Republic XF-91 Thunderceptor - Design and Development

Design and Development

During the development of the XP-84, Republic, under the guidance of Alexander Kartveli looked at the installation of rockets to fighters. The company was inspired by German war time aircraft: the rocket-powered Messerschmitt Me 163 and the experimental Messerschmitt Me 262C Heimatschützer (home protector) series of interceptor prototypes, which added rocket boosters to a turbojet aircraft.

The Thunderceptor design was one of two swept-wing modifications based on the original Republic F-84 Thunderjet, the other being the Republic F-84F Thunderstreak which was developed later. A serious problem with most swept wing designs of the era was dangerous performance at low speeds and high angle of attack. The stagnant airflow over the wing tended to "slide" towards the wingtips, which caused them to stall before the rest of the wing at high angles of attack. In this situation the center of lift would rapidly shift forward relative to the center of mass, pitching the nose up and leading to an even greater angle of attack or, in extreme cases, end-over-end tumbling of the aircraft. Aircraft caught in this regime would often stall and crash, and a rash of such accidents on the F-100 Super Sabre led to the term "Sabre dance". The most famous incident was the loss of F-100C-20-NA Super Sabre 54-1907 during an attempted emergency landing at Edwards AFB, California on 10 January 1956 which was caught by film cameras set up for an unrelated test. The pilot fought to retain control as he rode the knife-edge of the flight envelope but fell off on one wing, hit the ground and exploded with fatal results.

The Thunderceptor's most notable design feature was intended to address this problem. The wings were built to have considerably more chord (distance from the leading edge to the trailing edge) at the tip than root, allowing them to generate more lift. This neatly addressed the problem of Sabre dance by delaying the point of stall on the tip to that of the entire wing. A side effect of this design was that the tips had more internal room, so the landing gear was mounted to retract outward with the wheels lying in the wingtips, using two smaller wheels in a tandem arrangement for each main gear strut, instead of one larger one. Another design change was the ability to vary the angle of incidence of the wing as a whole, tilting it up for low speed operations during takeoff and landing, and then "leveling it off" for high-speed flight and cruise. This allowed the fuselage to remain closer to level while landing, greatly improving visibility.

In keeping with its intended role as an interceptor, the nose was redesigned to incorporate a radar antenna, forcing the air intake for the engine to be moved from its original nose-mounted position to a new intake below it. The fuselage was otherwise very similar to the F-84's. The first prototype did not include the radome, although this was fitted to the second prototype.