## 1 Introduction  
Volleyball is one of the most popular sports in the world. The key factors to victory are the skills and strategies of blocking, attacking, serving and receiving. The serve is important not only for scoring points directly, but also for gaining an advantage in the match by breaking the receiver's posture and reducing the power of the opponent's attack [1, 2]. There are two main ways to serve that are considered to be effective. The first type is the jump serve, in which the ball is hit with topspin and dropped at high speed into the opponent's court. The second type is the floater serve, which is the subject of this study. In the floater serve, the ball is hit with no or low rotation, which causes an irregular change in the trajectory of the ball. This irregular change in trajectory has been reported to be caused by the fluid forces generated by the three-dimensional twin vortices formed behind the ball [3, 4]. The irregular changes in the position of the twin vortices cause irregular changes in the trajectory of the ball.  
The fluid force on the ball has been investigated in wind tunnel experiments [5; 6], with a hitting robot [7], and computational calculations [8; 9; 10]. The drag coefficient applied to the ball is large at low velocities, and decreases drastically with the transition from laminar to turbulent flow as the speed of the ball increases. This decrease in the drag coefficient due to the change in flow is called drag crisis, and it plays an important role in volleyball because the ball decelerates in flight to a speed range where the drag crisis occurs [11; 12; 13; 14]. The fluid force characteristics of a ball vary depending on the type of ball and the panel surface [15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27]. Measurements using a wind tunnel showed that the drag coefficient, the magnitude of lateral force, and the speed range at which drag crisis occurs differs depending on the type of ball and the panel surface. This indicates that the type of ball used in a game influences the tactics of the players. Similar phenomena were observed with balls used in other sports, and it is important to focus on the panel shape and orientation when discussing the fluid forces on balls. However, the drag coefficient measurements of the ball surface fixed in the wind tunnel is not sufficient to describe the trajectory of the ball, because the speed of the ball decelerates during flight and the force applied to the ball changes irregularly. For a more detailed understanding, it is necessary to serve a large number of balls under the same conditions to clarify the relationship between actual flight and wind tunnel measurements [7]. We are also interested in how much the actual trajectory of the ball is affected when a person serves the ball while paying attention to the panel surface.  
In this study, in order to clarify more directly the effects of the ball type and panel surface on the actual flight of the serve, we measured the drag coefficient using a wind tunnel and the flight distance using an ejection machine for three types of balls used in competitions: MVA200, V200W, and FLISTATEC. We also compared the human serve with the ejection machine, and measured and clarified the effect of the panel surface on the human serve. Figure 1 shows the characteristics of the three types of balls measured in this study. The MVA200, MIKASA, is the ball used in the 2012 and 2016 Olympic Games, consisting of eight panels with a dimpled surface. V200W, MIKASA, is the ball used in the 2021 Tokyo Olympic Games, consisting of 18 panels with a dimpled surface. FLISTATEC, MOLTEN, is the ball used in the NCAA. It has an uneven surface with a hexagonal honeycomb structure and consists of 18 panels. The flight distance of MVA200 was measured while changing the initial speed of the ball. In addition, the effect of selecting the panel orientation on the actual serve was clarified by comparing the ejection by the machine and the human serve.  
Figure 1: Three types of balls in the measurement.