## 4 Discussion of Simulation Results
### Glide Simulation Research  
#### 4.2.1 Gliding Research with Blade Extension  
In the blade extension state, the Kappel propellers were combined with the underwater glider in order to carry out the simulation under the gliding state with the speed of 1 kn and 0.5 kn and the glide angle of 12\({}^{\circ}\), and the simulation physical time was at least 25 s. In the gliding mode with a speed of 1 kn, the total drag D_total, propeller drag D, total lift L_total, and the lift L generated by the propeller of the hybrid-driven underwater glider are shown in Table 9.  
As can be seen from the table, propeller resistance accounts for a large proportion of the overall resistance, about 47-49%. However, the negative impact of the propeller on the overall lift is small, and its value is basically 4.3-5.1% of the overall lift. The existence of the propeller not only increases the overall drag of the hybrid-driven underwater glider, but also weakens the overall lift of the hybrid-driven underwater glider, which is not conducive to the gliding performance of the hybrid-driven underwater glider in the gliding mode. Figure 24: Flow field distribution at 6 kn speed self-propulsion point, UG + Kap00 (**a**), UG + Kap01 (**b**), UG + Kap02 (**c**), UG + Kap03 (**d**), UG + Kap04 (**e**), UG + Kap05 (**f**). The axial velocity distribution diagram of the flow field of the hybrid-driven underwater glider gliding at 1 kn speed is shown in Figure 25. In this gliding state, the flow field velocity distribution of each hybrid-driven underwater glider is basically the same, and there are high-speed areas in the upper turning point of the bow curve, the lower turning point of the stern curve, and the blade clearance of the underwater glider. And, there are low-speed zones in the front of the bow of the underwater glider, near the surface of the stern, and behind the blade, in which the low-speed zone at the back of the blade has the smallest axial speed, and even shows the low-speed flow in the direction of the reverse flow. According to Figure 25, as the bending degree of the Kappel propeller end plate increases, the distribution range of the low-speed region (blue area) in the wake of the Kappel propeller gradually decreases. The distribution range of the wake high-speed area (yellow and red areas) gradually increases and shows a trend of expanding towards larger radius positions. This indicates that, as the bending degree of the Kappel propeller end plate increases, the axial kinetic energy of the Kappel propeller wake gradually increases, and the axial kinetic energy of the wake is the main contributing component to achieving propeller propulsion, which has a positive significance for the thrust output of the propeller. This further indicates that the combination of the Kappel series propellers can effectively improve the propulsion performance of hybrid-driven underwater gliders, especially when paired with the Kapp5 propeller with a higher degree of end plate bending. In gliding mode at a speed of 0.5 kn, the total drag D_total, propeller drag D, total lift L_total, and the lift L generated by the propeller are mixed to drive the underwater glider, as shown in Table 10. Among them, the proportion of propeller resistance in the whole resistance is basically in the level of 43\(\sim\)46%, and the propeller negative lift is basically 2.7\(\sim\)4.1% of the overall lift. The proportion of drag force and negative lift force caused by the propeller decreases in the glide condition with a speed of 0.5 kn compared with the glide condition with a speed of 1 kn.