342 FLIGHT Development §ai per char A Review with Aspect O-DAY the centrifugal type of supSscjiarger is "e temperature rise, that is, the increase in mixture universally for the aspiration of aircraft engine! temperature on passing through the supercharger, is T Its advantages over the "positive displacement" dependent on the pressure ratio and the efficiency, and can type, such as the Roots compressor, lie in its adaptability therefore also be said to be primarily a function of the to the engine structure, compact size, lightness and dur- tip speed. ability. The inherent characteristics of this type of super- The horse power required to drive the supercharger is charger have led to many methods of drive being evolved, governed by the pressure ratio, the inlet temperature, the and it is the object of this article to trace the various weight of air and the efficiency. The efficiency, when developments—from the inception of the single-speed, dealing with compressors of any type, is usually given single stage unit to the complicated two-speed, two-stage by the ratio of the adfebatic temperature rise to the actual unit of the present day—with temperature rise. The adiaba^ic special reference to the equipment Pi-ATMOSPHERIC temperature is that temperatui\ >wthe Royal Air Force. PRESSURE which would have been the result Ti -ATMOSPHERIC A diagrammatic sketch of a TEMPERATURE of compression without the addi- supercharger with high-speed drive tion or subtraction of heat from by means of spur gears is shown THROTTLE external sources. (Adiabatic com- on this page. The various terms, pression). In practice the tempera- referred to later, are indicated in ture rise is always greater than P2-VcENTRY the sketch. PRESSURE AT FULL adiabatic, due to skin friction and THROTTLE fe (APPROX) eddying. Adiabatic efficiencies for Principle of Operation ~ EQUALS Pi modern superchargers lie between SEMI-SHROUDED 65 per cent, and 75 per cent. These, The revolving impeller displaces IMPELLOR COMPOUND SPUR GEAR ON then, are the characteristics of this the mixture (or pure air in the case LAYSHAfT ot solid fuel injection) outwards, type of supercharger and must be imparting to it a high tengential IT?-Tt)-TEMPERATURE RISE borne in mind when reviewing its velocity. On leaving the periphery history. of the impeller the kinetic energy ~ = PRESSURE RATIO of the mixture, present by virtue of arly Obstacles its velocity, is converted into pres- Diagrammatic sketch of a single-speed spur :perirnental superchargers were sure energy by the expedient of gear-driven supercharger. produced during the last war. The slowing it up. In order to conduct mechanical drive type developed by this process in an efficient manner, with reg^d to the Brown, Boveri, at Mannheim, in 1917, and the exhaust- limited -space .available, diffuser vanes are used to regu- gas turbine designed by E. H. Sherbondy, and fitted to a late the transition period—-velocity to pressure. The degree Liberty engine, were but two among many. It was not, of compression of the mixture., or pressure ratio, is a however, until 1926 that the first superchargers came into fuction of the impeller tip speed. A given tip speed will squadron service in this country. This delay was primarily give the same pressure ratio for the general run of designs. due to? firstly, the mechanical difficulties of the drivfe, and secondly, the ill effort- of high rotational speed on % SPEED REGULATED TO the impeller and its bear- 'GIVE PRESSURE RATIO ings. The chief difficulty in the case of the drive CONSTANT BOOST-FULL THROTTLE / _.„ . CRITICAL ALTITUDE lay in the design of a satis- 10OO —mt. \ DETERMINED BY STAGE N. /PRESSURE & MAXIMUM BOOST factory slipping clutch, by means of which the im- peH€r might be '' left behmd" during rapid 9OO- "THROTTLED TO acceleration and decelera- CONSTANT BOOST / tions. Without the provi- THROTTLED TO sion of such a clutch the CONSTANT SPEED impeller would be subject to shock loading and changes of inertia which would quickly result in SINGLE SPEED either its destruction or « TWO SPEED that of the driving gears. < o VARIABLE SPEED MECHANICAL 7OO Again, owing to the + EXHAUST TURBINE high rotational speeds, considerable centrifugal loading is applied to the impeller blades. This in- feOO I G.L 5£>OO IO.OOO 20,000 25POO 30,000 ' 35,000 troduces the fear of elonga- ALTITUDE (FEET) tion, "permanent set," Graph showing characteristics of various types of single-stage superchargers. Engine speed and finally disintegration. is constant throughout. Disintegration may also be SEPTEMBER 23RD, 1943 FLIGHT 345 DEVELOPMENT OF THE AIRCRAFT SUPERCHARGER flying-boat classes, with their take- Limits of Single-stage off and low-level performance re- Operation requirements. In view of this, a With the take-off power looked "moderately" supercharged en- after by the low-speed gear, and gine was added to the range of 'with the introduction of 100 ocj»wrtr engine types made, and the high- fuel, making permissible a further altitude engines became known as increase in boost pressures, tip 'fully" supercharged. speeds have been pushed up to some The logical development from 1,300 ft./sec. Pressure ratios in thig point was the introduction of the region of 3 to 1 have been a/fwo-speed gear giving the advan- achieved, which with the aid of air- tages of both systems to one unit. intake "ram," supplied by the The first engine so equipped to go high speed of modern aircraft, have into service in this country was the resulted in '' all-out-level'' alti- Armstrong Siddeley Tiger, Mark 8, tudes as high as 22,000ft. as fitted to the Mark 2 Armstrong These high-pressure ratios are Whitworth Whitley in 1936. The also in part due to increases in effi- gearing in this case was epicyclic, ciency. The original type of fan changes in speed being effected by with unflanked blades was first re- locking or freezing the outer cage, placed by the "semi-shrouded" Mhe actuating mechanism being type having a web or complete spc*- hydraulic. This engine had a tion on the face remote fronr^the moderate gear giving a rated alti- entry. This increased the strength tude of 6,250ft. and a full gear and reduced the losses due to leak- giving 12,750ft. Later came the age between the fan and the blower Bristol Pegasus Mark 18 and the casing. Many engines to-day em- Rolls-Royce Merlin. Mark 10. ploy impellers of the '' fully Apart from tye increase in take- Impeller casing removed to show the blade shrouded'' type; this permits off and low-level power, the two- arrangement on the Bristol Jupiter VII greatly increased working clear- speed supercharger gives advan- engine. ances, together with the elimination tages in economy. In the first place of leakage on either face of the fan the specific fuel consumption is lower with the blade. moderate gear, owing to the smaller percentage With the open type of im- of power absorbed in driving the supercharger. peller the clearance between In the second/place the aircraft can be flown the fan and the casing was at full throttle in this gear on reduced r.p.m. always more than that de- (C.S. airscrew assumed) at quite moderate alti- manded by maximum effi- tudes—high b m.e.p. and low r.p.m. being the ciency, since under high- sine qua non,6f if^iel economy. 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 •346 FLIGHT SEPTEMBER 23RD, 1943 DEVELOPMENT OF THE AIRCRAFT .SUPERCHARGER to the Daimler Fluid Flywheel, whereby the impeller speed may be INTERCOOLER 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 CUIDES hydraulic clutch. Jfi this way the \S ^^^\ INLET BRANCH minimum temperature rise is created, -^~ mA OF INTERCOOLER 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 MODERATE CEAR CLUTCH & CEAR (or air) coming from the first is fur- DRIVE ther compressed by the second. In FULL CEAR theory, it each impeller had a pres- CLUTCH & CEAR sure of 3 to 1, the final charge would DRIVE be compressed in the ratio OIA to 1.