From concept to reality, it has taken a long time for the tiltrotor to earn its wings - more than 50 years will have elapsed between the first flight of an aircraft with tilting rotors and entry into service of the first operational tiltrotor.
Transcendental Aircraft was first to try tilting rotors, its Model 1G making the first conversion between vertical and forward flight in December 1954. Weighing 800kg (1,750lb), the single-seat 1G was powered by a single piston engine driving two three-bladed rotors at the wingtips. The rotors took 3min to transition through 82°.
Transcendental challenged Bell in a US Army/Air Force contest for an experimental tiltrotor. Bell won, and its XV-3 flew as a helicopter in August 1955, but crashed before a full conversion. After a change from three-blade fully articulated to two-blade semi-rigid rotors to improve rotor stability, the second single-engined XV-3 made a full conversion in late 1958. Tilting the rotors through 90° took 10s.
There followed a gap of almost 20 years before Bell rolled out the proof-of-concept XV-15 - the first truly successful tiltrotor. Built for the US Army and NASA, the XV-15 was powered by two turboshafts in tilting wingtip nacelles, driving three-blade proprotors and cross-shafted in case of engine failure. First hover was in May 1977, first conversion in July 1979, and the XV-15 reached a maximum speed of 345kt (640km/h).
Bell and Boeing Vertol, which had competed to build the XV-15, teamed in 1982 to offer a tiltrotor for the JVX joint advanced vertical-lift programme. A year later, they were selected to develop the V-22 Osprey. The aircraft first flew in March 1989, but it took many more years to produce a safe and effective operational tiltrotor. The V-22 was to survive cancellation in 1989, crashes in 1991 and 1992, and two more accidents in 2000 before finally making the grade.
Meanwhile, Bell began to develop the first civil tiltrotor, the BA609, in a joint venture with AgustaWestland. The six- to nine-seat aircraft first flew in March 2003, but certification is not expected until 2010. Bell and Boeing are studying the Quad Tilt Rotor (QTR), a C-130-sized design with tandem wings and four proprotors that will contest the US Army-led Joint Heavy Lift (JHL) requirement.
Source: Flight International
