## 4 Aerodynamic Design Method Investigation
### Influence of Key Section Airfoils on the Aerodynamic Performance of the Configuration
#### 4.3.2 Kink Airfoil Aft-Reflexing Design  
Considering the constraint of a fixed airfoil area, three configurations--Kink-reflex1, Kink-reflex2, and Kink-reflex3--are designed. Figure 20 depicts the comparison of the shape of the three aft-reflexing airfoils and the initial airfoil at the kink profile, as well as the incremental camber of each airfoil. Figure 21 depicts the comparison of the aerodynamic characteristics of the aft-reflex and initial configurations. As shown, the aft-reflexing kink airfoil reduces the lift and drag of the aircraft. The larger the aft-reflexing, the higher the reduction in lift and drag. The aft-reflexing design of the airfoil has a significant influence on the longitudinal moment of the aircraft. The larger the aft-reflexing, the higher the increment in the nose-up moment. Owing to the aft-reflexing of the airfoil, the lift-to-drag ratio of the aircraft decreases when \(\alpha<4^{\circ}\). However, the reduction in the lift-to-drag ratio of the aft-reflexed configurations are not as large as those of Twist2 or Twist3. Therefore, in comparison with the wing-twist design method, the aft-reflex kink airfoil design has lesser influence on the lift-to-drag ratio of the aircraft, while providing an almost identical increment in the nose-up moment. Figure 17: Aerodynamic characteristic of the configurations with different fore-loading kink airfoils. Ma = 0.85, H = 11 km, \(\alpha=-2^{\circ}\sim 8^{\circ},\Delta\alpha=2^{\circ}\). (**a**) Lift coefficient, lift to drag ratio; (**b**) Pitching moment. Figure 18: The increments of C\({}_{m,c}\) and K obtained by fore-loading the kink airfoil at cruise state. (**a**) Pitching moment coefficient; (**b**) Lift-to-drag ratio. The cruising angle of attack of the aircraft is increased by adopting the aft-reflexing airfoil. However, the increment is small and increases with the increase in the extent of aft-reflexing. According to Figures 14 and 22, during cruise, the maximum increment in the direction of the aircraft is \(\alpha=-2^{\circ}\sim 8^{\circ}\), \(\alpha\alpha=2^{\circ}\). The maximum increment in the direction of the aircraft is \(\alpha=-2^{\circ}\sim 8^{\circ}\), \(\alpha\alpha=2^{\  
pitching moment with Kink-reflex3 is about 38%, which is greater than that of Twist3 (34%). The reduction in the lift-to-drag ratio is only about half that of Twist3. The aft-reflexing design is a more effective method that not only provides an optimal nose-up moment, but also has a negligible influence on the lift-to-drag ratio of the aircraft. Figure 23 depicts the comparison of the chordwise pressure distribution along the Y/B = 0.5, 0.75, and 0.92 sections of the wing of the initial and aft-reflexing configurations at \(\alpha\) = 2\({}^{\circ}\). As shown, aft-reflexing has a negligible effect on the pressure distribution near the front of the wing, and only has a significant impact on the pressure distribution near the rear of the wing. Consequently, it does not generate lift or a weak negative lift, thereby reducing the nose-down moment. From the above analysis, it is observed that the fore-loading and aft-reflexing airfoils at the kink section can increase the nose-up moment and improve the longitudinal performance. According to Figures 18 and 22, the increment in the nose-up moment is almost proportional to the increment in camber at the same superposition location of the camber. The efficiency factor \(\kappa\) of camber trimming describes the longitudinal trimming efficiency of fore-loading or aft-reflexing, and is defined as follows:  
\[\kappa=\frac{\Delta C_{m}}{\left[CamVal\right]}\,, \tag{4}\]  
Figure 23: Pressure coefficient distributions at typical sections of the different aft-reflexed configurations. Ma = 0.85, \(\alpha\) = 2\({}^{\circ}\). Figure 22: The increments of C\({}_{m,c}\) and K obtained by aft-reflexing the kink airfoil at cruise state. (**a**) Pitching moment coefficient; (**b**) Lift-to-drag ratio. where, \(\Delta C_{m}\) is the increment in the nose-up moment, and _CamVal_ is the maximum value of the superimposed camber.