XF-12 Rainbow
Encyclopedia
The Republic XF-12 Rainbow was an American
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
four-engine, all-metal prototype
Prototype
A prototype is an early sample or model built to test a concept or process or to act as a thing to be replicated or learned from.The word prototype derives from the Greek πρωτότυπον , "primitive form", neutral of πρωτότυπος , "original, primitive", from πρῶτος , "first" and τύπος ,...
reconnaissance
Aerial reconnaissance
Aerial reconnaissance is reconnaissance that is conducted using unmanned aerial vehicles or reconnaissance aircraft. Their roles are to collect imagery intelligence, signals intelligence and measurement and signature intelligence...
aircraft designed by the Republic Aviation Company in the late 1940s. Like most large aircraft of the era, it used radial engine
Radial engine
The radial engine is a reciprocating type internal combustion engine configuration in which the cylinders point outward from a central crankshaft like the spokes on a wheel...
s—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.Although highly innovative, the postwar XF-12 Rainbow was fated to compete against more modern jet engine
Jet engine
A jet engine is a reaction engine that discharges a fast moving jet to generate thrust by jet propulsion and in accordance with Newton's laws of motion. This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, pulse jets...
technology and was not to enter production.
Development
The original proposal for the aircraft, delivered in late 1943, came from the USAACUnited States Army Air Corps
The United States Army Air Corps was a forerunner of the United States Air Force. Renamed from the Air Service on 2 July 1926, it was part of the United States Army and the predecessor of the United States Army Air Forces , established in 1941...
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
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
and key enemy installations. During World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
, 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, US President Franklin D. Roosevelt
Franklin D. Roosevelt
Franklin Delano Roosevelt , also known by his initials, FDR, was the 32nd President of the United States and a central figure in world events during the mid-20th century, leading the United States during a time of worldwide economic crisis and world war...
'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 ft, 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 was purchased in any quantity by the Air Force (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
Water injection (engines)
In internal combustion engines, water injection, also known as anti-detonant injection, is spraying water into the cylinder or incoming fuel-air mixture to cool the combustion chambers of the engine, allowing for greater compression ratios and largely eliminating the problem of engine knocking...
, it suddenly assumed tremendous importance in the eyes of both the U.S. Air Force
United States Air Force
The United States Air Force is the aerial warfare service branch of the United States Armed Forces and one of the American uniformed services. Initially part of the United States Army, the USAF was formed as a separate branch of the military on September 18, 1947 under the National Security Act of...
and the State Department
United States Department of State
The United States Department of State , is the United States federal executive department responsible for international relations of the United States, equivalent to the foreign ministries of other countries...
. 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 fuselageFuselage
The fuselage is an aircraft's main body section that holds crew and passengers or cargo. In single-engine aircraft it will usually contain an engine, although in some amphibious aircraft the single engine is mounted on a pylon attached to the fuselage which in turn is used as a floating hull...
. Aviation Week
Aviation Week & Space Technology
Aviation Week & Space Technology, often abbreviated Aviation Week or AW&ST, is a weekly magazine owned and published by McGraw-Hill...
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
Fairchild Camera and Instrument
Fairchild Camera and Instrument Corporation was a company founded by Sherman Fairchild. It was based on the East Coast of the United States, and provided research and development for flash photography equipment...
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
Aspect ratio (wing)
In aerodynamics, the aspect ratio of a wing is essentially the ratio of its length to its breadth . A high aspect ratio indicates long, narrow wings, whereas a low aspect ratio indicates short, stubby wings....
for maximum efficiency. The engines featured a sliding cowl
Cowling
A cowling is the covering of a vehicle's engine, most often found on automobiles and aircraft.A cowling may be used:* for drag reduction* for engine cooling by directing airflow* as an air intake for jet engines* for decorative purposes...
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
Propeller (aircraft)
Aircraft propellers or airscrews convert rotary motion from piston engines or turboprops to provide propulsive force. They may be fixed or variable pitch. Early aircraft propellers were carved by hand from solid or laminated wood with later propellers being constructed from metal...
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
Nacelle
The nacelle is a cover housing that holds engines, fuel, or equipment on an aircraft. In some cases—for instance in the typical "Farman" type "pusher" aircraft, or the World War II-era P-38 Lightning—an aircraft's cockpit may also be housed in a nacelle, which essentially fills the...
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
P-47 Thunderbolt
Republic Aviation's P-47 Thunderbolt, also known as the "Jug", was the largest, heaviest, and most expensive fighter aircraft in history to be powered by a single reciprocating engine. It was heavily armed with eight .50-caliber machine guns, four per wing. When fully loaded, the P-47 weighed up to...
(also built by Republic). Each engine featured twin General Electric turbocharger
Turbocharger
A turbocharger, or turbo , from the Greek "τύρβη" is a centrifugal compressor powered by a turbine that is driven by an engine's exhaust gases. Its benefit lies with the compressor increasing the mass of air entering the engine , thereby resulting in greater performance...
s, 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
Contra-rotating propellers
Aircraft equipped with contra-rotating propellers, also referred to as coaxial contra-rotating propellers, apply the maximum power of usually a single piston or turboprop engine to drive two propellers in contra-rotation...
, 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 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
Edwards Air Force Base
Edwards Air Force Base is a United States Air Force base located on the border of Kern County, Los Angeles County, and San Bernardino County, California, in the Antelope Valley. It is southwest of the central business district of North Edwards, California and due east of Rosamond.It is named in...
, and climbed westward to gain altitude over the Pacific Ocean
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
. 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 covering a 490-mile-wide field of vision. The aircraft landed at Mitchel Field at Garden City
Garden City, New York
Garden City is a village in the town of Hempstead in central Nassau County, New York, in the United States. It was founded by multi-millionaire Alexander Turney Stewart in 1869, and is located on Long Island, to the east of New York City, from mid-town Manhattan, and just south of the town of...
, Long Island
Long Island
Long Island is an island located in the southeast part of the U.S. state of New York, just east of Manhattan. Stretching northeast into the Atlantic Ocean, Long Island contains four counties, two of which are boroughs of New York City , and two of which are mainly suburban...
, 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
Life (magazine)
Life generally refers to three American magazines:*A humor and general interest magazine published from 1883 to 1936. Time founder Henry Luce bought the magazine in 1936 solely so that he could acquire the rights to its name....
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
B-50 Superfortress
The Boeing B-50 Superfortress strategic bomber was a post-World War II revision of the Boeing B-29 Superfortress, fitted with more powerful Pratt & Whitney R-4360 radial engines, stronger structure, a taller fin, and other improvements. It was the last piston-engined bomber designed by Boeing for...
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.
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
Acrylic glass
Poly is a transparent thermoplastic, often used as a light or shatter-resistant alternative to glass. It is sometimes called acrylic glass. Chemically, it is the synthetic polymer of methyl methacrylate...
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 each, 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
Cabin pressurization
Cabin pressurization is the pumping of compressed air into an aircraft cabin to maintain a safe and comfortable environment for crew and passengers when flying at altitude.-Need for cabin pressurization:...
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 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
American Airlines
American Airlines, Inc. is the world's fourth-largest airline in passenger miles transported and operating revenues. American Airlines is a subsidiary of the AMR Corporation and is headquartered in Fort Worth, Texas adjacent to its largest hub at Dallas/Fort Worth International Airport...
and Pan-Am
Pan American World Airways
Pan American World Airways, commonly known as Pan Am, was the principal and largest international air carrier in the United States from 1927 until its collapse on December 4, 1991...
) 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
Lockheed Constellation
The Lockheed Constellation was a propeller-driven airliner powered by four 18-cylinder radial Wright R-3350 engines. It was built by Lockheed between 1943 and 1958 at its Burbank, California, USA, facility. A total of 856 aircraft were produced in numerous models, all distinguished by a...
and the Douglas DC-6
Douglas DC-6
The Douglas DC-6 is a piston-powered airliner and transport aircraft built by the Douglas Aircraft Company from 1946 to 1958. Originally intended as a military transport near the end of World War II, it was reworked after the war to compete with the Lockheed Constellation in the long-range...
. 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
C-54 Skymaster
The Douglas C-54 Skymaster was a four-engined transport aircraft used by the United States Army Air Forces and British forces in World War II and the Korean War. Besides transport of cargo, it also carried presidents, British heads of government, and military staff...
. 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 1300 hrs. while returning to Eglin Air Force Base
Eglin Air Force Base
Eglin Air Force Base is a United States Air Force base located approximately 3 miles southwest of Valparaiso, Florida in Okaloosa County....
, 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. Airframe impacted two miles south of the base, in the Choctawhatchee Bay
Choctawhatchee Bay
Choctawhatchee Bay is a bay in the Emerald Coast region of the Florida Panhandle. The bay, located within Okaloosa and Walton counties, has a surface area of 129 mi2...
. 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 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 Air Force inventory and ended up as a target on the Aberdeen Proving Ground
Aberdeen Proving Ground
Aberdeen Proving Ground is a United States Army facility located near Aberdeen, Maryland, . Part of the facility is a census-designated place , which had a population of 3,116 at the 2000 census.- History :...
, Maryland
Maryland
Maryland is a U.S. state located in the Mid Atlantic region of the United States, bordering Virginia, West Virginia, and the District of Columbia to its south and west; Pennsylvania to its north; and Delaware to its east...
.
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.
