## 6 Conclusions and Future Works  
In this study, the flexible cable-connected unknown payload, which is a challenging engineering problem, is described. The challenging problems are related to the swinging unknown payloads and point-based reference trajectories. For this challenging engineering problem, a proportional-integral-derivative (PID) controller, type-1 Fuzzy PID controller (T1-FPID), and interval type-2 Fuzzy PID controller (IT2-FPID) are tuned and implemented on a real UAV. The control algorithms are generic and can be applied to different types and sizes of UAVs, although the experiments are performed using a DJI Tello UAV. The performance of the IT2-FPID is also compared against a PID and a T1-FPID controller. After set-point tracking and robustness testing via disturbing the payload with a stick and changing the reference points in coverage path planning (CPP), causing the payload to swing, the experimental results show that the IT2-FPID is better than the controllers. There are different aims for each test, such as minimum task time and RMSE. Disturbing effects of the flexible cable-connected payload on the UAV are shown throughout the experiments. These adverse effects can be eliminated by the three compared controllers to some point, but the results show the superiority of the IT2-FPID. As an example from the experiments, although the PID controller may solve the trajectory-tracking problem, the uncertainties, such as the unknown payload connection via flexible cable and disturbing the payload, lead to highly detrimental results for the PID controller. Thanks to implementing the IT2-FPID controller, these problems are eliminated considerably. Consequently, the system stability is guaranteed without considering the changes in the system dynamics equations and redesigning the controller.  
After the controller design, the sensor part of the UAV will be improved to achieve better accuracy. In order to improve the sensor accuracy, the noise distribution will also be investigated, and then the system will be tested both in the simulation environment and in real time. According to the type of distribution, a filter design will be included to eliminate such noise and general disturbances. Lastly, a theoretical analysis of the performance of the proposed approach under internal and external disturbances is a topic of future work.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/a16060273/s1](https://www.mdpi.com/article/10.3390/a16060273/s1)]([https://www.mdpi.com/article/10.3390/a16060273/s1](https://www.mdpi.com/article/10.3390/a16060273/s1)).  
The authors' contributions are conceptualization, F.C., O.F.D., L.M. and M.M.; methodology, F.C. and O.F.D.; software, F.C.; validation, F.C. and O.F.D.; formal analysis: all authors; investigation: F.C., L.M., T.K., M.M.; resources: F.C., L.M., M.M. and T.K.; data curation: F.C.; writing original draft preparation: F.C.; writing--review and editing: F.C., L.M., T.K. and M.M.; visualization: F.C.; supervision: L.M. and M.M. All authors have read and agreed to the submitted version of the manuscript.  
This research received no external funding.  
Not applicable.  
We acknowledge the Turkish government for funding the doctoral scholarship of [PERSON]. [PERSON] was supported by the BAGEP Award of the Science Academy and the TUBA GEBIP award from the Turkish Academy of Sciences.  
The authors declare no conflict of interest.