On several aerial vehicles such as the AgustaWestland AW609 and V-22 Osprey, a pair of three-bladed proprotors have been used. Both the proprotors and engines are mounted on load-bearing rotatable pylon at the wingtips, allowing the proprotors to be positioned at various angles. 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. Furthermore, it is typical for flight controls, such as blade pitch, to both resemble and function akin to their counterparts on conventional rotorcraft, easing the transition of conventional helicopter pilots to such vehicles. Proprotors can be designed to fold for storage purposes. However, in the case of the V-22, in order to facilitate proprotor folding, the proprotor's diameter had to be constrained to a diameter of 38-foot, five feet less than optimal for vertical takeoff; this difference has been attributed for causing relatively high disk loading. In a typical implementation, both proprotors must be rotating in order to maintain flight in helicopter mode. To guard against instances of single engine failure, on both the V-22 and AW609, both engines are connected by drive shafts to a common central gearbox so that one engine can power both proprotors if such a failure occurs. Despite this provision, the V-22 is generally not capable of hovering on a single engine. If a proprotor gearbox fails, that proprotor cannot be feathered, and both engines must be stopped prior to an emergency landing. The autorotation characteristics are poor partly due to the rotors' low inertia.
