Wind Modeling and Lateral Control for Automatic Landing

Abstract

For the purposes of aircraft control system design and analysis, the wind can be characterized by a mean com- ponent which varies with height and by turbulent components which are described by the von Karman correlation model. The aircraft aerodynamic forces and moments depend linearly on uniform and gradient gust components obtained by averaging over the aircraft's length and span. The correlations of the averaged com- ponents are then approximated by the outputs of linear shaping filters forced by white noise. The resulting model of the crosswind shear and turbulence effects is used in the design of a lateral control system for the automatic landing of a DC-8 aircraft. NE of the more challenging problems facing an autopilot designer is the synthesis of feedback logic which will achieve the desired attitude and position of an aircraft at touchdown in the presence of turbulent crosswinds. During the approach phase of an automatic landing, the desired flight path is maintained by heading slightly into the prevailing crosswind (crabbing). However, at touchdown, the aircraft centerline must be aligned with the runway to prevent lateral skidding of the wheels on the landing gear. The transition from the approach attitude to the touchdown attitude, termed the maneuver, is usually carried out by the pilot, using the rudder to reduce the heading deviation, and ailerons to dip the windward wing tip sightly so as to maintain the lateral position in the presence of the resulting aerodynamic sideslip. When visibility is reduced to the point where the pilot cannot maintain the required visual reference, it is desirable to perform the decrab maneuver automatically. In order to carry out an effective automatic landing, ac- curate information on the aircraft's attitude and position is required. In the last several years, accurate and reliable navigation systems have been developed to provide this in- formation. Such systems include inertially augmented ILS systems,1'2 microwave landing systems, MLS,3 and various attitude reference systems.4 It is anticipated that further im- provements in system accuracy and reliability will be made in the future. Touchdown errors in automatic landing are also caused by turbulent winds. In contrast with the navigation errors, there is little hope of reducing these disturbances. In fact, as lan- ding speeds are lowered, as in STOL aircraft applications, the wind effects become more pronounced.