Republic XF-12 Rainbow

The Republic XF-12 Rainbow was an American four-engine, all-metal prototype reconnaissance aircraft designed by the Republic Aviation Company in the late 1940s. Like most large aircraft of the era, it used radial engines, specifically Pratt & Whitney R-4360 Wasp Major corncob engines. The XF-12 was referred to as "flying on all fours" meaning: four engines, 400 mph cruise, 4,000 mi range, at 40,000 ft.

Minimizing drag was a primary consideration throughout the design of the XF-12. Many features came from Republic's experience with fighter aircraft. Unusually, no compromises to the aerodynamics were made in the shape of its fuselage. Aviation Week was quoted as saying "the sharp nose and cylindrical cigar shape of the XF-12 fulfills a designer's dream of a no compromise design with aerodynamic considerations."

For its reconnaissance role, the XF-12 had three photographic compartments aft of the wing. One vertical, one split vertical, and one trimetrogon each using a 6 in (150 mm) Fairchild K-17 camera. For night reconnaissance, the XF-12 had a belly hold which accommodated 18 high-intensity photo-flash bombs to be ejected over the target. All bays were equipped with electrically operated, inward retracting doors designed for minimum drag and camera lenses were electrically heated to prevent frost build-up. The XF-12 also carried complete darkroom facilities to permit developing and printing the film while still airborne augmented by adjustable storage racks to handle any size of film container and additional photo equipment. This allowed immediate access to the intelligence after landing without the usual processing delay.

Only two prototypes were built as the requirement evaporated after World War II ended, while the cheaper off-the-shelf Boeing B-29 Superfortress and Boeing B-50 Superfortress could temporarily fill the role until the jet-powered Boeing RB-47 Stratojet entered service. The XF-12 was the fastest aircraft of its day to use four reciprocating engines, and the only one to exceed 450 mph in level flight.

The engines used a sliding cowl to facilitate engine cooling instead of cowling flaps, which caused too much drag. There was also a two-stage impeller fan directly behind the propeller hub. These refinements allowed the engines to be tightly cowled for aerodynamic efficiency, while still keeping the engines adequately cooled. When the sliding cowl ring was closed during flight, the cooling air was ducted through the nacelle to the rear exhaust orifice increasing thrust, rather than adding drag as is usually the case.

Air for engine intakes, oil coolers and intercoolers was drawn through the leading edge of each wing between the inboard and outboard engines. This reduced drag compared to using individual intakes for each component. In addition, because the air was taken from a high-pressure area at the front of the wing, this provided a ram air boost for increased power at high speeds, and more effective cooling of the oil and intercoolers.

Although innovative, the jet engine and the end of World War 2 made it obsolete, and it therefore did not enter production.