The basic development started .with the Northrop design of an allweather ground attack fighter incorporating General Electric TG180 axial-flow gas-turbine engines and many of the desired features of penetration and interceptor fighters. Engineers chose to place the horizontal stabilizer well above the turbulent exhaust from the two jet engines. This gave the proposed aircraft the appearance of an angry scorpion, its tail raised to strike. It influenced the selection of a nickname.
The Army Air Forces set general requirements-known in later years as Advanced Development Objective-in the spring of 1945 and on 28 August asked aircraft manufacturers to submit design proposals conforming to the tentative military characteristics listed in these general requirements. The specifications confronting the competitors called for a conventional (propeller-driven) aircraft that could fly at 525 mph (45b.8 kn) at 35,000 feet, 550 mph (477.6 kn) at sea level, climb to 35,000 feet in 12 minutes, and have a 600-mile (521.7 nm) combat radius. A capability for launching air-to-air rockets would also be included.
Six aircraft manufacturers entered the competition (Bell, Consolidated, Curtiss, Douglas, Goodyear, and Northrop), and most submitted designs for a jet-propelled model instead of the propeller-driven type originally sought by the AAF. Although Curtiss had already been given a contract to develop its entry (a jet-propelled development of the A-43, subsequently known as the XP-87), one of the four designs actually submitted by Northrop was selected. This design also called for the use of jet-propelled engines.
Northrop received a $4 million letter contract for two experimental, two-place, twin-engine, turbojet propelled P-89 fighters. Afterseveral change orders requesting modifications of the aircraft's basic design, the LC of June 1946 was superseded. Procurement negotiations for the two XP-89s finally ended on 21 May 1947, with the execution of the first definitive contract. This $5.6 million contract-an increase of $1.6 million from the LC's amount--called for delivery of the first XP-89 within the next 14 months, ie., not later than mid-1948.
The Air Materiel Command was not favorably impressed with the mockup presented by Northrop in September 1946. The AMC inspection team wanted the radar operator moved closer to the pilot, the canopy redesigned, aluminum substituted for magnesium in the wings and something done about unsatisfactory fuel and oil systems. After another mockup session in December, Northrop was authorized to proceed with construction of the first XP-89 on the basis that certain other changes would also be made in order to improve the safety of the aircraft.
Despite the contractor's efforts, following the mockup inspections of 1946, an engineering acceptance inspection in June 1948 revealed that many discrepancies remained in the first XP-89. Foremost was the aircraft's instability (caused by tail flutter) and buffeting, the latter generally attributed to the airframe's basic design. Structural integrity also was still questioned. Further modifications and development changes would have to be incorporated in the second XF-89 in order to produce a satisfactory aircraft.
The flight took place 9 months later than planned, but the ensuing flight tests conducted by the contractor's pilots at Edwards AFB divulged no special problems. The first XF-89 finally appeared airworthy and functionally dependable. On Aug. 16, 1948, the XF-89 made its stately first flight at Edwards (then, Muroc Air Force Base). The glossy black airplane that Northrop test pilot Fred Bretcher took into the air that day was no fire-eater, but it was an interesting example of how requirements dictate aircraft design.
A pair of Allison J35 axial-flow turbojet engines was slung beneath a narrow fuselage with a long tandem cockpit for the pilot and radar operator. An up-rearing tail assembly with a high-mounted stabilizer, combined with an all-black paint job, made the Scorpion's nickname almost inevitable. The sole XF-89 was fitted with removable wingtip tanks (made permanent on all later models) and a blunt nose to accommodate a swiveling four-gun turret required by the original specification. Fortunately, the heavy turret was soon replaced by six 20mm cannon.
Supporting all of this was an immense slab-like straight wing with a remarkably thin (9 percent) airfoil. Shoulder-mounted, its spars could not pass through the rear cockpit, so the wing roots were butted directly onto a torsion box built into the fuselage. The thin airfoil yielded good high-altitude performance, but left very little room for the main landing gear. Northrop engineers designed a large-diameter wheel with a very thin, high-pressure tire that solved that problem, but that reduced space for wing fuel tanks. This in turn required a large fuselage fuel tank in the only available space: just above the hot section of the thirsty J35s.
As awkward as that all might seem, the Scorpion's basic design was sound, if basically conventional, and it could easily be upgraded. The jet had some difficulties with tail flutter, but its fundamental problems were the low-powered engines of the day and an inefficient radar-armament system.
Comparisons with three possible all-weather interceptors - the Curtiss XF-87, -the Lockheed XF-90, and the Navy's Douglas F3D - showed none to be really satisfactory, with the F-89 perhaps the least unsatisfactory. The successful flight of the Northrop experimental aircraft clinched the Air Force decision. The XF-89 had won the All-Weather Fighter competition over the Navy's XF3D-1 Skyknight and the hapless Curtis XF-87 Blackhawk primarily because its conservative design had greater potential for development. In November 1948, concurrent with Secretary of Defense James Forrestal's endorsement of the Air Force decision, Curtiss' 4-month old contract for 88 F-87 Blackhawks was cancelled.
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