Computational Fluid Dynamics and Doublet-Lattice Calculation of Unsteady Control Surface Aerodynamics (2009)


With the increased interaction among the aerodynamic, structures, and controls disciplines in advanced aircraft, it is increasingly important to predict accurately the aerodynamic forces due to control surface deflection. The aerodynamic behavior of oscillating control surfaces has historically been difficult to predict analytically. The doublet-lattice method (DLM)1 has been the de-facto standard unsteady aerodynamic analysis tool in the subsonic regime for decades, but because it is based on the linear potential equation, it has had limited success in computing control surface aerodynamics. In practice, the aerodynamic results from the DLM (or a similar process) were modified using (sometimes large) correction factors to improve correlation with test data. These correction factors are typically based on steady wind-tunnel data and are applied to steady and unsteady conditions alike. This is undesirable because the steady corrections will not necessarily be representative of the unsteady flow and cannot correct the unsteady phase lag.

With the advent of low-cost, high-power computers, the use of unsteady computational fluid dynamics (CFD) for the calculation of unsteady control surface aerodynamics has recently become more attractive. In this study, CFL3D.AE-BA2 was used to compute unsteady Navier–Stokes solutions of oscillatory control surface motion. This model includes not only the effects of compressibility and transonic shocks, but also accounts for viscous effects. To reduce the computational time of the CFD solutions, the control surface deflection (as a function of time) has the form of an exponential pulse (as opposed to a pure sinusoid), and the frequency-domain unsteady aerodynamics were computed using a Fourier transform.

 For cases with small angle of attack and no deflected control surface, linear lifting surface theory has been shown to be valid outside the transonic regime. This approach is no longer valid when analysis is required for cases involving transonic flow or significant viscous effects, or when modeling of details such as control surface gaps is important. Upstream propagation of information for cases in the transonic regime also cannot be predicted by linear theory. A classic example of such flow  characteristics occurs in the shock wave present for flows with a supersonic pocket at the surface of the wing. There is a significant difference in the magnitude and phase results obtained from linear theory and those obtained from test and CFD in the region upstream of the transition back to subsonic flow. It is the purpose of this study to investigate the benefits of higher fidelity aerodynamic theories in improving the accuracy of unsteady aerodynamic predictions.