The Bell X-22 tilt-rotor was a joint-service research effort sponsored by the US Navy to investigate the feasibility of an arrangement of four tilting duct power plants for VTOL aircraft. It was developed as the D2127 light transport and ordered in November 1964. It was an unusual configuration, two engines mounted close in to the forward fuselage, and two on a mainplane mounted near the rear of the aircraft. From 1966 to 1980, some 200 flight test hours were conducted on two Bell X-22 quad tilt duct demonstrators. Although the first X-22 managed to complete a series of vertical and short takeoffs, a hydraulic failure on 8 August 1966 resulted in the total loss of the machine. All test duties were then transferred to the second example. The X-22 was finally retired in October 1984 after completing over 500 flights.
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