X-31 Vector Aircraft, Low Speed Stability & Control, Comparisons of Wind Tunnel Data & Theory (Focus on Linear & Panel Codes)

This paper is via the initiative of the NATO RTO-AVT-161 Task group. The group is aimed at assessment of Stability and Control Prediction Methods for NATO Air and Sea Vehicles (essentially complex configurations). The assessments need to include the absolute values of forces and moments as well as the various symmetric and asymmetric stability derivatives, both steady and unsteady. This paper relates to the topic of the canard-delta X-31 “Vector” aircraft. Low speed experimental data is available on an X-31 model tested by DLR. The main focus of this paper is in the use of linear theory and Surface Singularity methods (Panel codes). Results are compared with experiment and other CFD methods. The main emphasis is on longitudinal stability aspects with component contributions. However, selected asymmetric effects are also considered. For the symmetric cases, the predictions have shown good agreement for Lift with experiment and CFD up to α about 15. For pitching moment, agreement with other CFD results is good up to α of 15. The experiments show a more discontinuous behaviour beyond α about 10. A series of longitudinal stabilty derivatives have been derived from the results. Longitudinal Trimming aspects are briefly discussed. For the asymmetric cases with sideslip, a limited set of stability derivatives have been obtained. These look reasonable but need to be compared with experimental data eventually. Further work is needed for estimation of roll and yaw derivatives. The work so far has been interesting and encouraging. It has led to an improved understanding of the complex configurations with strongly interacting and separating / vortical flows. It is remarkable that low-order theories apply reasonably well up to α of 15 or so, with results being comparable with high-order CFD solvers. In the wider perspective, for the future, an understanding has evolved for either exploitation or avoidance of the complex flows. This is an important motivation.