## 3 Results and Discussion
### Study on Basic Aerodynamic Characteristics of Vertical Tail Model
Under the condition of \(\beta=-10^{\circ}\) and \(Re=2.12\times 10^{5}\), the pressure coefficient curve of the rudder surface at different rudder deflection angles is shown in Figure 9. Since the preset sideslip angle is \(-10^{\circ}\), the upper surface of the rudder is windward when the rudder deflection angle is 0\({}^{\circ}\) and 5\({}^{\circ}\), and the surface pressure is positive. When the rudder deflection angle is 15\({}^{\circ}\), the pressure curve is consistent with that without the sideslip angle, and the surface pressure coefficient curve presents a \"negative slope\". When the rudder deflection angle is greater than 20\({}^{\circ}\), the curve of the rudder surface pressure coefficient is basically consistent with that without the sideslip. In addition, after the rudder deflection angle is greater than 25\({}^{\circ}\), flow separation occurs on the rudder obviously. Each pressure-measuring hole is in the flow separation zone, and the measured pressure coefficient changes within the range of 0.1. The rudder surface pressure data can only measure the surface flow field information and cannot measure the spatial flow field of the vertical tail model. The PIV experiment can determine the source of the vertical tail lateral force growth and obtain the control law of the vertical tail oscillating jet. Figure 10 is the velocity cloud diagram and streamline diagram of the upper wing surface of the vertical tail with different rudder deflection angles at a 0\({}^{\circ}\) sideslip angle. When the rudder deflection angle is 15\({}^{\circ}\), the flow separation phenomenon can be observed on the rudder. The dimensionless velocity is 0.21 at the trailing edge of the rudder. The flow separation starts to develop from the trailing edge of the rudder. In addition, the wing space flow field has a partial acceleration area on the rudder hinge. This phenomenon is caused by the deflection of the rudder surface, which changes the flow field in space and is also unobservable in measuring the surface pressure. When the rudder deflection angle increases to 30\({}^{\circ}\), the separated vortex structure on the upper surface of the rudder is more abundant. In addition, the separation area is extended to the full chord length of the rudder. We can be seen from the velocity cloud diagram that the velocity in the flow separation area on the rudder is close to 0. In contrast, the spatial flow field on the stabilizer is unchanged with the change of the rudder deflection angle. Figure 11 is the velocity cloud diagram and streamline diagram of the upper surface of the vertical tail with different rudder deflection angles at a \(-10^{\circ}\) sideslip angle. The velocity of the mainstream on the stabilizer decreased significantly to about 0.86. When the rudder deflection angle is \(15^{\circ}\), the flow separation phenomenon occurs on the upper surface of the rudder, and the near-wall streamline cannot cling to the rudder surface. Due to rudder deflection, the mainstream speed on the rudder increases to 0.96. When the rudder deflection angle is increased to \(30^{\circ}\), the flow on the rudder is wholly separated, and the change law of the spatial flow field is consistent with that when the sideslip angle is \(0^{\circ}\). However, the degree of flow separation on the rudder is lower than when the sideslip angle is \(0^{\circ}\).