A tilt-rotor aircraft combines the maneuverability of a helicopter with the speed of a turboprop aeroplane. It achieves the speed at the expense of payload, so that typical tilt rotors carry about half the payload of typical helicopters. For example, the BA-609 civil tilt rotor has two 2000 horsepower (1,500 kW) class engines and weighs about 11000 lb (5 t) empty weight to carry 6 to 8 passengers. The UH-60 Black Hawk has the same power and empty weight, and can carry over 20 passengers. Tilt rotors are more efficient in cruise flight, burning much less fuel that helicopters. This reduces the payload disadvantage somewhat at longer ranges, but the tilt rotor is not capable of matching a helicopter's payload at any range. The cruise speed of a tilt rotor is substantially faster than a helicopter, about 250 knots (460 km/h) versus 150 knots (280 km/h) for a modern helicopter. With the payload disadvantage of the tilt rotor, its transport efficiency (speed times payload) does not exceed that of a helicopter. The speed advantage is significant in some military missions, for example, if the mission extends to 500 nautical miles (900 km), a typical tilt rotor can arrive in 2 hours, where a helicopter might take as long as 3 1/2 hours. The 1 1/2 hours saved could be very valuable tactically, and is the principal virtue sought by military forces that advocate the tilt rotor.

As the name implies, it uses tiltable (rotating) propellers, or rotors, for lift and propulsion. For vertical flight the rotors are tilted to blow down, providing lift. In this mode of operation the craft is essentially identical to a helicopter. As the craft gains altitude, the rotors are slowly tilted to point towards the rear, eventually becoming perpendicular to the ground. In this mode the wing provides the lift, and the wing's greater efficiency helps the tilt rotor achieve its high speed. In this mode the craft is essentially a propeller aircraft.

In vertical flight, the tiltrotor uses controls very similar to a twin-rotor helicopter. Yaw is controlled either by applying differential power the rotors, and by tilting its rotors in opposite directions. Vertical motion is controlled with conventional pitch and collective controls, just like a helicopter.

The tilt rotor's advantage is primarily speed. In a helicopter the maximum forward speed is defined by the speed that the rotor turns at, at some point the helicopter will be moving forward at the same speed as the backward-moving side of the rotor is spinning, so that side of the rotor sees zero airspeed, and stalls. In reality the maximum speed is even less than this. However with the tiltrotor this problem is avoided, because the rotors are perpendicular to the motion in the high-speed portions of the flight regime. This means the tiltrotor has relatively high maximum speed, up to 300 knots (560 km/h) has been demonstrated in the two types of tilt rotors flown so far. Tilt rotors require about 50% more installed power as a helicopter of the same lifting capacity, meaning that the tiltrotor requires larger engines for a given mission, increasing its cost and reducing the time it can hover (for a given fuel load). However, the wing of the tilt rotor is significantly more efficient than the rotor of a helicopter, so that the tilt rotor uses considerably less fuel per unit distance when at high speed. This helps offset the smaller payload of the tilt rotor, especially at longer ranges. These qualities are of particular interest to the military, where speed is quite important.

It is commonly assumed that tilt rotors have more range than helicopters, but the comparison of like aircraft (same power and empty weight) shows that the lesser payload of the tilt rotor so significantly reduces the fuel it can carry that it never exceeds the range of a helicopter. Past comparisons that stated otherwise typically compared much smaller helicopters to much larger tilt rotors, for example when tilt rotor supporters would say that the V-22 tilt rotor will "go twice as far, twice as fast" as the helicopter it replaces. This pitted the 1960's 21,000 lb (9.5 t), 2,000 horsepower (1,500 kW) CH-46 helicopter with the 50,000 lb (23 t) 13,000 horsepower (9,700 kW) V-22. When the V-22 is compared with a helicopter with the same power and the same empty weight, it carries half the payload about the same distance, and flies about 50% faster. Tiltrotors have a greater number of expensive components and structure than airplanes and helicopters. Their two rotors require all the fundamental parts of a twin rotor helicopter, they also have a full set of airplane controls, and they have a critical tilt mechanism that slews the lifting rotors (while carrying flight loads). This means that the typical tltrotor has about three times the number of flight-critical components, adding to its cost and complexity. Several designs of such aircraft have been built, starting with the introduction of large turbine engines in the late 1950s. Two particularly successful designs were the Canadair CL-84 Dynavert tiltwing and the LTV XC-142 tiltwing. Both aircraft were technical successes, but neither entered production due to other issues. Another design philosophy was that instead of turning the wing, engine pods, or propeller shafts to horizontal and vertical, the entire aircraft could do the same. This resulted in the Ryan X-13 tailsitter, which never went into production. It was a ZLTO VTOL aircraft.

However Bell Aircraft was the primary keeper of the tiltrotor flame, with major designs from almost every decade back to the 1950s. They are also the only company to have produced a production tiltrotor aircraft, the Boeing-built V-22 Osprey. The Osprey has had a chequered history, but the reasons for this are not entirely clear – earlier projects were just as challenging and worked, and Bell's earlier models leading up to the V-22 were generally very successful. It appears that these problems were due to the natural problems that appear when a new configuration is fielded, where the flaws in the concept are defined and mitigated. With the bugs being worked out of the design, Boeing is now moving on to commercial tiltrotor designs, and Bell is studying larger four-rotor military models that would replace the Lockheed C-130 Hercules.

The V-22 has had several very serious accidents, leading to threats of cancellation. Additional testing and re-planning the program has reportedly helped understand and eliminate the accident causes. Most serious was the loss of an aircraft full of Marines, where the aircraft lost control and crashed upside down, killing all aboard. The cause was found to be an undiscovered propensity for the rotor to enter a vortex ring state, where its lift sharply reduces when it descends into its own downwash. While all rotorcraft can experience this, it becomes critical for tiltrotors because the loss of lift on one rotor created asymmetry in lift, causing it to go upside down. Helicopters lose lift during such events, but generally remain under control. This problem has been carefully studied, and is reportedly able to be controlled through additional pilot instruments and reduced maneuvering capabilities.

List of tiltrotorcraft

• V-22 Osprey
• Bell/Agusta BA609
• XV-15
• XV-3
• HV-911D Eagle Eye

See also

• Tiltwing
• Tailsitter

External links

NASA's Tiltrotor research http://www.simlabs.arc.nasa.gov/library/tiltrotor/ctr20th.html

Boeing V-22 Osprey http://www.boeing.com/rotorcraft/military/v22/flash.html

Bell-Agusta 609 http://www.bellagusta.com/html/theAircraft/ba_609/