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Aviation History
1931
1931 - 1060.PDF
FLIGHT, OCTOBER 2, 1931 ON THE TEST BENCH: One of the "R" engines in the special engine : .-.:•• test house at Derby. " R " Engine Development As might be expected, a large number of engine problems somewhat reactionary, but no furLhe»-failures at all were experienced on the engines after fitting them. The oil consumption rose to terrificfigures at the 1931 speeds and powers, partly owing to great quantities lostthrough the breathers. On one 25-min. run the oil consumption was at therate of 112 gallons an hour ; the test house was a sight after it! By weeksof work on combinations of scraper rings; and crankcase breathers, by modi-fication of the scavenging system, and by the final adoption of a deeper sumpwhich filled all the available space in the machine, the consumption was re-duced to about 14 gallons an hour for the final race engines. It also effecteda considerable reduction in the oil tem- perature rise through the engine ; oilentered the engine at about 80 deg. C. and came out at about 140 deg. C.The oil used was pure castor. The fuel to be used was the subjectof many tests ; a compromise had to be effected between power and specificconsumption. In the experiments on fuels very valuable assistance was ren-dered by the Ethyl Export Corpora- tion. The plugs used in both 1929 and1931 were Lodge, type X170, which gave very little trouble indeed, in spiteof the increased boost and power, and weaker mixture. During the course ofdevelopment it was found that a run on the engine was a more severe testfor insulation, etc., than anything the makers could impose. It was decidedto give all plugs an endurance test in an engine before sending them toCalshot. After this run they were re- turned to the Lodge firm, whoexamined the inners, re-polished them and fitted new outer bodies, beforesending them off to Calshot. A few doubtful cases were found in this way,which might have caused trouble in the machine. Actually, no plugtroubles of any description were ex- perienced in the machines. The sameplugs could be used for warming up and taxying about as for the fullthrottle flying. It is of interest to note that of the plugs that did fail on the test-bed, the majoritywhen examined microscopically were found to contain had to be solved before the required power output and small metallic inclusions, presumably out of the engine. standard of reliability were attained. It was decided thata one-hour run at full power was to be the first milestone that had to be passed, before the engines could be deliveredas airworthy. By the end of April, the experimental engines would usually last about 20 min. before some kindof failure. By the middle of July they would still only do about half an hour non-stop. On August 3 a run of58 min. was done at 2,360 b.h.p. and the hour non-stop was finally accomplished on August 12, at 2,350 b.h.p. As part of the re-designing of the engine, an entirely new type of connecting rod was designed and developed, and very considerable modifications were effected in the crank- shaft and crankcase, to withstand the terrific loads. Some idea of the magnitude of the crankchamber stresses may be gained from the fact that the load on the centre main bearing due to centrifugal and inertia forces was no less than nine tons. Alternative bearing metals were tried, but white metal was finally made to stand up. An obscure trouble experi- enced at one time was side-lashing of the big-end bearings against the webs, which caused failure of the white metal on either side of the big ends. Considerable trouble occurred with valve springs. Rather more so than was expected, considering that the speed was only raised 300 r.p.m. In fact, at one time two or three would be found broken after a ten-minute run. Extensive tests were done on a valve spring rig consisting of one cylinder block, and all aspects of design and material were thoroughly gone over. The final springs arrived at were 990 Both B.T.H. and Watford magnetos were used on therace engines. Some trouble was experienced with magnetos on the test-bed, due to the vibration to whichthey were subjected. Every nut on the magnetos had to be split-pinned, and certain parts had to be stiffened up.The magnetos are spigot-mounted, and in several cases engine oil worked its way right along the rotor into thecontact breaker and caused pitting of the points. All engines were tested on a hangar with a propellerfitted before going away, for opening and for tick-over. The tick-over speed was set at 475 r.p.m., and theengines were remarkable as racing engines for their tick- over and flexibility. Considerable trouble was taken withthe carburetters to reduce flooding and piling up troubles and improve the handling in the air. Gas starters werealso tested on the hangar. When in the machine the only method of starting is by compressed air or with the" Bristol " gas starter. Mixture distributors were pro- vided on the engines for this purpose. Hidunimium alloys (R.R.50 series) were used for allaluminium parts. The life of these engines being shor . forged aluminium was used to replace bronze and steel inmany parts of the engine. Much valuable experience was gained with stronger materials for such parts :<sconnecting rods. From the experience gained from many smashed-up engines, a " Life of Parts " list was drav nup, and pieces were thrown away after their allotted sp-n, whether broken or not. This was found to be cheaperand quicker than re-building smashed engines. ^eI£ is hardly a single component part in the engine v.'"cnhas not received design attention and been improved m
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