XF-12 Rainbow

infobox Aircraft
name = XF-12 Rainbow
type = Strategic reconnaissance
manufacturer = Republic Aviation

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caption =
designer = Alexander Kartveli
first flight = 4 February 1946
introduced = December 1945
retired = June 1952
produced =
number built = 2 Prototypes (S/N 44-91002 & S/N 44-91003)
status = Cancelled
unit cost = $1.25 Million (1946 Dollars)
primary user = United States Army Air Force
more users = American Airlines, Pan-Am (Republic RC-2 variants were never built)
developed from =
variants with their own articles = The 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.Marrett 2005, p. 23.] Although highly innovative, the postwar XF-12 Rainbow was fated to compete against more modern jet engine technology and was not to 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 nm). 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 was there 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, US 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, at a speed of 470 mph, over a range of 4,500 miles so it met and exceeded the design goals for which it had been designed. Neither the XF-11 or the XF-12 were purchased in any quantity by the Air Force (two each), as their need evaporated after hostilities ended in World War II. [ Machat 1994, p. 12.]

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. [Aviation week, 10 November 1947]

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, that accommodated 18 high-intensity photo-flash bulbs, that 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 electrically heated camera lenses were ground from optical glass 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. [Machat 1994, p. 16.]

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 provided 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 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 superchargers, situated at the aft 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 feet. [Machat 1994, p. 14.]

The original design of the XF-12 called for counter-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. [Machat 1994, p. 10.]

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. They 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. [Machat 1994, p. 50.]

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 designation was later converted to XR-12, when the Army Air Force 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 Air Force Flight Test Center at Muroc, California, and climbed westward to gain altitude over the Pacific Ocean. Upon reaching its 40,000 foot 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 covering a 490-mile-wide field of vision. The aircraft landed at Mitchell field at Garden City, Long Island, New York, completing a flight lasting six hours and 55 minutes (at 361 mph average speed). 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 Air Force Association Convention in New York.

At the time this record flight was made, the Air Force had already canceled the entire XF-12 program. The primary reason for its demise was the availability of both B-29 and B-50 types to meet the long-range photo-reconnaissance requirement until the far more capable RB-47 was brought into service. The B-29 and B-50 gave the Air Force less costly "off the shelf" options. Marrett 2005, p. 26.]

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, by adding 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 Air Force version. The engines also would have only had one General Electric turbo supercharger, instead of the dual arrangement on the 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. [Machat 1994, p. 51.]

Without an order from the 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 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.

In October 1948, prototype number two (s/n 44-91003) crashed on takeoff from Eglin Air Force Base, Florida. The #2 engine (inboard engine on the left wing) exploded shortly after takeoff. The pilot (Lynn Hendrix) was unable to maintain control due to violent buffeting, and he ordered the crew to bail out. Six of the seven man crew were able to bail out successfully (including the pilot Lynn Hendrix). That aircraft crashed into the Gulf of Mexico. The first prototype, which returned to service in 1948, continued the flight testing and development phase. After the 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 (s/n 44-91002) was retired (having flown just 117 additional hours from 1949-1952), was stricken from the Air Force inventory and ended up as a target on the Aberdeen Proving Ground, Maryland. [Machat 1994, p. 50-51.]

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 such ever to exceed 450 mph in level flight. [Machat 1994, p. 9.]

pecifications (XF-12)

aircraft specifications

plane or copter?=plane
jet or prop?=prop
crew=
length main=93 ft 9 in (RC-2 version 98 ft 9 in)
length alt=28.56 m
span main=129 ft 2 in (RC-2 version 129 ft 2 in)
span alt=39.37 m
height main=28 ft 1 in (RC-2 version 29 ft 11 in)
height alt=8.55 m
area main=1,640 ft² (RC-2 version 1,640 ft²)
area alt=152.36 m²
empty weight main=65,000 lb (RC-2 version 67,000 lb)
empty weight alt=29,483 kg
loaded weight main= 101,400 lb (RC-2 version 114,200 lb)
loaded weight alt= 45,994 kg
max takeoff weight main=101,400 lb (RC-2 version 114,200 lb)
max takeoff weight alt=45,994 kg
engine (prop)=Pratt & Whitney R-4360-31 (RC-2 version Pratt & Whitney R-4360-59)
type of prop= Curtiss Electric
number of props=4, four bladed propellers
power main=3,250 hp
power alt=2,423 kW each

max speed main=470+ mph (RC-2 version 450 mph)
max speed alt=756 km/h
range main=4,500 miles (RC-2 version 4,100 miles)
range alt=7,242 km
ceiling main=45,000 ft (RC-2 version 40,000 ft)
ceiling alt=13,716 m
climb rate main=5,000+ ft/min (RC-2 version 5,000+ ft/min)
climb rate alt=1,524 m/min
loading main=lb/ft²
loading alt= kg/m²
power/mass main= hp/lb
power/mass alt= kW/kg

References

Notes

Bibliography

* "Aviation Week", 10 November 1947.
* "Life magazine", November 1948.
* Machat, Mike. "Somewhere, Under a Rainbow." "Wings" Volume 24, No. 2, April 1994.
* Marrett, George. "Flights Into the Future." "Wings" Volume 35, No. 12, December 2005.

External links

* [http://www.aeroplaneart.com.au/Images/JB_Republic_Rainbow.jpgA picture of an XF-12 Rainbow in flight]
* [http://aeroweb.brooklyn.cuny.edu/specs/republic/xf-12a.htm more pictures from Aeroweb]
* [http://www.air-and-space.com/Republic%20XF-12.htm Republic XF-12 from air-and-space.com]

ee also

aircontent
related=
*Republic RC-2

similar aircraft=

lists=

see also=