Prediction of aerodynamic loads and stability derivatives using the unsteady Source and Doublet Panel Method for preliminary aircraft design

Preliminary aircraft design is increasingly carried out in combination with optimization procedures in order to arrive at highly efficient configurations before the detailed design stage. Such procedures require a large number of fast aerodynamic calculations to estimate aircraft performance; as the cost of Computational Fluid Dynamic (CFD) calculations is too high for such a large number of simulations, panel methods are used, typically the Doublet Lattice Method (DLM). This work presents the development of closed form expressions for the aerodynamic stability derivatives of wing and aircraft flying at subsonic compressible conditions using the unsteady subsonic 3D Source and Doublet Panel Method (SDPM). The stability derivatives are obtained directly from the frequency domain version of the nonlinear unsteady subsonic Bernoulli equation and can be calculated for negligible additional computational cost around the true geometry of the wing or aircraft, for each frequency of interest. The methodology is validated by applying it to an experimental test case of a straight tapered wing oscillating in yaw and sideslip in the wind tunnel, demonstrating that the values of the aerodynamic stability derivatives obtained from the SDPM are in good agreement with the experimental data at low and moderate angles of attack. Then, the methodology is applied to a Blended Wing Body Unmanned Aerial Vehicle configuration and the SDPM stability derivative predictions are compared to estimates obtained from both steady CFD calculations and the USAF DATCOM methodology. It is shown that the SDPM approach is accurate and can yield more information about the stability of the aircraft than the DATCOM, given that the latter was developed for conventional aircraft configurations.