Republic XF-91 Thunderceptor
1949
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Republic XF-91 Thunderceptor

The Republic XF-91 Thunderceptor, ordered in 1946, was by far the fastest US Air Force fighter of its time. It was also one of the most innovative of the many experimental aircraft of the late 1940s and early 1950s. With a mid-wing configuration and conventional fuselage design almost identical with that of the F-84F Thunderstreak, the XF-91 was built to test the use of rocket power to boost a jet fighter in combat. In addition to a 2359kg afterburning thrust General Electric J47-GE-3 turbojet, the two XF-91s each employed four 680kg thrust Reaction Motors XLRII-RM-9 rocket motors mounted two-each above and below the jet exhaust. With all five powerplants burning, the XF-91 was supersonic in level flight, attaining 1812km/h.

The first XF-91 made its initial flight on 9 May 1949 on jet power alone. By late 1949, evaluation of the rocket boost powerplant began. But the rocket engines were not the only unusual feature of the Thunderceptor. Its 35-degree swept wing could be adjusted to vary the incidence to the most effective angle for take-off, cruise and landing. And the wing planform was of inverse taper (the only time this concept was tried on a USAF fighter) with the thickest and widest portion of the wing at the tip instead of the root. This provided greater lift outboard and reduced the tendency of the wingtip to stall at low speeds. Because the wing was too thin inboard to accommodate the tandem bogie-type main landing gear, the undercarriage retracted outward to fit into the thickened tips.

The first XF-91 was refitted with a nose radome housing APS-6 radar above the engine intake, in the same manner as the F-86D Sabre. The second machine was retrofitted with a V-shaped butterfly tail and tested with this configuration after it was determined, in 1951, that the XF-91 would not be placed into production. This second airframe was eventually destroyed while the first has been retained by the Air Force Museum in Dayton, Ohio. Republic pilots of the period felt that the XF-91 was one of the truly outstanding fighters of its day, even if the jet-rocket combination may have been impracticable for operational use, and believed that the airplane should have reached squadron service.

Republic XF-91 Thunderceptor


Specification 
 MODELXF-91
 WEIGHTS
    Take-off weight10800 kg23810 lb
    Empty weight7190 kg15851 lb
 DIMENSIONS
    Wingspan9.53 m31 ft 3 in
    Length13.18 m43 ft 3 in
    Height5.69 m19 ft 8 in
    Wing area29.73 m2320.01 sq ft
 PERFORMANCE
    Max. speed1812 km/h1126 mph
    Ceiling14000 m45950 ft
    Range1600 km994 miles

3-View 
Republic XF-91 ThunderceptorA three-view drawing (800 x 561)

Comments
Guy Leida, 29.03.2014

stan and greg, all i know is that I and a crew took those heavy damn wings off normal empenage conf.AND THEY CAME AND TOOK IE AWYA IN BOXES THEY BUILT ON THE HANGAR FLOOR, AND














I HELPED LOAD IT IN A TRUCK TO WPAFB in 1954.

Guy Leida, 29.03.2014

STAN I WAS ASSIGNED TO EDWARDS 1954 CARGO MISC AS C/C ON A T6 PLAY TOY FOR TEST PILOTS TO GO SOMEWHERE FOR FUN OR GIRLS.I GOT A LOT OF STICK TIME. THERE WAS A STRANGE AIRCRAFT IN THE CENTER OF THE HANGAR. AN XF-91. call me 973-492 3739. GUY.

guy leida, 29.03.2014

STAN,






























I arrived at Edwards July 1954. I was assigned to cargo misc.section. In the middle of the hangar, against the wall was a strange aircraft. It looked like it had two engines c/L of aft fuselage,1 over the other. I was assigned C/C on a T-6, chas/pace

Stanley Pasternack, 12.09.2012

The XF-91 had a slab wing (one piece) and was hinged at the rear and had two very large hydraulic jacks that could move the entire wing + 6 degrees and up to -2 degrees, during flight, changing the angle of attack thus varying the lift capability.

Klaatu, 26.06.2011

"Republic pilots of the period felt that the XF-91 was one of the truly outstanding fighters of its day, even if the jet-rocket combination may have been impracticable for operational use, and believed that the airplane should have reached squadron service."

That statement sounds a bit contradictory! Why would anyone advocate putting an aircraft into squadron service that they deemed "impractical for operational use"?

Stanley Pasternack, 08.02.2011

The second XF-91 was sent back to Republic Aviation in Farmingdale, NY and was rebuilt and returned to Muroc Air Force Base for further testing. Does anyone know what airspeed it finally maintained? It would be apprecited. Stan

Dr Greg Varhall, 07.05.2010

Mr Pasternak is correct: That sentence should be reworded because it conveys the wrong impression. "The second airframe was destroyed when it crashed on takeoff during testing in 1952."

Stanley Pasternack, 28.08.2009

I was in the ground crew for the first XF-91 @ Muroc and witnessed the first 23 flights, with Carl Bellinger at the controls. I am trying to find out what happened to the second XF-91? Chuck Yaeger said that Carl had to re-land the second one after a 90 second flight, due to a fire that completely destroyed the aircraft. The above description just says that the second one was (eventually) destroyed making it sound like it was something deliberate. If anyone doubts Chuck Yaeger's description, please email me. I believe Yaeger really knew as he was flying the chase plane that day and Bellinger credits him for saving his life.

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FACTS AND FIGURES

The spade-like wings were inverse taper - they had greater chord at the tips than at the roots.

The XF-91's wing was also variable incidence and could be adjusted in flight to give a high angle of attack for take-off and landing and low angle of attack for high-speed flight.

Unlike all other F-84 derivatives, the XF-91 had a twin-wheeled main undercarriage.

The first XF-91 had a simple nose intake as on the straight-wing F-84G, while the second example had a chin intake and a radar nose.



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