Flight Test Results from the NF-15B Intelligent Flight Control System (IFCS) Project with Adaptation to a Simulated Stabilator Failure

Abstract

Adaptive flight control systems have the potential to be more resilient to extreme changes in airplane behavior. Extreme changes could be a result of a system failure or of damage to the airplane. A direct adaptive neural-network-based flight control system was developed for the National Aeronautics and Space Administration NF-15B Intelligent Flight Control System airplane and subjected to an inflight simulation of a failed (frozen) (unmovable) stabilator. Formation flight handling qualities evaluations were performed with and without neural network adaptation. The results of these flight tests are presented. Comparison with simulation predictions and analysis of the performance of the adaptation system are discussed. The performance of the adaptation system is assessed in terms of its ability to decouple the roll and pitch response and reestablish good onboard model tracking. Flight evaluation with the simulated stabilator failure and adaptation engaged showed that there was generally improvement in the pitch response; however, a tendency for roll pilot-induced oscillation was experienced. A detailed discussion of the cause of the mixed results is presented. Nomenclature

Aircraft Flight Envelope Determination Using Upset Detection and Physical Modeling Methods

“…This behavior was observed in Ref. 10, where the roll axis gains for a precision (pitch axis) tracking task following the failure of one stabilator introduced the tendency for a pilot induced oscillation in one piloted evaluation case. While adaptive control technology offers flexibility, it should be recognized that performance/robustness of the overall system may be improved with outer-loop "supervision" from a system that diagnoses potential impairment in parallel with recovery command generation.…”

“…Specifically, it is desirable to develop estimation and identification methods to identify/diagnose anomalous conditions and to determine appropriate recovery commands, which can consist of pilot indicators/guidance cueing and/or active feedback/feedforward commands. Some research and development that has been performed in recent years toward this end includes (1) fault detection, isolation, and reconfiguration (FDIR) methods for identifying failures in critical flight control components, such as sensors and actuators, and providing reconfiguration to maintain sufficient closed-loop functionality [3][4][5][6] ; (2) envelope limiting control systems where corrective commands are generated as operational limits are approached to prevent excursions into potentially dangerous flight regimes [7][8][9] ; and (3) adaptive control methods where feedback/feed-forward commands are determined to counteract changes in the aircraft system and/or operational environment 10,11 . Note that this (brief) literature survey is not intended to be comprehensive but is used to indicate the diversity of research and development that has addressed flight control approaches for improving aircraft safety, in particular during degraded flight operations.…”

Nonlinear Dynamic Inversion Baseline Control Law: Architecture and Performance Predictions

“…This architecture is designed specifically to be a baseline controller upon which advanced control elements can be easily added. This architecture will enable further control research into adaptive controls [3][4][5][6][7][8][9][10] and the control of flexible structures. 11,12 This baseline control law is a first step toward building a working environment in which design changes and new research objectives can be quickly brought to flight and their real behavior ascertained.…”

Nonlinear Dynamic Inversion Baseline Control Law: Flight-Test Results for the Full-scale Advanced Systems Testbed F/A-18 Airplane

“…The control law is hosted in a research processor with build-in protection restricting the available envelope. This architecture is designed specifically to be a baseline controller upon which advanced control elements can be easily added and will enable further control research into adaptive controls [3][4][5][6][7][8][9][10] and the control of flexible structures. 11,12 The NDI control law is a first step toward building a working environment in which design changes and new research objectives can be quickly brought to flight and their real behavior ascertained.…”