# Design of Wing Root Rotation Mechanism for Dragonfly-Inspired Micro Air Vehicle
## 2 Flight Characteristics of Dragonflies
### Aerodynamics of Dragonfly Flight  
The systems used by organisms for swimming or flying are divided into drag-based systems and lift-based systems, depending on the type of force [14; 15]. The lift-based system uses lift force in the manner of most fish or flying insects. Lift forces are obtained through up-down strokes, whereas drag force is generated in the opposite direction with the each up-down stroke, and the net force generated for one cycle is close to zero, as shown in Figure 2a. The drag-based system is based on the drag force, used in rowing locomotion or jellyfish swimming. In order to generate enough drag force to fly or swim, dragonflies flap their wings, and with each down-stroke maintain a large angle of attack  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Angle of Attack**} & \multicolumn{2}{c}{**Stroke Amplitude**} & \multicolumn{2}{c}{**Stroke Angle**} \\ \hline \multirow{3}{*}{Angle} & \multirow{3}{*}{Down stroke} & Hover & Forward & \multirow{3}{*}{\(-40^{\circ}\)\(-40^{\circ}\)} & \multirow{3}{*}{\(60^{\circ}\)} \\  & & 75\({}^{\circ}\) & & & \\ \cline{1-1} \cline{3-5}  & \multirow{2}{*}{Up stroke} & 15\({}^{\circ}\) & & & \\ \cline{1-1} \cline{3-5}  & & Mean angle of attack & \(30^{\circ}\) & \(0^{\circ}\) & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Kinematics of dragonfly for experiment.  
Figure 1: Coordinate system of flapping motion.  
to increase the drag force. In contrast, the upstroke minimizes the drag force by flapping with a small angle of attack, as shown in Figure 2b. This method allows the creature to generate force in the direction of movement through the drag force.  
[PERSON] [14] mentioned that the drag-based system is more stable than the lift-based system in stationary flight, while the lift-based system has superior performance in forward flight. In addition, [PERSON] [15] experimentally proved that the drag-based system is more stable in stationary flight. Unlike most insects that use a lift-based system, dragonflies use both a drag-based system in hovering flight and lift-based system in forward flight. This system conversion is achieved by changing their angle of attack, which gives them more stable and superior flight abilities.  
[PERSON] experimentally defined lift and drag coefficients for when the thin wing moves in low Reynolds fluid as the following equations.  
\[C_{L}=0.225+1.58\sin{(2.13\alpha-7.20)}, \tag{1}\]  
\[C_{D}=1.92-1.55\sin{(2.04\alpha-9.82)}. \tag{2}\]  
where \(C_{L}\) is the lift coefficient, \(C_{D}\) is the drag coefficient, and \(\alpha\) is the angle of attack. The equation shows that the drag coefficient is a function of the angle of attack. In the relationship between the angle of attack and drag force, in order to use the drag-based system at hovering, the dragonly increases the drag force using a large angle of attack at the downstroke and minimizes the drag force using a small angle of attack at the upstroke. On the other hand, in forward flight, the dragonfly flies by generating lift force using the same angle of attack in the up-down stroke like other insects. Thus, the rapid conversion from a drag-based system to a lift-based system without changing the stroke angle is achieved by changing the mean angle of attack.