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Aviation History
1943
1943 - 2328.PDF
•346 FLIGHT SEPTEMBER 23RD, 1943 DEVELOPMENT OF THE AIRCRAFT .SUPERCHARGER to the Daimler Fluid Flywheel, whereby the impeller speed may be varied independently of the engine speed. With such a device the engine can be run at full throttle at ground level by the expedient^of slipping the hydraulic clutch. Jfi this way the minimum temperature rise is created, which with the small power absorp- tion to drive the supercharger (due to the reduction in its pressure ratio), gives considerable increases in power obtainable for a given boost pres- sure. Two-stage Superchargers The stage has now been reached when theJtfpspeed is in the region of. and often considerably in excess of, the speed of sound in air. This forms the useful limit, in the inter- ests of efficiency, to the performance obtainable from the single-speed compressor. (France is the only country to have mentioned the exist- ence ofa three-speed, single-stage supercharger—a device which would prove redundant unless a further in- crease in tip speed were aimed at.) However, the demand for altitude has not stopped at 20,000ft.; rated altitudes of 30,000ft., and over are the aim of many new types, and already the reality of several opera- tional aircraft. These aircraft all employ a two- stage supercharger. This is simply two impellers in series; the mixture (or air) coming from the first is fur- ther compressed by the second. In theory, it each impeller had a pres- sure of 3 to 1, the final charge would be compressed in the ratio OIA to 1. The temperature rise will, m course, be prohibitively high, as it will be virtually the addition of the indi- vidual rise through each stage. For this reason an inter- cooler is alw"ays employed with a two-stage supercharger. The first successful two-stage supercharger of which details were published in this country was that employed by the high-altitude Bristol Pegasus engine, installed in the Bristol 138A, which obtained the world's height record in 1937 (53»937ft.)- This engine had the usual single-speed "built-in" supercharger, plus an auxiliary (first-stage) supercharger driven from the rear cover by a cardan shaft. Air entered the^uxiliary supercharger by two ducts, one on each side or the fuselage, passed through an intercooler into the Garourettor, and thence through the main super- charger (second-stage) into the engine. A clutch was incor- porated in the auxiliary supercharger drive, permitting this to be disengaged for take-off, thereby eliminating the temperature rise through, and the power absorbed by, the first stage. The overall pressure ratio was 5.5 to 1, and the rated altitude 44,000ft. To-day we have the Rolls-Royce Merlin 61. (Described in Flight of December 17th, 1942.) The drive in this case is similar to that employed on the two-speed, single-stage unit; the extra impeller is mounted,, as it were, on an extension of the original shaft. This' layout results in a great saving of space, a factor of vital importance in the single-seater fighter Both stages draw through the car- burettor, and the intercooler is situated between the second stage and the engine ; a pressure ratio of 6 to 1 is afthieve'd. The two-speed gear will give performance at low altitude INTERCOOLER CUIDES \S ^^^\ INLET BRANCH -^~ mA OF INTERCOOLER MODERATE CEAR CLUTCH & CEAR DRIVE FULL CEAR CLUTCH & CEAR DRIVE AMAL FUEL PRESSURE KDUCINC VALVE A sectioned sketch of the two-stage two-speed Rolls-Royce|tMerlin 61 supercharger. in excess of, or at least equal to, that obtained from the single-stage, two-speed unit, by virtue of the intercooler and the reduced temperature rise—the temperature rise for a given pressure ratio being lower for a two-stage com- pressor than for/a single-stage. Exhaust-gas Turbine It must be pointed out that the Americans were the first to* use two-stage superchargers in large numbers. Their • system, perfected by the General Electric Company, incor- porates an exhaust-gas turbine. The exhaust turbo-blower has received little attention here, since hitherto no British- made example has appeared in service with the Royal Air Force. A considerable amount of experimental work has, however, been carried out, and as long ago as 1926 an aircraft fitted with an exhaust turbo-blown engine, a Napiei: Lion, reached 32,000ft. fully laden. v Reverting to the American plant, the first stage is driven by the turbo wheel and the second by fixed gearing, this stage being /me usual "built-in" type of supercharger. The speed of the turbo wheel is regulated by a "waste gate" or throttle in the exhaust manifold, rated altitude being reached when the turbo wheel is doing its maximum r.p.m. A similar system to this was patented in France in 1929 (Rateau), but, like many other types of exhaust
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