## 6 Conclusions and Prospects  
This paper designs a soft transition control method for a compound-wing UAV VtoIA7 based on the implicit NDI and INDI control law, which solves the safety problems in the transition phase from fixed-wing mode to rotary-wing mode. Moreover, this paper proposes the normal overload as a physical quantity that reflects the switching quality, which has guiding significance for the evaluation of soft switching, and reflects the value of soft switching for compound-wing aircraft. The transition phase is a rather dangerous stage for compound-wing aircraft. This paper verifies and solves these problems in the transition phase:  
1. The normal overload is too large, which affects the passenger comfort and equipment safety in future UAM applications;
2. Sensitivity to model uncertainty. Affected by its adverse effects, it poses a threat to flight safety;  
Figure 22: Histogram of extreme overload statistics from MC simulation. * Poor pitch attitude and altitude command tracking;
* Throttle command and rotor command saturation. This paper designs a soft switching control by using the method of rotor pre-start to compensate for lift, which realizes the smooth switching from fixed-wing mode to rotary-wing mode in the landing preparation phase. Through 100 Monte Carlo simulations with 20% model uncertainty, it is verified that soft switching can reduce the limit normal overload by 89%. Designing a soft switching strategy can reduce the adverse effects of model uncertainty, which not only ensures the safety of the aircraft, but also can be used to improve the passenger comfort, and provides a new idea for future UAM vehicles. This paper finds the relatively suitable transition time by comparison, and finds that extending the duration of the transition switching mode to a certain extent is beneficial to reduce the adverse effects of model uncertainty. Therefore, finding out the appropriate duration of the transition phase is a necessary work, and is a process of optimizing the transition phase control strategy. In the transition phase, the artificial setting of the duration of the switching mode is usually only applicable to the specific flight state of the specific aircraft. For various compound-wing UAVs, if a specific parameter is added to the autopilot module to determine whether the switching conditions are met, in other words, if it is up to the flight control system to determine the duration of the switching mode, then it is expected to better solve the robustness and safety problems of the transition phase of compound-wing UAVs. We intend to take this problem as a future research topic, to design a more widely applicable transition segment control method. The transition phase control of compound-wing UAVs is a challenging and innovative work, and also a promising research direction. We hope to see more progress and achievements in the future. Conceptualization, Y.C., H.L. and Z.G. ; Methodology, Z.Y. and Y.C. ; Software, Z.Y. ; Validation, Z.Y. ; Data Curation, Z.Y. ; Writing--Original Draft, Z.Y. ; Writing--Review and Editing, Y.C. and P.C. ; Visualization, Z.Y. and P.C. ; Supervision, J.W. All authors have read and agreed to the published version of the manuscript. This research was funded by Aeronautical Science Foundation of China (Grant No. 2019 ZA052001) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). Not applicable. The data presented in this study are available on request from the corresponding author. The data are not publicly available due to the data is based on a UAV model that involves some commercial content and these data need to be protected. The authors declare no conflict of interest. The following nomenclatures and Abbreviations are used in this manuscript: