Stability and Control Analysis for an Unmanned Aircraft Configuration Using System-Identification Techniques

Abstract

Different methods are described to determine dynamic derivatives of an unmanned combat air vehicle configuration called SACCON (for “stability and control configuration”). These methods can be applied to both experimental and computationally obtained data sets. The first method assumes a linear derivative model and is based on a least-squares curve-fitting technique and a subsequent evaluation step to actually compute the derivatives themselves. Based on the unsteady simulation obtained by computational fluid dynamics, the routine is able to recover the major trends of vehicle performance with reasonable agreement for pitching stiffness and damping. Lift-related quantities do show a discrepancy, particularly at high angle of attack. The second approach also assumes a linear derivative model. In this case, however, the static pitching stiffness terms are defined explicitly from the static test results and then subtracted from the dynamic results to give the residual effect of the damping terms. A least-squares fit of these is used to determine the damping derivatives. Using this approach, it is demonstrated that the linear-derivative assumption falls down at higher angle of attack, and a more generalized modeling paradigm is required. The final approach enables the use of nonlinear model equations and is therefore applicable to the entire tested angle-of-attack and angle-of-sideslip regime, generating a single set of nonlinear derivatives. Thus, the hysteresis loops of the coefficients derived from dynamic wind-tunnel tests can be reproduced satisfactorily with most of their inherent significant changes depending on angle of attack and forced oscillation frequency.