SACCON Stability and Control Analysis Applying System Identification Techniques

Kurzfassung

Different methods are described to determine dynamic derivatives of an UCAV configuration named SACCON (Stability And Control CONfiguration). These methods can be applied to both experimental and computationally obtained data sets. The method applied by DSTO assumes a linear derivative model and is based on a least-square curve-fitting technique and a subsequent avaluation step to actually compute the derivatives themselves. Based on the unsteady simulation obtained by computational fluid dynamics, the DSTO routine is able to recover the major thrends 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 BAE Systems 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 (AoA) and a more generalized modeling paradigm is required, e.g., as developed by DLR. The DLR 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.