North American P-51D Mustang

The P-51 Mustang was perhaps the most famous fighter of World War II, and, many would say, the best all-round piston-engined fighter produced by any of the combatants during that conflict. Total production of all Mustangs amounted to 15,575 in the USA and 100 in Australia, ranking only behind the P-47 Thunderbolt in being the fighter manufactured in greatest numbers for the USAAF. Mustangs accounted for 4950 of the 10,720 air combat victories claimed by the USAAF in Europe, and 4131 of the 8160 ground strafing claims made in the same theatre, accounting for 48.9 percent of total losses inflicted on the enemy. They shot down more than 230 V-1 "buzz bombs", and they even managed to score some kills against Luftwaffe jet fighters.

Following the outbreak of war in Europe, the British Purchasing Commission, headed by Sir Henry Self, was posted to New York to determine if American combat aircraft could be of any use to the Royal Air Force. The Bell P-39 Airacobra and the Curtiss P-40 Warhawk were ordered in substantial numbers, even though they were not up to the performance standards of the latest British and German fighters. One of the corporations that Self had contacted had been the North American Aviation corporation. North American had already been building NA-16 trainers, and the British ordered a number of them for the RAF as the *Harvard*. In April of 1940, Kindelberger of North American Avation, was summoned by the British Air Purchasing Commision and asked to manufacture the Curtiss Hawk 87 (P-40D) under license for the RAF. Kindelberger responded that NAA could do that if it were really required, but countered that he and his company could build a better fighter than the P-40 and that they could design a REAL fighter in the same time that it would take to put the P-40 into production. The British commission felt that they could take Kindelberger at his word, and on April 10, 1940 they accepted his proposal on the condition that the first prototype be ready in 120 days. The design was assigned the company project name of Model NA-73.

At that time, the USAAC reserved for itself the right to block any foreign aircraft sales that it regarded as not in the Army's interest, for whatever reason. On May 4, 1940, the US Army reluctantly agreed not to block the British sale, but they added a condition. Two examples of the initial NA-73 lot for Britain were to be transferred to the USAAC for testing free of charge.

The NA-73X prototype contract was signed on May 23, 1940. The British insisted that a heavy eight-gun armament be fitted. NAA had actually been quietly working on such a fighter project since the summer of 1939, and by that date they had actually already completed much of the detail design. On May 29, a provisional RAF procurement was issued for 320 aircraft, contingent on satisfactory testing of the prototype. NAA agreed to start deliveries in January 1941. RAF serial numbers were assigned and the aircraft was given the name Mustang I in RAF service.

A sort of urban legend has grown up about Edgar Schmued, the designer of the NA-73X, which claims that he had once worked for Willy Messerschmitt and that the Mustang was heavily influenced by the Bf 109. Another urban legend surrounding the Mustang is that it owed a great deal to the Curtiss XP-46 and, in fact, stole numerous design features from that fighter. It is true that the British had insisted that since NAA had no fighter experience they should secure all current data from Curtiss about both the P-40 and the XP-46. Although NAA did pay $56,000 to Curtiss for technical aerodynamic data on the XP-46, there was only a very broad resemblance between the XP-46 and the NA-73X. The Curtiss aircraft shared only a similar radiator/ oil-cooler configuration with the NA-73X, and did not have laminar flow wings. In point of fact, the development of the XP-46 lagged behind that of the NA-73X, and prototypes were not ready for flight until February of 1941. In addition, preliminary design of the NA-73X was completed before NAA gained access to the Curtiss material. It could even be argued that the XP-46 data was most useful to NAA in guiding them in what NOT to do. The NA-73X appears to owe virtually nothing to any previous fighter design. Nevertheless, despite convincing denials from both Edgar Schmued and aerodynamicist Edward Horkey, the full magnitude of the contribution of Curtiss to the NA-73X design remains controversial to this day.

The NA-73 featured an all-metal stressed-skin structure, with the wing having a sheet web main spar and an almost equally strong rear spar to carry the ailerons and the flaps. Special attention was paid to features which would make the aircraft simple and inexpensive to manufacture. The leading and trailing edges were straight lines to the extent possible, and the underlying structure was simple to construct. The two wing spars had to be far enough apart to accommodate the length of a 0.50-in machine gun, with only the barrel protruding ahead of the main spar. Most previous NAA aircraft had left and right wings bolted to a horizontal center section, but the Mustang had the wings meeting on the centerline, with dihedral emanating from that line. A special NACA laminar flow wing profile was adopted for the Mustang, the undersurface being almost a mirror image of the upper. This wing was much more "slippery" than the old profiles, and provided less aerodynamic drag at high speeds than did more conventional aerofoils. However, it also had less lift at low speeds, so the NA-73X had to have large and powerful flaps to keep landing speeds from being impractically high.

The main landing gear members had a track of almost 12 feet, which made landing much easier than in such fighters as the Spitfire and Bf 109. The main wheels retracted inward into wheel wells in the wing's forward leading edges, the leading edges being kinked forward at the fuselage join to provide sufficent room. The retracted wheels were covered by doors hinged near the aircraft's centerline, and were closed again by their own jacks when the landing gear was fully extended. The tailwheel was fully retractable into a compartment with twin doors. The tailwheel was steerable and was linked to the rudder.

The British also specified that a liquid-cooled inline engine be used, and the Allison V-1710 twelve-cylinder Vee was the only American-built engine which fit the bill. The Allison V-1710 was a little bigger than the Merlin, slightly lighter, and similar in power at low altitudes. However, at higher altitudes the Allison suffered from a rapid drop in power in comparison to the Merlin. NAA briefly considered using a turbosupercharger to improve high-altitude performance, but ruled against it on the grounds of a tight schedule. The Allison engine had a downdraft carburetor, so the ram inlet of the NA-73X was located above the cowling. Radiators for cooling the ethlyene glycol and lubricating oil were located in a single heat-exchanger installed underneath the rear fuselage in a streamlined duct. The duct actually had the ability to add some propulsive thrust, by adding heat energy to the incoming air and expelling it out the back at a higher velocity. The drawbacks of such a cooling arrangement were the extra weight and the added combat vulnerability of the long pipes that led to and from the engine. Fuel was housed in two self-sealing tanks housed in the wing spars, one in each inboard wing. Total capacity was 180 US gallons, almost twice the fuel capacity of a Spitfire.

Final assembly and engine installation began on September 9, 1940, 102 days after the initial British order. In a contract approved on September 20, 1940, it was agreed that the fourth and tenth production NA-73s would be the planes diverted to the Army. The designation XP-51 was to be assigned to these two planes. On September 24, 1940, the RAF increased their Mustang I order to 620 planes.

The NA-73X prototype emerged from Inglewood plant in only 102 days, thus meeting the 120-day deadline with time to spare, although the airplane rolled out of the factory without an engine, which had been delayed at the Allison factory. The reason for the delay in engine delivery was because it was "government-supplied equipment" that was furnished on an as-available basis. Since the NA-73X was a private venture it was not allocated a very high priority in comparison with P-40s that were then rolling off the production lines. The engine that was eventually installed was an un-turbosupercharged Allison V-1710-F3R liquid-cooled Vee, rated at 1100 hp.

In December 1940, the RAF ordered 300 more of the Mustang Is which embodied only minor modifications. These were designated NA-83 by the factory and differed from the NA-73s only in having broad fishtail ejector exhausts.The P-51 was an immediate success. It outperformed even the Spitfire, but the Allison engine placed limitations on its 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.

The P-51D with its bubble-top canopy was perhaps the best-known version of the Mustang. One of the problems encountered with the Merlin-powered P-51B/C was the poor view from the cockpit, particular towards the rear. The "Malcolm hood" fitted to the P-51B/C was an early attempts to correct this deficiency. However, a more lasting solution was sought, and it was thought that the Mustang might be amenable to being equipped with a bubble-type all-round vision canopy similar to those fitted by the British to later marks of the Spitfire. North American Aviation secured an agreement with the Army to test a canopy on a Mustang in order to improve the pilot's view from the cockpit. The rear fuselage of the Mustang had to be cut down. However, the amount of retooling needed to accomplish this was not extensive, and very little restressing of the fuselage structure was necessary. Having proven the concept, NAA diverted two P-51Bs from their Inglewood production line and completed them as NA-106s with a cut-down rear fuselage and bubble canopy. These two aircraft were redesignated P-51D.

The machine guns in each wing of the Mustang had been tilted over at quite sharp angles, pinching their feed belts and resulting in frequent gun jams. And on the P-51D the gun installation was completely redesigned and the number of guns was increased from four to six. The 2 inboard .50 caliber Browning MG53-2 machine guns each had 400 rpg, and the other four each had 270 rpg. However, Mustang users had the options of removing two of the guns and having four, with 400 rounds each. Some pilots actually selected this option.

Another visible change introduced in the P-51D was in the increase of the wing root chord. The main landing gear was strengthened in order to accommodate the additional weight, but the wheels maintained the same diameter of 27 inches and the wheel bays and doors were modified and the "kink" in the wing leading edge was made much more pronounced. There were problems with the installation of the 85-gallon tank in the rear fuselage of the P-51B and its adverse effects on the directional stability. Things got still worse for the P-51D, in which the cutting down of the top line of the rear fuselage caused a lot of keel area to be lost. In order to provide for better directional stability, a dorsal fin was added ahead of the rudder during the production run of the P-51D. The extra weight and drag caused by this fin was quite small, but it helped a lot in improving the directional stability, especially when the rear fuselage fuel tank was full. The P-51D introduced the K-14 computing gyro gunsight, based on a British (Ferranti) design. When it first appeared, it was considered almost miraculous. The pilot needed only to dial in the wingspan of the enemy aircraft he was chasing and then feed in the target range by turning a handgrip on the throttle lever. Everthing was then done by an analog computer. All that the pilot had to do then was to get the wingtips of his target lined up on the bright ring projected on the gunsight, and press the trigger. This sight played a major role in the P-51D's impressive score of aerial victories. All P-51Ds had underwing hardpoints not only for bombs and fuel tanks but also for various types of rocket launchers. These included zero-length stubs for six 5-inch rockets or as many as ten if no drop tanks were carried.

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