Aircraft Control and Computers

D’ Azzo and Houpis (24) give a good explanation of the complexity of what is needed for an aircraft control system. The feedback control system used to keep an airplane on a predetermined course or heading is necessary for the navigation of commercial airliners. Despite poor weather conditions and lack of visibility, the airplane must maintain a specified heading and altitude in order to reach its destination safely.

In addition, in spite of rough air, the trip must be made as smooth and comfortable as possible for the passengers and crew. The problem is considerably complicated by the fact that the airplane has six degrees of freedom, which makes control more difficult than control of a ship, whose motion is limited to the surface of the water.

A flight controller is used to control aircraft motion. Two typical signals to the system are the correct flight path, which is set by the pilot, and the level position of the airplane. The ultimately controlled variable is the actual course and position of the airplane. The output of the control system, the controlled variable, is the aircraft heading.

In conventional aircraft, three primary control surfaces are used to control the physical three-dimensional attitude of the airplane: the elevators, the rudder, and the ailerons. A directional gyroscope (gyro) is used as the error-measuring device. Two gyros must be used to provide control of both heading and attitude of the airplane. The error that appears in the gyro as an angular displacement between the rotor and case is translated into a voltage by various methods, including the use of transducers such as potentiometers, synchros, transformers, or microsyns.

Selection of the method used depends on the preference of the gyro manufacturer and the sensitivity required. Additional stabilization for the aircraft can be provided in the control system by rate feedback. In other words, in addition to the primary feedback, which is the position of the airplane, another signal proportional to the angular rate of rotation of the airplane around the vertical axis is fed back in order to achieve a stable response. A rate gyro is used to supply this signal. This additional stabilization may be absolutely necessary for some of the newer high-speed aircraft.

In reading through this example, it should be obvious that as the complexity of the control feedback system of the aircraft increases, a need for computer processing to evaluate the feedback and to adjust or recommend flight control adjustments exists. Additional feedback may come from global positioning, from ground-based navigation systems through radio inputs, and from other aircraft. The computer is able to integrate these inputs into the onboard flight control inputs and provide improved recommendations for stable flight.