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The Bell XV-15 was a successful experiment into tilt-rotor technology and played a major role in the development of the Bell Boeing V-22 Osprey.
Bell already had some experience with tilt-rotor technology, having developed the Bell XV-3. This aircraft had its engine in the fuselage and rotors at the end of its wings, and had successfully moved from vertical to horizontal flight in 1958. Four years of flight tests followed, proving that the basic concept was valid.
In 1973 Bell was given a joint NASA and US Army contract to prove that tilt rotor technology could work.
The XV-15 was a high wing monoplane, with a fuselage that resembled a lengthened helicopter more than a conventional aircraft. It had high mounted wings, located just behind the cockpit. Unlike on the XV-3, this time the engines were located in nacelles at the end of the wings, and the entire engine and propeller rotated between horizontal and vertical positions. It took 12 seconds to rotate from one position to the other. In case of engine failure driveshafts ran along the full length of the wings, connecting the two engines, so that both propellers could be powered by a single engine.
The fuselage and tail assembly was built by Rockwell and delivered to Bell in October 1975. The first aircraft rolled out on 22 October 1976. A careful series of tests followed. The first simulated transition was made on 3 March 1977. The first untethered flight came on 3 May 1977.
The second prototype made its maiden flight on 23 April 1979. The first full transition in free flight came on 24 July 1979, and on 21 April 1980 prototype no.2 reached 302mph. Over the next year the aircraft logged 40 hours of tests, which proved that the basic configuration worked.
The first prototype then went to NASA and the US Army to investigate possible operational uses of the type. In October 1981 the second prototype began a series of tests at NASA to find the limits of its flight envelope.
The first prototype was used to test out its suitability for special electronic missions. On 2-5 August 1982 it made 54 landings on the USS Tripoli (LPH-10), an amphibious assault ship, to see if it was suitable for use at sea. By the end of August 1982 the two aircraft had flown for 289 hours.
During the 1980s the two prototypes were used to evaluate the military and civil uses of the type, and in particular to help with the long development of what became the Bell Boeing V-22 Osprey. In 1987 the second prototype was given new carbon fibre and Nomex rotors as part of that process.
The first prototype was eventually returned to Bell, but it was written off after a crash on 20 August 1992. The second prototype continued to be used to support the V-22 programme until 2003. It was then given to the Smithsonian National Air and Space Musueam.
Engine: Four Avco Lycoming LTC1K-4K turboshafts
Power: 1,550hp each
Span: 35ft 2in (over engine nacelles)
Length: 42ft 1in
Height: 15ft 4in
Empty Weight: 9,570lb
Max VTOL Weight: 13,000lb
Max STOL Weight: 15,000lb
Maximum Speed: 382mph at 17,000ft
Cruising Speed: 349mph at 16,300ft
Climb rate: 3,150ft/ min
Range: 512 miles