Raptor-informed feathered drone reveals tail-twist functions in avian turning manoeuvres

Banked turn is a common flight manoeuvre observed in birds and aircraft. To initiate the turn, whereas traditional aircraft rely on the wing ailerons, most birds use a variety of asymmetric wing morphing control techniques, validated in engineered replicas, to roll their bodies and thus redirect the lift vector to the direction of the turn. Nevertheless, when searching for prey, soaring raptors execute steady banked turns without exhibiting observable wing movements apart from tail twisting around the body axis. Despite the role as a vertical stabilizer in traditional aircraft, the reasons why birds twist the tails in banked turn are still not well understood. Here, we use an avian-inspired feathered drone to find that the tail located in proximal arrangement behind the wings enters wing-induced asymmetric flow region during twisting and generates asymmetric lift that results in both roll and yaw moments sufficient to coordinate banked turns. Moreover, twisting the tail induces a nose-up pitch moment that increases the angle of attack of the wings, thereby generating more lift that compensates for losses caused by the banking motion. Flight experiments confirm the effectiveness of tail twist to control not only steady low-speed banked turns but also high-speed sharp turns by means of coordinated tail twist and pitch with asymmetric wing shape morphing. These findings contribute to the understanding of avian flight behaviours that are difficult to study in controlled laboratory settings, and provide effective control strategies for agile drones with morphing aerial surfaces. One sentence summary Raptor-informed feathered drone reveals that twisting the tail located at the trailing edges of the wings generates aerodynamic control forces caused by wing-induced asymmetric flow to let birds execute both steady banked turns and high-speed sharp turns.