Influence of spanwise twisting and bending on lift generation in MAV-like flapping wings

Abstract

A numerical-simulation tool is developed that is well suited for modeling the unsteady and nonlinear aerodynamics of flying insects and small birds as well as biologically inspired flapping-wing micro air vehicles (MAVs). The tool consists of a combination of (1) an aerodynamic model that is an extension of the widely used three-dimensional (3D) general unsteady vortex-lattice model, and (2) a general kinematic model that is capable of describing multiple deformation patterns of lifting surfaces, such as spanwise twisting, in-plane and out-of-plane bending, and any combination of these. Moreover, the present tool offers an attractive compromise between computational cost and fidelity and is ideally suited to be combined with computational structural dynamics to perform aeroelastic analyses. The present tool was successfully validated by comparing some of the present results with those obtained from existing numerical models based on both Euler equations and vortex-lattice codes and with some experimental data. Using the numerical framework developed and for the deformation mechanisms analyzed here, two distinctly different effects were found: the wing span's twisting and in-plane bending affect the lift in specific zones of the stroke cycle (called "local behavior"); and the wing span's out-of-plane bending affects the lift throughout the stroke cycle (called "global behavior"). In addition, the results found show that the wing's flexibility certainly affects the lift production, at least for some flights at small scales. These findings definitely suggest the strong likelihood that the unsteady vortex-lattice method combined with a general kinematic model can be a very accurate and efficient tool for future aeroelastic studies.