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Aviation History
1953
1953 - 0371.PDF
TELESCOPIC ANCHOR STRUT GEAR BOX DRIVE ACCESSORY GEAR BOX FLIGHT, 20 March 1953 369 EXTINGUISHER BOTTLES This sketch shows the engine cowling arrangement and main services. A small strut separates the central cowling panels when those of both engines are open. THE STORY OF THE DART Engine Components.—Behind the airscrew blade roots is an annular intake which leads the air to the eye of the first compressor-stage. The nose cowling has also a single-entry oil-cooler intake, situated at the top of its periphery. The oil-cooler itself is mounted on top of the engine~and above the oil tank, which is cast around the air intake ducting to form pan of a self-contained lubrication system. The hot oil assists in keeping the air-intake passage free of ice, and the form of the oil tank effects an appreciable saving in weight. The leading edge of the cowling and the oil-cooler air intake are protected against icing by rubber-covered electrically heated strips (shown in the sketch). The two-stage centrifugal compressor is a logical outcome of Rolls-Royce development of this type of compressor for piston engines. The first stage receives 20 lb of air per second, directed by rotating steel guide-vanes between the 19 rotor blades, from which it is impelled at high speed. After passing through a diffuser, the flow is directed—through curved passages formed by the compressor casings and inter-stage guide-vanes—into the second-stage impeller. This is of smaller diameter, as it deals with air already partially compressed in the first stage, and the final pressure of air leaving the compressor is about five and a half times that at the intake. From the second-stage impeller, the air passes to the seven straight-flow combustion chambers. These are similar to those of the Derwent and Nene turbojets, and each consists of a cast expansion chamber, an aluminized-steel air casing and a Nimonic 75 flame-tube. Primary air for combustion is metered into the flame-tube through a series of holes and a set of fixed swirl-vanes in the head of the tube, and the remaining holes in the flame-tube walls are positioned to provide an even gas-stream temperature. A further airflow passes over the outer wall of the flame-tube and, after reducing the gas temperature at the turbine inlet and acting as an insulating stream between flame-tube and combustion casing, joins the burnt gases at the nozzle ring of the first-stage turbine. Torch igniters in No. 3 and No. 7 chambers provide initial ignition, while all chambers are interconnected. The turbine is of two-stage axial-flow design, and consists of a 131-blade S.62 stainless-steel disc for the first stage and a 103- blade disc for the second. Both high- and low-pressure blades are machined from Nimonic 80, and are mounted on fir-tree serrated roots in their disc rims. Each turbine disc is cooled on its front and rear face by air tapped from both compressor-stages, OIL COOLER WATER METHANOL PIPE DE- ICING PADS FUEL PUMP OIL TANK FILLER FUEL CONTROL UNIT An early Dart undergoing one of many special-category tests at the Rolls-Royce factory in 1948. In the following year the RDa 1 was rated at 1,250 s.h.p. plus 300 lb thrust. while labyrinth and steel-strip seals prevent inter-stage gas leakages. The coupled turbine discs are bolted to a drive-shaft, Which is connected to the compressor shaft by a spherically seated transfer- coupling. From the compressor shaft the drive is transmitted forward through a quill shaft to a high-speed pinion in the reduction gear. The helical pinion-teeth engage with three lay- shafts to provide the first speed reduction, while at the forward end the layshafts engage, on spur teeth, with a rotating annulus wheel locked to the airscrew-shaft. This second step-down provides the final reduction to a ratio of 0.106:1 of the engine speed. The electric starter-motor shaft engages with a bevel gear on the reduction gear main input shaft. Other drives included in the reduction-gear assembly are those for the starter motor, oil pumps, fuel pump, airscrew controller and torquemeter. The gearbox for the aircraft accessories, which is mounted in the nacelle, is driven by a shaft extending aft from a top-mounted turret immediately behind the compressor. The universal coupling is driven through a spur gear train from a gear bolted to the engine mainshaft. Normally this accessory gearbox serves such items as the tachometer, generator, hydraulic pump, vacuum pump, and air compressor. The lubrication system is integral with the engine, providing spray lubrication for the anti-friction bearings of the mainshaft and for the reduction gear, and also an oil flow to the various driving gears and support bearings. Oil pressures of approxim ately 70 lb/sq in and 30 lb/sq in are provided, through the control of a dual oil-pressure relief-valve: the higher figure is for the air screw controller unit, where pressure is further raised to operate the pitch-change mechanism, and the lower is for engine lubrica tion. Fire protection for the power plant consists of air insulation, stainless steel ducting for electric cables, and the provision of a Graviner spray extinguisher system, which is manually or auto matically operated by pilots' switches and inertia impact-switches respectively. A Graviner supply bottle in the nacelle is connected to the turbine casing and the two spray rings: the forward ring serves the engine access bay while the rear one, attached to the engine casing, serves the combustion bay. In these two bays are located respectively six and seven fire detection switches. Fuel System.—On the Viscount, port and starboard wing fuel-tanks are interconnected by a cross-feed pipe and cock, enabling fuel to be fed from either side. As related in our airframe description, provision is made for under-wing pressure refuelling as well as for normal gravity-type filling. Booster pumps feed the fuel at 5-10 lb/sq in into twin non return valves, and thence to the engine master cocks. From these cocks the fuel is led through flowmeters and low-pressure filters to the engine-driven high-pressure pumps mounted on the lower port side of each engine. A fuel-control unit—consisting of filter, barometric pressure control, throttle valve and shut-off cock—decides the fuel flow and pressure between the variable-stroke swashplate pump and the burners. Fuel from the booster pumps enters the unit through the barometric pressure chamber to the pump intake. The fuel passes from the pump outlet through the throttle and pressure- control valves to the burners. Fuel pressure at the burners varies from about 40 lb/sq in at idling power to 950 lb/sq in at take-off r.p.m. Should failure of the constant speed unit occur, an over-speed governor fitted to the high-pressure pump prevents the engine r.p.m. from exceeding 15,200. The high-pressure fuel shut-off cock is connected to the air screw-feathering control, so that the engine is automatically F
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