History
The development of the Consolidated Vultee XP-81 began in response to the USAAF’s 1943 requirement for a long-range fighter, specifically for Boeing B-29 escort operations in the Pacific. The requirement’s specifications called for a twin-engined aircraft with a 1,250-mile operating range, a ceiling of 37,000 feet, and a maximum speed of 500 mph.
In September 1943, Convair’s Vultee Field Division submitted a preliminary proposal using a General Electric TG-100 turboprop engine in the nose and an I-40-GE turbojet engine in the back, fed by intakes shoulder-mounted on the rear fuselage. TG-100 had a closely cowled annular intake located around the four-blade propeller spinner. The TG-100 turboprop and I-40 turbojet were eventually redesignated as XT31 and J33, respectively. The turboprop would be used for normal flight and cruising, while the turbojet would augment extra thrust for high-speed engagement. The cockpit was designed to be pressurized from air bled from the TG-100, and the ejection seat developed for the XP-54 was also used in the XP-81. The aircraft would be made of all-metal construction with heavy aluminum-alloy skin flush riveted to the forward of the wing, which enabled two underwing hardpoints capable of carrying a payload of 3,200 pounds. The aircraft was to be provided with a provision for six 0.50-cal. M2 Browning machine guns, three in each wing, or six 20-mm T-31 cannons. These guns can be mixed in any combination.
Two prototype XP-81s 44-91000/91001) were ordered on February 11, 1944, and the contract included thirteen YP-81s. While the TG-100 had development issues, the prototype was rolled out at Downey in January 1945 with a temporarily substituted Packard Merlin V-1650-7 with P-51 accessories and a P-38J-style beard radiator inlet installed. This aircraft made its first flight in February 1945, flying out of Muroc Army Airfield Base. The aircraft reached a maximum speed of Mach 0.74 and climbed to 30,000 feet in 7 minutes. Handling was reportedly acceptable. However, the aircraft suffered a marginal issue with its directional stability, so it was later fitted with a 15-inch fin extension and a short ventral fin. The second XP-81 was also fitted at the factory with a rounded fin extension and a long ventral fin. The aircraft would later conduct the J33 turbojet tests in air starts.
The TG-100 development was setbacked with delays, so it postponed the first turboprop flight until December 21, 1945, four months after the Victory over Japan Day. For the time being, the prototypes were only useful as a research vehicle, and the YP-81 orders were canceled after the American invasion of Guam and the Saipan islands, allowing the existing fighters to operate in their ranges.
On December 21, the initial prototype flew with its first turboprop engine installed but was only capable of producing 1,650 shp instead of the expected 2,300 shp. As a result, it offered no overwhelming performance gains over the Packard Merlin engine previously used on the XP-81. The XP-81 with the turboprop reached 492 mph against the predicted 507 at 30,000 feet, barely 30 mph faster than with the Merlin. Propeller vibration and oil leaks were reported. The engine-propeller combination had a substantial lag in throttle response. The turbine wheel had to be replaced frequently.
The second prototype flew in early 1946 and was subjected to exploring the potential results for mixed propulsion. It featured some aerodynamic refinements that included NACA inlets in place of straight troughs ahead of the shoulder-mounted turbojet intakes. Yet, both propeller vibration and oil leaks occurred during the testing.
After 69 flights totaling 42 hours with the turboprop, the XP-81 program was terminated on May 9, 1947, once the high thrust-to-weight turbojets and turboprops became available. Both surviving prototypes were redesignated ZXF-81 in 1948, indicating research and development test beds. However, there was no information that further flights had taken place before the prototypes were scrapped of useful parts. Remains of stripped XP-81 parts are currently stored at the National Museum of the United States Air Force near Dayton, Ohio.
