Identifiability investigation of the aerodynamic coefficients from free flight tests

Abstract

The identification of the aerodynamic coefficients, based on free flight measurements, remains a difficult task for flying vehicles like space vehicles, munitions, UAV. 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, the estimation of the model parameters must be processed with caution.In this paper, we propose a new procedure for the identification of the aerodynamic coefficients, and more precisely the pitch damping coefficient, Cmq of a re-entry space vehicle. This approach is based on system identification techniques and several steps are required, like the polynomial description of the coefficient as a function of the Mach number and the total angle of attack, the a priori and a posteriori identifiability study, followed by the estimation of the parameters in question based on real experimental free flight measurements. This model-based method improves the accuracy of the estimated coefficient.

The Analysis of Vehicle’s In-Flight Behaviour Using Quasi-LPV and Nonlinear Models

“…The nonlinear model, based on the 6-degrees-of-freedom Newton-Euler laws of motion [19], and its representation in a quasi-LPV form [9], are shortly presented hereafter.…”

“…where a is the speed of sound, and V is the total velocity, i.e. V = [19]. The output vector y represents the measurements obtained by available sensors and observers: embedded magnetometers, accelerometers and gyroscopes, and an on-ground trajectory radar.…”

“…The full form of the nonlinear model equations in the variable-roll reference frame is described by the following set of equations, based on the Newton-Euler laws of dynamics (refer to [19]):…”

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

“…Results obtained previously in [12] have shown that the polynomial description for the pitch damping coefficient seems to be better adapted for angles α t inferior to 10 degrees. However, for larger α t , the description of the pitch damping coefficient was adjusted and spline functions were used in order to avoid the disadvantages of the polynomial representations.…”

Hardware-in-the-loop experimental setup development for a guided projectile in a wind tunnel

“…Determination of the aerodynamic coefficients is usually done using a combination of the following techniques: free-flight tests [5], wind tunnel balance measurements [6], Computational Fluid Dynamics (CFD), empirical and semi-empirical codes such as PRODAS (PROjectile Design/Analysis System) or DATCOM. However, these techniques are usually expensive in terms of time and cost and may lack precision for some airframe configurations or flight conditions.…”