## 3 Results and Discussion
### Study on Flow Control of Vertical Tail in Different Spanwise Positions  
The flow on the three-dimensional swept-back vertical tail model has an obvious three-dimensional effect. Because the chord length of each section is different, the control effect of the actuator in different spanwise positions must be different. The influence of the actuators in different spanwise positions of the vertical tail model is studied quantitatively. The experimental Reynolds number for the group-independent control study is 2.12 \(\times\) 10\({}^{5}\), and the sideslip angle of the vertical tail model is set to 0\({}^{\circ}\). The force measurement is carried out on the model of group I, II, and III actuators when they are supplied with air separately.  
Figure 15: Lateral force increment of vertical tail model under different sideslip angles (Cu = 0.332%, \(Re=2.12\times 10^{5}\)).  
Figure 17 shows the lateral force coefficient increment curve under different momentum coefficients and different control positions. When the momentum coefficient of the actuator is 0.083%, it can be seen that the vertical tail lateral force increment when the actuator group II is controlled alone is significantly higher than that when the other two groups are controlled alone. Under this momentum coefficient, the control effect of the actuator at the wing tip position and the wing root position is close. When the momentum coefficient of the actuator is 0.166%, the control effect of the group II actuator at the middle section of the vertical tail is the best, and the control effect of the group I actuator at the wing root is slightly lower than that of the group II actuator. The control effect of the group III actuator at the wing tip is the worst. When the momentum coefficient of the actuator increases to 0.332%, the control effect of the actuator at the middle section of the vertical tail is close to that at the wing root, and the control effect at the wing tip is still the worst.  
Regardless of the momentum coefficient input by the oscillating jet actuator, the actuator control at the wing tip has the worst effect on increasing the vertical tail lateral force. The actuator in the midspan can provide a stable and considerable increment for the vertical tail's lateral force. As for the actuator at the wing root, the control effect is poor when the momentum coefficient is small. With the increase in the momentum coefficient, its impact on the vertical tail lateral force increases significantly. The above differences are mainly related to the chord lengths of airfoils at different spans of the 3D model. The chord length of the airfoil at the wing root is considerable, and the area of the vertical tail is much larger than that at the wing tip. When the momentum coefficient of the actuator is small, the flow separation in a large area cannot be wholly suppressed. We cannot achieve such a significant lateral force increase. However, with the rise of the momentum coefficient of the actuator, the separation area near the wing root on the rudder decreases gradually, and the lateral force increases significantly. Similarly, the rudder area at the wing tip is minimal, and the separation flow suppression here cannot significantly contribute to improving the vertical tail lateral force. The above is why the control effect is quite different when the control position is different. By studying the influence of spanwise control position on the flow control law of a vertical tail oscillating jet, we can optimize the flow control strategy to achieve a better flow control effect.  
In conclusion, the oscillating jet can significantly improve the lateral force of the vertical tail under typical working conditions. Especially under harmful sideslip conditions, the oscillating jet flow control can substantially improve the heading control ability of the vertical tail. We can enhance the lateral force coefficient of the vertical tail model by applying flow control alone at different spanwise positions, but the control effect is different.