The Boeing Vertol BV-347 (also known as the Model 347) is an experimental helicopter built by Boeing Vertol from a converted CH-47A Chinook. It was used to test potential upgrades for the Chinook, several of which were later incorporated into subsequent versions of the CH-47.
Design and development
Development of the BV-347 began in January 1969, when Boeing Vertol was contracted by the United States Army to develop an improved CH-47 Chinook. The Army loaned a single CH-47A, serial number 65-7992, to be modified in exchange for exclusive rights to the project's research data. The modifications, which were funded by Boeing, were carried out in two phases.[1] Phase I began with redesigned rotors, which had four blades of increased diameter compared to the Chinook's three-blade rotors, as well as a 30-degree offset flapping hinge.[2] The fuselage was stretched by 110 inches (280 cm), reducing the rotor overlap from 35% on the original Chinook to 22%, and provisions were added to mount small wings to the upper fuselage at a later time. The rear pylon was raised 30 inches (76 cm) to increase rotor clearance and decrease noise. The engines were replaced with uprated T55-L-11s, which would later be standard on the CH-47C.[1][3] The cockpit layout was improved, with thicker glass being added, as well as a bulkhead with an entrance door and other improvements to reduce cockpit noise.[1]
For Phase II, a pair of small wings was added to the fuselage. These wings served to provide extra lift and allowed the BV-347 to achieve a 60 degree banked turn. They had a variable incidence from 10 degrees down to 85 degrees up, and featured full-span flaps. During hover, the wings would be tilted to 85 degrees to minimize interference with the rotor downwash.[1]
The BV-347 made its first flight in Phase I configuration on May 27, 1970.[1] During Phase I test flights, the BV-347 demonstrated a transmission-limited maximum true airspeed of 177 knots (204 mph; 328 km/h), excellent stability and flight characteristics, and low vibration and sound levels. Flight testing of the Phase II configuration began in December 1971.[1]
Boeing Vertol used the BV-347 as part of the development of its XCH-62. As part of the program, the BV-347's hydro-mechanical rotor control system was replaced with a fly-by-wire system, becoming the first helicopter in history to be controlled with such a system. The BV-347 was also fitted with a retractable ventral gondola, with its own set of controls, which was used to operate the load recovery system. A sidearm controller originally developed for the XCH-62 was installed in the gondola. The new control system was noted to be easy for novice pilots to fly. During demonstrations in Washington DC, the BV-347 was flown by over 125 pilots, 100 of which had never flown an aircraft of any kind before, with some even being capable of performing normally difficult maneuvers.[1]
Flight testing of the BV-347 was completed in 1975.[3] Although the Army could not feasibly upgrade its entire CH-47 fleet with the stretched fuselage and wings of the BV-347, the latter did influence some of the improvements made to the CH-47D, such as the flight control systems and reliability improvements.[1][2]
Variants
Head on view of a BV-347
BV-347 Phase I
Original configuration with stretched fuselage.
BV-347 Phase II
Fitted with small wings on the mid-fuselage.
Civil BV-347
A civil variant of the BV-347 considered by NASA.[4]
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