# Design of Wing Root Rotation Mechanism for Dragonfly-Inspired Micro Air Vehicle
## 1 Introduction  
Small birds and insects are good objects to mimic for developing a micro air vehicle (MAV) for stable flight at low Reynolds fluid [1; 2; 3; 4; 5; 6; 7]. The RoboBee developed by researchers at Harvard [1], the robot hummingbird of DARPA [2], the beetle robot of Konkuk University [3], and the tailless aerial robot inspired by the flies of Delft [7] are representative robots developed by mimicking the flight of small birds and insects. Among the insects mimicked, the dragonfly has the most stable hovering ability, the ability to switch flight modes without changing posture, and ability to fly backwards [8; 9]. In order to achieve this high maneuverability, the dragonfly uses characteristics such as the phase difference between the forewing and hind wing [8; 10; 11; 12], independent control of each wing [13], and a drag-based system in hovering flight [14; 15]. Among these characteristics, we focus on the drag-based system. Most insects, except the dragonfly, use a lift-based system in which they stroke their wings in the horizontal direction of the body during hovering. On the other hand, dragonflies use a drag-based system in which they stroketheir wings in an inclined direction to the body. This drag-based system not only provides more stable flight but also improves maneuverability by maintaining posture when changing flight mode [14; 15]. Dragonflies that use a drag-based system rely on drag force for 76% of the force required for hovering, and most vertical forces for hovering flight are obtained on the downstroke [15]. For this reason, in order to maximize their vertical force, dragonflies increase the drag force through a large angle of attack during the downstroke, while minimizing the drag force through a small angle of attack in the upstroke [16; 17]. The aforementioned angle of attack is an important factor that enables most insects, including dragonflies, to control the magnitude and direction of the force generated when flapping their wings [18]. Most small birds and insects produce an angle of attack by rotating their wings in the longitudinal direction when the direction of the flapping stroke is reversed, such as from the upstroke to the downstroke, or from the downstroke to the upstroke [19; 20]. These wing rotations are divided into passive rotation using aerodynamic force and inertial force and active rotation that is directly controlled through the muscles. It is not known exactly what type of wing rotation insects use, but it has been proven that passive rotation explicitly occurs during flapping [21]. A variety of mechanisms for generating wing rotation for the FW-MAV (flapping wing micro air vehicle) have been proposed in studies mimicking the flight characteristics of insects. These mechanisms are divided into passive [22] and active rotation mechanisms [23; 24] like insect wing rotation. A passive rotation mechanism has been produced by creating a wing frame using a flexible material or loose membrane. For the active rotation mechanism, a method of artificially generating rotation using a spring, and a method of simultaneously implementing flapping and rotation with a single actuator using a spatial four-bar link mechanism have been presented. Various attempts have been made to develop a wing rotation mechanism, but there is a clear limit to mimicking the drag-based system of dragonflies. In the case of the active rotation mechanism, since wing rotation is directly generated using the power of an actuator, it is suitable to implement the desired movement through mechanical design. Moreover, the flapping pattern is easily switched from an asymmetric flapping pattern to a symmetric flapping pattern or vice versa by adding degrees of freedom to the MAV. However, this mechanism is difficult to apply in practical MAV development. The reason is because not only is the structure complex, such that the weight of the airframe increases, and friction and torsion occur to a large extent, but additional energy is also required to generate wing rotation.