JULY 5TH, 1945 FLIGHT Survey of Britain's Most Powerful Radial Engine : An Example of . r l^OgiCal Layout to Achieve Compactness with Power T has been eommbn knowledge far some time past that •power figure of 3,500, the b.h.p. /litre of both Hercules the Bristol Aeroplane Co., Ltd., have been engaged with Centaurus is 46.5, although we know that the latter engine the production of a larger and improved model in the is somewhat better than this, as may be rough!y indicated age of radial, air-cooled, sleeve-valve engines with which by the figures for b.h.p./sq. in. of piston area, these ey have for so long enhanced their reputation. This being respectively:. Hercules 4.93. and Centaurus better mmon knowledge—the result of unofficial "leaks"— than 5.34. • f ibraced the facts that the new engine was an 18-cylinder : Cooling Arrangements lit of over 2,000 h.p. and was called the Centaurug.. ther than this nothing much was generally known until As an indication of refinement in the design of the cowl- nctioned reference, to the engine was made with the ing, if. we take as a datum the frontal area of the Hercules lease of the Short Shetland flying boat (Flight, May 17th, at 2,122 sq. in. and give it the value of unity, then the 145), when it was revealed that the Centaurus was of Centaurus frontal area of 2,402 sq. in. gives a comparative 'er 2,500 h.p. ratio of 1.13:1, which is well below the relative h.p..ratio Even now we are not permitted to give any indication of 1.385 :1, itself a conservative figure. Put another way, the power output other than this means that for an increase in at it is well in excess of power output of 38.5 per cent, over 500 h.p., so that, in assessing the Hercules, the Centaurus has only it'cific qualities' relating a 13 per cent, greater frontal area, >wer, one can its specific' power thus being ily use the 150.5 h.p./sq. ft. ;ure of 2,500 as It must be appreciated that 'datum and the goal of really close cowling ite that the ...- . ,.., . -•.. is hindered in at- uated result is '."'•''/ tainment by the- mewhat lower very critical fac- an the factual tors of cooling, due. and Bristol's Probably the achievement is ost impressive rendered the more ature to strike deserving of praise e casual ob- by virtue of their having obtained, rver of the with it, an en- mtaurus is the hanced cooling mpactness of efficiency. To a e unit in rela- - certain extent the )n to its power itput, and, as a employment of a irollary, the achieve- cooling fan driven from the air- ent of a clean and efn- screw spinner can be deemed ent fairing by refine-. partially responsible for the in- ent of design. It is, crease in cooling efficiency— but by no means wholly. course, obvious that every miponent in the engine is the As the Centaurus is designed- as a suit of months, in some cases power plant, the cowling, fairing and baffling arrangements naturally come ;ars, of development, and that Compactness without complication. c only means by which such WOLIC- Note the backswept exhaust stack into the picture in addition to the anship can bear fruit is by way of cooling muffs and the collected exhaust engine per se, so whilst on the sub ie hard road of technical ability fol- tail pipes. ject of cowling we might as well deal wed strongly by the incessant trial" with it before going on tc the engine' id error of testing. By such means a tremendous fund proper. The subject is well worthy of study. E data is amassed, such data being the "history" only The main body of the engine in way of the cylinder uough which the future may be discerned. Looking -'banks is closely cowled by four panels, two per side, hinged ack in order to look forward applies equally in the tech- together at top and bottom on the fore and aft centre ical as in the ordinary world. • • » plane of the engine and secured together with three toggle- type fasteners on each side. The latter are recessed to Specific Powers give a smooth surface line. Forward of the cowling panels From the aspect of arrangement analysis the Centaurus is the-aerofoil-section annular nose fairing which, with the basically similar to its immediate forerunner, the Her- very unusual spinner extension fairing (fitted around the jles, with the chief differences that it has four more reduction gear casing), provides a smoothly contoured, but flinders and that entirely separate individual gear trains surprisingly small, annular intake duct in the entry of which re embodied front arid rear for driving the sleeves in the rotates the- 18-blade cooling fan fitted to the rear of the -spective cylinder* banks. Swept volume is naturally spinner. irger, but so is the unit swept volume as, although the It may thus be seen that cooling air entrained is ob- ylinder bores remain the same at 5.75m., the stroke of the structed not at all until it reaches the front cylinders. entaurus is 7.0m. as compared with the 6.5m. of the Having done its due amount of fin circulation the air is lercules. This gives the new engine a capacity of 53.6 then allowed to escape to the outside atmosphere through tres (3,270 cu. in.) in contrast to the Hercules' 38.7 litres cooling gills in the skirt ring of the main cowling panels. 2.360 cu. in.), an increment of 38.5 per cent. In terms of In consideration of the amount of development design apacity and power output, at the restricted Centaurus which has gone into producing the Centaurus, the instance 1*4 FLIGHT BRISTOL CENTAURUS ; cylinder and head finning is interesting. By experiment has been shown that the optimum pitch for fin location in the region of o.iin.-^the pitch on the Centfturus is [izin. and the cooling area of finning to each cylinder id head is no less than 32.47 sq. ft. Not the least novel feature of the cooling arrangements the fact that, unexpectedly enough, the front,row of flinders, 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, across he cylinders is the rather high figure of 10 inches of water, WWmB 3d, very shrewdly, this is put to incidental, yet none the •i*J\\.--:\ ••'•• :T: BALANCED ' *%£- useful, advantage in cooling the rear-swept front-" ylinder exhaust pipes. These are shrouded' with tubular jufis, open at each end, which extend nearly to the trans-. jterse centre plane of the engine, and the pressure differ- mum ential between front and rear open ends induces an air- Bow through the muffs which effectively cools the exhaust Wipes whilst, at the same time, isolating their cooling ajf prom the ordinary flow to the rear cylinders. Limits of Cooling The cooling efficiency of the Centaurus is nearly as high it 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 closer pitching of the cooling fins. Nevertheless, it would appear that the ultimate is at- least within sight, and from this aspect it is a reasonable assumption that some limit will sooner or later enforce a barrier against the use of more potent fuels and increase of engine rating by stepping up the b.m.e.p. This reflection excludes consideration of the (use of water/methanol injection and kindred devices which conceivably might have an increasingly large part to play. The reduction gear is the well-tried Bristol-Farman type I bevel 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 annular Iseatings, the bearing faces of which are spherically curved. I This measure ensures that the«driving and sun bevels can- [each rock slightly and so permit the load to be distributed I 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 supported" I in a bearing-in the forward end of the crankshaft. Airscrew' — thrust is carried by a large-diameter ball-bearing housed in the" front of the r.g. casing forward of the gear itself. The crankcase of the Centaurus is unorthodox in that it is composed of three sections—front, centre, and rear— the mating faces of which are disposed on the transverse , centre planes of the cylinder banks..' Each section has a diaphragm, those of the front and rear supporting the sleeve drive bearings and isolating the gear trains from the crank throws, and that of the centre provides a sup- port for the main central crankshaft bearing. The front and rear sections of the casing are secured to the central section .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 production efficiency. Both sets oi sleeve drives are composed of spur gear two-bolt maneton joints. Each of the balance weights trains powered from a common input gear mounted on .contain two 2.875m.