sentences:  In the case of the AW609's, while flown in helicopter mode, the proprotors can be positioned between a 75- and 95-degree angle from the horizontal, with 87 degrees being the typical selection for hovering vertically; and in airplane mode, the proprotors are rotated forward and locked in position at a zero-degree angle, spinning at 84% RPM.STOL rolling-takeoff and landing capability is achieved by having the nacelles tilted forward up to 45°.Typically, flight control software would perform much of the complex transition between the distinct helicopter and airplane modes; while automated systems are usually provided to inform crews on the optimal tilt angle and air speed to pursue.