What is a Tiltrotor?
A “tiltrotor” is a unique type of aircraft that has the cruise speed and efficiency to achieve the range and endurance of a turboprop airplane and can takeoff, hover, and land vertically to provide the runway independence of a conventional helicopter. A tiltrotor has a wing with lift/propulsive proprotors at each wing tip. These proprotors are aerodynamically designed to function both as propellers and as rotors. The proprotors, along with their engines and reduction gearboxes, are mounted in the wingtip nacelles. They rotate from a horizontal position (airplane or cruise mode) to the vertical position (helicopter or hover mode). In helicopter mode, the proprotors provide all lift and control with the required engine power being the highest when hovering. As the aircraft transitions to airplane mode, the power and thrust of the engines are reduced (as the wing takes on more of the lift). As a result, the tiltrotor can achieve a very efficient cruise performance in airplane mode. Additionally, the tiltrotor can stop the transition and fly in the conversion mode, a flight regime unique to the tiltrotor.
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.
A Tiltrotor is not A Tiltwing
In the tiltwing configuration, the aircraft’s engines and propellers are rigidly mounted to the wing. The entire propulsion system and wing are tilted between the vertical and horizontal positions. Propellers, instead of proprotors, provide the propulsion. They have no cyclic pitch-control in the vertical flight mode. Due to the concerns of wing stall (during conversion), the envelope of safe tilt angles for different airspeeds is very small. The tiltwing flies well in the airplane mode; but is inefficient at low speeds and during hover. Tiltwings avoid hovering for long periods of time due to their heavy fuel consumption.
A Tiltrotor is not an Airplane
An airplane’s lift is produced from its wing(s). An airplane creates lift by moving its wing through the air fast enough to generate the necessary lift to overcome the aircraft’s weight. An airplane uses propellers or jet engines to maintain the speed necessary to sustain flight. At low speeds the airplane wing stalls and can no longer maintain lift. The airplane’s direction of flight is controlled through the use of ailerons, elevators, and rudders. In airplane mode, the tiltrotor functions the same as a typical airplane. In forward flight, the tiltrotor is supported by the lift of its wing. Its turboshaft engines drive the proprotors in order to sustain its speed. Its controls are ailerons, elevators and rudders, and function like those of a conventional airplane. However, for low speed or hovering flight, a tiltrotor can rotate its nacelles (i.e., its thrust direction) from the horizontal to the vertical position — something no airplane can do. Finally, a tiltrotor’s proprotors are much larger than an airplane’s propeller. Consequently, the proprotors can generate the same amount of thrust as an airplane at a much slower RPM. The lower tip speed of the tiltrotor makes it very quiet in cruise flight (even at high speeds) which is a significant advantage when operating in urban or high threat environments.
Flying a Tiltrotor
The pilot controls both flight modes with a single set of controls. The conventional airplane stick, rudder pedals, and thrust lever automatically function like a cyclic stick, yaw pedals, and collective control in a helicopter. The flight control system is designed to smoothly change the flight control functions between helicopter and fixed wing modes automatically and transparently as the aircraft speed increases or decreases during conversion from one mode to the other. In a tiltrotor, the pilot also controls the nacelle angle. Using the nacelle control provides an additional method to accelerate forward or aft, and to control aircraft pitch attitude – it is used as complementary control with the longitudinal cyclic stick, and includes automatic conversion corridor protection control.
Transition and Conversion
The process of rotating the nacelles between helicopter and airplane mode is called “transition”, and the reverse from airplane mode to helicopter mode, “conversion”. Transition and conversion procedures are simple, straightforward, and easy to accomplish. The amount and rate of nacelle tilt can be manually controlled by the pilot or can be performed automatically by the flight control system. The V-22 can perform a complete transition from helicopter mode to airplane mode in as little as 16 seconds. Conversions and transitions can be continuous, stopped partway through, or reversed as desired. A tiltrotor can fly at any degree of nacelle tilt within the authorized conversion corridor envelope. During vertical takeoff, the conventional helicopter controls are utilized. As the tiltrotor gains forward speed, the wing begins to produce lift and the ailerons, elevators, and rudders become more effective. Between 40 and 80 knots, the rotary-wing controls begin to be phased out by the flight control system. Once in airplane mode, the wing is fully-effective and pilot control of cyclic pitch of the proprotors is locked out. Because the nacelle angle can be commanded separately from the primary pitch controls of rotor cyclic and tail elevator, the conversion corridor (the range of permissible airspeeds for each angle of nacelle tilt) is very wide (about 100 knots). In both accelerating and decelerating flight, this wide corridor means that a tiltrotor can have a safe and comfortable transition or conversion, offering the combined advantages of speed and maneuverability for low level flight.