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The North American P-51 Mustang

In late 1939, with the likelihood of full scale war in Europe a major concern, the British Royal Air Force was looking seriously at methods of quickly increasing its fighter strength. In April 1940, the British Air Purchasing Commission approached North American Aviation with the intent of having them build P-40's for the R.A.F. Instead, North American offered to build an entirely new fighter using the same Allison V-1710-39 engine as the P-40. The British agreed only on the stipulation that a prototype be on hand within 120 days. North American designers Raymond Rice and Edgar Schmued immediately set about meeting the requirements. Schmued had been a part of Willy Messerschmitt's design group in Germany; no doubt the somewhat angular lines of the new fighter came from this relationship.

The Allison-powered prototype NA-73 was assembled within the specified period, but the engine was not yet ready, causing a delay of some six weeks before the NA-73 could fly. In the meantime, on May 4, 1940, the U. S. Army released the design for export sales with the condition that two of the planes be delivered to them for evaluation. At this time the NA-73 was assigned the XP-51 designation. The first and tenth airframes were sent to the Army for testing; these were given the serial numbers 41-38 and -39. An order for 150 P-51's followed. These planes were named "Apache" for a short time, but later the name "Mustang" was adopted for the P-51.

The P-51 was an immediate success. It outperformed even the Spitfire, but the Allison engine placed limitations on the performance. In England, a mock-up was devised to use the Rolls Royce Merlin in the P-51 airframe. One concept was to locate the new engine behind the cockpit, but this idea was rejected and the Merlin was mounted in the conventional position in the nose. Four airframes were adapted in England to take the Merlin engine. These planes had deep intakes below the engine for carburetor air. In the meantime, North American had undertaken a similar conversion project and was building two Packard Merlin-powered Mustangs. The results of the British tests were passed on to North American; and even before the Army's Merlin -powered Mustangs had flown, the U. S. Army ordered 2,200 of the more powerful fighters. For a short time, this model was designated P-78, then reclassed as P-51B.

To say the Merlin Mustangs were successful would be an understatement. The P-51 became one of the aviation world's elite. The total number of 14,819 Mustangs of all types were built for the Army. American Mustangs destroyed 4,950 enemy aircraft in Europe to make them the highest scoring U. S. fighter in the theater. They were used as dive-bombers, bomber escorts, ground-attackers, interceptors, for photo-recon missions, trainers, transports (with a jump-seat), and after the war, high performance racers.

The Merlin -powered P-5lB and its Dallas-built twin, the P-51C, began operations in December 1943. A further improvement to the Mustang was introduced when a graceful teardrop canopy was installed to eliminate the dangerous blind area created by the faired cockpit. First tested on two P-51B's, they became standard on the P-51D and all later models. The P-51D became the version produced in the greatest quantities, 7,954 being completed. The "D" model carried six .50 cal. machine guns instead of the four mounted in the "B's"; and other refinements, such as moving the wing forward slightly and providing for rocket launchers, were included. The first "D" types were delivered without dorsal fins but this feature was added to compensate for keel-loss when the bubble canopy was adopted.

Later developments to the P-51 series included the final production type, the P-51H with several changes which made it the fastest production variant with a maximum speed of 487 mph at 25,000 feet. Five hundred fifty-five P-5lH's were delivered before VJ Day led to cancellation of the P-51 production program.

The P- 51 D represents the typical Mustang configuration. It had a 37-foot wingspan with an area of 233 square feet and was 32 feet 3 inches long. Height was 13 feet 8 inches. The Packard-built Merlin V-1650-7 was capable of delivering 1,695 hp which provided a speed of 437 mph at 25,000 feet. Weights were 7,125 lbs. empty and 10,100 lbs. normal gross, but an additional 2,000 lbs. could be carried. Internal fuel capacity was 105 gallons, giving a range of 950 miles at 362 miles per hour at 25,000 feet. Armament was six .50 cal. wing-mounted machine guns with 1,880 total rounds.

The P-51 was one of the first fighters to use a laminar-flow airfoil, a high-speed shape which became standard on most later high performance fighters.

The P-51 was designed as the NA-73 in 1940 at Britain's request. The design showed promise and AAF purchases of Allison-powered Mustangs began in 1941 primarily for photo recon and ground support use due to its limited high-altitude performance. But in 1942, tests of P-51s using the British Rolls-Royce "Merlin" engine revealed much improved speed and service ceiling, and in Dec. 1943, Merlin-powered P-51Bs first entered combat over Europe. Providing high-altitude escort to B-17s and B-24s, they scored heavily over German interceptors and by war's end, P-51s had destroyed 4,950 enemy aircraft in the air, more than any other fighter in Europe.

Mustangs served in nearly every combat zone, including the Pacific where they escorted B-29s to Japan from Iwo Jima. Between 1941-5, the AAF ordered 14,855 Mustangs (including A-36A dive bomber and F-6 photo recon versions), of which 7,956 were P-51Ds. During the Korean War, P-51Ds were used primarily for close support of ground forces until withdrawn from combat in 1953.

The ultimate version of the Mustang was the P-51H, which was the fastest Mustang variant to see service and one of the fastest (if not the fastest) piston-engined fighters to enter production during the Second World War. However, it was destined never to see any combat, having entered service too late to participate in the final action against Japan.

The P-51H was an outgrowth of the experimental XP-51F and G lightweight Mustang projects of early 1944. Rather than commit the F or G versions to production, the USAAF decided instead to produce a version powered by the uprated Packard Merlin V-1659-9 engine. This engine had the Simmons automatic boost control for constant manifold pressure maintenance and was equipped with a water injection system which made it possible to overboost the engine to achieve war emergency powers in excess of 2000 hp for brief periods. North American Aviation gave the project the company designation NA 126, and it was ordered into production as the P-51H in June of 1944 even before much of the initial design work was done.

The weight-savings program which produced the XP-51F and XP-51G was put to good use in the design of the P-51H. The fin and rudder were significantly increased in height and the rear fuselage was lengthened to produce an overall length of 33 feet 4 inches (nearly two feet longer than the P-51D). Other features were taken directly from the XP-51F project--it had the same shallower carburetor air intake underneath the nose and modified cowling with integral engine mounting, the same simplified undercarriage with smaller wheels and disc brakes, and it had the same broad-chord wing (without the leading edge "kink"). However, the cockpit canopy was much smaller than that of the XP-51F, being more nearly equal in size to that of the P-51D. The profile of the canopy was somewhat different from that of the P-51D, with the top of the hump being much closer to the front just above the pilot's head. The rqdiator installation was increased in depth and the matrix was increased in size. The front edge of the inlet duct was vertical as it was in the lightweight versions, and the bottom line downstream was almost straight rather than bulged. The fuselage was modified in order to raise the cockpit to give an 8-degree gunsight deflection angle looking down along the top line from gunsight to spinner. Armament returned to six machine guns with 1880 total rounds, although alternative installations of four guns with 1600 total rounds could be fitted. Provisions were made for normal loads of external stores, similar to that which could be carried by the P-51D/K. Access for gun servicing was improved by redesign of the wing doors and ammunition feed system, and by making the ammunition boxes removable. The fuselage fuel tank was restored, but its capacity was fixed at 50 US gallons, giving a total internal fuel capacity of 255 US gallons.

The first P-51H-1-NA was flown by Bob Chilton on February 3, 1945. There were 20 P-51H-1-NAs built, all with the XP-51F tail. The distinctive taller tail was installed on the P-51H-5-NA and later production block aircraft and was later retrofitted to earlier P-51H-1-NAs. This new tail once and for all eliminated the yaw instability problem which had been characteristic of all earlier Merlin-powered Mustangs.

Along with the Republic P-47N Thunderbolt, the P-51H was intended to be the leading USAAF fighter used during the upcoming invasion of Japan. 2000 P-51Hs were ordered, made up of 555 NA-126s and 1445 NA-129s with minor differences. All of these planes were to be built at the Inglewood factory. 1629 more examples were ordered from NAAs Dallas plant under the charging number of NA-124, these being designated P-51M by the USAAF. The P-51M differed primarily in having the V-1650-9A engine, which had a lower war emergency rating by virtue of having the water injection deleted.

One P-51H was given to the RAF for evaluation at Boscombe Down. Its serial was KN987.

A P-51H (44-64420) was borrowed by the US Navy In August of 1945 for trials to determine the type's suitability as a carrier-based fighter. The earlier P-51D had been deemed to be unsuitable because of the lack of adequate rudder control at low speeds, especially at high angles of attack. The tests proved that the P-51H did indeed provide adeuqte rudder control, but since the war was already over, the possibility of a carrier-based P-51H was not considered any further. A second P-51H 44-64192 was acquired by the Navy in 1948 for tests of various aerofoil shapes at transonic speeds at the Grumman Aircraft Corporation. While in Navy service, the plane became BuNo 09064. After the tests were over in 1952, the plane was transferred to the Air National Guard.

The P-51H was too late to see action in the war in Europe. By the late summer of 1945, some P-51Hs had been issued to a few operational units. These units were in the process of working up to operational status when the war in the Pacific ended with the Japanese surrender. None had the opportunity to see any combat. At the time of V-J Day, 555 P-51Hs had rolled off the Inglewood production lines. With the coming of peace, orders for 1445 more P-51Hs were cancelled, along with the entirety of the order for the Dallas-built P-51Ms after only one example (45-11743) had been completed.

Also cancelled was an order for 1700 P-51Ls (company designation NA-129). They were to have been similar to the P-51H but were to be equipped with the more powerful V-1650-11 engine with a Stromberg speed/density injection-type carburetor, rated at a peak power of 2270 hp with water injection. None were built.

The last P-51H rolled off the production line in 1946.

Pilots generally found the P-51H to be even more delightful to fly than the D model. However, some pilots were distrustful of the H's lighter structure, preferring the greater sturdiness of the D. Consequently, it was not considered as being suitable for combat operations in Korea.

Parker Information Resources
Houston, Texas
E-mail: bparker@parkerinfo.com

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