Birds produce tail control torques to maintain their attitude by unfolding their tails in the wing induced flow at hover. This mechanism is termed as wing-tail interaction. This study aims to apply this mechanism to the pitch control of a flapping wing aircraft. Momentum theory is adapted to analytically calculate the flapping wing induced flow and its distribution along the body is also presented. The proposed theory is experimentally tested on our self-designed flapping wing aircraft and it holds when the tail is installed within 1.5 c (wing chord) distance from the leading edge at frequency above 10 Hz. The tail of our aircraft is mounted at the experimentally identified maximum tail control torque position to enlarge the control effect. An accurate tail control model of the aircraft is established with the validated theory and it is utilized to design frequency dependent tail controller. The controller can realize good pitch stabilization by including the flapping wing induced flow. The proposed theory can be easily incorporated into the tail controller design of flapping wing aircraft and has promising application potential in the similar tailed aircraft.

This article intends to give a comprehensive model to a brand new micromechanical flapping-wing aircraft inspired by wing–tail interaction. In this aircraft, the flapping-wing induced flow contributes to the tail control torque generation. However, this also makes the wing–tail interaction nonnegligible, and this flow would be disturbed by the relative flow produced by the body motion. The dominant unsteady aerodynamic effects of the flapping wing are considered. The flapping-wing induced flow over the tail is included when analyzing the tail aerodynamics. Especially, the aerodynamic–dynamic coupling effect of both the wing and the tail is considered to account for the force due to the body motion. A nonlinear and time-periodic simulation model of the aircraft is, thus, established. The model is averaged and linearized around hover status. Because of the combination of the wing–tail interaction with the aerodynamic–dynamic coupling, the state and control matrixes, including both the induced flow effect and the body motion effect, can be obtained. The pitch dynamics are analyzed, and it is found that the tail can push the two unstable oscillatory poles closer to the imaginary axis. The role of the tail is fully demonstrated by considering the induced flow. Cascade controllers are also designed according to the root-locus plot based on the built model. Due to the inclusion of the wing–tail interaction and the aerodynamic–dynamic coupling, the controllers are proved to be efficient in the attitude stabilization during statistic hover, descent, and ascent flights. Our proposed model can be easily applied to the modeling of other flapping-wing aircraft with similar structures, and it makes the controller design process more efficient and targeted.

Flapping-wing aircraft dynamics features with periodic characteristics. The widely-used averaging theory simplifies the model to some extent, but it also fails to uncover the dynamic evolution in the cycle. The accuracy of the solution is poor and the derived controller might fail. This study proposes a scheme for the stabilization controller design of a special wing-tail interaction inspired flapping-wing aircraft by considering the periodic characteristics. A periodic longitudinal dynamic model is first built by including the main periodic issues existed in the aircraft. With this model, the equilibrium status of the aircraft is trimmed as a limit cycle by using a multiple shooting method and its periodic stability is analyzed. The aircraft is unstable, and its periodic controllability is proved in ahead. Stabilization controllers based on averaged and periodic models are separately designed. It is proved that the controllers derived from root locus based on the averaged model are not always able to stabilize the aircraft in simulation, while the periodic based controller can give a more sufficient stabilization conclusion by giving a novel stability criterion. The final experiment also proves that the periodic-based controller can successfully stabilize the aircraft. This firmly supports our proposal that the periodic characteristics should be included in the dynamic analysis of flapping-wing aircraft.