A tiltrotor aircraft combines the vertical lift capability of a helicopter with the speed and range of an airplane.
As the name implies, a tiltrotor aircraft uses tiltable rotating propellers, or proprotors, for lift and propulsion. For vertical flight the proprotors are angled to direct their thrust downwards, providing lift. In this mode of operation the craft is essentially identical to a helicopter. As the craft gains speed, the proprotors are slowly tilted forward, with the blades eventually becoming perpendicular to the ground. In this mode the wing provides the lift, and the wing's greater efficiency helps the tiltrotor achieve its high speed. In this mode, the craft is essentially a turboprop aircraft.
The tiltrotor's advantage is significantly greater speed than a helicopter. In a helicopter the maximum forward speed is defined by the turn speed of the rotor; at some point the helicopter will be moving forward at the same speed as the spinning of the backwards-moving side of the rotor, so that side of the rotor sees zero or negative airspeed, and begins to stall. This limits modern helicopters to cruise speeds of about 150 knots / 277 km/h. However, with the tiltrotor this problem is avoided, because the proprotors are perpendicular to the motion in the high-speed portions of the flight regime (and thus never suffering this reverse flow condition), meaning that the tiltrotor has relatively high maximum speed - over 300 knots / 560 km/h has been demonstrated in the two types of tiltrotors flown so far, and cruise speeds of 250 knots / 460 km/h are achieved.
This speed is achieved somewhat at the expense of payload. As a result of this reduced payload, a tiltrotor does not exceed the transport efficiency of a helicopter. Additionally, the tiltrotor propulsion system is more complex than a conventional helicopter due to the large, articulated nacelles and the added wing; however, the improved cruise efficiency and speed improvement over helicopters is significant in certain uses. Speed and, more importantly, the benefit to overall response time is the principal virtue sought by the military forces that are using the tiltrotor. Tiltrotors are inherently less noisy in forward flight (airplane mode) than helicopters. This, combined with their increased speed, is expected to improve their utility in populated areas for commercial uses and reduce the threat of detection for military uses. Tiltrotors, however, are typically as loud as equally sized helicopters in hovering flight.
Tiltrotors also provide substantially greater cruise altitude capability than helicopters. Tiltrotors can easily reach 6000 m / 20,000 ft or more whereas helicopters typically do not exceed 3000 m / 10,000 ft altitude. This feature will mean that some uses that have been commonly considered only for fixed-wing aircraft can now be supported with tiltrotors without need of a runway. A drawback however is that a tiltrotor suffers considerably reduced payload when taking off from high altitude.
The V-22 Osprey is a multi-mission, military, tiltrotor aircraft with both a vertical takeoff and landing (VTOL) and short takeoff and landing (STOL) capability. It is designed to perform missions like a conventional helicopter with the long-range, high-speed cruise performance of a turboprop aircraft. Its mission is to conduct long-range infiltration, exfiltration and resupply missions for special operations forces. The USMC variant received the MV-22 designation and the Air Force variant received CV-22.
This versatile, self-deployable aircraft offers increased speed and range over other rotary-wing aircraft, enabling Air Force Special Operations Command aircrews to execute long-range special operations missions. The CV-22 can perform missions that normally would require both fixed-wing and rotary-wing aircraft. The CV-22 takes off vertically and, once airborne, the nacelles (engine and prop-rotor group) on each wing can rotate into a forward position.