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Aviation History
1943
1943 - 2327.PDF
SEPTEMBER 23RD, 1943 FLIGHT 345 DEVELOPMENT OF THE AIRCRAFT SUPERCHARGER flying-boat classes, with their take- off and low-level performance re- requirements. In view of this, a "moderately" supercharged en- gine was added to the range of engine types made, and the high- altitude engines became known as 'fully" supercharged. The logical development from thig point was the introduction of a/fwo-speed gear giving the advan- tages of both systems to one unit. The first engine so equipped to go into service in this country was the Armstrong Siddeley Tiger, Mark 8, as fitted to the Mark 2 Armstrong Whitworth Whitley in 1936. The gearing in this case was epicyclic, changes in speed being effected by locking or freezing the outer cage, Mhe actuating mechanism being hydraulic. This engine had a moderate gear giving a rated alti- tude of 6,250ft. and a full gear giving 12,750ft. Later came the Bristol Pegasus Mark 18 and the Rolls-Royce Merlin. Mark 10. Apart from tye increase in take- off and low-level power, the two- speed supercharger gives advan- tages in economy. In the first place the specific fuel consumption is lower with the moderate gear, owing to the smaller percentage of power absorbed in driving the supercharger. In the second/place the aircraft can be flown at full throttle in this gear on reduced r.p.m. (C.S. airscrew assumed) at quite moderate alti- tudes—high b m.e.p. and low r.p.m. being the sine qua non,6f if^iel economy. Impeller casing removed to show the bladearrangement on the Bristol Jupiter VII engine. Limits of Single-stage Operation With the take-off power looked after by the low-speed gear, and 'with the introduction of 100 ocj»wrtr fuel, making permissible a further increase in boost pressures, tip speeds have been pushed up to some 1,300 ft./sec. Pressure ratios in the region of 3 to 1 have been achieved, which with the aid of air- intake "ram," supplied by the high speed of modern aircraft, have resulted in '' all-out-level'' alti- tudes as high as 22,000ft. These high-pressure ratios are also in part due to increases in effi- ciency. The original type of fan with unflanked blades was first re- placed by the "semi-shrouded" type having a web or complete spc*- tion on the face remote fronr^the entry. This increased the strength and reduced the losses due to leak- age between the fan and the blower casing. Many engines to-day em- ploy impellers of the '' fully shrouded'' type; this permits greatly increased working clear- ances, together with the elimination of leakage on either face of the fan blade. With the open type of im- peller the clearance between the fan and the casing was always more than that de- manded by maximum effi- ciency, since under high- depression conditions, such as the throttled dive, there was a tendency/for the blower casing to dist«fit, thereby shrinking the clearance. A further re- finement has been the intro- "duction of '' Inlet Guide Vanes." These reduce the entry-shock or turbulence caused by the sudden direc- tional change of the mixture entering the impeller. Another form of this refinement is the " bent tip " impeller. In Germany a hydraulic ••>? coupling has been evolved, ^' working on a similar principle \ Features of the Bristol Hercules two- speed supercharger. (Left) The complete supercharger partly sectioned. (Top) Close-up of the impeller and difiuser vanes. (Right) The main shaft with spring drive spline driven from crankshaft
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