Tiltrotor
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A tiltrotor aircraft combines the maneuverability of a helicopter with the speed of a turboprop aeroplane.
As the name implies, it uses tiltable (rotating) propellers, or rotors, for lift and propulsion. For vertical flight the rotors 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 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 tiltrotor 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 to the rotors, or by tilting its rotors in opposite directions. Vertical motion is controlled with conventional pitch and collective controls, just like a helicopter.
The tiltrotor'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 backwards-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, meaning that the tiltrotor has relatively high maximum speed - up to 300 knots (560 km/h) has been demonstrated in the two types of tiltrotors flown so far.
This speed is only achieved at the expense of payload, so that typical tiltrotors carry about half the payload of typical helicopters. For example, the V-22 tiltrotor has an empty weight of about 32,000 lb (14,500 kg), has two 6,150 shp (4.6 MW) engines and carries 24 troops. To illustrate the relatively greater efficiency of a helicopter, the CH-53E Super Stallion has the same empty weigh and the same engine power (3x 4300 shp), but can support a 50% higher maximum weight. This allows the CH-53E to carry 55 troops, and results in a payload that is more than twice that of the V-22. Because of the helicopter's greater payload, it has greater range than the tiltrotor, since it can carry much more fuel. The V-22, however, cruises 50% faster, and at much higher cruise altitudes. Because of this payload disadvantage, the tiltrotor's transport efficiency (speed times payload) does not ever equal that of a helicopter. The speed advantage alone, however, is significant in some military missions. For example, if the mission extends to 500 nautical miles (900 km), a typical tiltrotor 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 the military forces that advocate the tiltrotor.
Though more complex both mechanically and to fly and as well as having a lower payload than fixed-wing aircraft, the advantages of the V/STOL capability of tiltrotors, particularly to the military, are debated. It is clear that for some military missions, such as special operations and long range rescue, the tactical advantage of speed might well be worth the reduced payload capability.
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 could 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
See also
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/)ja:ティルトローター