## 4 Simulation Results and Discussion
From the figure of tracking error, it can be concluded that the 11 s (starting with a change in sweepback) dive process has no significant impact on the tracking of the flight-path angle, and the tracking error still maintains a continuous decay state, indicating that the proposed ADSC control strategy has a certain degree of robustness. Obviously, as the flight-path angle increases during the entry process, the convergence time of the tracking error also lengthens, which means that when the HAUV enters the water at a large flight-path angle, the controller needs more time to adjust to keep up with the ideal trajectory. Figure 5: Under the phenomenon of plunge\(-\)diving, complete the change in variable sweepback fully and then ingress the water. Flight\(-\)path angle tracking results and tracking errors of the variable\(-\)swept HAUV in the plunge\(-\)diving process at different change rates of flight\(-\)path angle in _case 1_: (**a**) \(\Delta\gamma=\frac{\pi}{12}\); (**b**) \(\Delta\gamma=\frac{\pi}{6}\); (**c**) \(\Delta\gamma=\frac{\pi}{4}\). (**a**) Tracking result and error when \(\Delta\gamma=\frac{\pi}{12}\). (**b**) Tracking result and error when \(\Delta\gamma=\frac{\pi}{2}\). (**c**) Tracking result and error when \(\Delta\gamma=\frac{\pi}{4}\). Figure 6 shows the angle of rudder reflection of the HAUV when it changes sweepback during diving. During the simulation process, consider the actual situation and limit the deviation angle of the rudder surface to within the range of \(\pm 30^{\circ}\). When analyzing, the main focus is on the vicinity of key time points in each subgraph. For case 1, the key time points are 5 s, 10 s, 11 s, etc. From the deflection of the rudder angle in 5 s (starting of the dive), it can be seen that the controller can quickly adjust the angle of rudder reflection and smoothly transition. There is no obvious control input near 10 s to 11 s, and it can be analyzed that the aerodynamic parameter changes caused by the variable sweepback process during the dive process have not had an impact on the control stability. Similarly, it is evident that as the flight-path angle increases, the input jitter of the rudder deviation control becomes more frequent, which means the controller has fast reaction to disturbance and aerodynamic parameter change. Figure 7 shows the change law of AOA and pitch angle rate under case 1, and with a different sweepback angle rate, we can conclude that when \(\Delta\gamma\) increase at the same time, AOA and pitch angle's oscillations become more apparent. Figure 6: The actual control input of angle of rudder reflection due to the variable\(-\)swept and plunge\(-\)diving process in _case 1_. Figure 7: Under the phenomenon of plunge\(-\)diving, complete the change in variable sweepback fully and then ingress the water. Change laws of AOA and pitch angle rate of the variable\(-\)swept HAUV in the plunge\(-\)diving process at different change rates of flight\(-\)path angle in _case 1_: (**a**) \(\Delta\gamma=\frac{\pi}{2}\); (**b**) \(\Delta\gamma=\frac{\pi}{2}\); (**c**) \(\Delta\gamma=\frac{\pi}{4}\). (**a**) Change law of AOA and pitch angle rate when \(\Delta\gamma=\frac{\pi}{12}\). (**b**) Change law of AOA and pitch angle rate when \(\Delta\gamma=\frac{\pi}{6}\). (**c**) Change law of AOA and pitch angle rate when \(\Delta\gamma=\frac{\pi}{4}\). Through the simulation process in _case 1_, it can be concluded that a low change rate of flight-path angle should be maintained as much as possible during the diving process under a certain water-entry time to determine the stability of the control system. _Case 2: During the level flight, it changes sweepback and then dives into the water._  
The relationship between ideal flight-path angle and sweepback over time in case 2 are as follows:  
\[x_{1d}= \begin{cases}0&t\leq 15\\ -\gamma_{e}\sin(\frac{\pi}{10}t+\frac{\pi}{2})&15<t\leq 20\text{ and }\chi= \begin{cases}\frac{\pi}{12}&t\leq 5\\ \frac{\pi}{4}\lg(t-4)+\frac{\pi}{12}&5<t\leq 14\\ \frac{\pi}{4}&t>14\end{cases}\end{cases}