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.

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.

3-View

A three-view drawing (800 x 561)