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?in this case, the Pratt & Whitney R-4360 Wasp Major. 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 mile 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 jet engine technology, and did not enter production.
Development
The original proposal for the aircraft, delivered in late 1943, came from the USAAC Air Technical Service Command, stationed at Wright Field. The proposal was for a reconnaissance aircraft which included a requirement for speed (400 mph), ceiling (40,000 ft) and range (4,000 nmi). Its primary objective was for high-speed overflights of the Japanese homeland and key enemy installations. During World War II, due to the extended range requirements of operating in the Pacific, existing fighters and bombers were being used for missions for which they were never intended. The need existed for an aircraft specifically designed for the photo-reconnaissance mission. The aircraft required adequate speed, range and altitude capabilities for its missions to be successful.

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 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 P&W 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 (14,000 m), at a speed of 470 mph (760 km/h), over a range of 4,500 mi (7,200 km), so it met and exceeded the design goals for which it had been designed. Neither the XF-11 or 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 bursts 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.
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 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.
Operational history
The first prototype was damaged in landing on 10 July 1947. The aircraft was undergoing maximum landing weight tests. During one particularly hard landing, the right main gear was severed at the engine nacelle. The aircraft bounced hard, and staggered back into the air. The test pilot was able to maintain control, and climb to a safe altitude. He continued to fly the aircraft to burn off excess fuel, to both make the aircraft lighter and lessen the chance of fire. Once excess fuel was burned off, the pilot landed on the left main gear and the nose wheel. The pilot touched down, and while keeping the right wing up, scrubbed off as much speed as possible before it touched down. During the incident the aircraft suffered significant damage. The right wing spar was cracked, and the #3 and #4 engines and props needed to be replaced due to the ground contact. The aircraft was repaired by Republic, and later returned to service.

The only external difference between the first and second prototypes was the addition of cooling gills on the upper engine cowlings. Internally, the second prototype was far more "finished." This included its full operational reconnaissance equipment suite, to allow for further testing.

The XF-12 was later re-designated XR-12, when the U.S. Army Air Forces separated from the Army and became the U.S. Air Force.

The most successful part of the XF-12 flight history is "Operation Birds Eye." The mission was conceived to demonstrate the newly designated XF-12?s ultimate photo capabilities. On 1 September 1948, the second prototype XF-12 departed U.S. Air Force Flight Test Center at Muroc, California, and climbed westward to gain altitude over the Pacific Ocean. Upon reaching its 40,000 ft cruising altitude, the XF-12 headed eastward and began photographing its entire flight path over the United States. The crew shot a continuous 325 foot-long strip of film composed of 390 individual photos (10 inches per photo) covering a 490-mile-wide field of vision. The aircraft landed at Mitchel Field at Garden City, Long Island, New York, completing a flight lasting six hours and 55 minutes at 361 mph average speed (approx. 1m4s per photo). The record-shattering flight was featured in the 29 November 1948 issue of Life magazine and the actual filmstrip went on exhibit at the 1948 U.S. Air Force Association Convention in New York.

At the time this record flight was made, the U.S. Air Force had already canceled the entire XF-12 program. The primary reason for its demise was the availability of both Boeing B-29 Superfortress and B-50 types to meet the long-range photo-reconnaissance requirement until the far more capable Boeing RB-47 Stratojet was brought into service. The B-29 and B-50 gave the U.S. Air Force less costly "off the shelf" options.

Republic had intended to also build an airline version of the aircraft to be known as the RC-2. This variant was supposed to be a "stretched" version of the XF-12, growing in length from 93 ft 9 in to 98 ft 9 in, with the addition of a fuselage "plug" in front of the wing. Also the complex Plexiglas nose section was supposed to be replaced with a solid metal nose with a bifurcated windshield. Fuel capacity would have been increased, and more powerful (at lower altitude) P&W R-4360-59s would have been substituted in place of the P&W R-4360-31s on the U.S. Air Force version. The engines also would have only had one General Electric turbosupercharger each, instead of the dual arrangement on the U.S. Air Force model. The aircraft would be lavishly appointed for the 46 passengers and seven crew. It would have been fully pressurized to sea level, air conditioned, with an electric galley providing hot meals and with an inflight lounge. It would have had the ability to cruise above the weather at 435 mph at 40,000 feet. No versions of this aircraft were ever built.

Without an order from the U.S. Air Force to offset the cost for development and tooling, the cost of building the civilian airliners went up exponentially. As a result, the two airlines (American Airlines and Pan-Am) that had originally placed tentative purchase orders, both cancelled due to the additional unit cost. Economically, the RC-2 wasn't as feasible as other designs available at the time, such as the Lockheed Constellation and the Douglas DC-6. Both of those aircraft could carry more people, at a lower cost per mile. In addition, after the hostilities ended in World War II, there were large collections of surplus military transports available for purchase, such as the Douglas C-54 Skymaster. These former transport aircraft lent themselves to be readily converted to airline service at a fraction of the cost of buying new aircraft. Without additional orders, Republic cancelled all further plans to build not only the XF-12 but also the RC-2, leaving just the two original prototypes.

On 7 November 1948, prototype number two, 44-91003, crashed at 13:00 while returning to Eglin Air Force Base, Florida. The number 2 (port inner) engine exploded as the aircraft was returning from a photographic suitability test flight. The pilot was unable to maintain control due to violent buffeting, and he ordered the crew to bail out. Five of the seven crew escaped safely, including pilot Lynn Hendrix, rescued by Eglin crash boats and helicopters. The airframe impacted two miles south of the base in the Choctawhatchee Bay. Sgt. Vernon B. Palmer and M/Sgt. Victor C. Riberdy were killed. The first prototype, which returned to service in 1948, continued the flight testing and development phase. After the U.S. Air Force declined to order any additional aircraft, and with the loss of the second prototype, the flight testing period wound down. In June 1952, the first prototype, 44-91002, was retired (having flown just 117 additional hours from 1949-1952), was stricken from the U.S. Air Force inventory and ended up as a target on the Aberdeen Proving Ground, Maryland.

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. According to Machat, 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 such ever to exceed 450 mph in level flight. The closest the USAF ever got to the Rainbow, the 44 (converted from bombers) Boeing RB-50Bs, could only reach 385 mph at 25,000 feet with exactly the same engines (Wasp Majors).


















General characteristics

Length: 93 ft 9 in (RC-2 version 98 ft 9 in) (28.56 m)
Wingspan: 129 ft 2 in (RC-2 version 129 ft 2 in) (39.37 m)
Height: 28 ft 1 in (RC-2 version 29 ft 11 in) (8.55 m)
Wing area: 1,640 ft? (RC-2 version 1,640 ft?) (152.36 m?)
Empty weight: 65,000 lb (RC-2 version 67,000 lb) (29,483 kg)
Loaded weight: 101,400 lb (RC-2 version 114,200 lb) (45,994 kg)
Max. takeoff weight: 101,400 lb (RC-2 version 114,200 lb) (45,994 kg)
Powerplant: 4 ? Pratt & Whitney R-4360-31 Radial engine, 3,250 hp (2,423 kW each) each
Performance

Maximum speed: 470+ mph (RC-2 version 450 mph) (756 km/h)
Range: 4,500 miles (RC-2 version 4,100 miles) (7,242 km)
Service ceiling: 45,000 ft (RC-2 version 40,000 ft) (13,716 m)
Rate of climb: 5,000+ ft/min (RC-2 version 5,000+ ft/min) (1,524 m/min)
Wing loading: 61.83lb/ft? (301.9 kg/m?)
Power/mass: hp/lb (kW/kg)