CRISIS-BORN FIGHTER PROJECTS, XP-48 TO XP-77
XP-54, XP-58, XP-60
Vultee’s XP-54 weight grew from 11,500 pounds in 1941 to 18,233 pounds when completed. When the first prototype order was approved on January 8, 1941, to be powered by a Lycoming XH-2470-1, a top speed of 510 mph at 20,000 feet was promised. But numerous changes resulted in a larger and heavier aircraft. By September 1941, turbosuperchargers and a pressurized pilot cabin had been added, and top speed was guaranteed at 476 mph at 30,000 feet with a 16,145 pound gross weight.
The original nose armament of six .50-caliber guns was replaced by two 37-mm guns with 120 rounds mounted in the nose below a pair of .50-caliber guns with 1,000 rounds. A compensating gun sight could elevate the entire nose to give more reach to the cannon. Full armor and fuel tank protection was included.
Largest single-seater offered the Air Force, the XP-54 had twin rudders suspended on tail booms extending from the gulled wing. The bullet-shaped fuselage stood so high on tricycle gear that the pilot was raised up through the bottom into the cockpit by an elevating seat. In emergencies, the seat dropped downward on a swinging arm to throw the pilot clear of the four-bladed propeller. The XP-54 had excellent pilot’s visibility, but its weapon system was never actually tested in flight.
More delay of the contract’s July 1, 1942, delivery date came from a change of the Downey company’s ownership that resulted in the XP-54’s designer, Chief Engineer Richard Palmer, leaving Vultee. Flight tests begun January 15, 1943, by Frank Davis, produced only a disappointing 381 mph at 28,500 feet. Although the contractor polished and waxed the XP-54 so it did reach the 403 mph of the revised specification, the Air Force notified Vultee on May 25 that no production could be considered, and that the 24-cylinder Lycoming engine was being abandoned. A second prototype made the first of its ten flights on May 24, 1944. Total cost of the XP-54 project was $1,497,000.
The Curtiss-Wright XP-55 was the first Air Force fighter with swept wings, which angled back 45 degrees with twin vertical tail surfaces placed near the tips Since the pusher propeller eliminated the usual tail assembly, the elevators were placed at the nose in a “canard” layout.
Such a radical configuration required much new engineering and wind tunnel testing, and construction of a full-scale wood and fabric flying model powered by a 275-hp Menasco. Flight tests of this CW-24 begun in secret at Muroc Dry Lake on December 2, 1941, justified a contract, approved July 10, 1942, with the St. Louis firm for three prototypes.
The 1,600-hp Continental XI-1430-3, which was supposed to give the XP-55 a 507-mph top speed, was replaced by the available 1,125-hp Allison V-1710-95. A dorsal fin above and ventral fin below the engine contained air intakes for the engine and radiator while the three-bladed prop could be jettisoned to make a pilot’s bailout safer. Armament consisted of four .50-caliber guns in the nose with 250 rpg and wing racks were provided for two 50-gallon drop tanks.
The first example flew on July 13, 1943, but was lost in an inverted spin November 15. The second XP-55 first flew on January 9, and the third on April 25, 1944, but an excessive takeoff run and dangerous stall behavior continued as problems. Over 1,000 pounds overweight, and costing $3,524,622, the XP-55 project did introduce the Air Force to the development of swept wing flight.
The third and most radical of these pushers was the XP-56, a tailless interceptor with a very short elliptical fuselage. The newly reorganized Northrop company had designed a flying wing, the N-1M, with a control system of “elevon” control surfaces on the wings and rudders replaced by air-operated split flaps on the drooped wingtips. An NS-2 fighter design was ordered as the XP-56 prototype on September 26, 1940, and by March 12, 1941, the original X-1800 engine was replaced by an R-2800-29 Wasp cooled by air from wing-root intakes and turning contra rotating propellers behind dorsal and ventral fins. Delivery was planned for March 1942, and guaranteed top speed was 467 mph at 25,000 feet.