(A) study on longitudinal flight stability and control of ornithopters considering fluid-structure interaction

Abstract

Biological flyers are considered and also has been witnessed as the most efficient and maneuverable flying object on earth. These fascinating creatures made scientists to think of another approach to develop a different type of aerial vehicles, namely ornithopters. The ornithopters, however, are still barely able to fly and its flying mechanisms including unsteady low Reynolds number aerodynamics around flexible wings, fluid-structure interaction between them, and flight dynamics/control are not yet understood thoroughly. Moreover, numerical approaches to simulate the flight dynamics of ornithopters are still in its infancy due to the extreme complexity and heavy computational cost. In the present study, an integrative ornithopter flight control simulation framework is proposed to numerically simulate and control the ornithopter longitudinal flight dynamics with efficiency. It takes account of unsteady low Reynolds number aerodynamics, anisotropic flexible wings, and its fluid-structure interaction through a modal-based flexible multi-body dynamic solver, and a semi-empirical reduced-order flapping wing aerodynamic model. It is found that the time-averaged longitudinal trim flight dynamics of ornithopters with respect to various flapping frequency show similar characteristics with biological flyers. In terms of time-varying characteristics, it is found that trim flight dynamics of ornithopters show nonlinear behavior characterized by limit-cycle oscillations of flight state variables. Its pitch attitude stability is enhanced through a pre-defined simple harmonic tail-wing motion phasically linked with main-wing motion. A simple altitude-hold flight controller is successfully implemented to the ornithopter model. Pitch directional stability augmentation system is found to be another effective way to stabilize pitch attitude oscillation. It is expected that one can use this proposed simulation framework as a tool of unraveling the complex characteristics of the ...