Merlin and Griffon Engines
Rolls Royce Merlin
The Merlin engine was essential to Britain's war effort, it not only powered the Spitfire, but also the Hurricane, Lancaster and Mosquito. The vast majority of Spitfires in the Battle of Britain were fitted with the Merlin III of 1,030 horsepower.
Developed as a replacement for the Kestrel engine that had powered the RAF's graceful Fury Biplanes in the `30s, and whose development had been spurred by the American Curtiss engines, it is perhaps surprising that the Merlin was designed at all. When the Merlin was on the drawing board a simple development of the Kestrel called the Peregrine promised to be very successful, and a 24 cylinder variant called the Vulture was hoped to give 1,700 horsepower. There was also the chance that the "R" engine that had powered the Supermarine S6 could be developed as a production engine (the Griffon). Thus the Merlin was seen as something of a stop-gap to fill the void between the Vulture and the Kestrel. It is just as well the Merlin existed, the Vulture engine had a very troubled time in development and two aircraft programmes based on it, the Avro Manchester bomber and the Hawker Tornado fighter had to be cancelled. The Peregrine overcame some early problems, but simply did not have the development potential of the Merlin, and the excellent Whirlwind fighter that was powered by a pair of Peregrines was only produced in small quantities. The Griffon only became available in quantity during the last two years of the war. Of course there was another manufacturer of in-line engines in Britain, D. Napier and Sons, but again both of their major engine projects, the Dagger of 1,000 hp and the Sabre of 2,000 hp, had problems. Napier persevered with the Sabre, but it was only during 1942 that they became available in any numbers to power the Hawker Typhoon. Thus it was the Merlin that had to meet all Britain's in-line aero-engine needs for the early war years.
The Merlin was at first designed to have a novel cooling system. Evaporative cooling was to be provided by condensers in the wings with a small retractable radiator for use at low speeds and when taxiing. Then Rolls-Royce adopted Ethylene Glycol as a coolant, which is more efficient than water, a radiator for the new coolant could be much smaller than those used with water-cooled engines, the wing condensers were then done-away with. The cooling system was vulnerable to damage by gunfire, particularly as the header tank was situated at the very front of the aircraft. A hit here by the rear gunner of a German bomber would cause a Spitfire pilot to have to break off his attack and land before the engine overheated. Even worse, pure Ethylene Glycol is flammable and added to the risk of the engine catching fire.
Designed by Rolls-Royce as a private-venture, the Merlin was able to take advantage of the new 100 octane fuel developed in the U.S.A. The Merlin only had one disadvantage when compared with German engines, the latter were fitted with fuel injection to deliver a precise charge of petrol to the combustion chamber. The Merlin still used a carburettor, which had the advantage of being much simpler and needing much fewer components, but it did cause the Merlin to "conk-out" if negative G forces were applied. Thus a German pilot with a Spitfire on his tail could simply pull negative G nosing into a dive and the Spitfire would fall behind until the engine picked up, only a matter of a second or two, but that second was all the German needed. Spitfire pilots developed a way around this by doing a half-roll before following into a dive. This meant that the force of gravity acted in the opposite direction and the Merlin was unaffected. In 1941 a carburettor modification, developed by Miss Tilly Shilling, enabled the Merlin to carry on working with short periods of negative G, a vital stop-gap until the introduction of true negative G carburettors in 1943.
Merlin development might have stagnated after 1940, any further increases in power needed a more efficient means of transferring the heat away from the engine. Rolls Royce responded with a mixture of water and Ethylene Glycol which was put under pressure. This mixture also reduced the fire risk associated with using pure Ethylene Glycol. This system was first used in the Merlin XII used in the Spitfire Mk II. The rapid introduction of this system was only made possible by everything Rolls Royce had learnt about pressurised cooling when developing the Goshawk and early Merlin condenser systems.
As piston engines get higher, they lose power because the air gets thinner. What is needed is a fan to suck more air into the engine and improve combustion, just like bellows in a furnace. Such a device is called a supercharger. It was the introduction of a more powerful two-stage supercharger to the Merlin that produced the leap in performance of the Mark VIII and IX Spitfires.
The Merlin was produced under licence in America by the Packard company. These engines were used in the Spitfire XVI, but they also found use as the powerplant that enabled the P51 Mustang to be transformed from a low altitude army co-operation fighter into the long range, high altitude nemesis of the Luftwaffe.
One thing that is often forgotten is that the capacity of the Merlin was quite small when compared to the opposition. The Merlin had a capacity of 27 litres, whereas the DB601 of the Messerschmitt was 39 litres and the BMW801 engine of the Focke-Wulf 190 had 42 litres. The superiority of the later Merlin engined Spitfires (i.e. Mk IX) over these Luftwaffe aircraft is all the more remarkable when this is remembered.
ROLLS-ROYCE MERLIN ENGINE.
TYPE- TWELVE CYLINDER 60 DEGREE UPRIGHT VEE LIQUID COOLED INTERNAL COMBUSTION ENGINE.
BORE x STROKE= 5.4in x 6in (137.3mm x 152.5mm) CAPACITY= 1,647 cu in (27 litres)
TAKE OFF POWER: 880 HP AT 3,000 REVS
INTERNATIONAL RATING: 990 HP AT 2,600 REVS AT 12,250 FT.
MAX POWER: 1,440 HP AT 3,000 REVS AT 5,500 FT.
WEIGHT: 1,375 LBS.
TAKE OFF POWER: 1,315 HP AT 3,000 REVS
MAX POWER: OVER 1,650 HP.
WEIGHT: 1,650 LBS
Although the Griffon entered service long after the Merlin, in many ways it is an older design, being based on the Buzzard which first ran in 1928 and which itself was a scaled up version of the Kestrel. The big Buzzard ran at only 2,000 rpm and was mostly used to power flying boats, but was developed into the "R" engine that ran at 3,400rpm for short periods. The "R" powered the Supermarine S6 to its Schneider wins in 1929 and 31. A derated version of the "R" was being developed in 1933 but this was dropped so that Rolls Royce could concentrate on the Merlin. It is perhaps surprising that work on the Griffon did not start again in earnest until 1939, 10 years after the "R" engine flew. However, once restarted, work on the Griffon proceeded at a fantastic rate and the new engine was put to good use in the Spitfire. The Griffon ran at 2,750 rpm, a remarkably high speed for such a big engine.
The first Griffons had single-stage superchargers, and were fitted to the Spitfire MK XII. These aircraft arrived just in time to take on the Focke-Wulf 190 "Tip and Run" fighter bombers that were attacking England's South Coast. Their impressive low-level performance was used to good effect.
For high altitude a two-stage supercharger was needed and these arrived in the Spitfire XIV and XVIII. This enabled the Spitfire to stay in the forefront of fighter performance until the end of the war.
Amongst the other aircraft which used the Griffon engine were the Firefly naval two-seat fighter, the Avro Shackleton Maritime patrol/AWACS aircraft, the very advanced Martin-Baker MB5 fighter prototype, and the Commonwealth Aircraft Corp CA15 prototype.
ROLLS-ROYCE GRIFFON ENGINE.
12 CYLINDER UPRIGHT VEE, LIQUID COOLED, INTERNAL COMBUSTION ENGINE.
BORE = 6 INCHES (152.4MM) STROKE = 6.6 INCHES (167.64MM)
GRIFFON II (SINGLE STAGE SUPERCHARGER)
TAKE OFF POWER = 1,720 HP.
MAX POWER = 1,730 HP AT 750 FT, 1,490 HP AT 14,000 FT.
WEIGHT = 1,800 LBS.
GRIFFON 61 (TWO STAGE SUPERCHARGER)
TAKE OFF POWER = 1,540 HP.
MAX POWER = 2,030 HP AT 7,000 FT, 1,820 HP AT 21,000 FT.
WEIGHT= 1,980 LBS.
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