| XF-12 Rainbow | |
|---|---|
 | |
| General information | |
| Type | Strategicaerial reconnaissance |
| National origin | United States |
| Manufacturer | Republic Aviation |
| Status | Cancelled |
| Primary user | United States Army Air Forces |
| Number built | 2 |
| History | |
| First flight | 4 February 1946 |
| Retired | June 1952 |
TheRepublic XF-12 Rainbow was an American four-engine, all-metalprototypereconnaissance aircraft designed by theRepublic Aviation Company in the late 1940s. Like most large aircraft of the era, it usedradial engines, specifically thePratt & Whitney R-4360 Wasp Major. The XF-12 was referred to as "flying on all fours" meaning: four engines, 400 mph (640 km/h) cruise, 4,000 mi (6,400 km) range, at 40,000 ft (12,000 m).[1] The aircraft was designed to maximize aerodynamic efficiency. Although innovative, thejet engine and the end ofWorld War 2 made it obsolete, and it did not enter production. A proposed airliner variant, theRC-2, was deemed uneconomical and cancelled before being built.
The original proposal from theUnited States Army Air Corps Air Technical Service Command in late 1943 was for a 400 mph (640 km/h) reconnaissance aircraft with a range of 4,000 nmi (7,400 km; 4,600 mi) and a ceiling of 40,000 ft (12,000 m). Its primary objective was high-speed overflights of theJapanese homeland and key enemy installations. DuringWorld War II, due to the range requirements of operating in the Pacific, existing fighters and bombers were being used but were poorly suited to the USAAF's needs. The requirement existed for dedicated photo-reconnaissance aircraft with speed, range, and altitude capabilities beyond what was then available.
In August 1943, U.S. PresidentFranklin D. Roosevelt's son, ColonelElliot Roosevelt, commander of aLockheed F-5 (a modified P-38 Lightning) "recon" unit, recommended the acquisition of a dedicated high-performance photo reconnaissance aircraft to provide pre- and post-strike target analysis intelligence as well asphoto interpretation to better allow commanders to make decisions for bombing raids. Republic Aviation submitted the XF-12 and it was competing against theHughes XF-11. Both were powered by the new P&W R-4360. The XF-12's first flight was made on 4 February 1946 and during flight testing, it reached an altitude of 45,000 ft (14,000 m) at 470 mph (760 km/h), and demonstrated a range of 4,500 mi (7,200 km), exceeding design criteria. The XF-12 could photograph in both daylight and night and under conditions of reduced visibility at high altitudes over long ranges and with great speed. This "flying photo lab" was capable of mapping broad stretches of the globe.[2]
Only two prototypes were built each of the XF-11 and the XF-12 were ordered into service by the U.S. Army Air Forces,[3] as the requirement evaporated after World War II ended, while the cheaper off-the-shelfBoeing B-29 Superfortress andBoeing B-50 Superfortress could temporarily fill the role until the jet-poweredBoeing RB-47 Stratojet entered service.[4] The XF-12 was the fastest aircraft of its day to use four reciprocating engines, and the only one to exceed 450 mph (720 km/h) in level flight[5]
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 itsfuselage.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 onetrimetrogon 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.[6]
Its wing had a straight taper with a highaspect ratio for maximum efficiency and squared tips. The engines used a slidingcowl to facilitate engine cooling instead of cowling flaps, which caused too much drag. There was also a two-stage impeller fan directly behind thepropeller 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 thenacelle 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 theleading 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 aram air boost for increased power at high speeds, and more effective cooling of the oil and intercoolers. The intakes made up 25% of the total wingspan and were extensively wind tunnel tested. After being used, the air was ducted toward the rear of the nacelle, to provide thrust. The entire engine nacelle was nearly as long as aRepublic P-47 Thunderbolt. Research showed that a force roughly equivalent to 250 hp (190 kW) was generated by each engine exhaust during high speed cruise while at altitude.[7] Each engine featured twin General Electricturbochargers at the rear of the nacelle and for brief bursts of additional power,water-methanol injection.
The XF-12 was originally intended to usecontra-rotating propellers similar to those used on the XF-11, However, due to delivery delays and reliability issues, they were never installed. They would have been twinned three-bladed propellers (rotating in opposite directions). As it was, the aircraft used standard four-bladedCurtiss Electric propellers.[8]
The only visible external difference between the first and second prototypes was the addition of cooling gills on the upper engine cowlings. The second prototype was fitted with the full reconnaissance equipment suite.[4]
The first prototype was damaged on 10 July 1947 while undergoing maximum landing weight tests when the right main gear was severed at the engine nacelle. After bouncing hard and staggering back into the air the test pilot climbed to a safe altitude where excess fuel was burnt off, to lighten the aircraft and reduce the risk of fire. The pilot landed on the left main gear and the nose wheel and despite losing as much speed as possible before the other wing dropped, the aircraft suffered significant damage. The wing spar was cracked, and engines and props needed to be replaced but it was repaired by Republic, and returned to service.[9] When the U.S. Army Air Forces became the U.S. Air Force the XF-12 was later re-designatedXR-12.
Operation Birds Eye was conceived to demonstrate the XF-12's capabilities. On 1 September 1948, the second prototype departed the U.S. Air Force Flight Test Center atMuroc, California, and climbed west to its 40,000 ft (12,000 m) cruising altitude over thePacific before heading east. It then photographed its entire flight path across the United States on 390 individual 10 in (250 mm) photos each covering 490 mi (790 km), which were jointed to form a continuous 325 ft (99 m) print.
They landed atMitchel Field onLong Island, New York after six hours and 55 minutes at an average speed of 361 mph (581 km/h). A photo was taken approximately every 66 seconds. The flight was featured in the 29 November 1948 issue ofLife magazine and the filmstrip exhibited at the 1948 U.S. Air Force Association Convention in New York.[1] The XF-12 program had already been canceled when this flight was made.
On 7 November 1948, the second prototype crashed while returning toEglin Air Force Base from a photographic suitability test flight after the number 2 (port inner) engine exploded, causing violent buffeting. Five of the seven crew escaped safely while two crew members were killed.[10]The first prototype continued flight testing after being returned to service in 1948 but with no orders forthcoming and with the second prototype lost, flight testing was wound down and the remaining prototype was retired in June 1952, having flown just 117 hours from 1949 to 1952. It was later expended as a target at theAberdeen Proving Ground,Maryland.[11]
Republic proposed an airliner version, theRC-2, which would be lengthened to 98 ft 9 in (30.10 m) with a fuselage plug ahead of the wing, and thePlexiglas nose replaced with a conventional nose. Pratt & Whitney R-4360-59s with only one General Electricturbosupercharger each would replace the R-4360-31s, providing more power at lower altitudes, and fuel capacity would be increased. The airliner would carry a crew of seven, and the lavishly appointed cabin for 46 passengers would featurepressurization to sea level with air conditioning, an electric galley providing hot meals, and an inflight lounge. The RC-2 would cruise above bad weather at 435 mph (700 km/h) at 40,000 feet (12,000 m).[citation needed]
American Airlines andPan Am made tentative orders, but without military F-12 orders to subsidize development and tooling costs, the RC-2's purchase price was higher than they would accept. The RC-2 also would have had higher operating costs per passenger than more capacious airliners with similar fuel consumption such as theLockheed Constellation and theDouglas DC-6, and the end of the war created a glut of surplus military transports such as theDouglas C-54 Skymaster which could be readily converted into airliners for a fraction of the cost of new aircraft. Thus, the RC-2 program was cancelled before any were built.[12]
Data from Jane's all the World's Aircraft 1947[13]
General characteristics
Performance
Aircraft of comparable role, configuration, and era
Related lists
