Republic XF-91B
Thunderceptor

by Phil Brandt

images by Milton Bell

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The radical lines of this Fifties interceptor mark the end of Republic's F-84 design evolution: inversely tapered, variable incidence wings; outward folding tandem main landing gear; and a powerful empennage-mounted rocket engine to bolster the thrust of its J-47 jet engine. Although two XF-91s were built, only one became the F-86D lookalike B model which was envisioned to meet an Air Force requirement for a radar interceptor to complement the F-104. The intended Curtiss-Wright rocket engine never made it into production, but had it become a reality, Mach 2 speeds were estimated.....in 1952! Instead, the less powerful, four-chambered rocket engine from the Bell X-1 was substituted, which still produced level speeds of mach 1.2. The B model survived testing and is on display at the USAF Museum where yours truly was able to take a group of closeup pix during one of his many pilgrimages to Fairborn, Ohio.

Although Bondo certainly owns his share of exceptionally-mastered-but-as-yet-unbuilt Tamiyagawa kits, there's still a masochistic attraction in his elderly bones to do exotic airframes, even though it's fer sure a plastic 'beating's' gonna go along with the project. And so I retrieved my ancient 1/48 Lindberg XF-91 and F-86D kits, neither of which would make most modeler's Top 1000 list, out of long term storage and began surgery. It's uncanny how closely the forward fuselage of the XF-91B resembles the Sabre Dog, but I'm here to tell ya, my nickname was fully borne out when coat after coat of epoxy/lacquer putty dried, was ground down and sanded! Especially taxing was the scratchbuilding of the under-radome intake trunk and the splitter section/nosegear well. One-off B-model nosegear doors were also scratchbuilt.

After the forward fuselage lines were beaten into submission, the aft rocket pack extension came in for its own re-profiling. I lengthened it and scratchbuilt the rocket exhaust outlets. The jet engine exhaust was kitbashed with a resin aftermarket one for the Academy Flanker. Just as in the case of the F-86D, a ventral rocket weapons tray was envisioned by Republic's designers, so I integrated the deployed tray from one of my two 1964 vintage Marusan 1/50 Sabre Dog kits.

The Lindberg cockpit could charitably be said to be non-existent, with a molded-in pilot's upper torso and seatback. I've always liked the Monogram F-84F, albeit the raised details, and they have a nicely 'busy' cockpit tub, seat, instrument panel and, best of all, the canopy raising mechanism, with canopy side detailing. I 'Dremmelled' out the existing Lindberg 'cockpit' and fuselage sides and applied a thin layer of A&B epoxy putty. Then I pushed the Monogram tub into place, withdrawing it to leave a perfect mounting impression for the later tub installation. The instrument panel coaming w/black boxes was shaped from the same component in the Monogram F-100. The area aft of the seat and canopy was scratchbuilt, using the F-84F canopy components integrated with the Lindberg XF-91 profile and the distinctive XF-91 canopy.

Per modeling custom in the Fifties, Lindberg provides only a gear door outline on the wing undersides. I cut out the gear doors and boxed in the wheel well sides, adding Evergreen strips as necessary for structural detailing. Another set of gear doors was cut out of a second XF-91 kit, and all door edges were "rabbitted" with the Dremel to give structural appearance. All wheels were widened with plastic sheet, discbrake calipers were scratchbuilt and gear mounts were fabricated within the wheel wells. The nosegear strut was accurized per my museum pix.

The distinctive external tanks were correctly molded by Lindberg, but new, more extended pylons were scratchbuilt, incorporating the downward incidence of the tank tips. A small "point" was added to the stabilator leading edge where it intersects with the vertical fin leading edge. This may seem a small thing, but, together with the serious stab fit problem in the Lindberg kit, occupied significant building time to fillet, putty and sand to the degree necessary for a natural metal finish.

The entire airframe was scribed per line drawings I had collected and enlarged from an old magazine feature. 

Many iterations of Blue Acryl, gray lacquer primer, and progressive sanding took place until the flat primer took on a gloss. Three shades of my favorite metallizer, Alclad II, were airbrushed, followed by some darker shades of Testors Metallizer. I always do the Testors on top of the Alclad because the Testors doesn't like to be masked. Baremetal foil was applied to the highly polished stainless friction area where the variable incidence wing rubs against the fuselage. Decals were minimal and were taken from various SuperScale sheets. Wash was not applied because as a minimally flown prototype, the bird was usually in very clean condition. In fact, the B model at the USAF Museum has been painted with silver lacquer.

It's been a real Dance Plastique, and Bondo probably won't subject himself to this routine for some time to come.....but then there's always the Airmodel PSM-6, the Combat Tradewinds and all those Mach 2 kits!

Phil Brandt IPMS 14091

Model and Text Copyright © 2001 by Phil Brandt
 Images Copyright © 2001 by Milton Bell
Page Created 30 May, 2001
Last Updated 04 June, 2007

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