Handling Qualities of Model Reference Adaptive Controllers with Varying Complexity for Pitch-Roll Coupled Failures

Abstract

Three model reference adaptive controllers (MRAC) with varying levels of complexity were evaluated on a high performance jet aircraft and compared along with a baseline nonlinear dynamic inversion controller. The handling qualities and performance of the controllers were examined during failure conditions that induce coupling between the pitch and roll axes. Results from flight tests showed with a roll to pitch input coupling failure, the handling qualities went from Level 2 with the baseline controller to Level 1 with the most complex MRAC tested. A failure scenario with the left stabilator frozen also showed improvement with the MRAC. Improvement in performance and handling qualities was generally seen as complexity was incrementally added; however, added complexity usually corresponds to increased verification and validation effort required for certification. The tradeoff between complexity and performance is thus important to a controls system designer when implementing an adaptive controller on an aircraft. This paper investigates this relation through flight testing of several controllers of vary complexity.

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

“…This undesirable aspect of the task forced an increase from one ship length of separation for the results presented in table 3 to two ship lengths of separation for the MRAC tests. 3,4 This increase in separation helped with repeatability; however, it changed some aspects of the task such as pilot aggressiveness effects. Additionally, it would have been more desirable to perform a tight formation task on the wing of the lead aircraft; however, the limited experience with the ARTS IV architecture and the limited verification and validation testing performed on the NDI control law precluded maneuvers during which the vehicles overlap longitudinally.…”

“…He compensated for this by consciously backing out of the loop slightly. Pilot A was asked to repeat this task with the NDI three months later at the conclusion of the MRAC flights 3,4 with a longitudinal separation of two ship lengths, and his ratings improved by one for both the gross acquisition (GA) and fine tracking (FT) tasks (GA 2, FT 3). His comments about the task changed slightly as well.…”

“…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.…”

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.…”

“…The choice of dynamic inversion was driven by the intuitive architecture, explicit model-following behavior, the ability to be used to introduce fundamental-level simulated failures within the aerodynamic model for testing the performance of advanced control elements, and by the fact that dynamic inversion can be included in the stability proofs for many advanced control schemes. 3 The focus of this paper is to describe the control law and provide insight into the design choices that facilitated the inclusion of adaptive elements 3,4 and any that had an effect on predicted handling qualities. Each component is described in detail, and the fundamentals explained.…”