# Improvement of Hexacopter UAVs Attitude Parameters Employing Control and Decision Support Systems
## 2 Related Work
### Multirotor Drones Stability Assessment Based on FEM Approach  
When discussing the hexacopter stability ([PERSON] et al., 2018; [PERSON] et al., 2018; [PERSON] et al., 2018; [PERSON] et al., 2018), aspects related to the structural components of a hexacopter platform have to be considered. In the case of FEM analysis, this frequently focuses on structural stiffness and stability and requires a detailed assessment of the hexacopter model in a synergic connection between the virtual CAD model and FEM codes. The results matter not only in the decision-making process regarding the design of the hexacopter structure but also to the flight stability and the position control on the trajectory. Reducing the drag force remains one of the main challenges in UAV aerodynamics research, as battery consumption can be significantly reduced if the drag decreases. [PERSON] et al. (2018) analyse the aerodynamic behaviour of a quadcopter equipped with a seeding device in order to determine the appropriate bank angles (\(0^{\circ}\), \(15^{\circ}\) and \(30^{\circ}\)) for take-off and drone flight evolution during the seeding operation. Moreover, the work aims to define a suitable flight plan to increase the battery range. Aerodynamic results demonstrate that for take-off, the \(30^{\circ}\) tilt represents the most favourable aerodynamic position, due to the lower drag force that occurs during climb. In terms of the drone's behaviour during seeding, the \(0^{\circ}\) tilt is the one that creates a lower frontal drag and a lower drag force coefficient, respectively. [PERSON] et al. (2018) discuss the aerodynamic performance of a hexacopter with different rotor spacings. The hovering flight efficiency of the drone is analysed by performing experimental tests and numerical simulations. A number of indices characterising the aerodynamic performance of the hexacopter are analysed theoretically, followed by tests and simulations on a hexacopter drone with different rotor-spacing ratios in relation to propeller size (i = 0.50, 0.56, 0.63, 0.71, 0.83). Using a custom-conceived test platform, the thrust, power load and hover flight efficiency of the hexacopter were obtained. Finally, CFD simulations were performed to obtain the fluid flow, pressure and velocity contour distributions of the hexacopter. The results show that the aerodynamic performance of the hexacopter drone varies by changing the rotor spacing. It was also observed that the thrust force increased by 5.61% and the overall efficiency increased by about 8.37% at i = 0.63 for the working mode (2200 RPM), indicating that the rotor spacing ratio at i = 0.63 achieved the best aerodynamic performance. The design and development of a hexacopter capable of lifting a high payload has been investigated by [PERSON] et al. [42] to ensure a stable attitude in flight. The evaluation focusses on the frame displacement and stress analysis to ensure the expected payload. In [85], experimental and CFD simulation tests for a small size UAV model to test wind influences at low speeds were reported. The study is comprehensive, but the prototype geometry as well as the flow regimes were found to be below the common level of UAV applications. The flow regimes of air streams from the upper to the lower surface at different fixed-point flight altitudes were simulated and analysed by [PERSON] et al. [86] for plant protection applications. Although the study is substantial, the results are particular to the field of agricultural engineering. In conclusion, many FEM simulation attempts have been reported on hexacopters, but the most interesting results for this type of platform have been achieved by software developers to demonstrate the capabilities of the solvers, as simulation in this case is still considered a challenge. In the case of the six rotors, the rotational domains of the propellers are so close to each other that the narrow space induces even more modelling and computational difficulties.