Republic XF-12 Rainbow

https://en.wikipedia.org/wiki/Republic_XF-12_Rainbow

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.

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.

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.


Role
Strategic aerial reconnaissance

National origin
United States

Manufacturer
Republic Aviation

First flight
4 February 1946

Retired
June 1952

Status
Cancelled

Primary user
United States Army Air Forces

Number built
2

Unit cost

$1.25 Million (1946 Dollars)

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 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.

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 one 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).

Specifications (XF-12)

General characteristics
Crew: 7
Capacity: (RC-2: 46 pax)
Length: 93 ft 10 in (28.59 m) (RC-2: 98 ft 9 in (30.10 m))
Wingspan: 129 ft 2 in (39.36 m)
Height: 28 ft 4 in (8.63 m) (RC-2: 29 ft 11 in (9.12 m))
Wing area: 1,640 sq ft (152 m2)
Empty weight: 65,000 lb (29,484 kg) (RC-2: 66,980 lb (30,380 kg))
Gross weight: 101,400 lb (45,994 kg) (RC-2: 114,200 lb (51,800 kg))
Fuel capacity: 5,000 US gal (4,200 imp gal; 19,000 l)
Powerplant: 4 × Pratt & Whitney R-4360-31 Wasp Major 28-cyl. four-row air-cooled
radial piston engines, 3,250 hp (2,420 kW) each
Propellers: 4-bladed Curtiss, 16 ft 1 in (4.9 m) diameter reversible-pitch
constant-speed propellers

Performance
Maximum speed: 470 mph (756 km/h; 408 kn) plus (RC-2: 450 mph (390 kn; 720
km/h))
Cruise speed: 400 mph (644 km/h; 348 kn) at 40,000 ft (12,000 m)
Stall speed: 104 mph (167 km/h; 90 kn)
Range: 4,500 mi (3,910 nmi; 7,242 km) (RC-2: 4,100 mi (3,600 nmi; 6,600 km))
Service ceiling: 44,000 ft (13,000 m) plus (RC-2: 41,000 ft (12,000 m))
Rate of climb: 5,000 ft/min (25 m/s) plus (RC-2: 1,680 ft/min (510 m/min))
Wing loading: 61.8 lb/sq ft (302 kg/m2) (RC-2: 69.6 lb/sq ft (340 kg/m2))
Power/mass: 8.4 lb/hp (5.09 kg/kW) (RC-2: 8 lb/hp (4.855 kg/kW))
Take-off distanceRC-2: 4,950 ft (1,510 m) to 50 ft (15 m))
Landing distance: (RC-2: 3,550 ft (1,080 m) from 50 ft (15 m))



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