Grey-box identification of the pitch damping coefficient from free flight tests

The identification of the aerodynamic coefficients, based on free flight measurements, remains a difficult task for flying vehicles like space vehicles, munitions, Unmanned Aerial Vehicles. This is mainly due to the nonlinear structure of the mathematical model describing the behavior of the vehicle in flight, the absence of an input signal, the unknown initial conditions and the nonlinear dependence of the aerodynamic coefficients on several state variables. Under these conditions, model parameter estimation must be processed with caution. In this paper, a detailed procedure for the estimation of the aerodynamic coefficients, and more precisely the pitch damping coefficient C_mq of a re-entry space vehicle, is presented. Taking into account a polynomial description of the previous mentioned coefficient, this approach is based on system identification techniques. Thus, several steps are required like the a priori identifiability study, followed by the estimation of the parameters in question, based on real experimental free flight measurements.