System identification of flexible aircraft in frequency domain

Purpose– The purpose of this paper is to identify the flexible aircraft model accurately from the frequency responses.Design/methodology/approach– The frequency domain output error method is used to estimate the aerodynamic (rigid body and elastic body)derivatives, and mode shape parameters in the process of identification of flexible aircraft model. The accurate identification of lightly damped lowfrequency rigid-body response modes requires a careful selection of the frequency sweep length and the fast Fourier transform (FFT) window size,as the FFT window length cannot be longer than any individual sweep records. To address this issue, an effort is made to derive the FFT windowlength for the application of frequency domain estimation approach.Findings– The investigations are initially made to select a suitable FFT window size for the accurate identification of the lightly damped lowfrequency rigid-body response modes of the flexible aircraft. Subsequently, frequency domain estimation approach is applied to simulated data offlexible aircraft. Besides the stability and control derivatives, the structural modes of the flexible aircraft are also estimated as part of state spacemodel identification, and it is shown that all the model parameter estimates are accurate. Identification of such flexible aircraft aerodynamic (rigidbody and elastic body) derivatives and structural mode shape parameters will lead to mathematical models of flexible aircraft that are accurate overa wide frequency range. The identified models are validated using the time response of frequency sweep data.Research limitations/implications– Aircraft system identification is an integral part of aerospace system design and life cycle process. Thisbecomes a complex process when the aircraft has significant effects of flexibility on the flight dynamics, especially as the frequencies of the elasticmodes become lower and approach those of the rigid body modes. Thus, an integrated mathematical model of flexible aircraft is required to develop,and it should be valid for a wide frequency range and relevant for the design of flight control system.Originality/value– This paper focuses on the application of frequency domain approach to identify the valid model of flexible aircraft by estimatingthe aerodynamic (rigid body and elastic body) derivatives and structural mode shape parameters of flexible aircraft. The unknown frequencies ofstructural modes are also able to identify accurately in frequency domain. This gives more value addition to analyze the flight data of flexible aircraft,as it is challenging problem in parameter estimation of flexible aircraft.