The process of rotating the nacelles to transition between helicopter and airplane modes is called conversion. This process is simple, straight forward, and easy to accomplish. The amount and rate of nacelle tilt can be completely controlled by the pilot or can be performed automatically by the flight control system. The minimum time to accomplish full conversion from hover to airplane flight mode is 12 seconds. A tiltrotor can fly at any degree of nacelle tilt.
During vertical takeoff, conventional helicopter controls are utilized. As the tiltrotor gains forward speed to between 40 and 80 knots, the wing begins to produce lift and the ailerons, elevators, and rudders become effective. At this point, rotary-wing controls are gradually phased out by the flight control system. At approximately 100 to 120 knots the wing is fully effective and cyclic pitch control of the proprotors is locked out.
The conversion from airplane flight to a hover simply reverses the process described above. Since the fuselage and wing are free to remain in a level attitude during the conversion, there is no tendency for the wing to stall as speed decreases. Rotor-borne lift fully compensates for the decrease in wing lift. Because there is great variability available between aircraft and nacelle attitude, the conversion corridor (the range of permissible airspeeds for each angle of nacelle tilt) is very wide (about 100 knots). In both accelerating and decelerating flight this wide corridor means that a tiltrotor can have a safe and comfortable transition, free of the threat of wing stall.
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