Use of Optimal Integral Control to Restore Trim in a Reconfigurable Flight Control System

Abstract

Abstract Description of State of the Art features . In a reconfigurable flight control system actuator or control surface failures are detected from output signals obtained from the system s motion sensors When such a failure occurs the control law of the flight control system is reconfigured in a manner that results in the aircraft retaining an acceptable degree of control and handling The reconfiguration is achieved by arranging for the new control law to use. sometimes in a manner different from custom, all the other appropriate serviceable actuators and control surfaces In practice, such reconfiguration schemes have greater effectiveness when used in aircraft which possess much greater control freedom than is usually found in conventional aircraft On fighter aircraft there are generally more control surfaces with a capacity to act independently, differentially, or collectively. The motion of such types of aircraft may be coupled quite strongly so that the loss of a Single control can have significant effects on the resulting closed-loop motion. In this paper the proposed reconfiguration system is developed for a generic fighter aircraft with eight control surfaces and whose lateral and longitudinal motion is to be controlled simultaneously. First, a baseline AFCS is studied and some typical responses from such a system for combinations of control surface failure, and for different actuator failures, are presented to provide some results against which to compare the performance of the reconfigurable flight control system in response to the same failures The reconfiguration scheme does not employ the control distribution technique proposed in some early papers (1. 2 & 3) because, incertain failure conditions, its use can lead to an unstable AFCS instead a scheme involving a weighted control distribution was used (4. 5) to achieve superior reconfiguration performance, even when the failures occurred in conditions of moderate atmospheric turbulence. Description of the New Contributions From the results obtained with the reconfiguration system it became evident that in using different control surface deflections to recover aircraft control, the aircraft did not recover to the original trim condition Work was carried out to improve the baseline AFCS by modifying its control law to use optimal integral control which was beneficial in removing the steady-state errors whenever control surface or actuator failures occurred Naturally, the effects of such failures on the performance of the new baseline AFCS were greater The control distribution matrix was then re-designed and the resulting reconfiguration scheme was studied. Acceptable flying qualities were achieved with this system even in the presence of several simultaneous failures and atmospheric gusts, without loss of the trimmed state for most failure cases An extensive investigation of the fullest extent of the system's capacity for reconfiguration was also undertaken A summary is given of the results obtained winch showed that when as many as five of the eight control surfaces had failed simultaneously, reconfiguration could still be achieved A greater number of simultaneous surface failures resulted, in almost every case, in loss of control of the aircraft.