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The devices and systems which govern the attitude of an aircraft and, as a result, the flight path followed by the aircraft. Flight controls are classified as primary flight controls, auxiliary flight controls, and automatic controls. In the case of many conventional airplanes, the primary flight controls utilize hinged, trailing-edge surfaces called elevators for pitch, ailerons for roll, and the rudder for yaw. These surfaces are operated by the human pilot in the cockpit or by an automatic pilot. In the case of vertically rising aircraft, a lift control is provided.
Controls to govern the engine power and speed, while not usually classified as flight controls, are equally important in the overall control of the aircraft. This is especially true if the engine exhaust can be directed to produce pitch or yaw motions.
Auxiliary flight controls may include trimming devices for the primary flight controls, as well as landing flaps, leading-edge flaps or slats, an adjustable stabilizer, a wing with adjustable sweep, dive brakes or speed brakes, and a steerable nose wheel.
Automatic controls include systems which supplement or replace the human pilot as a means of controlling the attitude or path of the aircraft. Such systems include automatic pilots, stability augmentation systems, automatic landing systems, and active controls. Active controls encompass automatic systems which result in performance improvement of the aircraft by allowing reductions in structural weight or aerodynamic drag, while maintaining the desired integrity of the structure and stability of flight.
The control system incorporates a set of cockpit controls which enable the pilot to operate the control surfaces. Because of the approximately fixed size and strength of the human pilot and the need to standardize the control procedures for airplanes, the primary controls are similar in most types of airplanes. The cockpit controls incorporate a control stick which operates the elevators and ailerons, and pedals which operate the rudder. Sometimes a column/wheel arrangement is used to operate the elevators and ailerons, respectively. The cockpit controls for auxiliary control devices are not as completely standardized as those for the primary controls.
Control systems with varying degrees of complexity are required, depending on the size, speed, and mission of an airplane. In relatively small or low-speed airplanes, the cockpit controls may be connected directly to the control surfaces by cables or pushrods, so that the forces exerted by the pilot are transmitted directly to the control surfaces. In large or high-speed airplanes, the forces exerted by the pilot may be inadequate to move the controls. In these cases, either an aerodynamic activator called a servotab or spring tab may be employed, or a hydraulic activator may be used . In some airplanes, particularly those with swept wings and those which fly at high altitudes, the provision of adequate static stability and damping of oscillations by means of the inherent aerodynamic design of the airplane becomes difficult. In these cases, stability augmentation systems are used. These systems utilize sensors such as accelerometers and gyroscopes to sense the motion of the airplane. These sensors generate electrical signals which are amplified and used to operate the hydraulic actuators of the primary control surfaces to provide the desired stability or damping. See Stability augmentation
The weight and complication of mechanical control linkages and the extensive reliance on electrical signals in automatic controls led to the development of control systems in which the control inputs from the pilot, as well as those from the stability augmentation sensors, are transmitted to the primary control actuators by electrical signals. Systems of this type are called fly-by-wire systems. The electrical signals are readily compatible with computers, typically digital, which can perform the functions of combining the signals from the pilot and the sensors. See Digital computer
Fly-by-wire systems can malfunction when exposed to high-intensity electromagnetic fields. The solution to this problem has been to shield the transmission media and extensively test the system before certifying it, adding cost and weight to the system. These difficulties, the intrinsic immunity of optical technology to electromagnetic interference, and the availability of optical-fiber-based transmission media from the communications industry led to the development of fly-by-light systems. These systems use optical fibers to transmit signals instead of wires; the interface units are replaced with optical-electrical converters. See Optical communications
There are three major reasons why digital flight control computers are used in modern airplanes. First, digital flight control computers can enhance the pilot's control of the airplane by optimizing the movement of the control surfaces in response to pilot commands, over the operating flight conditions of the airplane. Second, as a result of their ability to rapidly monitor and interpret multiple sensor inputs, digital flight control computers can often exceed the performance of an unassisted pilot in compensating for critical situations which might otherwise result in loss of airplane control. Third, digital flight control computers permit input directly from remote control and navigation devices such as digital automatic landing systems, assisting the pilot in zero-visibility conditions or freeing the pilot for other airplane management tasks.
A digital flight control computer evaluates its inputs based on precomputed models of the airplane's expected behavior under various conditions of flight in order to produce command signals for the control surface actuators. Typically, wind-tunnel data are used to derive and verify the precomputed models used in the computer. Test flights and computer simulations are also used extensively to verify computer operation. See Aircraft testing, Wind tunnel
The flight of an aircraft may be controlled automatically by providing the necessary signals for navigation as inputs to the control system. In practice, automatic pilots are used to relieve the human pilot of routine flying for long periods, and automatic control systems are used to make precision landings and takeoffs under conditions of reduced visibility.