## 4 Discussion of Simulation Results
### Glide Simulation Research
#### 4.2.2 Gliding Research When Blade Folded  
Through the simulation analysis, it is found that, after the hybrid-driven underwater glider is equipped with Kappel propellers, the propeller significantly increases the overall resistance of the hybrid-driven underwater glider and weakens the overall lift of the hybrid-driven underwater glider. In order to deal with this problem, this paper puts forward the strategy of folding the blade in order to reduce the adverse effect of the propeller on the gliding performance of the hybrid-driven underwater glider. Taking the combination of propeller Kap05 and lozenge wing underwater glider as an example, the propeller blade in the geometry model is set to rotate and fold 60\({}^{\circ}\) in the direction of the stern, and the simulation is carried out under the gliding conditions of 1 kn and 0.5 kn and gliding angle of 12\({}^{\circ}\) by using the SST \(k\)-\(\omega\)+\(\gamma\) transition turbulence model. The calculation results of total drag D_total, propeller drag D, total lift L_total, and lift L generated by propeller of the hybrid-driven underwater glider are shown in Table 11. Where the value of L is negative, the direction is vertical downward, opposite to the direction of the total lift L_total. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Gliding Speed (kn)** & **D\_Total (N)** & **D (N)** & **L\_Total (N)** & **L (N)** \\ \hline
1.0 & 5.7351 & 1.7166 & 13.7148 & 0.0944 \\
0.5 & 1.4997 & 0.3872 & 3.7141 & 0.0291 \\ \hline \hline \end{tabular}
\end{table} 
Table 11: Gliding calculation results of hybrid-driven underwater glider with propeller blade folding in working condition. Figure 25: Flow field distribution of hybrid-driven underwater glider with propeller blade extension (speed of 1 kn, gliding angle of 12\({}^{\circ}\)), UG + Kap00 (**a**), UG + Kap01 (**b**), UG + Kap02 (**c**), UG + Kap03 (**d**), UG + Kap04 (**e**), UG + Kap05 (**f**). Comparing the calculation results in Tables 9 and 11, it can be seen that, if the propeller Kap05 blade folds by 60\({}^{\circ}\), the total drag D_total of the hybrid-driven underwater glider decreases by 22.43% and the total lift L_total increases by 1.64% under the glide condition of 1 kn speed. The propeller drag D decreased by 50.59%, and the negative lift L amplitude generated by the propeller decreased by 85.46%. In the gliding condition with a speed of 0.5 kn, compared with Tables 7 and 9, it can be seen that the total drag D_total of the hybrid-driven underwater glider decreased by 20.01%, the total lift L_total increased by 10.77%, the propeller drag D decreased by 54.62%, and the negative lift L amplitude generated by the propeller decreased by 78.74%. Thus, it can be seen that the measures of folding blades can indeed weaken the adverse effects of propellers on the gliding performance of the body, and effectively help reduce the gliding resistance and negative lift caused by propeller blades, with a significant overall effect. When the propeller Kap05 is combined with the underwater glider and folds the blades, the axial speed distribution cloud map under the glide conditions of 1 kn and 0.5 kn speed is shown in Figure 26. Compared with the unfolded state, the low-speed zone of the stern when the blade is folded is no longer dispersed in the rear of each blade and shaft, but presents centralized; that is, there is only one low-speed zone in the stern. equipped with a series of Kappel tip pitch propellers gradually reduces the wake fraction at the waypoint as the bending degree of the propeller end plate increases. The hybrid-driven underwater glider equipped with the Kappel propeller 05 has the lowest wake fraction and performs the best under autopilot conditions, which helps to improve the actual propulsion performance of Kappel-type propellers in the wake field. Meanwhile, the speed also affects the actual propulsion efficiency of Kappel-type propellers. The lower the cruising speed at the waypoint, the higher the actual propulsion efficiency of Kappel-type propellers. 4.