The Bell XV-15 was a joint U.S. Army/NASA proof-of-concept tiltrotor that demonstrated practical conversion between helicopter-mode vertical flight and efficient airplane-mode cruise. Its tilting wingtip nacelles and cross-shafted proprotors established the technology path to the V-22 Osprey and later civil tiltrotors; two Bell-built aircraft supported wind-tunnel, flight, shipboard, and operational research from 1977 onward.
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