# A Novel Drone Design Based on a Reconfigurable Unmanned Aerial Vehicle for Wildfire Management
## 7 Static Structural Analysis
### Structural Simulation Results
#### UAV Body  
Analysing and estimating the strength of a UAV's airframe is critical in the development of the UAV. Various forces act on the structural elements of UAVs, including thrust forces from the propulsion system, gravitational forces from the payload, impact loads, and wind-induced loads. The airframe must maintain structural integrity while preserving size and weight under both dynamic and static loading conditions. In essence, the ideal airframe for any UAV should be lightweight while also being robust. The sub-assembly of this UAV was simulated with the following considerations:  
* Structural support: A fixed constraint was applied, as shown in Figure 20a;
* Loads: The loads applied to the aluminum joints due to motor thrust were calculated as described in Figure 20b. The sketch above (Figure 20b) is a representation of the UAV arm from the moving part of the aluminum joint (left) to the motor/propeller end where the thrust is applied (right). The maximum thrust from the 32\" propeller (100%) is 16.5 kg (Table 1), which is approximately 165 N. To calculate the reaction forces at points A and B, we apply the formulas for equilibrium on each axis. Following that, the three forces (Ax, Bx, By) are transferred to the stationary part of the aluminum joint, resulting in opposite directions (Figure 21). Figure 19: Forces acting on the UAV’s body. Each of the horizontal forces is applied across two equal surfaces, so half of the force is considered for each surface (9547 N/2 \(\approx\) 4785 N). The simulation results for the frame are presented below, with an emphasis on stress and displacement (Figure 22). The outcome shows that the stresses and displacements are within acceptable limits. The maximum stress occurs on the outer bolts of the upper side, which are high strength, grade 12.9 (meaning a tensile strength of 1200 MPa and a yield strength of 0.9 \(\times\) 1200 = 1080 MPa), making them more than adequate to withstand the stresses. The stress on the CFRP upper plate is relatively low, while the maximum stress on the small reinforcing CFRP plate is larger but still within allowable range. Similarly, the maximum stress on the stationary part of the aluminum joint is within the permissible threshold. It is important to highlight that the aluminum joint itself was a purchased part, and its ability to withstand the required stresses has been physically tested from the beginning, prior to integration into the UAV (Figure 23). In this test, the arm joint-CFRP tube assembly was repeatedly subjected to a tensile load of up to 18 kg at a distance of 920 mm from the joint. It is important to highlight that the joint was turned upside down to facilitate force application, with the force direction being downwards. Additionally, in this test, the CFRP tube wall thickness was only 1 mm. However, a final wall thickness of 2 mm was selected to minimize deformations. Figure 21: Forces acting on the aluminum joint: (**a**) By; (**b**) Ax; (**c**) Bx. Figure 20: UAV body: (**a**) structural support; (**b**) load calculations. Telescopic Arms and Joints  
Despite the plan for the drone's motors to operate at half of their maximum thrust across all three configurations, in this study, the system was tested under full power to validate the integrity of the structure. The authors simulated two of the three configurations of the UAV, as these are the two scenarios where the UAV's arms experience the highest force. Regarding the co-axial quadcopter configuration, the following considerations were taken into account:  
Figure 22: Stress analysis for the UAV frame: (**a**) assembled UAV frame; (**c**) upper plate; (**d**) small reinforcing CFRP plate; (**e**) aluminium joint. Displacement analysis for the UAV frame: (**b**) assembled UAV frame. 1. Structural support: a fixed support was assumed at the ends of the tubes at the connection with the aluminum joint (note that the tube end-aluminum joint connection had already been physically tested);
2.