On this day in aviation history, February 11, 1945, the Consolidated XP-81 made its first flight. Intended to be a long-range escort fighter, the Consolidated XP-81 also represented a departure from conventional aircraft, but the rapidly evolving nature of both the Second World War and jet engines would see the XP-81 becoming nothing more than a footnote in American aviation history.
In May 1943, the Boeing B-29 Superfortress was officially adopted into service with the United States Army Air Force. Around this time, the Army Air Force began considering that it would require a long-range, high-altitude escort fighter for the Pacific theater. Around this time, the USAAF was also developing its first jet fighters, but knew that all-jet aircraft at this time were short-ranged and burned fuel quicker than propeller-driven aircraft, and so the optimal design was to feature a propeller-driven engine in the front for long range performance and a jet engine in the tail for high speed in combat. The specifications then drawn up by the USAAF called for a 1,250 mile (2,012 km) operating radius, fuel capacity for 20 minutes of combat plus a reserve for landing, a cruising speed of 250 mph (402 km/h) at 25,000 ft (7,620 m), maximum speed of over 500 mph (804 km/h), a climb rate of 2500 fpm (feet per minute) / 762 mpm (meters per minute) while at 27000 ft (8230 m), a service ceiling of 37,500 ft (11,430 m), two engines, and 12 ° angle of vision over the nose.
Th company that would rise to the occasion for this project was the Consolidated Vultee Aircraft Corporation (later known as Convair), which appointed the Vultee Division’s chief engineer Charles R. Irving and Vultee’s chief test pilot Frank W. Davis to lead the project’s design team. With the design designated the Consolidated Vultee Model 102, the biggest obstacle was finding the engines for this design. While a piston engine was considered for the nose-mounted engine, it was decided to select the first turboprop engine designed and built in America, the General Electric T31 (company designation TG-100), which was to turn a four-bladed propeller on the Model 102. Meanwhile, the General Electric J33 (later developed by Allison as the Allison J33) was selected to be the rear-mounted engine. The aircraft also featured a pressurized cockpit, and considerations were made for it to be armed with six 20mm cannons or six 12.7mm machine guns. Consolidated-Vultee submitted the design of the Model 102 to the US Army Air Force’s Air Material Command in September 1943, and the Model 102 was approved for further development as the XP-81.
Work on two XP-81 prototypes (serial numbers 44-91000 and 44-91001) got underway in January 1944 at the Vultee Division’s plant in Downey, California, part of the Los Angeles metropolitan area, with the company being expected to build a further 13 YP-81s. However, production delays in the TG-100/T-31 engine arose, and so it was decided that the first prototype, XP-81 44-91000, was to be temporarily fitted with a Packard V-1650 Merlin engine in the nose with a P-38J-style beard radiator inlet until a T-31 engine could be procured. Unfortunately, the Packard Merlin was heavier and less powerful than the T-31, but it was the only option.
The Packard-equipped first XP-81 prototype, 44-91000, was finally completed in January 1945, and shipped to Muroc Dry Lake (now the site of Edwards Air Force Base), where on February 11, 1945, Frank Davis took the XP-81 for its first flight. The aircraft exhibited poor directional stability (which was solved through the later addition of a ventral tail fin) at low speeds and oil sometimes splattered on the canopy’s windscreen. On May 18, 1945, the T-31 turboprop was finally ready and by June 11, the aircraft was ready to begin testing with its intended engine at Muroc. Ground testing from June to December 1945, however, found the T-31 difficult to start, and once started, difficult for the pilot to control. Furthermore, the engine was unreliable, and the turbines in the engine had to be replaced constantly from war and tear. The throttle controls also suffered from control lag, with Frank W. Davis stating, “The pilot had about a 10 second lag when he wanted to go and about 2 seconds lag when he wanted to stop, with both thrust and drag being powerful and non-adjustable when they did occur.”
It was not until December 21, 1945, four months after Japan’s surrender had ended WWII, that the first flight of the XP-81 with its intended T-31 turboprop engine was made on what was the aircraft’s 47th flight. While the general performance was satisfactory, Davis noted that the T-31 had an excessive oil consumption rate. Later test flights also showed that of the T-31’s original estimate of 2,3oo hp, it only produced around 1,400 hp, while the J33 engine installed produced 250 lbs less thrust than needed. These combined factors led to a slower top speed for the aircraft than expected, but the XP-81 still had descent handling and climb rate and was light on the controls.
In November 1946, Convair produced the second XP-81 prototype, 44-91001, which was ready for flight testing at Muroc by February 1947. This second prototype differed from the first in that it had a longer ventral fin than the one added to the first prototype and was fitted with a Hamilton Hydromatic propeller as opposed to the original prototype’s Aeroproducts propellers, which engineers felt was the source of many of the first prototype’s issues.
By this point, despite efforts by Convair to propose adding upgraded engines for the design, the USAAF was losing interest in the XP-81’s development, and after cancelling the production orders of the 13 YP-81s on January 27, 1947, the XP-81 program itself was cancelled on May 9, 1947. The Army Air Force took the two XP-81s from Convair and with the formation of the United States Air Force later that year, the two prototypes were redesignated ZXF-81s and kept in storage until 1949, when they were stripped for their engines and instruments and placed on the photography and bombing range at the newly designated Edwards Air Force Base.
Perhaps one day, the time will come when the two XP-81s are given due credit and restored to their former glories, but for now, we ought to consider ourselves lucky that both examples of this unique aircraft have survived to the present day.
Today in Aviation History is a series highlighting the achievements, innovations, and milestones that have shaped the skies. All the previous anniversaries are available HERE
