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.
Specs of the P-51H-5-NA:
One Packard Merlin V-1650-9 twelve-cylinder Vee liquid cooled engine rated at 1380 hp for takeoff and a a war emergency power of 2218 hp at 10,200 feet and 1900 hp at 20,000 feet with water injection. Performance: Maximum speed was 444 mph at 5000 feet, 463 mph at 15,000 feet, and 487 mph at 25,000 feet. Range in clean condition was 755 miles at 359 mph at 10,000 feet, 1975 miles at 239 mph at 10,000 feet. Range with two 62.5 Imp. gall. drop tanks was 1150 miles at 339 mph at 10,000 feet and 1530 miles at 243 mph at 10,000 feet. An altitude of 5000 feet could be reached in 1.5 minutes, 15,000 feet in 5 minutes. Service ceiling was 41,600 feet. Weights: 6585 pounds empty, 9500 pounds normal loaded, and 11,500 pounds maximum. Dimensions: Wing span was 37 feet 0 inches, length was 33 feet 4 inches, height was 8 feet 10 inches, and wing area was 235 square feet.
Serials of the P-51H and P-51M:
44-64160/64179 North American P-51H-1-NA Mustang 44-64180/64459 North American P-51H-5-NA Mustang 44-64460/64714 North American P-51H-10-NA Mustang 45-11743 North American P-51M Mustang