Accelerated Flight Envelope Expansion Using Near Real Time Stability Margin Estimation

Abstract

A novel technique for estimating the stability margins in near real time has been developed by the authors [1]. This paper shares the invaluable flight test experience gained in using it for flight envelope expansion of India’ s first digital fly-by-wire Light Combat Aircraft (LCA). Use of this technique has resulted in reducing the overall flight test effort and cost without compromising safety. Using the technique, it was possible to estimate the stability margins on a desktop computer using real time telemetry data within approximately 60 seconds of completing the test. The flight test results for ‘Fixed’ (restricted flight envelope) and ‘Scheduled’ (full flight envelope) gain control laws, for manually generated pilot 3-2-1-1 inputs and pre-programmed 3-2-1-1 inputs generated using the onboard Flight Test Panel (FTP) are presented. During the LCA flights, the stability margins were found to be satisfactory throughout the flight envelope. In order to establish that the same technique would work equally well when the stability margins are marginal, typical results were also generated using simulation data and are included in this paper. By artificially increasing the plant gain / delay in the simulation model it is shown that even if the stability margins are low, the same technique correctly estimates the relative stability.