Republic XF-12 Rainbow - Design

Design

Low drag was a primary consideration throughout the design of the XF-12. Many of its features were taken directly from Republic’s considerable experience with fighter plane design. In an extremely rare case of design direction, absolutely no compromise with aerodynamics was 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."

To fulfill its reconnaissance role, the XF-12 contained three separate photographic compartments aft of the wing. One vertical, one split vertical, and one trimetrogon each using a six inch Fairchild K-17 camera. For night reconnaissance missions, the XF-12 had a large hold in the belly which accommodated 18 high-intensity photo-flash bombs; these were ejected over the target area. All of the bays were equipped with electrically operated, inward retracting doors (again designed for maximum aerodynamic cleanliness). The camera lenses were electrically heated to eliminate distortion. All of this combined to allow full photo operations during high speed flights. The XF-12 also carried a variety of photographic equipment, including complete darkroom facilities to permit the development and printing of films in flight. This was augmented by adjustable storage racks, able to handle any size film containers and additional photo equipment. This allowed the Army Intelligence units to have immediate access to the intelligence the aircraft was able to collect, with no delay in processing.

The Rainbow featured a wing of straight taper with squared tips and high aspect ratio for maximum efficiency. The engines featured a sliding cowl arrangement to facilitate cooling airflow instead of the normal cowl flaps, which caused too much drag. At the front of the cowls, the engines were also fitted with a two stage "impeller fan" directly behind the propeller hub and prop spinner. This allowed the engines to be tightly cowled for aerodynamic efficiency, but still provide the cooling airflow the engines required. When the sliding cowl ring was closed (during flight), the air used for cooling the engine was ducted through the nacelle to the rear exhaust orifice for a net thrust gain, as opposed to the usual cooling drag penalty.

All of the air for the engine intakes, oil coolers and intercoolers was drawn through the front of each wing between the inboard and outboard engines. This allowed less drag than with 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" benefit for increased power at high speeds, and more effective cooling of the oil and intercoolers. The intake portion of the wing comprised 25% of the total wingspan. They were extensively wind tunnel tested for intake efficiency and inlet contour efficiency. This cooling air, after being utilized, was ducted toward the rear of the nacelle, to provide additional net thrust. The entire engine nacelle was the length of a P-47 Thunderbolt (also built by Republic). Each engine featured twin General Electric turbochargers, situated at the aft end of the nacelle.

All of the exhaust from the P&W R-4360 was ducted straight out of the back of the nacelles. This provided additional thrust. Research showed that approx 250 equivalent horsepower was generated by each engine exhaust during high speed cruise at 40,000 ft.

The original design of the XF-12 called for contra-rotating propellers, similar to those used on the original XF-11. However, due to the added complexity and reliability issues, the propellers were never installed. They would have been twin three-bladed propellers (rotating in opposite directions). As it was, the aircraft used standard four-bladed Curtiss Electric propellers for all flights.