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Aviation History
1937
1937 - 2142.PDF
SUPPLEMENT TO FLIGHT 1326 2 THE AIRCRAFT ENGINEER JULY 29, i937 100 eo X 40 20 — POWER. ABSORBED BY CENTRIFUGAL BLOWER. ENGINE RATING 900H.P. AT 15 OOO FEET SPEED 2GOO R.P.M. B0OST + ) LB eo' HI a gped a5 ui 4o' h TE J .MPERATURE RISE HROUGH BLOWER. ASSUMING 65 "/. EFFICIENCY 200 400 600 BOO IOOO ROTOR. TIP SPEED FT/SEC I6OOCV H 12000 UJ i E 200 400 600 800 1000 ROTOR. Tip SPEED 8000 4000 O:L. ROTOR TIP SPEED AND ALTITUDE FOR +• 1 LB BOOST 200 400 600 8OO IOOO ROTOR. TIP SPEED FT/SEC Curves 1-3, showing relationships between rotor tip speeds and power absorbed, temperature rise, and altitude respectively. It can be seen that, with the two-speed supercharger in low gear at ground level, an additional 137 h.p. is available. This is due to the lower mixture temperature and reduction in power absorbed. In addition, the engine conditions will be generally improved by the lower mixture temperature. As the power absorbed by the blower rises more rapidly than the increase in performance, and the effect of high intake temperatures becomes increasingly serious, it is evident that the advantages of a two-speed supercharger become still more apparent when the performance of engines of higher duty is considered. Probably the most important advantage of the two-speed supercharger is the saving in fuel that can be effected by cruising at the working altitude of the engine. For its use when cruising, an engine is required to supply 2/3 of its maximum power, at about the same altitude as that given for maximum power. W-'-h a single-speed supercharger, this condition can only be met by throttling. With a fully supercharged engine, with a single-speed blower rated St say i5,ooo;ft., the power absorbed by the blower is still large under cruising conditions and the mixture temperature high. The effect of these two factors is to raise the specific fuel consumption, and in practice it is not found possible to reduce this much below about .54 pt./b.h.p./hr. how Consumption With a two-speed blower, however, instead of throttling back to get cruising power at altitude, this condition can be achieved at or near full throttle by changing into low gear. The condition is then equivalent to running a moderately supercharged engine with a rating of about 5,000 ft. at 15,000 ft., with the important difference that maximum power is instantly available, if and when required, by changing into high gear. Under these con ditions the supercharger is just doing the work required to give cruising power and no more ; the power absorbed by the blower is small and the mixture temperature is low and it is thus possible to reduce the specific fuel consumption to about .49 pt./b.h.p./hr. This represents a saving of 10 per cent, on the fuel consumption under cruising con ditions, directly due to the provision of a two-speed supercharger, with a consequent valuable increase in cruising range for the aircraft. Provision of a two-speed drive to the supercharger can be made in various ways. One form consists of two independent sets of gearing, one or the .other being used as required by suitable clutch mechanism. In the Armstrong Siddeley units high gear is obtained by a simple train or back gearing, which for low gear is allowed to function as an epicyclic system. This compact design has proved to be light in weight, and mechanically sound, while reducing to a minimum the necessary control mechanism. The first two-speed supercharger unit designed and made by Armstrong Siddeley was completed in 1930. Fitted to the Panther engine it successfully passed its tests on the bench and in flight. An illustration of this early unit is given below. Two trains of gears were provided, which in any case OIL FEED TO_ CLUTCH OPBJAT1N& PISTONS Sectional view of the first experimental two-speed supercharger. would be normally required for a single-ratio blower. The high-gear drive consisted of three radially disposed sets of back gears, driven through a spring drive mechanism from the rear of the crankshaft. The three layshafts with their planet wheels were mounted in a cage which could be allowed to rotate around the axis of the unit.
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