Aerodynamics and Control of an Ornithopter with a Bird-Like Tail

Ornithopters or bird-like flapping wing aerial vehicles are often equipped with a vertical tail along with the existing horizontal tail or ‘V’ tail to achieve stability and control during flight. Birds, in contrast, are very proficient in directional control by using their tail in which there is no vertical tail or neither a ‘V’ tail. The absence of a vertical tail in a conventional aircraft would cause severe control and stability problems. Different types of tails evolved in different birds like graduated, pintail, shallow fork and deep fork shapes. It has been found from other studies that longer tails with a shallow fork are aerodynamically more efficient hence in this paper, we investigate the characteristics of a bird tail with no vertical stabilizer using an experiment and a mathematical model of the bird’s tail it can roll and pitch about the body axis. Three different tail models of differing sizes and spans are fabricated and fitted on a flapping bird replica in a wind tunnel. The interchangeable tails can be set to a certain orientation while the whole model is mounted on a 6-component dynamic load cell in a wind tunnel to measure the roll, pitch and yaw moments generated due to the tail. The theoretical model predicts the rolling, pitching and yaw moments with the given rotations of the tail during its flying conditions. The nature of the moments from the mathematical model is verified with the wind tunnel measurements. The paper also presents a design criterion for sizing the tail for an ornithopter based on which the control authority is characterized. Based on the findings, an ornithopter is fabricated and successfully tested with good handling quality using this natural bird tail.