A Tiltrotor is not A Helicopter
Although a tiltrotor aircraft can hover and has excellent maneuverability and handling qualities in vertical flight like a helicopter, it has other capabilities that greatly exceed those of a helicopter. Consequently, it is misleading and inaccurate to refer to a tiltrotor as a helicopter. A helicopter’s wing is its rotor (or rotors). The rotor blades are rotated about a shaft above the aircraft. As the rotors pass through the air, they create the lift so the aircraft can remain stationary over the ground – the process is called hovering. Varying the pitch of the rotor blades (as they rotate) provides control of the helicopter. The helicopter can move vertically up and down by increasing or decreasing pitch on all rotor blades simultaneously – a process called collective pitch control. The helicopter can control its movement over the ground by varying the pitch of individual blades, increasing or decreasing lift at selected points during blade rotation – a process called cyclic pitch control. The combination of collective and cyclic pitch-control gives the helicopter its excellent control characteristics in the hover. The helicopter’s thrust is almost always pointed upward. The helicopter achieves forward flight by tilting the plane of its blade rotation forward; thus, slightly tilting its thrust in the desired direction of flight. Tilting the thrust direction is an inefficient method for generating forward thrust. Consequently, it requires a great deal of power to achieve high speeds, while sustaining level flight. This characteristic of the helicopter, along with the high drag of its rotor system, and the problem of retreating blade stall (at higher speeds) accounts for the speed limitations of a helicopter. The tiltrotor achieves its lift and control in hovering flight in exactly the same way as a helicopter: proprotor system lift, collective pitch-control, and cyclic pitch-control. This gives the tiltrotor its excellent hover and slow flight-handling characteristics. However, the tiltrotor can tilt its proprotors from vertical to horizontal for providing thrust while relying on its wing for lift. In this mode, the tiltrotor overcomes many of the helicopter’s high speed limitations: Some of these helicopter’s limitations include: • High rotor system drag • Retreating blade stall • High vibratory loads A tiltrotor has characteristics uncommon to conventional single rotor helicopters or conventional airplanes. Of particular significance are the following: • The counter-rotating proprotors eliminate the yawing movement due to primary lift, which drives the requirement for an anti-torque device in single rotor helicopters. • The interconnecting driveshafts automatically deliver power to both rotors following the loss of one engine, which eliminates asymmetrical thrust during single engine operation.
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