## 3 Results and Discussion
### Study on Basic Aerodynamic Characteristics of Vertical Tail Model  
At Reynolds number 2.12 \(\times\) 10\({}^{5}\), the flow measurement experiment of the vertical tail model without flow control was conducted. The sideslip angle range was 0\({}^{\circ}\)--10\({}^{\circ}\), and the rudder deflection angle range was 0\({}^{\circ}\)-40\({}^{\circ}\). Figure 6 shows the curve of the vertical tail without control aerodynamics changing with the rudder deflection angle at \(\beta\) = 0\({}^{\circ}\), and the rudder change step is 2\({}^{\circ}\). Because of the possible declination of the wind tunnel's airflow and the deviations in model making, the rudder deflection angle is hard to set at 0 degrees in the air flow coordinate system. This may be the cause of the non-zero force at 0 degrees yaw and 0 degrees rudder deflection. When the rudder deflection angle is less than 18\({}^{\circ}\), the lateral force coefficient curve keeps the same slope. When the rudder deflection angle is in the range of 18\({}^{\circ}\)-22\({}^{\circ}\), the slope of the lateral force coefficient curve decreases obviously, and the lateral force coefficient increases  
slowly. We can see from the drag coefficient curve that when the rudder deflection angle is less than \(18^{\circ}\), the drag coefficient of the vertical tail model increases slowly, and the curve slope is slight. When the rudder deflection angle is greater than \(20^{\circ}\), the drag coefficient of the vertical tail model rises significantly. While the rudder drag increases significantly, the direct force has a component in the positive Y direction, which is the main reason for the further increase in the lateral force coefficient when the rudder deflection angle is large. The study of the characteristics of the vertical tail lateral force under the typical conditions of \(-5^{\circ}\) and \(-10^{\circ}\) sideslip angles continued. Figure 7 shows the curve diagram of the lateral force coefficient of the vertical tail model under different sideslip angles when no control is applied. It can be seen that when the sideslip angle is \(-10^{\circ}\), the rudder deflection angle within the range of \(0^{\circ}\)-\(37^{\circ}\) cannot produce the lateral force in the positive direction. This means that under this working condition, the vertical tail cannot balance the yaw moment caused by a large crosswind only by the rudder deflection, which is a huge threat to flight safety. The reasons for the change of the lateral force coefficient are explored through the study of rudder surface pressure. Under the condition of \(\beta=0^{\circ}\) and \(Re=2.12\times 10^{5}\), the  
Figure 6: Aerodynamic curve of uncontrolled vertical tail model: (**a**) C\({}_{\rm Y}\) curve; (**b**) C\({}_{\rm D}\) curve (\(\beta=0^{\circ}\), \(Re=2.12\times 10^{5}\)). Figure 7: Lateral force coefficient of vertical tail model under different sideslip angles (\(Re=2.12\times 10^{5}\)). pressure coefficient curve of the rudder surface at different rudder deflection angles is shown in Figure 8. It can be seen from the figure that when the rudder deflection angle is less than 15\({}^{\circ}\), the surface pressure coefficient curve presents a significant \"negative slope\", and the negative pressure gradually decreases along the flow direction. There is a pressure gradient due to the attached flow on the rudder. With the rudder deflection angle increase, the overall pressure coefficient curves \"move up\". The negative pressure at the rudder trailing edge increases. However, when the rudder deflection angle is increased to 20\({}^{\circ}\), especially after 25\({}^{\circ}\), the slope of the pressure coefficient curve is entirely different from that when the rudder deflection angle is slight, showing a slope close to \"0\". Flow separation occurs on the rudder, and the pressure taps are all in the flow separation zone. The change of the pressure data curve corresponds to the gradual slope of the lateral force coefficient curve above.