F-81 / XP-81 Vultee
The Consolidated F-81 (also identified as the XP-81) Vultee was a low-wing monoplane built to satisfy the AAF escort fighter requirements of September 1943. The high-fuel consumption of early jet fighters prompted Convair to equip the XP-81 with a turboprop and jet combination. A Rolls-Royce Merlin V-1650 engine, manufactured by Packard, replaced the yet to be available General Electric TG-100 (XT-31) turboprop during the initial tests.
As envisioned the F-81 would use the turbojet in combat and during takeoff while the turboprop would be relied on at all other times in flight. Testing produced poor results and the F-81 was ultimately cancelled.
Most people know that the United States developed two jet fighters during World War II-the broad-winged P-59 Airacomet, closely followed by the sleek P-80 Shooting Star. The Airacomet turned out to be slow and stately in the air, more suited to training duties, and the war ended before the speedy P-80 could enter combat. Hidden under the shadow of the first two jets, however, was a third-the XP-81. In several ways, it was the most interesting of the three.
For one thing, the XP-81 was a twin-engine jet hybrid, powered by one turbojet as well as the country's first turboprop. For another, it was designed for a revolutionary main engine, well before it was fully developed. When the United States Army Air Forces (USAAF) issued a specification in 1943 for a long-range escort fighter, it was already apparent that the brand-new turbojet engine offered great speed, but very short range. Consolidated Vultee Aircraft (CONVAIR) explored the advantages of a turboprop, a jet engine geared down to turn a conventional propeller.
Convair engineers designed an airframe that would combine a General Electric I-40 centrifugal-flow turbojet with a G.E. TG-100 axial-flow turbojet in a push-pull configuration. In 1943, however, neither engine was near being ready for mass production. Nevertheless, the concept offered much greater fuel economy at a cost of somewhat reduced speed. The economical turboprop would take the fighter long distances across the Pacific, and the pilot could cut in the jet engine when speed was needed for takeoff and combat maneuvering.
Convair delivered two XP-81s to Muroc, and the first flight took place on February 7, 1945. General Electric's turboprop engine was not available, so the first flight took place with the I-40 in the rear and a 12-cylinder Allison V-1710 in the nose. Teething troubles with the turboprop delayed its delivery until June, and America's first turboprop flight finally took place on December 21, 1945. In the meantime, Great Britain had gained the honors for the world's first turboprop flight some two months earlier, when a converted Meteor took to the air, powered by two Rolls Royce R.B. 250 "Trent" engines.
The XP-81s were large and very clean airplanes, approximately the size of the P-38 and weighing nearly as much (24,650 lbs. gross) as the P-61 Black Widow. Air for the I-40 was supplied by two saddle-mounted air intakes, and the turboprop was fed by an annular intake around a slim cowling made possible by the TG-100's narrow diameter. Nearly all of its power was delivered via the propeller; the exhaust, directed through a large vent beneath the belly, contributed only some 600 lbs. of thrust.
The XP-81s handled well in the air, with a good rate of climb and light controls that were well balanced. Unfortunately, the turboprop proved incapable of delivering its planned horsepower, and the two dissimilar engines were never harmonized properly. The plane was also subject to gearbox problems and heavy propeller vibration.
These problems, coupled with generally lackluster performance, resulted in the termination of the program on May 9, 1947. The two airframes were placed in caretaker status, then stripped of their usable parts and relegated to the Muroc photo range. There, they languished in sun-blasted neglect until August 1994, when Air Force Flight Test Center (AFFTC) Museum Curator Doug Nelson acted to salvage the hulks.
|Join the GlobalSecurity.org mailing list|