Benchmarking the prediction of dynamic derivatives: Wind tunnel tests, validation, acceleration methods

The dynamic derivatives are widely used in linear aerodynamic models which are considered to determine the flying qualities of an aircraft: the ability to predict them reliably, quickly and sufficiently early in the design process is more and more important, in order to avoid late and costly component redesigns. This paper describes some experimental and computational activities dealing with the determination of dynamic derivatives. The work has been carried out within the FP6 European project SimSAC. Numerical and experimental results are compared for two aircraft configurations: the generic civil transport aircraft, wing-fuselage-tail configuration DLR-F12 and a generic Transonic CRuiser (TCR), which is a canard configuration. Static and dynamic wind tunnel tests have been carried out for both configurations and are briefly described. The data base generated for the DLR-F12 configuration includes force and pressure coefficients obtained during small amplitude pitch, roll and yaw oscillations while the data base for the TCR configuration includes force coefficients for small amplitude oscillations, dedicated to the determination of dynamic derivatives, and large amplitude oscillations, in order to investigate the dynamic effects on nonlinear aerodynamic characteristics. The influence of the canard has been investigated too. Dynamic derivatives have been determined on both configurations with a large panel of tools, from linear aerodynamic (Vortex Lattice Methods) to CFD (unsteady Reynolds-Averaged Navier-Stokes solvers). The study confirms that an increase in fidelity level enables dynamic derivatives to be better calculated. Linear aerodynamics (VLM) tools can give satisfactory results but are very sensitive to the geometry/mesh input data. Although all the quasi-steady CFD approaches give very comparable results (robustness) on steady dynamic derivatives, they do not allow the prediction of unsteady components of the dynamic derivatives (angular derivatives w.r.t. time): this can be done with either a fully unsteady approach (with a time-marching scheme) or with Frequency Domain solvers, both of them giving very comparable results for the DLR-F12 test case. As far as the canard configuration is concerned; strong limitations of linear aerodynamic tools are observed. A specific attention is paid to acceleration techniques in CFD methods, which allow the computational time to be dramatically reduced while keeping a satisfactory accuracy.