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The XF-81 was designed to solve the very limited range problem of early jet fighters. The aircraft had two engines: a General Electric XT-31 gas turbine turboprop of 1,650 hp. and an Allison J-33 turbojet of 3,750 lbs. thrust. The concept was to use use the turbojet only during combat and takeoff and the turboprop for cruise.

Two XF-81's were ordered (S/N 44-91000, 44-91001) on 11 Feb 44. The first flight was 11 Feb 45; however, because of delays with the turboprop engine, a Packard V-1650 "Merlin" engine was fitted to the aircraft instead. Convair retrofitted the first aircraft for the XT-31 which flew for the first time with the design configuration on 21 Dec 45.

Performance with the turboprop was barely better than with the Merlin and disappointing overall. An order for 13 YF-81's was canceled, but the XF-81's continued as test aircraft until 29 Sep 47 when they were declared obsolete.

 

TYPE XF-81 YF-81

Number built/Converted 2 0

Remarks 1st AF turboprop fighter 13 canceled

 

SPECIFICATIONS
Span: 50 ft. 6 in.
Length: 44 ft. 10 in.
Height: 14 ft. 0 in.
Weight: 24,650 lbs. max.
Armament: Designed for six .50-cal. machine guns or six 20mm cannon and two 1,000 lb. bombs
Engines: One Allison J-33 and one General Electric T-31 gas turbine
Crew: One

PERFORMANCE
Maximum speed: 507 mph
Cruising speed: 275 mph (turboprop only)
Range: 2,500 miles
Service Ceiling: 35,500 ft

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The Convair XP-81

 

Joe Baugher

In March of 1943, the Consolidated Aircraft Corporation merged with Vultee Aircraft Incorporated, forming the Consolidated Vultee Aircraft Corporation. The name of the new merged corporation was often abbreviated to *Convair*, although that name did not get officially registered as a trademark until 1954.

One of the first products of this new merged corporation was the XP-81 mixed-power escort fighter. The primary drawback of most early jet fighters was their high fuel consumption, leading to a relatively limited range and a rather short endurance as compared to piston-engined fighters. In 1943, the USAAF was interested in figuring out a way to couple the improved performance offered by jet propulsion with the long endurance that was demanded by the Pacific campaign, and issued a requirement for a long range escort fighter to be powered by a combination of turboprop and jet engine. The turboprop engine would be used for cruising flight, with the turbojet engine being turned on for takeoff and for high speed flight. The specification called for a 1250-mile operating range and a maximum speed of 500 mph.

Charles R. 'Jack' Irvine, Vultee's chief designer and chief test pilot Frank W. Davis collaborated with the USAAF and came up with an all-metal cantilever low-winged monoplane powered by an experimental General Electric TG-100 (later redesignated XT31) turboprop mounted in the nose and provided with a ventral exhaust, as well as by an Allison-built J33-GE-5 turbojet in the rear fuselage fed by a pair of dorsal intakes. A retractable tricycle undercarriage was fitted. The pressurized cockpit was housed underneath a bubble canopy. The project bore the company designation of Model 102.

Convair began detailed design work on the Model 102 on January 5, 1944. The USAAF ordered two prototypes of the design from Convair on February 11, 1944. The designation XP-81 was applied, and the serials were 44-91000 and 44-91001. The contract was subsequently modified to include thirteen service-test YP-81s. The YP-81 was to have been powered by the lighter and more powerful TG-110 turboprop, the wing was to have been moved aft ten inches, and an armament of six 0.5-inch machine guns or six 20-mm cannon was to have been fitted.

The TG-100 turboprop engine encountered an extensive series of teething troubles and was not yet available when the XP-81 (44-91000) was ready for its first flight. A Packard Merlin V-1560-7 power package from a P-51D Mustang was supplied in its place. A P-38J-style beard radiator inlet was mounted below the propeller spinner. The Merlin-powered XP-81 was trucked to Muroc Dry Lake, where it was flown for the first time on February 11, 1945. The handling proved to be exceptionally good. However, the directional stability was marginal and a 15-inch fin extension and a short ventral fin were added. The second XP-81 was fitted at the factory with a rounded fin extension and a long ventral fin.

In the meantime, the war against Japan had progressed to the point where the capture of such islands as Guam and Saipan had largely eliminated the need for long-range, high-speed escort fighters. The 13 pre-production YP-81s were cancelled shortly before V-J Day, but work continued on the two XP-81s.

The first XP-81 was flown back to Vultee Field to be fitted with the TG-100, but the work was not finished until after V-J Day. The first flight with the TG-100 took place on December 21, 1945. The TG-100 was supposed to deliver 2300 e.h.p, but actually delivered only 1400 e.h.p. Consequently, the performance of the turboprop-powered XP-81 was no better than that of the Merlin-powered version. In addition, excess propeller vibration and persistent oil leaks became important problems.

The XP-81 program was officially terminated on May 9, 1947. Both prototypes were redesignated ZXF-81 in 1948, indicating a testbed status. In 1949, they were both stripped of useful parts and placed on a photographic and bombing range at Edwards AFB. The remains of both machines were stored for a long time at the Flight Test Museum at Edwards. They have now been moved to the USAF Museum at Wright Patterson AFB for eventual restoration and display.

Specification of the XP-81:

One 2300 ehp General Electric XT31-GE-1 (TG-100) turboprop and one 3750 lb.st. Allison J33-GE-5 turbojet. Proposed armament (never fitted) was six 0.50-inch machine guns or 6 20-mm cannon and two 1600-pound bombs. Weights were 12,755 pounds empty, 19,500 pounds loaded, 24,650 pounds maximum overload. Dimensions were wingspan 50 feet 6 inches, 44 feet 10 inches, height 14 feet 0 inches, 425 square feet wing area. The following estimated performance figures were based on an assumption of a full 2300 hp output from the turboprop: Maximum speed, 478 mph at sea level, 507 mph at 30,000 feet. Range 2500 miles at 275 mph at 25,000 feet. Initial climb rate was 5300 feet per minute. Service ceiling was 35,500 feet.

Joe Baugher

Sources:

1. General Dynamics Aircraft and their Predecessors, John Wegg, Naval Institute Press, 1990.
    
2. War Planes of the Second World War, Fighters, Volume Four, William Green, Doubleday, 1964.
    
3. American Combat Planes, Third Enlarged Edition, Ray Wagner, Doubleday, 1982.
    
4. E-mail from Martin Keenan on current status of XP-81 airframes.

 

The 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.

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