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The Bell XV-3 (Model 200) was a convertiplane powered by rotors mounted at the end of the wings, which could switch between vertical and horizontal positions. Although it was produced in the 1950s, it was also an early step in the development of the Bell Boeing V-22 Osprey, which entered development in the 1980s and service in the 2000s.
In 1950 the US Army launched a design competition for a workable convertiplane - something that would combine the VTOL capabilities of the helicopter and the level speed abilities of a conventional aircraft. In 1951 Bell was given a contract to produce their design. Phase I, which began in May 1941, covered the theoretical and engineering problems. This was followed by Phase II, which covered the production of a prototype of the Bell design, with the designation XH-33-BF. This was later changed to XV-3-BF.
The first mock-up of the Bell Model 200 was completed in June 1952, as was a quarter-scale model for wind tunnel tests. These lasted from June to November 1952, and in October 1953 a contract was awarded for the production of two prototypes.
The Model 200 had a conventional aircraft fuselage, but with a heavily glazed forward section which more resembled a helicopter. A single 450hp Pratt & Whitney R-985-AN-3 nine-cylinder radial engine was mounted behind the cockpit, with its air intake above the fuselage. This powered two rotors, one at the end of each wing. The rotor assemblies could rotate from the horizontal position to a nearly horizontal position in 10-15 seconds. The idea was to use gain enough height with them in the vertical position to allow for any loss of lift while forward speed was being gained. Once the rotors were in the horizontal position a gear mechanism would be engaged to reduce the rotor speed. If the engines failed the rotors could be manually returned to the vertical position to allow for a normal helicopter auto-rotative landing.
The first prototype was completed by January 1955, and a series of rotor whirl tests and static ground tests began, lasting to the end of 1955. This period also saw the aircraft make its first flight, a hover test on 23 August 1955. Flight tests continued until 23 August 1956, when the aircraft was damaged in a crash landing.
The second prototype was completed in April 1955. It had two-bladed rotors, which were installed in January 1958. The aircraft was used for full scale wing tunnel tests in October 1958, before on 18 December 1958 the aircraft finally made its first full transition in flight. Bill Quinlan, the Bell test pilot, described the transition as 'smooth and comfortable'.
In January-March 1959 the XV-3 underwent three months of Air Force evaluation at Edwards AFB. The Air Force test pilots found the XV-3 to be generally flexible and forgiving, but there were problems when hovering the aircraft within the ground effect zone (at very low altitude), especially because of the low power of the engine. It was also found to be statically and dynamically unstable below 34.5mph.
The first gear shift was made on 13 April 1959, after the aircraft had been returned to Bell. A total of 250 test flights followed, with 110 full conversions, before the flight programme ended in 1962. For the next three years Bell engineers worked on some of the faults that had been revealed by the test programme, but the aircraft was then badly damaged when the left pylon failed during wind tunnel tests in 1965, and work on the XV-3 came to an end.
This didn’t end Bell's interest in the basic concept. In 1972 Bell was awarded another contract for a VTOL aircraft, leading to the XV-15. This time the engines were also placed in the wing tips,
Engine: Pratt & Whitney R-985-AN-3 nine-cylinder radial engine
Power: 450hp
Crew: 4
Span: 31.3ft (excluding rotors)
Length: 30.3ft
Height: 13f 6in
Empty Weight: 3,600lb
Loaded Weight: 4,800lb
Maximum Speed: 180mph at 12,000ft
Climb rate: 1,400ft/ min