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Aviation History
1954
1954 - 1296.PDF
FLIGHT, 7 May 1954 THE TWO Rs . . . run on October 15th, a number of weaknesses were revealed, and it was not until July 1934 that a 100-hr type-test could be com pleted. This first unit gave 625 h.p. at 2,500 r.p.m. for take-off, and 790 h.p. at 12,000ft. Rated boost was plus 2 lb, and weight 1,177 1b. In The Magic of a Name, Harold Nockolds relates how early bench tests resulted in persistent cracking of cylinder jackets and failures of the double-helical reduction gear. The substitution of straight spur-gears cured the reduction-gear trouble, but strengthening of the big integral cylinder blocks and top half of the crankcase was not regarded as the complete answer to the cylinder-jacket defects. Experiments with different types of cylinder heads were made, and the integral cylinder block and upper half of the crank- case were replaced by separate castings for the two components. The Rolls-Royce engineers then justifiably felt that they were on the right road, but an attempt in 1935 to pass a 50-hr civil type-test with the Merlin C (as the engine incorporating these changes was called) ended in failure, and it was not until December of that year that the test was completed. The rating was 955 b.h.p. at 2,600 r.p.m. at 11,000ft, with a maximum output of 1,045 b.h.p. at 3,000 r.p.m. at 12,000ft, and the engine (Merlin F) was put into production as the Merlin I. In September 1937 Flight published the first description of the Mks I and II, remarking that engines of this series had then flown for over 2,000 hr and that they had shown a marked superiority over the early Kestrels in respect of the rough treat ment they would stand. It was disclosed that the chief difference between the Merlin I and II (formerly Merlin G) lay in the cylinder heads. Whereas in the Merlin I these were of the detachable "ramp" type, the Merlin II had blocks and heads cast in a unit, following earlier practice. Both models had four valves per cylinder, each with two concentric return springs. There were two sodium-cooled exhaust valves on the outside of the head and two inlet valves on the inside. On the Merlin II all four were parallel to the centre-line of the block, but the two inlet valves in the detachable head of the Merlin I were inclined at about 45 deg to the exhaust valves. In both engines the latter had phosphor-bronze guides, and high-silicon-chrome steel seat- ings were screwed into the heads. A fixed-datum automatic boost regulator maintained a con stant induction-pipe pressure without continual reference to the boost gauge and throttle adjustment. The hollow crankshafts were carried in seven special lead- bronze bearings, and the reduction gear was of 0.477:1 ratio. Half the casing for the gearing was cast integrally with the crank- case; in this respect the Merlin differed from the Kestrel. Oil pumps carried on the lower half of the crankcase took their drive from the wheelcase through an idler gear. The dry-sump system was employed, and two scavenge pumps drained the front and rear ends of the crankcase. The pistons and the floating steel gudgeon-pins, which had phosphor-bronze bushes, were splash- lubricated, a baffle in the lower half of the crankcase preventing excess oiling. Sandwiched between the supercharger and the crankcase at the rear of the engine was a wheelcase from which a full comple ment of drives was taken. The Rolls-Royce/S.U. carburettor was of the twin-choke tube, updraught type, with a separate diffuser to each choke placed at right angles to the airstream. The semi-automatic, two-stage mixture-control device was operated by air intake pressure, boost and/or a cockpit lever. International power of the Merlin I and II was 950/990 h.p. at 2,600 r.p.m. at 12,250ft, and the maximum take-off output was 890 h.p. at 2,850 r.p.m. When some of the first figures for the Merlin were published in Flight during May 1937, a note was appended on the develop ment by Rolls-Royce, Ltd., of compact "power plant" assemblies, wherein the mounting was arranged to permit the radiator being carried close to the crankcase. Moreover, by mounting the header tank round the nose of the reduction gear the amount of piping was reduced to a minimum. Advantage was taken of then-recent research in the reduction of cooling drag by enclosing the radiator in a low-drag cowling, wherein the cooling was done by air at relatively low velocity, and from which the flow through the matrix was controlled to suit various flight conditions by an adjustable flap at the exit. A tribute was paid also to Rolls-Royce's special experimental flight at Hucknall, where, on April 12th, 1935, a P.V.12 engine had first been flown in a Hawker Hart. (Other early machines used at Hucknall were the High Speed Fury II, a Gloster Gnat- snapper, a Hawker Horsley, a Heinkel He 70A, and a Fairey Battle.) "Valuable work in connection widi the initial proof tests of new engines, the development of low-drag cooling systems, efficient exhaust systems, and flight problems associated with the use of variable-pitch airscrews, has already been done," it was reported. 577 During June 1937 a Merlin II, mounted in a Horsley, began a 400-hr flight endurance test at Farnborough, and a specially rated "racing" engine was developed from it with a view to installa tion in the special Speed Spitfire, with which an attack on the world's speed record was contemplated. The engine used was a Merlin III, which differed from the Merlin II in having a standardized de Havilland/Rotol airscrew shaft and dual accessory-drive. It was taken from stock and was fitted with strengthened pistons, gudgeon-pins and connecting rods to with stand the extra load. "The power output of the standard engine," writes Harold Nockolds, "was 1,030 b.h.p. at 3,000 r.p.m. at 10,250ft with plus 6i lb boost. "Solely by opening the throttle, raising the supercharger pressure, and using fuel of a higher octane," he goes on [the petrol normally used at that time was 87 octane], "the engine was made to develop no less than 2,160 b.h.p. at 3,200 r.p.m. with the supercharger giving 27 Ib/sq in boost. This was a phenomenal performance, for it meant that a power- to-weight ratio of 0.621 lb per horsepower had been achieved— a considerable improvement on the 0.71 lb per horsepower of the 1931 R engine. "This tremendous output, which was admittedly only attained for a short period, nevertheless gave ample proof of the inherent possibilities of the Merlin. But Elliott and Hives were perhaps even more satisfied with a 15-hr endurance run at 1,800 b.h.p., 3,200 r.p.m. and 22 lb boost accomplished during the development period. After this they felt perfectly satisfied that the Merlin would be capable of meeting all the demands that might be made of it. How right they were!" How right, indeed, for the story of Merlin development we now have to outline is one of the most inspiring in the history of British aero-engineering. The Merlin II and III were installed in the Spitfire I, Defiant I, Hurricane I, Sea Hurricane I, and Battle I, and were—as will always be remembered—vital factors in the winning of the Battle of Britain. The Merlin IV had pressure-water cooling in place of the glycol cooling of the earlier models, and was developed for installation in the Armstrong Whitworth Whitley IV bomber. The Mk VIII, installed in the Fairey Fulmar I, was a medium- supercharged unit rated at 1,010 h.p. at 2,850 r.p.m. at 6,750ft, and, using 100-octane fuel, delivered 1,080 h.p. at 3,000 r.p.m. for take-off. The Merlin X—installed in the Halifax I, Wellington II and Whitley V and VII—represented a very important advance in that it had a two-speed supercharger to improve take-off, low- altitude performance during climb or level flight, and fuel economy under cruising conditions. The speed change was effected through an oil-pressure system, the actual changeover under full power taking about a second. In low gear the Merlin X gave 1,145 h.p. at 5,250ft, and in high gear 1,010 h.p. at 17,750ft. The Merlin XII, driving a Rotol three-blade constant-speed airscrew, was installed in some Spitfire lis; its maximum output was 1,150 h.p. at 3,000 r.p.m. at 14,000ft and it had a 0.477:1 reduction gear. The next production-type engine was the Merlin XX, which, compared with the X, delivered a greatly increased power at height. The two units were, however, interchangeable. Describ ing the engine in great detail during 1942, Flight remarked: "So far-seeing and gifted were the designers in evolving the original design that the successive types up to the latest and far more powerful Merlin XX remain substantially the same. Even the crankshaft and bearings, the pistons, cylinder blocks, crankcases, are basically the same as in the original conception, though naturally innumerable detail improvements have been effected as constant study and experiment and metallurgical improve ments have made possible." The two-speed supercharger of the Merlin XX was of improved design, incorporating a modified form of central entry which gave a freer flow of air to the blower. The low-gear ratio was 8.15:1 and the high gear 9.49:1. Amendments were
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