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RR Merlin Aircraft Applications

Courtesy of Eric Mann, Unlimited Excitement, LLC.

The Rolls-Royce Merlin engine is recognized as the powerplant that won the battle of Britain during WWII. The Merlin powered Hurricane and Spitfire were instrumental in neutralizing the German Luftwaffe's attempt to gain air superiority over Britain, reducing the effectiveness of German bombing and preventing an Axis invasion.

The North American P-51 Mustang became an outstanding fighter capable of long-range bomber escort when the Merlin was adopted as the standard powerplant. One of the significant differences between the Allison V-1710 and the Rolls-Royce Merlin was the Allison relied upon a GE turbocharger to maintain high power at altitude, while the Merlin used two speed (and eventually two stage) supercharging.

GE was unable to produce the turbochargers in sufficient quantity to equip both bombers and fighters, so aside from the P-38 Lighting which was equipped with turbochargers, most Allison-equipped planes were limited to relatively low altitude operation -- under 20,000 ft.

When powered by the Merlin, the Mustang was able to achieve excellent performance at altitudes above 30,000 ft which allowed it to effectively combat Axis aircraft while performing high altitude bomber duties.

The Merlin was one of the most significant military aircraft engines in history, being installed in a wide variety of aircraft including fighters, bombers, transports, reconnaissance, patrol, and attack planes. Models powered by Merlins include the Battle, Spitfire, Defiant, Sea Hurricane, Halifax, Wellington, Whitley, Hurricane, Lancaster, York, Mosquito, P-40F, Barracuda, Fulmar, Seafire, Mustang, Welkin, Lincoln, and P-82. Some of the most significant aircraft are described below.

Supermarine Spitfire and Seafire

One of the best know fighter planes to use the Merlin engine was the Supermarine Spitfire, with the Fleet Air Arm (FAA) version called the Seafire. This plane is recognized by the British as one of the key weapons which successfully defended England during the air war with the Luftwaffe in 1940 which has come to be known as the Battle of Britain.

The prototype was called the Supermarine Type 300 and was developed in 1934 by R.J. Mitchell. The prototype first flew in 1936, being powered by a Merlin"C" engine. In 1937, the Mk. I Spitfire entered squadron service, being powered by the more advanced Merlin Mk II.

The plane was a low-wing monoplane with an aluminum monocoque fuselage, the engine being mounted in a chrome-moly tubular frame attached directly to the firewall which transferred loads to the upper fuselage with two upper longerons, the lower mounts were part of the lower wing spar, transferring loads directly to the wing. An elliptical wing gave the Spitfire its characteristic planform. The radiators (oil and coolant) were located under the wings at approximately mid-span, the large rectangular coolant radiator was mounted on the right wing and the smaller circular oil cooler on the left in order to counter some of the torque from the clockwise-turning propeller.

During the battle of Britain, the Spitfire's primary role was to combat the high-performance German fighters, namely the Messerschmitt Bf109E and the Bf110. Following the Battle of Britain, Germany introduced the Focke-Wulf 190 which outperformed the Spitfires until the Merlin-60 series powered Spitfire IX was introduced. In addition to the European theater, Spitfires saw action in the North African, China-Burma-Indian, and the Italian theatres.

The Seafire's first operational mission was supporting the US Army invasion of North Africa. The Seafire proved to be a difficult aircraft to operate from carrier decks -- at the Salerno landings in Italy 60 aircraft were lost in 5 days. As soon as more effective Fleet aircraft became available (such as the Vought Corsair), the Seafires began to be phased out.

Hawker Hurricane

The Hurricane was a low-wing monoplane using older construction techniques -- it featured a fabric covered chrome-moly steel tubular frame fuselage with metal panels covering the fuselage forward of the cockpit. The wing was a classic two-spar design with stressed skin outer panels, the rest was fabric. Armament consisted of either eight 0.303-in machine guns or four 20-mm cannon.

The Hurricane first flew in November 1935 -- this was also the first flight of a Merlin engine.

In service, the Hurricane was an important component of the British Expeditionary force sent to France in 1939. While most Spitfires remained in Britain, the Hurricane was the lead British fighter during the Battle of France. When the Axis eventually captured France, Hurricanes joined Spitfires as the planes which fought the brunt of the battle. The British employed tactics that maximized the effectiveness of the planes during the battle -- Hurricanes attacked German bombers while Spitfires battled German fighters, both being guided by newly invented radar. While the Spitfire had better performance than the Hurricane, the Hurricane featured a simpler design which made it easier to service and repair.

North American P-51 Mustang

When North American Aviation was approached by the British Purchasing Commission in 1940 to become a second-source for the Curtiss P-40, North American proposed building a better aircraft based on new NACA wing data. The British said if the plane could be designed and built quickly they would agree. North American met the urgent British need with the Allison-powered P-51 Mustang, designed and built in just 100 days! First flight was October 1940, and the plane quickly entered service performing low-level reconnaissance. The Allison's lack of altitude performance was remedied when Rolls-Royce was given the opportunity to fit an altitude-rated Merlin to an airframe -- the resulting significant improvement in performance created the plane called the Mustang II (or P-51/B). The U.S.A.A.F. became interested in the plane which had originally only been intended for export -- at their request North American built a similar plane powered by a Packard-built Merlin.

The Mustang (as named by the British) used a laminar flow NACA wing. The aluminum monocoque fuselage featured very low drag. Even more impressive was the low drag cooling system -- the belly mounted radiator/oil cooler/inter- and after-coolers took advantage of the expanding air as it passed through the coolers to offset some drag with thrust from the expansion. A novel monocoque engine mount was used which reduced weight and increased strength. The P-51/D introduced a Plexiglas bubble-canopy which dramatically improved visibility for the pilot.

The very efficient laminar flow wing and sleek fuselage of the P51 combined with the altitude performance of the Merlin provided the range and performance to provide fighter escort for bombers all the way to Berlin. In the hands of the Eight Air Force, the Mustang dramatically reduced losses of bombers to Axis action, enabling the Allies to inflict almost relentless massive bombing missions resulting significant damage to strategic German facilities and cities, which had a substantial impact on the Nazi's ability to wage war.

Avro Lancaster

The Avro Lancaster evolved the twin engine Avro Manchester which was intended to be powered by two Rolls-Royce 24 cylinder Vulture engines. When development problems with the X-24 Vulture became critical and it was determined they could not be solved in the short-term, the decision was made to replace the two Vultures with four Merlins. While the fuselage was essentially unchanged, the wing required extensive redesign to accommodate the additional engines. As the Battle of Britain raged, completion of the Lancaster was a top priority. The prototype flew eight months after program launch. The Lancaster featured semi-monocoque stressed skin construction with internal bracing to compensate for the relatively thin skins.

An important feature of the Lancaster was its very large bomb-bay. This made it suited for delivering remarkable weapons such as the rotating bombs used by the 617th "dam busters" to breach dams in the Ruhr Valley, the 12,000 lbs Tallboy bomb which was used to sink the German pocket battleship Tirpitz in a Norwegian fjord, or the 22,000 lbs Grand Slams which were used (along with Tallboys) to damage heavily fortified U-boat pens and similar structures (while 5,000 lbs bombs would apparently barely damage these structures, the Grand slam would penetrate through as much as 35 ft of reinforced concrete before exploding, sending a large shockwave through the ground when they exploded.

De Havilland DH98 Mosquito

The Mosquito was a small, fast twin-engine bomber which relied on speed rather than armament for defense. Conceived in 1938 and first flown in November 1940, it faced considerable resistance from the RAF during its development phase. During its service evaluation by the RAF the only complaint was there were not enough planes available soon enough! Fighter and reconnaissance versions of the Mosquito were soon developed.

The twin-Merlin airplane featured a radical construction technique, featuring one of the original composite structures for an aircraft. The wooden Mosquito had a monocoque fuselage composed of a sandwich of two very thin skin layer -- 0.035 to 0.080 inch plywood which was loaded in tension. The skins sandwiched a layer of balsa which typically ranged from 3/4 to 1-1/4 inch thick, the grain of the balsa oriented to span outer skins thus taking advantage of Balsa's high compressive strength in this orientation.

The Mosquito had relatively high radiators which necessitated "reverse" cooling flow, which meant the coolant flowed from the cooling pump to the radiator and then to the cylinder banks and back to the pump, rather than the conventional pump to bank to radiator flow.

The Mosquito was played a significant role in the European theatre. One of the key missions was pathfinder -- particularly at night it was difficult for the heavy bombers to find targets, so Mosquito's would drop colored incendiaries while flying fast low-altitude passes which would illuminate the target for the 4-engined heavy bombers which were flying at considerably higher altitudes. This tactic dramatically improved bombing accuracy.

The Mosquito was also used for precision bombing, and for special missions like delivering the 4,000 lbs "cookie" bomb.

Curtis P-40F

The Curtis P-40 was introduced in the late 1930's , becoming the first aircraft to see large scale production powered by an Allison V-1710 engine. Even as it entered service, however, it was being eclipsed by contemporary German and British aircraft. Its 350 mph speed at 12,000 ft put the P-40 at a serious disadvantage, thus the P-40 was primarily exported. In order to achieve more performance, P-40 designer Don Berlin asked to have the P-40 fitted with two-stage Merlin engines. The two-stage engines were in demand for more critical applications, but the Packard-built V-1650-1 Merlin was fitted to the plane, becoming the P-40F (P-40L's used the same engine).

The French ordered a large number of planes, but before they could be delivered Germany had occupied France. The planes were redirected to the British, who became the first to use the planes in combat. The planes lacked sufficient armor plating and self-sealing fuel tanks since these were not specified by the French. The planes were could not challenge the German aircraft in Europe, but they were found to perform very well in North Africa where they performed well against both the Italian Air Force and the Luftwaffe.

The P-40 is perhaps remembered best as the plane of General Claire Chennault's famous "Flying Tigers" American Volunteer Group (AVG). The small AVG group racked up an incredible kill ratio against the Japanese in China. The AVG was only in existence for eight months, yet in that time they scored an incredible 286 kills with no more than 80 AVG pilots at any given time. In July 1942 the AVG became the twenty-third fighter group of the USAAF.

During the attack on Pearl Harbor, a few P-40C's were able to get airborne and kill some of the attacking Japanese planes. While obsolete by 1941/1942 standards, this was the best fighter the Air Corps had in service at the time. Production ended in November 1944.

Fairey Barracuda

The Barracuda was originally intended to use the Rolls-Royce Exe engine, but when development problems with the Exe developed, this single-engine torpedo bomber was fitted with the Merlin instead. The change required a major redesign to accommodate the Merlin, the prototype monoplane first flying in early 1941 with a single-stage, two-speed Merlin 30 of 1300 hp. The production aircraft used a more powerful Merlin 32 which delivered 1645 hp.

The Barracuda was the first Fleet Air Arm monoplane torpedo bomber to have retractable landing gear and was significantly more advanced than its contemporaries. The plane featured an all-metal monocoque fuselage with provisions for a two-man crew. One of the novel features was a Youngman flap -- a flap behind and below the wing which essentially had a slot between the wing and the flap. The angle of the flap could be changed for take-off, landing, cruising, and dive-bombing.

The plane was effective in the Pacific against the Japanese and also played a significant role in sinking the German pocket battleship Tirpitz on the 3rd of April 1944 when the ship was hiding in a Norwegian fjord.

Armstrong Whitworth Whitley

The Whitley was a twin-engine Medium bomber which dated back to 1934 when its Air Ministry specification was issued. It was obsolete at the start of the war, but managed to perform satisfactorily throughout the war. The prototype first flew with 14-cylinder air-cooled Armstrong Siddeley Tiger engines, but Whitley Vs made up most of the production run and they were equipped with Merlin X's.

The Whitley was used extensively during the build-up to hostilities by flying leaflet raids and other propaganda missions. Once war broke out, the Whitley was quickly withdrawn from front-line duties due to the superiority of Axis fighter aircraft, leaving the Whitley to fill secondary roles such as glider towing, convoy protection, training, general reconnaissance, anti-submarine/convoy protection duty, Coastal Command, and others.

Boulton Paul Defiant

The Defiant was a single engine fighter conceived in the 1930's as the "wave of the future". The innovation this fighter would sport was a powered gun turret behind the pilot. The single engine monoplane had a strong first campaign protecting the British Army during its withdrawal from France. Thirty-seven German aircraft were shot down in one day with no Defiant losses. The initial success did not last long, however. The Germans realized the Defiant was vulnerable to attack from the front, since the pilot had no forward facing guns for defense. The Defiants were soon suffering unacceptable losses, so the plane was withdrawn from daytime front-line service. The defiant was next used as a night-fighter, and was eventually relegated to target towing.

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