Tiltrotor
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A tiltrotor aircraft combines the vertical lift capability of a helicopter with the speed of a turboprop aeroplane.
As the name implies, it 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, 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.
In vertical flight, the tiltrotor uses controls very similar to a twin or tandem-rotor helicopter. Yaw is controlled by tilting its rotors in opposite directions. Roll is provided through differential power or thrust. Pitch is provided through rotor cyclic or nacelle tilt. Vertical motion is controlled with conventional rotor blade pitch and either a conventional helicopter collective control lever (as in the Bell-Agusta BA-609) or a unique control similar to a fixed wing engine control called a thrust control lever (TCL) (as in the Bell-Boeing V-22 Osprey).
The tiltrotor's advantage is significantly greater speed than a helicopter. 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 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. The two production tiltrotors flown so far have about half the payload of a helicopter with the same power and empty weight. As a result of this reduced payload, a tiltrotor does not exceed the transport efficiency (speed times payload) of a helicopter (reference 1). 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.
The advantages of the V/STOL capability of tiltrotors, particularly to the military, are still being evaluated. However, it is clear that for some military missions, such as rapid troop insertion/extraction and long range combat rescue, the tactical advantage of speed might well be worth the reduced payload capability. Tiltrotors also provide substantially greater cruise altitude capability than helicopters. Tiltrotors can easily reach 20,000 ft or more whereas helicopters typically do not exceed 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. Based on the approved flight manuals for each, the 50,000 lb class (22,600 kg) V-22 Osprey carries the same payload as the 22,000 lb class (9,950 kg) UH-60L Black Hawk helicopter when both operate from a landing zone at 10,000 feet above sea level.
Tiltrotor proprotors require all the fundamental parts of a twin rotor helicopter. They also have a full set of airplane controls, and they have a tilt mechanism that rotates the lifting rotors (while carrying flight loads). This means that the cost of a tiltrotor is typically 50 to 100% more than a helicopter of the same power and empty weight. For example, one V-22 Osprey is reported to cost more than $80 million without including development costs.
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 Vertijet tailsitter, which never went into production. It was a ZLTO VTOL aircraft.
However, Bell Helicopter has been dominant in tiltrotor development with major designs from almost every decade back to the 1950s. They are currently partnered with Boeing on the first production tiltrotor aircraft, the jointly developed and manufactured, Bell-Boeing V-22 Osprey. Bell is developing commercial tiltrotors like the Bell-Agusta BA-609. Bell-Boeing is studying larger Quad Tilt Rotor military models for possible application to the US Army's Joint Heavy Lift program.
List of tiltrotorcraft
See also
External links
- NASA's Tiltrotor research
- Bell-Boeing V-22 Osprey
- Bell-Agusta 609
- Reference 1 - Tiltrotor/Helicopter Payload comparison and transport efficiency shown at "Naval Expiditionary Logistics: Enabling Operational Maneuver from the Sea", Commission on Physical Sciences, Mathematics, and Applications, figs D.3 and D.4, Page 82 at: [1]de:Kipprotor