## 3 Result and Discussion
### Ejection Experiment  
Figure 9 shows the distribution of landing points when MVA200, V200W, and FLISTATEC were ejected at a speed of 60 km/h using an ejection machine. The panel characteristics of the ball flight direction were distinguished by the panel orientation, logo position, and ball valve position. While varying these factors, 12 patterns for MVA200, 24 patterns for V200W, and 26 patterns for FLISTATEC were measured five times each. The different types of plots represent differences in the orientation of the panels, the logos, and the positions of the valves. The red line indicates the end line of the court, and any ball that flies farther than this line is an out ball. Receivers are usually placed at a range of 6.5 to 7.5 m. The area enclosed by the red square is where it is difficult to return the ball, close to the net or end line. As shown in Figure 9, the distance of the ball varies, but is characterized to some extent by the type of ball and the panel surface. The average landing point for each ball was 7.5 m for the FLISTATEC, 7 m for the V200W, and 6.8 m for the MVA200, which is consistent with the order of decreasing drag coefficient in the fluid force measurement shown in Figure 8.  
Figure 5: Relationship between Reynolds number and drag coefficient on MVA200. Figure 8: Relationship between Reynolds number and averaged drag coefficient on each ball. Figure 6: Relationship between Reynolds number and drag coefficient on V200W. Figure 7: Relationship between Reynolds number and drag coefficient on FLISTATEC. In the case of FLISTATEC, the difference in distance depending on the panel surface was larger than that of MVA200 and V200W. For this reason, it is important to pay attention to the panel face of the FLISTATEC in order to make the ball fall at the target position. Figure 10 shows the standard deviations in the x and y directions for each panel of the balls. Since each panel was measured only five times, it cannot be said to be a statistically sufficient amount, but it was found that there is a difference in the variability of each ball and panel. In the previous fluid force measurement using a wind tunnel [7; 11], the magnitude of the force applied to the ball varied with time, and it was reported that some panels had large amplitudes while others had small amplitudes. The results of the present study, which show that the variation in the drop point varies from panel to panel, are consistent with the previous result, indicating that the blurring of the floater serve varies from panel surface to panel surface. Figure 10: Standard deviation of the drop position by the panel in the x and y directions. Figure 9: Landing point of MVA200, V200W, and FLISTATE C that changes the orientation of the panel and eject at the initial speed of 60 km/h. The different types of plots represent differences in the orientation of the panels, the logos, and the positions of the valves. Figure 11 shows the suggested effective positioning of the receivers based on the trend of the ball flight distance. It is considered better to place the receivers in a horizontal row because the MVA200 has relatively small variation in the y direction. On the other hand, V200W and FLISTATEC have a large variation in the y-axis direction, so it is desirable to protect the front and back with a zigzag receiver formation instead of a simple horizontal formation. The effect of initial velocity was investigated for the V200W. Figure 12 shows the landing point of the V200W as the velocity changes. When ejected with an initial velocity of 60 km/h, the V200W landed on the court with a probability of 96%. As the initial speed is increased to 60 \(\pm\) 1.5 km/h and 70 \(\pm\) 1.5 km/h, the probability of landing on the court decreases to 65% and 24%, respectively. In case of the V200W serve with the hitting angle 5.5\({}^{\circ}\), it is important to hit at 60 to 65 km/h while considering the panel.