Research of the gust response and gust alleviation based on direct force control.

The paper presents a methodology for calculating the unsteady aerodynamic forces characteristics and gust response of the unmanned air vehicle (UAV) model using computational fluid dynamics (CFD) tools, and the efficiency of gust alleviation based on direct force control are also studied. Based on unsteady Euler equations, dynamic responses in vertical gust flow perturbation are investigated. The grid velocity method, which is used to model unsteady flow via grid movement, is introduced to simulate the gust influence. This approach avoids numerical instabilities and decouples the step change in the angle of attack from a pitch rate. And the dynamic chimera grid method is used to simulate the rudder movement of the gust alleviation effect. Numerical results for the NACA0006 airfoil are validated by comparison with analytical results for two dimensional indicial responses. The initial and final lift values of the indicial responses closely match the exact analytical values as given by piston theory and quasi steady thin airfoil theory respectively. The solutions at small times match quite closely the exact solution which is known only for small times given by linear theory. The application of the same method to the UAV model reveals important characteristics of the three-dimensional vertical discrete gust response for the One-minus-cosine gust profile and gusts cause step changes in the angle of attack, and important characteristics of the gust response and resulting flow features are explained. Furthermore, the flow perturbations under dynamic rudder motion are investigated using moving chimera grid technique. Our study shows that with certain designed motions, the flapping rudder can effectively alleviate the freestream gust fluctuation and ensure a stable flight. However, no single parameter can determine whether the flapping rudder can reduce the gust effect. Finally, unsteady aerodynamic loads are computed for the wing of model with the designed rudder motion, the significance of the gust propagation effects is shown. It is found that the direct calculation of the gust response using CFD gives quite accurate results and provides a rich database. In general, this paper shows that the well-designed rudder movement can effectively alleviate the freestream gust fluctuation and ensure high flight qualities. [ABSTRACT FROM AUTHOR]