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Aviation History
1923
1923 - 0118.PDF
the other the driving boss is run at crankshaft speed, tobe used in conjunction with a chain-driven propeller. This is, we think, a very wise precaution on the part of the designersof the " Cherub,*" as it is at any rate conceivable that, especially for experimental purposes, gear ratios other thantwo-to-one may be required. For instance, a small machine THE BRISTOL " CHERUB " : Above details ofcamshaft, rocking-shafts, etc., and below the crank- gg *-•••-.•_- shaft and connecting rod assembly. designed with the object of getting a fairly high top speed, i.e.,with a reasonably heavy wing loading, might employ the two- to-one internal gearing and still get quite good propellerefficiency On the other hand, a different type of machine might be intended for very low landing speed, light wingloading, and good climb, but not with a very high maximum speed—say, 45 m.p.h. or so. In that case, the runningof the propeller at 1,250 r.p.m., with a forward speed of only 45 m.p.h., would scarcely result in very good propellerefficiency, and it might be advisable to choose a 3-to-l. or even lower gearing. This could be very easily and cheaplydone by fitting the sprocket type of engine, and then experi- ment with different reduction ratios. Thus a large-diameter,low-pitch propeller could be used, giving very good propeller efficiency. The accompanying illustrations show clearly the generaldesign of the Bristol " Cherub." Two opposed cylinders are attached to a box-shaped crank-case of aluminium. Thecylinders, which have a bore and stroke of 85,mm. and 94 mm. respectively, have detachable alluminium alloy heads, heldon to the cylinder barrel by five bolts. The valves are inclined in the head, and are operated by a somewhat unusualvalve gear. A single camshaft with four cams integral with the shaft liesinside the crankcase, and is driven by very robust gearing from the crankshaft. The cams operate fingers, which in turnoperate cranks, on the inner ends of shafts running parallel to the cylinders. These shafts are not push-rods, as theyare not displaced lengthwise, but oscillate or rock under the action of the cams. At their upper, or more correctlyspeaking outer, ends these rocking shafts carry other cranks, •bearing on the ends of the valve stems, or rather on thewashers which cap the valve springs. The rocking shafts are enclosed in tubes, and the whole mechanism is enclosedand automatically lubricated. Thus comparatively silent MARCH 1, 1923 running is obtained, while the arrangement is such that theclearance between rocking shafts and valves does not alter with expansion of the cylinders. The only occasion foradjustment should therefore be to take up any slight wear that may develop after prolonged running. This shouldbe so small that, for all practical purposes, the valve mechanism should very rarely need attention.The crankshaft, which is, of course, of the two-throw 180° type, is of ample dimensions, and is very stiff and robust.It is carried in three ball bearings, with a thrust bearing in front. The connecting rods-are threaded on to the crankshaft,and run on case-hardened crankpins with -j^-in. rollers. The big ends of the connecting rods have case-hardenedbushes inserted. From the photograph it will be seen that the whole crankshaft and connecting rod assembly areof very ample proportions, and special attention has been given to the connecting rod roller bearing assembly, so thatvery long life can be guaranteed under continuous working conditions. The crank-case is a one-piece aluminium casting, having atthe rear end a very large opening which enables the connecting rod assembly to be inserted complete. An induction ductis formed inside the top portion of the crank-case, and extends in the form of an inlet pipe at each end to the inlet valves inthe cylinder head. A short bent induction pipe communicates with the duct, and carries a Zenith carburettor, drawing airheated by the exhaust pipe. Ignition is by a B.T.H. twin- cylinder magneto, mounted on a platform on top of thecrank-case, near the right-hand side. Single sparking plugs are fitted centrally in the cylinder heads. Lubrication is automatic throughout. Oil is positivelydelivered to the timing gears, camshaft, rocker mechanism, etc., and pressure feed is directed on to the connecting rodassembly, from which the cylinders are lubricated by splash, A gallon of oil is carried in the sump in the lower portionof the crank-case, which floods the plunger pump. An oil filler and filter is arranged on the front of the crank-case, andis visible in the front view of the engine. The main details of the Bristol " Cherub " are as follows :—Two-cylinder, horizontally opposed ; bore, 85 mm. ; stroke. 25 i5 G. I (0 10 0. .-. .. • ••;•' / • • .. - &M.EP - ••.'.-.-.C-^'^T — 90 1000 -fSOO 2000 ^500 3000 R PM Power curve of the Bristol " Cherub." 94 mm. ; R.A.C. rating, 8.95 b.h.p. ; rated h.p., 18 at 2,500r.p.m. ; weight of engine complete, 85 lbs. ; petrol consump- tion per hour, 12 pints ; oil consumption per hour, £ pint.It will be seen that the " Cherub " is not particularly light, and there is no doubt that, had they so wished, thedesigners could have reduced the weight considerably. They have chosen, however, to produce a robust engine whosereliability could be guaranteed, rather than go for extremely light weight. The accompanying power curve shows thatat 2,500 r.p.m. the engine develops 20 h.p. The makers 118
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