# Bio-Inspired Morphing Tail for Flapping-Wings Aerial Robots Using Macro Fiber Composites
## 3 Tail Concept and Design
### Final Model  
After iterating several times, it was decided that the best shape should try to maximize the surface while maintaining a triangular shape that is similar to the one found in nature. The final model is a triangle in which the actuation structure consists of ten MFC for performing morphing. For adding some more rigidity to the system, scotch tapes were added to maintain the tail straight. Figure 5 shows the model. According to [42], different tail configurations can be found in nature to control the bird flight. In Figure 6, the tail developed in this work is presented in the different configurations found in nature for a better understanding. Figure 6 represents the asymmetric YAW control configuration. In this configuration, one actuator of the tail is raised when the other stays at the rest position. This situation generates a moment around the Z-axis of the system to control the YAW. Figure 6 represents the symmetric PITCH control configuration, where the configuration symmetrically activates both actuators generating a moment around the Y-axis of the system to control the pitch. Finally, Figure 6 represents the asymmetric Air-Brake control configuration. In this configuration, both of the actuators are activated proportionally in opposites directions. For proving the improvement of the developed tail, it was compared with the tail of a commercial Ornithopter. By doing so, the advantages that were obtained when using the morphing tail instead of a classic mechanical actuated tail were shown. Figure 7 shows a classical mechanical tail and the tail developed in this work. When comparing both tails, one key element is their surface, as it is very important to generate aerodynamics forces. More surface will allow for a better control and aerodynamic behavior when actuating the tail and performing a change of direction. The commercial tail has a surface of approximately 462 cm\({}^{2}\), while the developed MFC tail has a surface of 1190 cm\({}^{2}\). The weight of the commercial tail is 67.1 g and the one of the MFC tail of 51.8 g, as it can be seen in Figure 7. Therefore, the weight/surface ratio is 0.445 in the MFC tail and 1.45 in the commercial one. This means that this ratio is 3.3 times better in the MFC tail. Because surface is a mayor property for the tail, the conclusion is an improvement of the qualities of the developed tail compared with the traditional approach. Figure 5: Final Developed Tail Model Prototype. (**a**) Tail upper site. (**b**) Tail bottom part. The displacement performed by the morphing of the tail is also more natural and smoother when compared with the mechanical displacement. Moreover, it allows a compliant movement, as the actuators can change their deformed shape without breaking when an external force is applied. An external deformation of the tail while blocked by the servomotor would cause the servo to break. Table 2 shows a resume of the tail properties comparison. In order to analyze the feasibility of the model, two different experiment configurations were performed several times for proving feasibility and repeatability. The first one is presented in Figure 8a, where the intention is to raise the tail against the gravity. The second experiment that is presented in Figure 9a tries to analyze the feasibility of the model to perform a twist move in static conditions. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameters** & **V-Tail** & **Morphing Tail** \\ \hline Weight (g) & 67.1 & 51.8 \\ Surface (cm\({}^{2}\)) & 462 & 1190 \\ Weight/Surface Ratio (Kg/m\({}^{2}\)) & 1.45 & 0.44 \\ Improvement Ratio (\%) & 100 & 330 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Weight/Surface Tails Comparison  
Figure 6: Bio-inspired Tail Control Configurations. (**a**) Asymmetric YAW. (**b**) Symmetric PITCH. (**c**) Air-Brake. Figure 7: Commercial Tail with Servos (**a**) Compared with MFC Tail (**b**). Figure 8: Tail Raise Feasibility Experiments. (**a**) Shows the potential behavior of the tail.