Research progress on the flight control of flapping-wing aerial vehicles

In nature, flying creatures flap their wings to generate lift, which is necessary for flight. Most birds change flight patterns by moving their wings using their wing muscles and adjusting their tail states. Insects, which are without tails, can achieve maneuverable flight using their chest and abdomen muscles and other structures such as hind wings. Owing to high mobility and high energy efficiency, researchers have developed various flapping-wing aerial vehicles according to the bionic principle to improve flight performance. However, a flapping-wing aerial vehicle is a nonlinear and time-variable system. The low Reynolds number and unsteady eddy are important characteristics of the flapping-wing aerial vehicle, and the values are different from those of traditional aircraft. The Reynolds number of the traditional aircraft is larger; thus, the air viscosity is small enough to be ignored. However, the air viscosity of the bionic flapping-wing aerial vehicle is high at low Reynolds number conditions. Adopting a conventional aerodynamic configuration will result in insufficient lift. In addition, the traditional aerodynamics theory cannot explain the high lift of the flapping-wing aerial vehicles, and the mature technologies in traditional aircraft design cannot be directly applied owing to the low Reynolds number. Owing to the periodic movement of the flapping wing, it is difficult for researchers to accurately analyze the aerodynamic model. The autonomous flight of a flapping-wing aerial vehicle is limited by several challenges. To solve this problem, researchers have studied the flight principle of birds and insects. Moreover, the attitude control, position control, and stability analysis of flapping-wing aerial vehicles have been studied. Several control strategies based on robust control, neural networks, and other methods have been proposed to realize the autonomous flight of flapping-wing aerial vehicles. Researchers have also adopted control methods such as adaptive controllers combined with linearization techniques to control attitude. Position control has been achieved using a hierarchical controller and other approaches. In addition, perturbation observation is used to deal with the uncertainty of the system to improve stability. In this paper, the flight control strategies of flapping-wing aerial vehicles of different scales are reviewed. The current research on the flight control of the flapping-wing aerial vehicle is mostly in the prototype phase. Most of these theories have not been verified in actual flight. Therefore, the flight control theory needs to be combined with actual missions to promote the application of the flapping-wing aerial vehicle. Finally, the future trend of the flight control of the flapping-wing aerial vehicle is highlighted.