TYPE: High-performance reconnaissance aircraft
ACCOMMODATION: Crew of seven
POWER PLANT: Four Pratt & Whitney R-4360-31 “Wasp Major” radial engines, rated at 3,250 hp each
PERFORMANCE: 470 mph
COMMENT: The Republic XF-12 “Rainbow” was an American four-engine, all-metal reconnaissance aircraft designed by the Republic Aviation Company in the late 1940s. The aircraft was designed with maximum aerodynamic efficiency in mind. The XF-12 was referred to as an aircraft that was “flying on all fours” meaning: four engines, 400 mph cruise, 4,000 miles range, at 40,000 feet. It is still the fastest piston-engined airplane of this size, exceeding by some 50 mph the Boeing XB-39 of 1944. Although highly innovative, the postwar XF-12 “Rainbow” had to compete against more modern turbojet engine technology, and did not enter production.
In August 1943, U.S. President Franklin D. Roosevelt’s son, Colonel Elliot Roosevelt, commander of the F-5 (modified P-38) “recon” unit, recommended the acquisition of a dedicated high-performance photo reconnaissance aircraft, capable of providing pre-strike target acquisition and photo interpretation. Followed by additional overflights to provide post-strike analysis of their subsequent destruction, this would give commanders the ability to make pivotal strategic decisions and set up subsequent raids. The XF-12 “Rainbow” was Republic Aviation’s attempt to meet those goals. Its primary competition during this time was the Hughes XF-11. Both were introduced at the same time, and both were powered by the new Pratt & Whitney R-4360. The XF-12’s first flight was made on 4 February 1946. During the XF-12’s subsequent flight testing and development period, it demonstrated the capability of operating at 45,000 feet, at a speed of 470 mph, over a range of 4,500 mi, so it met and exceeded the design goals for which it had been designed. Neither the XF-11 nor the XF-12 was purchased in any quantity by the U.S. Army Air Forces (two each), as their need evaporated after hostilities ended in World War II.
When the XF-12 was modified with increased “all weather” equipment and outfitted with its new engines capable of providing short burst of extra power, it suddenly assumed tremendous importance in the eyes of both the U.S. Air Force and the State Department. As a potent intelligence weapon, the XF-12 had the ability to obtain photographs both in daylight and under conditions of restricted visibility at high altitudes over long ranges and with great speed. In theory, operating from northern bases (Alaska and Canada), this “flying photo laboratory” was capable of mapping broad stretches of territory in the Arctic regions performing reconnaissance with near-invulnerability.
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, the sharp nose and cylindrical cigar shape of the XF-12 fulfilled 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 XF-12 “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 Republic P-47 “Thunderbolt”. Each engine featured twin General Electric turbochargers, situated at the aft end of the nacelle. All of the exhaust from the engines was ducted straight out of the back of the nacelles. This provided additional thrust. Research showed that roughly 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.
Had the XF-12 “Rainbow” been available in 1944, it almost inevitably would have been ordered in quantity, and along with its civilian counterpart, the whole postwar structure of aircraft markets might have been altered. As it was, the XF-12 disappeared into oblivion, despite its graceful lines and high performance. The “Rainbow” remains the ultimate expression of multi-engine, piston-powered aircraft design. Its high speed, near-perfect streamlined form, and neatly cowled engines make it a design classic, often unappreciated, and not very well known. The XF-12 was the fastest, four engine pure piston-powered aircraft of its day, and the only one ever to exceed 450 mph in level flight. (Ref.: 24).