# Aerodynamic Optimization and Characterization of a Ducted Tail for a Box-Launched Aircraft
## 5 Optimization Result Analysis
### Discussion of Optimization Results for Ducted Tail Position
#### 5.1.3 Effect of L4 and H1 on Tail Aerodynamic Performance  
The effects of the preferred parameters on the aerodynamic performance of the tailplane are shown in Figure 10. It can be seen that the multi-objective genetic optimization algorithm proposed in this paper has carried out comprehensive optimization of parameters H1 and L4. It preferred the H1/L4 combination parameters and made a compromise between parameters H1 and L4, which both shortened the length of the air ducts and increased the width of the air ducts, which resulted in the enhancement of lift coefficients of the tail, and the linearization of lift coefficients has been significantly improved. From Figure 10a, it can be seen that with the simultaneous increase of parameters L4 and H1, the interference of the fuselage on the tail airflow is reduced due to the shortening and widening of the ducted area formed by the tail and the fuselage, and the degree of linearization of the tail lift line is improved with an increase in the slope. Meanwhile, the increase of parameter H1 leads to the increase of the tail chordwise loop value, which effectively suppresses the decrease of the lift coefficient due to the shortening of the duct area and effectively improves the tail lift coefficient. From Figure 10b, it can be seen that with the simultaneous increase of H1 and L4, the duct area is shortened and widened, the airflow obstruction inside the duct is weakened, the airflow interference from the fuselage to the tailplane is reduced, the chordal annulus flowing through the tailplane increases, and the lift-to-drag ratio of the tailplane is significantly improved, especially in the interval of the angle of attack from \(-2^{\circ}\) to \(4^{\circ}\); the lift-to-drag ratio is more obviously improved. From Figure 10c, it can be seen that the pressure coefficient difference between the upper and lower surfaces of the tail at the \(6^{\circ}\)  
Figure 10: Effect of L4 and H1 on lift and drag characteristics. (**a**) Lift coefficient comparison; (**b**) lift-to-drag ratio comparison; (**c**) pressure coefficient at \(6^{\circ}\) angle of attack. angle of attack is obviously increased after parameter optimization, which further verifies that the degree of linearization of the tail lift coefficient, the lift-to-drag ratio, and the lift value are effectively improved after the parameter optimization due to the weakening of the airflow disturbance and the increase in the tail chordwise loop volume. As shown in Figure 11, comparing and analyzing the pressure cloud of sample 2 and sample 6 on the symmetry plane, due to the smaller relative parameter H1/L4 of the tail and fuselage in sample 2 which results in a narrow and long duct area, the fuselage's interference effect on the tail airflow is strengthened with the increase of the angle of attack. This is prone to form airflow obstructions, and there is no obvious change to the low-pressure area in the upper airfoil of sample 2. On the other hand, due to the improvement of the duct area, the low-pressure area on the upper airfoil of sample 6 is obviously larger than that of sample 2, and the range of the low-pressure area gradually increases, which makes the pressure difference between the upper and lower airfoils of sample 6 larger than that of sample 2. In addition, where the tilted section of the tail intersects with the horizontal section, the sudden change in the shape leads to the emergence of a dead zone in the airflow, and a high-pressure region is found to exist, which is coupled with the upper airfoil of the tail wing; this is unfavorable to the enhancement of the tail lifting force. Figure 11: Comparison of pressure cloud images between sample 6 and sample 2. (**a**) AOA = 0\({}^{\circ}\); (**b**) AOA = 4\({}^{\circ}\); (**c**) AOA = 8\({}^{\circ}\); (**d**) AOA = 12\({}^{\circ}\). As shown in Figure 12, the velocity cloud diagrams of sample 2 and sample 6 on the symmetry plane are compared and analyzed.