A New System Identification Method Using Short Duration Flight Test Inputs

Flight test activities that expand the flight envelope are inherently expensive and at times dangerous, particularly when approaching stability and/or flutter boundaries. With the computational power now available, on-line software applications have emerged that can clear test points while a flight is in progress. For flying qualities evaluations, it is important to identify accurate frequency responses from which the parameters associated with commonly used criteria are obtained. Typically, these frequency responses are generated from long duration test inputs such as frequency sweeps that may be minutes long depending on the frequency range of interest. To meet goals set by the Air Force Flight Test Center, a new system identification method – the Narrowband Signature (NBS) – has been developed to obtain accurate frequency responses from short duration (five seconds or less) test inputs. A flying qualities flight test database was created using the Calspan Corporation Learjet In-Flight Simulator to exercise the new identification method. Comparisons were made against industry standard fast Fourier transform (FFT)-based identification techniques for both long and short duration inputs with a longitudinal axis cruise configuration that featured well-damped short period dynamics as well as a lateraldirectional approach configuration that featured a lightly damped Dutch roll mode. The identification results for the cruise configuration found that all methods identified the system accurately with minimal adjustment of the default identification parameters for the long duration input. For those configurations with lightly damped modes, even with the long duration inputs, adjustment of the recommended window sizes was necessary for proper identification of the lightly damped mode. For the short duration input cases, NBS showed a clear advantage over the FFT-based techniques for both configurations. To demonstrate accuracy of the methods, the results are compared against transfer function models of the selected configurations.