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 ! temperature on passing through the , 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 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- 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. AMAL The temperature rise will, m course, FUEL PRESSURE be prohibitively high, as it will be KDUCINC VALVE virtually the addition of the indi- vidual rise through each stage. For this reason an inter- A sectioned sketch of the two-stage two-speed Rolls-Royce|t cooler is alw"ays employed with a two-stage supercharger. Merlin 61 supercharger. The first successful two-stage supercharger of which details were published in this country was that employed in excess of, or at least equal to, that obtained from the by the high-altitude Bristol Pegasus engine, installed in the single-stage, two-speed unit, by virtue of the intercooler Bristol 138A, which obtained the world's height record and the reduced temperature rise—the temperature rise in 1937 (53»937ft.)- This engine had the usual single-speed for a given pressure ratio being lower for a two-stage com- "built-in" supercharger, plus an auxiliary (first-stage) pressor than for/a single-stage. supercharger driven from the rear cover by a cardan shaft. Air entered the^uxiliary supercharger by two ducts, one Exhaust-gas Turbine on each side or the fuselage, passed through an intercooler It must be pointed out that the Americans were the first into the Garourettor, and thence through the main super- to* use two-stage superchargers in large numbers. Their • charger (second-stage) into the engine. A clutch was incor- system, perfected by the General Electric Company, incor- porated in the auxiliary supercharger drive, permitting porates an exhaust-gas turbine. The exhaust turbo-blower this to be disengaged for take-off, thereby eliminating the has received little attention here, since hitherto no British- temperature rise through, and the power absorbed by, the made example has appeared in service with the Royal Air first stage. The overall pressure ratio was 5.5 to 1, and Force. A considerable amount of experimental work has, the rated altitude 44,000ft. however, been carried out, and as long ago as 1926 an To-day we have the Rolls-Royce Merlin 61. (Described aircraft fitted with an exhaust turbo-blown engine, a Napiei: in Flight of December 17th, 1942.) The drive in this case Lion, reached 32,000ft. fully laden. v is similar to that employed on the two-speed, single-stage Reverting to the American plant, the first stage is driven unit; the extra impeller is mounted,, as it were, on an by the turbo wheel and the second by fixed gearing, this extension of the original shaft. This' layout results in a stage being /me usual "built-in" type of supercharger. great saving of space, a factor of vital importance in the The speed of the turbo wheel is regulated by a "waste single-seater fighter Both stages draw through the car- gate" or throttle in the exhaust manifold, rated altitude burettor, and the intercooler is situated between the second being reached when the turbo wheel is doing its maximum stage and the engine ; a pressure ratio of 6 to 1 is afthieve'd. r.p.m. A similar system to this was patented in France The two-speed gear will give performance at low altitude in 1929 (Rateau), but, like many other types of exhaust SEPTEMBER 23RD, 1943 FLIGHT 347

DEVELOPMENT OF THE AIRCRAFT SUPERCHARGER turbo-blower produced in that country, does not appear to have been a suc- cess. In Germany, too, the exhaust turbo-blower would appear to be the most popular type, and the early Junkers was interesting in that it em- ployed a step-up gear between the turbo wheel and the impeller, kater V.D.M. patents also incorpor>rfe this featuie. The characteristics of the exhaust turbo are quite different from the mechanical supercharger and, gener- ally speaking, much better. Its merits are not derived alone from the facili- ties for speed variation, in itself a great advantage, but also from the fact that the means of driving the compressor ate obtained from a hitherto untapped urce—the latent power in the ex- ust gases. These advantages, The exhaust-driven G.E.C. turbo-supercharger on a Wright Cyclone of a Fortress. giving increased power and economy below the rated altitude, are somewhat offset, in the case been taken out covering/" mechanical cooling"; that is, of military aircraft, by installation requirements, loss of cooling the actual tuj#5o components by means of an air- exhaust thrust effect and inability to flame-damp and stream applied to hollow rotors and blades. silence the exhaust. However, the installation on the Finally, it is interesting to note that the demand tor Thunderbolt shows a great improvement from Ule aesthetic, increased pressure ratios has been largely due to, and, flame-damping and silence points of view over the earlier dependent on, the rapid development of aircraft fuels. installations, such as the Boeing B-17C with its totally Also, granted the demand, the majority of obstacles in the exposed turbo wheel. # path of these higher tip speeds have been constructional Due to the high operating temperatures of the exhaust or metallurgical. Most of the design features that have gases (up to 1,100 deg. C.) it is necessary to supply some lent added efficiency during recent years were well under- means of either cooling these gases or the working parts stood at the time of the inception of the supercharger, but of the turbine itself. The American system is understood were not practicable at that stage of the art due to material to employ "gas cooling" by the expedient of exposing or production difficulties. Any further advances will have the exhaust manifold to the airstream, and in the earlier to be accompanied by parallel aircraft development, since case referred to above, by also -exposing the turbo wheel greater operational ceilings than those alrqiray in reach will and its casing. In Germany a number of patents have call for the use of pressurised cabins on all types of aircraft.

Newfoundland and Aviation f I '^HE value of speedy communications between New- transcontinental service to Montreal, Ottawa, Toronto. \ foundland and Canada was given dramatic em- Winnipeg and Vancouver. The route, 730 miles, is by 4**- phasis when the Knight of Columbus Hut in St. way ot Sydney, Nova Scotia, 223 miles from Moncton. John's was destroyed by fire. For the relief of the many This flight takes five hours, as compared with 65 hotns injured in that catastrophe large shipments of plasma for by rail and steamer, when connections are made. blood transfusions and other medical supplies were rushed Between 70 and 90 per cent, of the travellers using the by Trans-Canada Air Lines from Montreal, Toronto, Win- air passenger service are on military duty, or are contrac- nipeg and other centres. Going by air express, the emer- tors or men actively engaged in war work. Extension or ' gency materials were in St. John's at noon the day follow- die TCA service to Newfoundland has brought the island ing the night they left. even more closely into/the orbit oWStnaja JMjL the U.S. Put into effect by treaty arrangements between the Either through air rfail or jrasoaqel ^j|iW*jt*Ts possible Government of Canada/and the Newfoundland Commis- for the NewfoundlaMer to-dmrtp dftrafrlete important war- sion of Government ni a wartime measure, the Canada- time businessfin\a^actioiy<*4(mrtirne previously needed. Newfoundland service inaugurated by Trans-Canada Air This develoryeA/jrflpr' ifaffne importance in the future Lines a year ago has proved successful. It arose out <.f development wOTJewfoundland. a demand for a closer contact between the island and the It is. apparmt that ,after the war Newfoundland will mainland, and from the beginning aircraft have been carry- have Wich 1# gain by maintaining this new relationship ing capacity passenger loads almos' t every mon " and the United Stated. To develop the waiting lists of reservations made sometimes t s resources will require substantial capital, for some three weeks in advance. > hich/at least, it will probably be necessary to go t:> • f Thep passengeg r load factor, 84 per cent. f