A tilt-rotor aircraft designed for military applications.
The Osprey is a tilt-rotor aircraft, a hybrid of a helicopter and fixed-wing aircraft. Its unique design allows it to take off and land vertically and hover, like a helicopter, and to fly at high forward speeds, like a turboprop airplane. The Osprey weighs 33,140 pounds and can carry almost 20,000 pounds of cargo. Its top speed is 340 knots and its maximum range is 700 miles. From nose to tail, the Osprey measures more than 57 feet. With its 38-foot-diameter rotors, it is almost 84 feet wide.
The Osprey, built under a team agreement between Bell Helicopter Textron and the Boeing Company, is the first tilt-rotor aircraft ever to be approved for production. There are three variants on the basic V-22 design: the MV-22, the CV-22, and the HV-22. The MV-22 was built for the U.S. Marine Corps as a replacement for its aging CH-46 Sea Knight helicopters, which performed combat-assault and assault-support missions. The CV-22 is built for U.S. Air Force long-range, special-operations missions. The U.S. Navy’s version, the HV-22, is intended for combat search and rescue, special operations, and logistics support.
The MV-22 was temporarily grounded in 2000 following a crash that killed nineteen Marines; flights were resumed after an investigation found that the helicopter had descended too quickly. Its future with the Marine Corps was placed in doubt.
In forward flight, the Osprey looks like a fixed-wing aircraft with very large propellers attached to the nacelles located at the wingtips. The nacelles contain the turboshaft engines and transmissions that provide power to the rotors. In the event of an engine failure, an interconnect shaft between the two nacelles allows one engine to power both rotors. The Osprey is unique compared to propeller-driven aircraft or helicopters because its nacelles are designed to pivot. When the nacelles are pivoted such that the rotors are pointed up like those of a helicopter, the Osprey can take off and land vertically or hover. In cruise flight, when the rotors are in their horizontal position, they provide propulsive force like propellers, while the wings provide the lift necessary to keep the aircraft aloft.
Although helicopters have superior performance for vertical takeoff and landing (VTOL) and hovering flight, they are limited in forward speed. A typical helicopter has a cruise speed of less than 150 knots, which is far more slow than that of many propeller-driven, fixed-wing aircraft. In order to achieve speeds approaching those of fixed-wing aircraft, aircraft designers since the 1950’s have investigated concepts for aircraft that can hover, take off, and land vertically and achieve high forward speeds.
In December, 1954, the Model 1-G, built by the Transcendental Aircraft Company, became the first tilt-rotor aircraft ever successfully to perform a transition from hover to forward flight. Before being lost in an accident, the Model 1-G flew more than twenty hours in more than one hundred flights. The Model 1-G was followed by the Model 2, which was tested in 1956 and 1957. Despite these accomplishments, the Air Force, which was supporting tilt-rotor development, chose to shift its support to the Bell Helicopter Company, which completed the first of two XV-3 prototypes in 1955.
In 1973, under contract to the National Aeronautics and Space Administration (NASA) and the Army, Bell Helicopter, by now a subsidiary of Textron, began the development of the XV-15 as a tilt-rotor technology demonstrator aircraft. The XV-15 made its first flight in May, 1977, and performed its first conversion in July, 1979. The two XV-15 aircraft built under this program have flown hundreds of research and demonstration flights and continue to be flown. The unprecedented success of the XV-15 contributed directly to the development of the V-22 Osprey.
Unlike helicopters and fixed-wing aircraft, the Osprey must operate in three flight regimes, cruise, hover, and transition. In cruising flight, it is flown in a manner similar to that of fixed-wing aircraft. Fixed control surfaces on the aircraft allow the pilot to change the aircraft’s attitude. A large elevator on the horizontal tail controls pitch; flaperons, which operate both as flaps and ailerons, on the wings control roll; and rudders on the twin vertical tails control yaw. In order to increase the forward speed of the aircraft, the pilot increases the pitch angle of the rotor blades, thereby increasing the thrust.
In hovering flight, the Osprey’s fixed control surfaces are not effective, because the aircraft has no forward velocity. Therefore, all of the control must come from the rotors. Collective pitch changes are obtained by changing the pitch angle of all rotor blades on one rotor by the same amount. To increase the altitude of the aircraft, the pilot increases the collective pitch on both rotors by the same amount. Roll is obtained with differential collective pitch, which involves increasing the collective pitch on one rotor while decreasing the collective pitch on the other. Changing pitch angle of each rotor blade in a sinusoidal pattern during each revolution effects cyclic pitch changes on a rotor. Unlike collective pitch, cyclic pitch does not change the total thrust produced by the rotor but does produce a movement about an axis perpendicular to a line between the points where the largest and smallest pitch angles are obtained. Control of the aircraft pitch in hover is obtained by changing the cyclic pitch of both rotors by the same amount. Yaw control is obtained by using differential cyclic pitch.
During the transition flight regime, the Osprey changes from being an aircraft that is controlled like a helicopter to one that is controlled like a propeller-driven, fixed-wing aircraft. Through control system software, which relies primarily on measurement of forward speed, the pilot’s control of the aircraft is gradually transitioned between the helicopter flight regime and the cruise flight regime. Once the cruise flight regime has been attained, with the rotors in their horizontal position, the rotation speed of the rotors is changed from its helicopter mode value of 397 revolutions per minute to a cruise value of 332 revolutions per minute.
Emert, P. R. Special Task Aircraft. Englewood Cliffs, N.J.: Silver Burdett, 1990. Specifications and uses of various aircraft designed for special tasks. Jackson, Paul. Jane’s All the World’s Aircraft: 2000-2001, Alexandria, Va.: Jane’s Information Group, 2000. The definitive source for aircraft photographs and specifications. Thornborough, Anthony. V-22 Osprey Bell-Boeing Tilt Rotor. Essex, England: Linewrights, 1990. Part of the publisher’s Aeroguides series, this volume is an in-depth study of the Osprey tilt-rotor aircraft.
Vertical takeoff and landing
The Osprey helicopter is designed to fly either vertically, like a helicopter, or horizontally, like an airplane. Unfortunately, the aircraft has been plagued by fatal crashes.