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Aviation History
1939
1939 - 0091.PDF
JANUARY 12, 1939 FLIGHT. 39 The airscrew shaft, viewed from the front end, rotates clockwise. It is carried on two bearings, there being a roller bearing immediately in front of the gears and a ball and roller bearing at the front of the nose-piece. It is geared to the two crankshafts by a single spur reduction gear having a ratio of 0.308 :1. In view of the increased speed of the engine and the con sequent modification of the reduction gear, it has been necessary to raise the airscrew shaft slightly above the centre line. Through the centre of the airscrew shaft there is an oil feed for the De Havilland constant-speed airscrew governor which is mounted on the nose-piece and takes its drive from the starboard crankshaft. The air screw-shaft is a standard S.B.A.C. No. 5 size, and will normally accommodate a three-bladed airscrew. Ignition There are two B.T.H. magnetos with separate 24-point distributors, all four components being mounted on the nose-piece and driven through gears from an extension of the starboard crankshaft. Breeze ignition harness is fitted as standard, and there is an automatic advance and retard unit. The drive for the timing gear, blower and accessories is in the form of a tubular shaft centrally placed and driven by the port crankshaft. Bevel gears provide the drives for the four overhead camshafts, working through vertical shafts at the rear of the cylinder banks. The rockers, through which the valves are operated from the camshafts, incorporate hydraulic tappets which eliminate any need for adjustment of clearances. The entire valve gear is en closed and lubricated continuously. NAPIER-HALFORD DAGGER VIII. Bore Stroke Compression ratio Capacity ... Net dry weight ... Rated power Maximum power rating... Take-off power ... Max. r.p.m. (T.V. dive)... Fuel consumption cruising 13,500 ft.) Fuel consumption (all-out 8,750 ft.) at | power level flight Oil consumption (max. cruising) Max. cylinder temperature for climbing at at 07 mm. 95 mm. 7.5 : 1. 16.8 litres. 1,300 lb. 890/925 h.p. at 4,000 r.p.m. at 9,000ft. 1,000 h.p. at 4,200 r.p.m. at 8,750ft. 955 h.p. at 4,200 r.p.m. at sea level. 4,800. 52 gal./br. 94| gal./hr. 8 to 14 pt./hr. 245 deg. C. The cylinder, head and valve assembly of the Dagger VIII. The up-draught carburettor is of special S.U. design and is mounted below the super charger. It is of the twin-choke fully automatic type and em bodies a two-position mixture control and a four-stage boost control, the latter being opera ted by a two-row cam worked by the throttle. Oil heating is provided for the chokes and butterfly valves. From the tubular centre shaft already mentioned the centri fugal supercharger is driven by spur gears through two lay- shafts. The impeller spindle carries a torque overload relief clutch. The new impeller is double-sided in view of the double-entry design of the supercharger. From the impeller the mixture is led through stationary diffuser vanes to the induction pipes, which are cast in the top and bottom of the timing gear casing. Thence it passes to two separate cast induction pipes ex tending for the length of the crankcase. The hot and cold air intake incorporates an Amal flame trap. Lubrication A high-pressure oil supply (50 lb. per sq. in.) lubricates the crankshaft journals and big ends. The reduction gears are lubricated by two jets from the high-pressure supply. The cylinders, small ends of connecting rods and gudgeon pins are splash-lubricated. A reducing valve supplies a low-pressure system (10 lb. per sq. in.) to lubricate the rest of the engine. Splash oil escaping from the bearings drains into the sump and is returned to the supply tank by' scavenge pumps. The high-pressure oil system has an adjustable pressure relief valve. There is one pressure pump and two scavenge pumps, all of the gear type. The former delivers under pressure to the working parts of the engine oil from the supply tank, while the suction pumps scavenge the engine and return the oil. At each end of the lower camshaft casings is a small vane-type pump to return surplus oil to the crankcase. Between the pressure pump and the engine are two Tecalemit filter elements fitted in parallel; there are two gauze filters for the scavenge oil. Provision can be made for high initial oil pressure for starting. Fuel is supplied to the carburettor by a gear-wheel pump on the starboard side. A fuel relief valve is incorporated, differentially controlled to allow for variations in atmo spheric pressure. The capacity of the pump is twice that of the maximum requirement of the engine. A Rotax Eclipse combined hand and electric starter is arranged to operate via bevels on the rear end of the port crankshaft, a ratchet device being supplied to prevent damage from backfires. Drives are provided for Rotax 12- volt 500-watt electric generators, B.T.H. air compressor, Pesco vacuum pump and hydraulic pump. In addition to those mentioned in the foregoing description, the following are among the firms contributing to the construction of the Dagger VIII: — British Aluminium Co., High Duty Alloys, Magnesium Castings and Products, Ltd., Birmingham Aluminium Casting Co. and Ster ling Metals (crankcase, piston, supercharger and other alloy cast ings), Firth-Derihon (connecting rods, cylinders, valves, gears and airscrew shaft), George Turton Piatt (cylinders), English Steel Cor poration (crankshafts), Integra) Auxiliary Equipment Co. (pump). BRISTOLS IN CANADA AN exemplary catalogue of current Bristol engines has been issued by British Aeroplane Engines, Ltd., of Montreal, Quebec, which company represents the Bristol concern in Canada. Apart from describing and illustrating in detail the Mercury and Pegasus range and the sleeve-valve Aquila, Perseus and Hercules it contains details of a type of Pegasus not so far seen in this country. This is the Mk. IX which, judging from performance figures, is a derated version of the Pegasus X, the rated power being 725 h.p. at 2,200 r.p.m. at 4,500ft , instead of 850 h.p. at 2,250 r.p.m. at 4,000ft. This model is being installed in the Blackburn Sharks manufac tured in the Vancouver works of Boeing Aircraft of Canada.
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