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First the name: it’s not for the wizard from the days of King Arthur. Rolls had a naming procedure for its engines that used birds of prey, and merlin is a specie of falcon. Kestrel, Hawk, and even Vulture were names of the company’s piston engines. The Kestrel with its twelve cylinders in a V configuration is regarded as a smaller displacement predecessor of the Merlin.
The development of the Merlin had typical growing pains. An initial bench test in 1933 had a 790 HP rating. The first flight test in 1935 pointed out that bearings, gearing and cylinder heads needed improvement. As corrections were made the horsepower increased. In 1945 it reached 2200 HP from the 790 HP in 1933.
The arrival of Stanley Hooker at Rolls from the British Admiralty Research Laboratories in 1938 was to bring great leaps forward. Hooker had academic backgrounds in both math and fluid dynamics. He found that the Rolls knowledge and development of supercharging woefully inadequate to downright incorrect. Design of superchargers – devices to add a pressure boost to the intake of an engine’s air/fuel mixture – had been around since World War One. Hooker’s improved designs dramatically brought more power. On design raised it 30%. Another innovation used modified engine exhaust stacks that created ejected thrust and boosted speed. A Supermarine Spitfire with the Merlin Model 61, running two superchargers in series, could claim a service ceiling of 40,000 feet and 70 more miles per hour. One hundred octane aviation gas added a reported 135 horsepower. The tanker Beaconhill from the U.S. had made it to port with 100 octane just prior to the Battle of Britain in 1940. That added to the Merlin’s potential to defend England.
The Merlin was liquid cooled, and fans of air-cooled radial engines cite this factor as a weakness for the engine. Pilots learned that the Merlin could take the abuse of exceeding operational limits and still get them home. Early Merlin fuel systems would flood out in a dive as the negative G would affect fuel flow. A flow restrictor fixed this; a single point fuel injection system was the ultimate cure.
The V-1650 was a Merlin model built under license by Packard in the U.S. It was the only engine of British design used in American warplanes during the conflict. In 1942 the Merlin was installed in the North American P-51 Mustang. With internal and drop fuel tanks, the plane gained an endurance of 7 ½ hours, more power and improved performance at low and high altitudes. It could fly with the heavy bombers through an entire mission. The Merlin-powered P-51 took the Allies to air superiority over Europe in 1944. The Merlin engine production run was 160,000 units, with Packard assembling 60,000 of these. There were 50 models, 21 power ratings, and 31 different aircraft that flew with Merlin power. AVIATION HISTORY was source material for this story.
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