## 3 Results and Discussion
### Aerodynamic Behavior
In the range of 2\({}^{\circ}\) to 10\({}^{\circ}\), the increase rate of the absolute amplitude is relatively slow, while the increase rate is relatively fast in the range of 10\({}^{\circ}\) to  
Figure 7: The amplitude of aerodynamic coefficient: (**a**) lift coefficient, (**b**) drag coefficient, (**c**) moment coefficient. 18\({}^{\circ}\). The relative amplitude of the drag coefficient is smallest at 2\({}^{\circ}\), changes little between 4\({}^{\circ}\) and 12\({}^{\circ}\), and increases rapidly at 14\({}^{\circ}\). However, the time-averaged value increases more rapidly, causing the relative fluctuation to decrease from 14\({}^{\circ}\). The absolute amplitude of the moment coefficient increases near-linearly with the angle of attack from 2\({}^{\circ}\). The relative amplitude increases with the increase in the angle of attack from 2\({}^{\circ}\), reaches its maximum at 14\({}^{\circ}\), and then decreases gradually. Therefore, the medium angle of attack (2\(\sim\)10\({}^{\circ}\)) should be selected as the design cruise angle of attack for the WIG craft. Within this range of angle of attack, the fluctuation amplitude of lift coefficient and moment coefficient is relatively small, and the fluctuation amplitude of drag coefficient changes little, thus avoiding the large bump of the WIG craft when flying near the wavy ground or water wave and facilitating the control of the craft. The 0\({}^{\circ}\) angle of attack should be avoided as the design cruise angle of attack at which the fluctuation of lift coefficient exceeds 100%, which can cause severe bumps during takeoff/landing and even endanger flight safety. The maneuverability of the WIG craft is deficient in the situation of a high angle of attack. If the WIG craft encounters a high angle of attack when it is flying near the wavy ground or water wave, the fluctuation in the aerodynamic coefficients will deteriorate the maneuverability of the aircraft. Figure 8 shows the time-averaged aerodynamic coefficient varying with the angle of attack in a period. The aerodynamic coefficients in the situation of free stream and flat ground are also given. Other conditions, except the ground boundary conditions, are the same for wavy ground, flat ground, and free stream. It can be seen from the figure that the time-averaged aerodynamic coefficients in the case of wavy ground are close to those in the case of flat ground, whether at small or high angles of attack. It can be concluded that, among all angles of attack, the wavy ground only causes a fluctuation  
Figure 8: Variation in the time-averaged aerodynamics with the angle of attack: (a) lift coefficient, (b) drag coefficient, (c) moment coefficient. in the aerodynamic coefficient compared with flat ground. However, it should be noted that the fluctuation amplitude of the aerodynamic coefficients varies at different angles of attack, there are no clearly predictable laws, and even the trend of aerodynamics is changed at a 0\({}^{\circ}\) angle of attack. The fluctuation in aerodynamics is the peculiarity of the aerodynamic characteristics of an airfoil moving over a wavy ground. There is an apparent difference between aerodynamic coefficients in the case of ground effect and those in the case of free stream. In the case of free stream, the lift coefficient curve has an obvious linear section when the angle of attack is less than 8\({}^{\circ}\), while there is no linear section of lift coefficient in the case of ground effect. Within the range of a 0-4\({}^{\circ}\) angle of attack, the change rate of the lift coefficient with the angle of attack is large, and then the change rate becomes smaller from 4\({}^{\circ}\). Within the range of a 0-12\({}^{\circ}\) angle of attack, the drag coefficient in the case of ground effect is almost the same as that in the case of free stream. As the angle of attack is higher than 12\({}^{\circ}\), the drag coefficient in the case of ground effect increases sharply, while the drag coefficient in the case of free stream increases slowly.