When World War II began, North American Aviation was a relatively small and new company, but that would change with the development of the P-51.
by Randy Wilson
Copyright © 2002 by the Commemorative Air Force and Randy Wilson. All rights reserved.
When World War II began, North American Aviation (NAA) was a relatively new and small aircraft company, having opened its factory at Mines Field in Los Angeles, Calif. in 1936. The company’s most-successful design was a single-engine military trainer, which became the standard Allied advanced trainer, the AT-6/SNJ Texan or Harvard in British service. North American was also building the O-47 observation plane for the U. S. Army Air Corps and was developing a medium bomber, which would become the B-25 Mitchell.
In early 1940, both Britain and France needed additional modern combat aircraft and wanted to purchase American-built Curtiss P-40 fighters. However, all Curtiss fighters were needed by the Air Corps, and the Anglo-French Purchasing Commission decided to find another American company to build P-40s for them. Commission members approached NAA president James H. “Dutch” Kindelberger about building the P-40 but he declined to build another company’s fighter, especially one that was already becoming dated. Kindelberger wanted North American to design a new fighter, and in April, the commission agreed. The contract was for 400 aircraft, with initial delivery by January 1941. When France fell in May 1940, Britain’s need for the new fighter became even more urgent.
North American gave the new fighter the designation NA-73, and work began on its design with a team lead by Chief Engineer Raymond Rice and Chief Designer Edgar Schmued. To get both high speed and long range required not only adequate power but also the elimination of as much drag as possible. The liquid-cooled Allison V-12 engine used in the P-38, P-39 and P-40 fighters was chosen due to its better streamlining than a radial engine and its 1,000-plus horsepower.
Because the new fighter was being developed for the British, the NAA design team was not restricted by a rigid set of specifications, as were many designs being developed for our own Air Corps. Thus, a new type of wing was tried. The laminar flow wing was developed by the National Advisory Committee for Aeronautics (NACA) and had its greatest thickness much further back from the leading edge than previous designs. The result was a smoother, less turbulent airflow over the wing, resulting in greater lift, reduced drag and higher speed.
To go with the laminar flow wing, the designers next turned to streamlining the fuselage. One of the greatest sources of drag on a plane with a liquid-cooled engine is the coolant radiator. In the 1930s water had been replaced in aircraft engines by chemical coolants, such as ethylene glycol, better know by the brand name Prestone. However, the design of an efficient but low-drag radiator in a high-speed fighter was still a challenge. The NA-73’s radiator was placed under the fuselage, aft of the cockpit, based partly on data that NAA had purchased from Curtiss on wind tunnel tests of the XP-46’s radiator. While the XP-46 turned out to be a failure, the NA-73’s radiator placement was a success.
Manufacture of the Texan trainer had taught NAA the value of designing a plane for efficient mass production. Most contemporary fighters, including the P-40 and British Spitfire, had a complex design requiring a great deal of hand assembly and fitting but the NA-73 was designed from the start for mass production.
The first NA-73X airframe was rolled out of the shop in early September 1940,
minus its Allison engine and on wheels borrowed from a Texan trainer, only 102
days after construction had begun. Delivery and installation of the engine were
completed in the next few weeks, and the NA-73X made its maiden flight on
October 26, 1940, piloted by Vance Breese, a free-lance civilian fighter test
pilot. Breese made four successful flights in the NA-73X before turning further
testing over to NAA test pilot Paul Balfour.
Part of the agreement between the Air Corps and the British, was that two early production Mustangs would be provided to the Air Corps for testing at no cost. These two aircraft were designated as XP-51s and were delivered in February and March 1941 but for various reasons did not arrive at the Air Corps test facility at Wright Field until August and December. Thus, the first Mustangs were not fully evaluated by the U. S. Army until after America’s entry in the war on December 7, 1941.
Despite not having yet evaluated the new plane, the Army placed an order for 150 Mustangs IAs armed with four 20 mm canons on June 7, 1941, but these planes were intended for British use, under the new Lend-Lease Act. Fifty-five of these were retained by the Air Corps and designated P-51s but due to a lack of funds, no further orders for fighters could be placed by the Air Corps that year. One solution to keeping the new fighter in production was to modify it into a dive bomber, thus with the addition of bomb racks and dive brakes the A-36 was born, and the Air Corps ordered 500 in 1942.
When the first Mustang Mk I arrived in Britain in October 1941, it was found to be faster than the Spitfire Mk V at altitudes up to 15,000 feet but that at higher altitudes, the Allison V-1710 engine’s mechanical supercharger could not match the power of the Spitfire’s Merlin engine. This was due in part to the Allison V-1710 having been designed from the start to have a turbo-supercharger in addition to the mechanical supercharger. However, only a few engines were so fitted in prototype and test aircraft when the decision was made by the Air Corps to limit the use of turbosuperchargers to heavy bombers, with the exception of the Lockheed P-38 fighter. Despite the lack of power at altitude, the Royal Air Force (RAF) pilots liked the handling qualities of the Mustang and used them effectively at lower altitudes for reconnaissance and ground attacks against German targets in occupied France.
The last Allison-engined Mustang was the P-51A, which entered service in mid-1943. Fitted with a more powerful V-1710-81 engine of 1,330 horsepower, and armed with four caliber .50 machine guns in the wings, the P-51A could also carry two 150-gallon under wing drop tanks. Fifty P-51As were given to the British and designated Mustang Mk IIs. Despite its greater power and range, the P-51A’s Allison engine still limited its high-altitude performance – but that was about to change.
In Great Britain, Rolls-Royce had been designing and manufacturing aircraft engines since before World War One, and in the 1920s developed a liquid-cooled V-12 called the Kestrel, named after a bird of prey, as were all Rolls-Royce Vee engines. The company learned a great deal about building efficient mechanical superchargers developing the engine for the Supermarine S6 racer, which won the final Schneider Trophy Race in 1931. By that date, the Kestrel was near the end of its potential and Rolls-Royce began the development of the PV-12 or Private Venture twelve-cylinder engine which would become the Merlin. The name came from a small falcon, the male of which sports a dark blue pattern on its back, reminiscent of the cape worn by the sorcerer of King Arthur’s court.
In April 1942, Rolls Royce test pilot Ronald W. Harker lobbied Rolls-Royce to mate a Merlin engine to the Mustang airframe, and despite some opposition about working with an American airframe, the company finally agreed. The American air attaché in London, Lt. Col. Thomas Hitchcock, reported very positively on the use of the Merlin engine, and North American began work on a Mustang powered by a Packard-built Merlin engine, first calling it the XP-78 but soon changing the designation to the XP-51B. The British won the race to fly the first Merlin-powered Mustang on Oct. 14, 1942 with the XP-51B taking to the air a few weeks later.
The improved performance of the XP-51B surprised many, having a top speed of 441 mph, more than 50 mph faster than the Allison-powered models. Production of the P-51B began at the Inglewood plant but was slow at first due to a delay in Packard’s production of license built Merlin engines, designated the V-1650 in the American system.
With both the Army Air Corps and RAF ordering Merlin-powered Mustangs in early 1943, North American opened a second assembly line at its factory in Dallas, Texas. The first P-51B to roll off the Inglewood line flew on May 5, 1943, while the first Dallas-built P-51C took to the air on Aug. 5. Despite the different designations, the P-51B and C were basically identical, with the initial production models having a Packard V-1650-3 engine of 1,680 horsepower and four caliber 0.50 machine guns in the wings.
The following excerpt is from a report of the Army Air Forces Board Project No. (M-1) 50 and is a summary of how the Merlin-powered Mustang and other American fighters compared in 1943:
The P-51B, from sea level to 11,000 ft, is some seven to ten miles per hour slower than the P-51A which is the fastest fighter at this altitude. Between 14,000 and 22,000 ft. the P-51B is about fifteen to twenty mph faster. From 22,000 ft the P-51B, in high blower, widens this speed advantage up to seventy-five mph at 30,000 ft. From sea level, the P-51B gradually gains on the P-38J and the P-47D until, at 16,000 ft. it has a speed of about 420 mph which is about ten mph faster than the P-38J and about twenty mph faster than the P-47D. Above 27,000 ft, the P-51B can no longer get war emergency power, but its speed of about 430 mph at 30,000 ft is equal to that of the P-47D and about twenty mph faster than the P-38J, both using war emergency power. The P-51B is capable of 400 mph at 40,000 ft.
The P-51B is by far the best climbing aircraft of all current American fighters. It takes about 4.5 minutes to get to 15,000 ft as against five minutes for the P-38J and about seven minutes for the P-47D. The P-51B maintains a lead of about .5 minute over the P-38J to 30,000 ft and reaches that altitude in about eleven minutes which is about 6.5 minutes faster than the P-47D.
In zooming the P-51B with the P-47D from level flight at cruising and high speeds, and from high speeds out of dives, the P-51B gains speed rapidly and leaves the P-47D far behind. In zooming the P-51B with the P-38J, from level flight at cruising speed, the fighters climb evenly at the start. However, the P-51B falls off while the P-38J keeps climbing. In zooms from high speeds (425 indicated air speed), the P-51B pulls away from the P-38J and its zoom ends considerably higher.
The diving characteristics of the P-51B are superior to those of any other fighter plane. It is exceptionally easy to handle and requires very little trimming. The P-51B dives away from all other fighters except the P-47D, against which the P-51B loses several hundred feet ahead in the initial pushover and then holds that position, apparently neither gaining nor losing distance.
The new seal-balanced ailerons of the P-51B give the fighter a faster rate of roll at all speeds than any other fighter except the P-47D with which it is equal at cruising speeds.
The search view of the P-51B is better than in the P-51A but is still obstructed above, to both sides, and to the rear, by the canopy construction. The view forward over the nose is considerably improved over the P-51A by the relocation of the carburetor air intake scoop, the elimination of the clear view panel on the left side of the windshield, and lowering of the nose of the engine one and one-half degrees.
The fighting qualities of the P-51B were compared with those of the P-47D-10 and the P-38J-5 and, briefly, with the P-39N-0 and the P-40N. The only maneuver the P-39 and P-40 have that is superior to the P-51B, is a slight advantage in a turning circle. In all other maneuvers, as well as performance, they are both far inferior. The P-51B has good performance at all altitudes, but above 20,000 ft the performance improves rapidly, and its best fighting altitude is between 25 and 35,000 ft. The rate of climb is outstanding, with an average of about 3,000 ft per minute from sea level to 25,000 ft. Above 20,000 ft. the overall fighting qualities of this aircraft are superior to those of all the other types used in the trials.
Later production P-51B/Cs were powered by the more powerful V-1650-7 engine, had
their internal fuel capacity increased from 184 to 269 gallons, and some mounted
six guns in the wings. Provisions were also made to carry two 110-gallon
under wing fuel tanks that could be dropped when empty.
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