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Survey of Britain s Most Powerful Radial Engine : An Example of. r l^ OgiCal Layout to Achieve Com pactness with PowerT has been eommbn knowledge far some time past thatthe Bristol Aeroplane Co., Ltd., have been engaged withthe production of a larger and improved model in theage of radial, air-cooled, sleeve-valve engines with whichey have for so long enhanced their reputation. Thismmon knowledgethe result of unofficial "leaks"ibraced the facts that the new engine was an 18-cylinderlit of over 2,000 h.p. and was called the Centaurug..ther than this nothing much was generally known untilnctioned reference, to the engine was made with thelease of the Short Shetland flying boat Flight,May 17th,145), when it was revealed that the Centaurus was of'er 2,500 h.p.Even now we are not permitted to give any indicationthe power output other than

at it is well in excess of500 h.p., so that, in assessingit'cific qualities' relating>wer, one canily use the;ure of 2,500 as'datum andite that theuated result ismewhat loweran the factualdue.Probably theost impressiveature to strikee casual ob-rver of themtaurus is thempactness ofe unit in rela-)n to its poweritput, and, as airollary, the achieve-ent of a clean and efn-ent fairing by refine-.ent of design. It is,course, obvious thatmiponent in the engine is thesuit of months, in some cases;ars, of development, and thatc only means by which such WOLIC-anship can bear fruit is by way ofie hard road of technical ability fol-wed strongly by the incessant trial"id error of testing. By such means a tremendous fundEdata is amassed, such data being the "history" onlyuough which the future may be discerned. Lookingack in order to look forward applies equally in the tech-ical as in the ordinary world. .

Specific PowersFrom the aspect of arrangement analysis the Centaurusbasically similar to its immediate forerunner, the Her-jles, with the chief differences that it has four more

flinders and that entirely separate individual gear trainsre embodied front arid rear for driving the sleeves in the-spective cylinder* banks. Swept volume is naturallyirger, but so is the unit swept volume as, although theylinder bores remain the same at 5.75m., the stroke of theentaurus is 7.0m. as compared with the 6.5m. of thelercules. This gives the new engine a capacity of 53.6tres (3,270 cu. in.) in contrast to the Hercules' 38.7 litres2.360 cu. in.), an increment of 38.5 per cent. In terms ofapacity and power output, at the restricted Centaurus

every

Compactness without complication.Note the backswept exhaust stackcooling muffs and the collected exhausttail pipes.

power figure of 3 500 the b.h.p. /li tre of both HerculesCentaurus is 46.5, although we know that the latter engineis somewhat better than this, as may be rough y indicatedby the figures for b.h .p ./ sq. in. of piston area, thesebeing respectively:. Hercules 4.93. and Centaurus betterthan 5.34. f: Cooling Arrangemen ts

As an indication of refinement in the design of the cowl-ing, if. we take as a datum the frontal area of the Herculesat 2,122 sq. in. and give it the value of unity, then theCentaurus frontal area of 2,402 sq. in. gives a comparativeratio of 1.13:1, which is well below the relative h.p..ratioof 1.385 :1, itself a conservative figure. Put another way,this means that for an increase inpower output of 38.5 per cent, overthe Hercules, the Centaurus has onlya 13 per cent, greater frontal area,its specific' power thus being150.5 h.p./sq. ft.

It must be appreciated thatthe goal of really close cowling...- . . ,.., . -.. is hindered in at-. / tainment by the-very critical fac-tors of cooling,and Bristol'sachievement isrendered the moredeserving of praiseby virtue of theirhaving obtained,with it, an en-hanced coolingefficiency. To a- certain extent theemployment of acooling fan driven from the air-screw spinner can be deemedpartially responsible for the in-crease in cooling efficiencybut by no means wholly.

As the Centaurus is designed- as apower plant, the cowling, fairing andbaffling arrangements naturally comeinto the picture in addition to theengine per se, so whilst on the subject of cowling we might as well dealwith it before going on tc the engine'proper. The subject is well worthy of study.

The main body of the engine in way of the cylinder-'banks is closely cowled by four panels, two per side, hingedtogether at top and bottom on the fore and aft centreplane of the engine and secured together with three toggle-type fasteners on each side. The latter are recessed togive a smooth surface line. Forward of the cowling panelsis the-aerofoil-section annular nose fairing which, with thevery unusual spinner extension fairing (fitted around thereduction gear casing), provides a smoothly contoured, butsurprisingly small, annular intake duct in the entry of whichrotates the- 18-blade cooling fan fitted to the rear of thespinner.It may thus be seen that cooling air entrained is ob-structed not at all until it reaches the front cylinders.Having done its due amount of fin circulation the air isthen allowed to escape to the outside atmosphere throughcooling gills in the skirt ring of the main cowling panels.In consideration of the amount of development designwhich has gone into producing the Centaurus, the instance

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BRISTOL CENTAURUS cylinder and head finning is interesting. By experimenthas been shown that the optimum pitch for fin locationin the region of o.iin.- the pitch on the Centfturus is[izin. and the cooling area of finning to each cylinderid head is no less than 32.47 sq. ft.Not the least novel feature of the cooling arrangementsthe fact that, unexpectedly enough, the front,row offlinders, due to the amazing control of the cooling air-low, actually runs a trifle hotter than the rear row.pressure drop in the cooling air through, or, rather, acrosshe cylinders is the rather high figure of 10 inches of water,3d, very shrewdly, this is put to incidental, yet none theuseful, advantage in cooling the rear-swept front-ylinder exhaust pipes. These are shrouded' with tubularjufis,open at each end, which extend nearly to the trans-.jterse centre plane of the engine, and the pressure differ-ential between front and rear open ends induces an air-Bow through the muffs which effectively cools the exhaust

ip s whilst, at the same time, isolating their cooling ajfprom the ordinary flow to the rear cylinders.Limits of Cooling

The cooling efficiency of the Centaurus is nearly as highit is possible to get. There are for the future the addi-tional refinements of a geared cooling fan running at higher[speed so inducting a greater mass flow,'and also the closerpitching of the cooling fins. Nevertheless, it would appearthat the ultimate is at- least within sight, and from thisaspect it is a reasonable assumption that some limit willsooner or later enforce a barrier against the use of morepotent fuels and increase of engine rating by stepping upthe b.m.e.p. This reflection excludes consideration of the(use of water/methanol injection and kindred devices whichconceivably might have an increasingly large part to play.The reduction gear is the well-tried Bristol-Farman typeIbevel epicyclic which is so well known as to render a[further description superfluous. One element is, however,[worthy of note; the rear driving bevel and the front[stationary (sun) bevel ring are both located on annularIseatings, the bearing faces of which are spherically curved.I This measure ensures that thedriving and sun bevels can-[each rock slightly and so permit the load to be distributedI equally among all three planet pinnions. The three[ spokes on which the planets are carried are integral[with the airscrew shaft, the tailpiece of which is supportedin a bearing-in the forward end of the crankshaft. Airscrew' thrust is carried by a large-diameter ball-bearing housedin the front of the r.g. casing forward of the gear itself.

The crankcase of the Centaurus is unorthodox in thatit is composed of three sectionsfront, centre, and rearthe mating faces of which are disposed on the transverse ,centre planes of the cylinder banks..' Each section has adiaphragm, those of the front and rear supporting thesleeve drive bearings and isolating the gear trains fromthe crank throws, and that of the centre provides a sup-port for the main central crankshaft bearing. The frontand rear sections of the casing are secured to the centralsection .by hollow bolts on each side of each cylinder.Mating faces are metal-to-metal, there being no gaskets,this in itself being a tribute to the Bristol productionefficiency.Both sets oi sleeve drives are composed of spur geartrains powered from a common input gear mounted on

the crankshaft. A description of one set suffices for both,as front and fear Urives are identical. Meshing with thecentral input gear are three equispaced intermediate gearseach of which drives the middle wheel of a separate trainof three, each of the three wheels driving a sleeve crank ;h three sleeve-driving wheels are connected in a train bydler wheels^one between adjacent pairs. 'A built-up, nitrided crankshaftalso in three sections6 employed,-the front and fear sections, which include thebalance weights, being attached to the central section by

mi J \ \ . : \ : T : BALANCED * -

two-bolt maneton joints. Each of the balance weights.contain two 2.875m. dia., 3.52 1b. Saloman ball-typevibration absorbers which have proved to be very effectivein de-tuning both torsional and flexural vibration. Ex-amination of the double curvature seat track of the ballenclosures has established that 'the ball movement citherlongitudinally or transversely does not exceed 0.75111.Big-end bearings carry-on the unique Bristol precedentof employing white-metalled sleeves shrunk on to thecrankpins, in contrast to the more usual floating-bushtype bearing. Each crankpin carries a master connectingrod to which the other eight very slim con-rods are articu-

31 i

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. .JOLY 5TH I945

SL VDRIVINGRANK

SLEEVE VALVEDRIVING GEARSFOR BACK ROWOF CYLINDERS

SUPERCHARGERDRIVING GEARCENTRIFUGES MOUNTINGFOR STARTER

ELF ALIGNINGCRANKSHAFT

ROLLER BEARINGS

IIMII T\ :PORT MAIN 1AIR ENTRYTO

SUPERCHARGER y < i

IGNITIONHARNESS

I-UELINJECTOR

UNIT

EXHAUSTOUTLETS

V

lated ; incidentally, the ordinary con-rods are wider acrossthe flanges than they are in depth of web. It is of interest .-., , . " , - . - . , .-.to note that the master-rod big-end bearing is of 12.05 j in. projected area, whilst the wrist-pin bearing area ofthe normal rods s 1.583 sq. in., as against a httle-end a r r a n g L e n t and structural details of the Centaurus. Carefularea of 1.575 sq. in. Oil ducts in the crankshaft lubricate profiling of the cooling air intake entry is a noteworthythe main and big-end bearings and also feed jets in the - feature of the design. The sectioned cylinder at the topbalance weights which spray the pistons, sleeves and little- . of the drawing clearly shows the relative disposition of piston "ends wrist-pins are lubricated from the big-end supply and rod assembly with cylinder, sleeve, head and porting. .

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16 LI H T JU LY 5TH, 1945

B R I S T O LC E N T A U R U S1throu gh spring-loaded oil retainers .The three high-capacity main bearingsare of large diameter and of thespherical roller, self-aligning type,crankshaft end thrust being restrainedby the central bearing. Maximumangularity of the master rods is13 deg. 38 min., but some of thenormal rods (which are not evenlyspaced on the ma ster) reach an angu- _..-;laruty of som ething over .20 deg. ' 'Cylinders are machined from forged billets, each barrelbeing retained by 16 large-diameter s tuds , the nu ts, forwhich are locked in position b y spring locking plates. Aspreviously mentioned, the heads are deeply indented anda r e addition ally, m ade in two sections. The top half is acasting (including fins) into which the forged and machinedbottom half is shrunk.

Sleeves are of the latest stiffened type and as masterrod sleeves have a harder time than the others, they areshell pressed, in contrast to normal sleeves, which are cast.There are five ports in each sleeve, two each for inlet andexhaust, an d one which is a common port for both. Port-ing areas are respective ly: inlet, 6^65 sq. in., ex hau st,4.00 sq. in.Pistons are very short-skirted and each is fitted with twowedge-section gas rings, a channel-section scraper ring anda norm al typ e bottom scraper ring. Projected area of thegudgeon pin in the piston is 3.23sq. in. and , as the rod little-end bearing area is 1.575 sq. in., theratio is 0.487 : 1 .The Centaurus is aspirated by atwo-speed, single-stage blower oper-ated in conjunction with a Hobson-R.A .E. fuel injector. Air is en-trained on both sides of the engineand ducted to the respective mainentries on each side of the drivecasing, where it is fed into a beau-tifully formed turbine-type entryto the double-shrouded impeller.This discharges through a diffuservane ring to the several inductionpipes serving the cylinders. All theinduction pipes are the same lengthin order to ensure equal distribu-tion, and terminate in cast light-alloy branch yokes which arebolted to the cylinders and dis-

siavtPOPftT

/

VALVEVALVt

// / EXHAUVi/

\ \

\ \\ \\

\ \. \

> 4Q 2 14 4 0Bristol engine development of specificpower and specific weight against year.

Cylinder baffles, by mean s of which the airis directed to cover the optimum fin area.

ENGINE

. These two details show the disposal of, head finning and the cooling air flowr^ -. ' pa ths.C Y L I N D E RS U E V E

mounted above the clutches and are readily accessible forcleaning. The clutches themselves are driven by a spring-drive gear on the tail of the crankshaft to protect the drivefrom cyclic torqu e variatio ns. Gearratios are: low, 6.761:1, and high,9.03:1, which, at 2,700 r.p.m.(crankshaft) result in an impellertip speed of respectively 1,012 and1,325 feet/s ec. Maximum blowercompression ratio is achieved at2,700 engine r.p.m. in S gearand equals 3-5:1 .Selection of blower gear isgoverned by a valve which variesthe supply of oil to the clutches asrequired, and the valve determinesthe gear according to the inductioncharge requirements of the engine,this being linked up with theHob son-R.A .E. injector. This in-jector was developed by Farn-borough and is manufactured byClaudel Hobson. It meters the cor-rect amount of fuel into the blowerentry according to* an integrationof blower and back pressure differ-,ential with charge temperatureexisting for a given engine speed.The injector nozzles are fitted inthe twin air entries to the blowerimmediately down-stream ofthe butterfly throttles. Mixtureratio for f ullpower is roughly 10 :1by weight, and for economic cruis-ing conditions approximately 16:1by weight.At the rear of the superchargerdrive shaft is a gear box contain-ing the drives for the superchargeritself and the various engine acces-sories, such as fuel and oilpumps, magnetos, tachometer, en-gine starte r, etc. The twin mag-netos which, externally, lookstrangely like little centrifugalblowers, are mounted on rakingfaces to port and starboard of

PLATE

OTTOM (-Y U N D t R IS KRKNG PLUGS

' ^ ' . '

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BRISTOL CENTAURUSj , : (mentioned gear bo x, whilst theengine s ta r te risi ted vert ically aboveitand engages thro ugha doggeding-e following major i tems on the Cen tau rus are supp liedie underm ention ed f irms : ignetos British Thomson -Houston.uition Ha rnes s Plessey orM arconi. ..arking Plugs Lodge . , ' -jxible Pipe s Supe rflexit or Avery. -J Mater ial: H igh Duty Alloys.

n s : \ Ma chining: Well worthy orBristol.ton Rings Brico, We lhvorth y,orHepworth andGrandage.~ .ector Hobson. _ ' .

BRISTOLType ...Bore and strokeCompression ratio

Swept volumeDiameter over cylindersAirscrew reduction gear ratio..Airscrew rotationAirscrew shaft size ... ..Fuel specificationOil specificationOil consumption(average)Engine weight (bare, dry)Firing order

C E N T A U R U S18-rylinder, two-row radial,air-cooled, sleeve-valve, withtwo-speed supercharger andHobson-K.A.E. injector.5.75 in.x7,0 in.7. 8 :1 . -S.-270 cu. in. (33.0 l i tres).55.3in.0.444 engine speed. ' -L.H. tractor.S.B.A.C. standard No.fi.D.E.D. 247;. (100/130 grade).D.E:D. 2472.2.5 g.p.h.at2,400 r.p .m .; 1.12f:.p.h.at 1,000 r.p.ni.. 2,780 1b.1, 12,5,16,ft,2,13, 0, 17, 10, 3, 14,7, 18, 11, 4, 15,8.

Something ofPast Effort andFuture Intentions Together withAnnouncements of NewEngines and AircraftOMPAKABLY with most other major concerns ,allof whichgo tom a k etheBrit ish aircraf t ind ustry ,onlya small proportionofthe work th e Bristol A ero-Co. , L td. , have been engaged upon dur ing the pas t3ars has cometol ight . How ever , as a resul tof anition staged at Br is tol onJun e 28 th , we are nowlosiriontogiveafair ly wide surve yoft he c om pa n y ' s .ties.the preceding pages will be found a descriptione latest and mo st powerful Bristol engine,theurus;bu t ,inaddi t iontoth e new:, three new modelsof thewell-Hercules are a lso annou nced,being designated. respectively00, 120 an d 130- All,ofcourse ,asically similar, the chiefdif-es lying in the superchargers .lark 100 is thela tes t mi l i ta ryles,and has thehighest max i-power output of thethreeatb .h .p . It e m ploysa two-speed:hargerofimproved ent ry shape npe.ller design together withan-K.A.E. fuel injector . Markercules also havethenew two-blowerandH o b s o n - R . A I E .in-

but opera teatsl ightly reduce d1, these engines hav ing been:ed primarily for high-a l t i tudeircraft, forw hic h e m ploym e ntare fatted with a special re arhaving an increased capa c i ty>ry drive capable of t r a nsm i t -pto 150 h.p . tocope withtheblowers necessaryforpressuris-Qie Hercules 130isac tua l lytheodd describedinthe May 24thfFlight, a ndisf i t ted only withle-speed supercharger , being designed foreconomicionatm ode r a t e a l t i t ude s .listonthe subjec tofaircraf t power un itsit isa ppr o-1to reveal th at Bristols hav e been in terestedforjtimein thevariou s forms of gas t u r b ine s , a nd ,in' carried outt r ia lson theexhaust -dr iven turbo -larger as early as 1923 and 1924. Theore t ica lwere cont inued, and were given addi t iona l impetusp. success of Air Comm odore W hi t t le ' s br i l l iant work on je t -propuls ion. It wasfe lt th a ttheidicated by the i r previous exper ience lay in the

C 6 S242220

fie1 6

U_

2129,o

642

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1920 22 4 6 8193 32 34 J63B 1940 42 44 46Increase in altitude at whichsuc-cessive engines have maintained boostequivalenttoJupiteratfull thro ttle,s_ea level.

larger andlonger-range aircraf t , andtheir design wo rk,which was directed towards the provisionof a gas- turbinefor these types, was concentrated along specif ic l ines earlyin 19\i. This has culmina ted in the product ionof a gas-tu r b ineinwhichthe bulkof the powerisutilised todr ivea variable-pitch airscrewofnormal typ e;means have a lsobeen provided in thedesign for therecupera t ion of he a tenergy, which would otherwisebelostinth ejetdischarge .T h a t thepowerform a ny type sof both civilandmili-ta ry a i rc ra f tofth e future willbeprovidedby jetp r opu l -sion or the gast u r b ine is cer ta in .There i s , however , much deve lopmenttobedon e before the gas turbin e can,for instance,be aneconomical propo-sition forair l ine ope ration , and it is,therefore , prudent toa s sum e t ha t t hereciprocating engine st i l ihas yearsofuseful life beforeit.The preference of a i r line opera tors ,for radial enginesinthe larger classesof civil transports was fair ly markedbefore thewar , and it would appe arthat this preference st i l l stands.The-listofnew aircraf t poweredbyBr is toli s im press iv e :

Cciitaurus, i&-cylinders. over 2,500 li.h.p.Short Shetland flying boat.Airspeed A.S.57 airliner.Bristol Type 167 (Brabazon I) airliner

Hercules, 14-cylinders. 1 675/1 800 b.h.p.Short Sunderland (lying boat.Handley Page Hermes airliner.Hand ley Page-Hermes t ransport .Handley Page Halifax transport.Vickers-Armstrong Viking airliner.Bristol Freighter.Bri-tol Wayfarer aircoach.O ne of the mer i t s of the Bristolengineist h a t theoriginal design wasgenerousinconcept ion wi th prec ise ly 'the need forincreas ing powers in m ind . T he H e r c u le s ,for instance, has been increased from 1,375 b.h.p.in1937to 1,800 b.h .p . to-day,againinpowerofover 30 per cen t. ,which has been obta ined at theexpenseofon ly a bou t10per cent, increase in engine weight . Bo th HercuJcsandCe nta ur us arecer ta inly capable of de ve lopm e nt to stillh igher outputs , and may conf ident lybeexpec tedtocoverthe 2 ,000-3,000 b.h.p . rangeinthe near future. Below thisrangeit isprob able th at Bristol will also offera deve lop-m e n tofth e Perseu s nine-cylinder sleeve-valve engine witha powerofa roun d 1,200 b.p .h . com pared w i th the pre-war