions such as a glide at altitude. During normal flight all the oil passed through the matrix and from the outlet banjo at the top was returned to the tank through an external pipe.

Scavenge Filter. An aluminium casing having inlet and outlet connections and, in the centre, a long stud, contained two cylindrical wire filters, one inside the other. The outer filter seated in a sheet metal sleeve in the casing, and a slightly tapered sheet metal cylinder having a shallow lip at its base was inserted between the outer and inner filters; a nut and a locking nut on the stud held this assembly in place. A bell-shaped cap spigoted over the casing and was locked by a wing-nut that threaded over the stud end. After passing through the filter, the oil was returned to the tank through an external pipe.

The pressure filter, which on later engines was inserted in the line between the pressure pump and the bearings, was of the same construction as the scavenge filter; its purpose was to eliminate all foreign matter from the oil before it circulated through the engine.

The exhaust cone was bolted to a flange on the turbine shroud ring and led the gas from the annular turbine discharge to the jet pipe. The exhaust cone assembly consisted of a truncated cone with flanges welded at each end for attachment to the engine at the large diameter, and to the jet pipe expansion joint at the rear small diameter, and a smaller internal cone that served as a fairing for the turbine disc. The inner cone was supported concentrically within the outer exhaust cone by four long studs, arranged along diameters in pairs, and faired off by sheet metal streamline struts. An insulation plate was secured by a large circlip and a central clamping bolt to the closing plate in the inner cone forward end; this insulated the turbine disc rear face from exhaust gas heat.

The expansion joint, which connected the exhaust cone to the jet pipe, was bolted to the outer cone rear flange, with a gasket between their two flanges. This joint had a spherical shoulder projecting rearwards from its flange to allow relative jet pipe movement while still providing a gas seal. The outer cone was lagged with aluminium foil retained in position by two sheet aluminium jackets, separated in the centre by the cabin heating jacket.

Hot air for cabin and gun heating was obtained from a jacket around the cone middle. The airframe connection was taken from a circular boss on the starboard or port side according to the particular installation, by a spring-loaded telescopic tube. The jacket was held in position by two bolts, which on progressive tightening clamped it firmly to the exhaust cone.

The jet pipe that led gas from the exhaust cone to the propelling nozzle, was made of welded stainless steel and, like the exhaust cone, was lagged for most of its length with aluminium foil, retained in position by a sheet aluminium jacket. The jet pipe front end had an outer flanged ring welded to it, which fitted over the expansion joint spigot. Two eyebolts, mounted on short studs welded to the pipe at the forward end, passed through holes drilled in the expansion joint and exhaust cone flanges, and secured it to the engine, while at the same time allowing horizontal clearance and alignment adjustment. Approximately mid-way along the pipe two steel blocks were bolted to provide a mounting for the two eye-bolts of the rear support. A detachable propelling nozzle was bolted to a flange at the pipe tail end, and welded in the wall were four union bodies used to house the four thermocouples that measure the exhaust gas temperature.

A fire extinguisher ring was fitted at the engine rear. This ring consisted of two perforated brass tubes joined at each end to T-unions, which provided alternative inlets from the aircraft fire extinguishing system. The tube assemblies were held by clips to the cooling air outlet pipes.

The cabin pressure manifold was fitted around the centre bearing casing just behind the rear air intake and air baffle plate. It consisted of a ring from which connections were taken to unions on each compressor delivery pipe. This ring had two T-unions for alternative, port or starboard airframe connections.

Cooling Air System. The centre and rear bearings and the turbine disc front face were cooled by a separate air stream provided by a small centrifugal fan mounted immediately forward of the centre main bearing. Air was admitted through short stub pipes fitted to the centre bearing casing front end and passed through the fan to the rear bearing housing inside. The air passed over and around the bearing and radially across the turbine disc front face, and forward through the casing and curved outlet pipes to the cooling air manifold, and finally away to atmosphere outside the engine cowling.

Instruments. An rpm indicator, jet pyrometer, oil pressure and oil inlet temperature gauges were installed in the cockpit to enable the pilot to ensure that the Operating Limitations were not exceeded. A burner pressure gauge also provided a fuel system cross check.

Controls. With the exception of the starting switches and low-pressure fuel cock, there were only two engine controls, the H.P. cock, which was used for stopping the engine, and the throttle. Thus the only control used during flight was the throttle control, under the action of which the aircraft behavior was similar to propeller-driven aircraft.