North American F-82 Twin Mustang Part I

At the beginning of the Second World War American radar technology and night fighter development was considerably inferior to that of the British. By 1943, however, American technology began to surpass British radar systems with the introduction of the SCR-720. Throughout the Second World War aircraft companies in the United States produced 900 night fighters for the U.S. Army Air Force (USAAF) while the U.S. Navy and Marine Corps operated a considerably larger number because existing front-line fighters were better suited for adaptation to the night fighter configuration. Late in November 1945, the USAAF approved military characteristics for a jet-propelled aircraft as a post-war successor to the Northrop P-61. At first the all-weather interceptor was conceived as an aircraft that would be effective in daylight as well as at night or during inclement weather. However, by 1946, Major General Curtis LeMay, Deputy Chief of Staff for Research and Development, indicated that this concept be revised due to the added weight of the radar gear which would limit aircraft performance. Because the heavy radar-equipped all-weather fighter would be no match for a small day fighter, ‘all-weather’ was to mean primarily night and inclement weather. Military performance characteristics were then revised to conform to this decision and designs for two experimental all-weather aircraft, the Curtiss XP-87 Blackhawk and Northrop XP-89 Scorpion, were selected for investigation.

Post-war apathy by the American public and government in the wake of the Second World War cut deeply into military appropriations and stifled night/all-weather fighter development, and the newly established U.S. Air Force (USAF) and the Navy continued to rely on piston-driven night fighter aircraft. Changing international situations in Europe and Asia resuscitated interest in night and all-weather interceptors. Soviet aggression with the establishment of communist-bloc countries of Eastern Europe, the Berlin Crisis of 1948, and the Soviet’s testing of a crude nuclear bomb in 1949, forced the U.S. military on a quest to develop superior strategic bombers and jet interceptors, which in turn, resulted in the development of air intercept radar and all-weather aircraft. Early attempts to develop jet-powered all-weather fighters ran into a series of snags and delays. The Air Force ordered the Curtiss XP-87 in December 1945, but it ran into developmental difficulties and the Air Force abandoned the project, which incidentally ended Curtiss’ thirty-year history as an aircraft manufacturer. The P-89 Scorpion seemed to hold greater promise, but it too ran into teething troubles and its acceptance and delivery did not occur until 1952.

Development of jet-powered all-weather aircraft would be slow and, until such time when this type became available, the U.S. Air Force needed an interim aircraft. In the immediate post-war period, the P-61 had formed the bulk of the night fighter force. Due to the lack of any suitable jet-powered replacement, the P-61 soldiered on for a few more years. However, manoeuvres held in the North-western United States early in 1948 quickly confirmed the Black Widow’s limitations and the Air Force deemed the aircraft as having no tactical value in defensive operations. In order to help fill the gap until the F-89 Scorpion became operational, North American Aviation (NAA) developed a night and all-weather adaptation of the piston-engine North American F-82 Twin Mustang.

Since the P-82, like the P-61, would be of little value in daylight operations, the Air Force assigned such jet interceptors as the Lockheed P-80 Shooting Star (later F-80) and Republic P-84 Thunderjet (F-84) to fighter-interceptor squadrons, which began replacing the single-engined, propeller-driven North American P-51 (F-51) Mustang in the Air Force inventory. These jet aircraft possessed the required speeds to combat enemy bombers of the Boeing B-50 type but both fighters lacked the electronic equipment to allow them to operate at any other time than during daylight (again, the weight of 1950s-era airborne radar technology precluded its use in a small high-speed interceptor). In turn, beginning late in 1949, the North American F-86A Sabre replaced the F-80 and F-84. By the end of 1950, of the 365 aircraft assigned to the U.S. Air Defense Command (ADC), 236 were F-86A and E models. Therefore, without a real all-weather interceptor, the Air Force had no other alternative but to place its reliance on a dual fighter force, with jet aircraft for daytime operations and radar-equipped F-82s for night and inclement weather interceptor missions.

Development

Possibly one of the oddest American aircraft to go into full production after the end of the Second World War, the P- (later) F-82 Twin Mustang series was the last mass production propeller-driven fighter acquired by the U.S. Air Force. Originally designed as a long-range bomber escort during the Second World War, it evolved into a night and all-weather fighter during the post-war years. The original concept of the XP-82 traces back to the development of North American Aviation’s XP-51 Mustang in 1940 and the production of subsequent variants of that aircraft as a long-range fighter to protect Allied bombers against the German Luftwaffe and, subsequently, to provide cover for Boeing B-29 bombers in the Pacific. However, the Air Force lacked an interceptor with an extreme-range capability. The USAAF fighters with the longest range during that conflict were the P-51D Mustang, with a maximum range of 1,600 miles, and the Lockheed P-38J Lightning with a maximum range of 2,200 miles. Both aircraft were capable of escorting B-17 and B-24 bombers from bases in England to the heart of Germany; but it was another story in the Pacific.

The vast distances of the Pacific, where missions could last eight to twelve hours, required a fighter with a far better range than any existing aircraft in the Air Force’s inventory at the time, but such a design would need a two-man crew consisting of a pilot and co-pilot/navigator to reduce workload stress and fatigue. The concept for such a radical aircraft design, closely resembling two P-51 Mustang fighters joined together with a common centre wing and horizontal stabilizer, belonged to Edgar Schmued, North American Design Chief, who proposed the idea in November 1943 at the company’s factory in Inglewood, California. Two months later on 7 January 1944, when General of the Army Air Force Hap Arnold was taking a tour of the factory, Ed Schmued showed him the revolutionary design; the General quickly endorsed the project. After viewing a mock-up and wind tunnel model of the design, Air Force brass issued development contract AC-2029 on 8 February 1944 ordering the construction of three experimental prototypes (serial numbers 44-83886 through 44-83888), with the NAA company designation NA-120.

XP-82

North American engineers developed the two airframes and wings for the NA-120 from the basic fuselage and wings of the experimental XP-51F, which was used to develop the P-51H production model. Although the P-51 and XP-82 were quite similar in appearance, very few components were interchangeable. External design changes included lengthening the aft fuselage sections by fifty-seven inches, internal strengthening of the centre and outer wing sections to allow for fuel and armament on external pylons, increasing the length of the ailerons, inclusion of slotted flaps along the trailing edge of the centre section, and enlarging the dual vertical stabilizers to improve stability. Hydraulically operated wing flaps extended from the aileron to the fuselage on each wing panel and between the fuselages on the centre section interconnected and controlled by a lever located in the pilot’s cockpit only. Interestingly the XP-82’s entire wing area of 408 square feet was less than two Mustangs (470 square feet) but the overall length was six feet longer than the P-51H; a nine-foot difference with the F-82G model. The XP-82 prototypes and the subsequent P-82B variant, used counter-rotating Aeroproducts propellers powered by two liquid-cooled, supercharged, two-stage, two-speed, Packard Merlin V-1650 engines with, on the left (portside), a V-1650-23 and right (starboard), a V-1650-25 to compensate for torque. The aircraft’s fuel system consisted of four internal self-sealing wing tanks; one consisting of two interconnected cells in each outer wing panel and two in the wing centre section with a total fuel capacity of 575 gallons. This arrangement allowed each engine to operate with a separate and complete fuel system with the two cells interconnected by a cross-feed line. Range and flight time could be increased by the addition of either two 110, 165, or 310-gallon drop tanks installed under each outer wing panel.

The first prototype, serial number 44-83886, rolled out of the Inglewood factory on 25 May 1945 and flew on 16 June 1945, with NAA test pilot Joe Barton and Edward M. Virgin at the controls. This was the second attempt; the month before it refused to get off the ground due to excessive drag caused by the props’ air flowing upwards to the centre wing section, an effect created by the rotation of the propellers – the left, counter-clockwise, and the right clockwise. Engineers solved the problem by switching the engines to the opposite sides resulting in successful flight testing. The second prototype, serial number 44-83887, made its initial flight of approximately one hour on 30 August 1945. The first and second prototypes showed the aircraft to have exceptional high speed, manoeuvrability, and rate of climb. The USAAF accepted the first prototype in August 1945 followed by the second the following month. The XP-82 was 39 feet in length with a wing span of nearly 52 feet and its two Merlin engines gave it a top speed of 468 mph, almost identical to the P-51H, and a maximum range of nearly 3,500 miles with internal and external fuel stores (over twice the range of the P-51).

The Twin Mustang’s command pilot’s port cockpit contained a full set of flight instruments and controls while the co-pilot/navigator position, although equipped with basic instrumentation to fly the aircraft, lacked other features. Both cockpits provided essential power plant and flight controls with the capability to transfer control of guns, fuel, engine heat, anti-icing gear, and the command radio by shift switches in each cockpit. However, the co-pilot had no control over landing gear and flaps (unless in an emergency), or engine ignition, surface control boost, bombs (except salvo), and rockets. In a situation where the command pilot might become incapacitated, NAA equipped both cockpits with an emergency landing gear release handle located at the right side of both cockpits. Pulling the handle unlocked the gear and fairing doors, and actuated a hydraulic pump valve, which allowed trapped hydraulic fluid in the landing gear and wheel door cylinders to return to the system reservoir. This allowed the doors to open and the gear to extend by gravity without hydraulic pressure. North American also included an option for turning the Twin Mustang into a single-seat fighter by removing cockpit equipment from the copilot’s position, and removing and fairing over the canopy.

The proposed armament package for the XP-82 consisted of a bank of six M-3 .50-caliber machine guns, with 400 rounds-per-gun, housed in the centre wing section. The prototype and subsequent production models allowed for the installation of underwing and centreline racks to carry up to twenty-five 5-inch High Velocity Aircraft Rockets (HVAR), 4,000lbs of bombs – two on each outer wing, two on the centre wing section, or a chemical tank (AN-M10 or AN-M33) which was used to lay smoke screens or dispense chemical agents and was installed on each outer wing rack. North American also provided for an optional removable pod for the centre section that could house eight additional .50-caliber machine guns, radar, or a photographic reconnaissance package. However, except for the radar pod, the gun pod never materialized for operational aircraft while the photographic reconnaissance package was fitted and tested on only one aircraft. Production models were equipped with the K-18 compensating gun sight installed on the instrument panel shroud of the pilot’s cockpit. The sight automatically computed the correct lead angle of fire and consisted of two optical systems, fixed and gyroscopic, contained within the sight. The reticle of the fixed sight, projected on the reflector glass, consisted of a seventy-mil circle with a small cross in the centre, and a rocket scale located below the cross. The fixed sight’s circle and cross was used only when strafing, firing rockets, or when the gyro sight malfunctioned. The reticle of the gyroscopic sight, also projected on the reflector glass, consisted of a circle with six diamond-shaped images surrounding a central dot. Both sights were used together to automatically compute the lead angle needed to fire on a hostile aircraft.

The Air Force accepted the first prototype on 30 August 1945 but NAA retained it for eighteen months and, after a series of airworthiness and stability flights, it was sent to Wright Field, Ohio, for evaluation by the Air Force’s Air Material Command. Afterward, the National Advisory Committee for Aeronautics (NACA) operated it until 1955 when it was scrapped. The Air Force accepted the second prototype on 11 September 1945 and like the first XP-82, NAA kept it for further flight testing until turning back over to the Air Force in March 1946. NACA received it in October 1947, operating it until February 1950 when it sustained major damage after sliding off a runway.

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