# Bio-Inspired Morphing Tail for Flapping-Wings Aerial Robots Using Macro Fiber Composites
## 3 Tail Concept and Design
### Design and Development  
The first decision about developing a tail is its geometry. The tail design is similar to the geometry found in nature, as the intention is to imitate animal behavior. Our design has a triangular shape, similar to the one of the commercial ornithopter of the Carbonsail company presented in [21], but with its structural part consisting of two fiber sheets coming from the base. Fiberglass is the material used for the basic structure, due to their low weight and high resistance. However, adding fiberglass in the full length of the tail would \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameters** & **Servomotor** & **MFC** \\ \hline Dimensions (mm) & \(22.8\times 12\times 29.4\) & \(101\times 20\times 0.5\) \\ Weight (g) & \(15.8\) & \(2\) \\ Blocking Force (N) & \(58.5\) & \(152\) \\ Power consumption & \(2\) & \(\sim\)0 \\ Energy Efficiency & \(85\) & \(100\) \\ Speed & Medium & High \\ Dust and Water resistant & No & Yes \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison between Servomotors and Macro Fiber Composites (MFC) Actuator. add too much weight and resistance. This would affect the displacement desired to be applied by the MFC. Other important restrictions are introduced by the properties of the actuator presented in Table 2. The isolation makes the MFC resistant to water and dust, but the necessity of being attached to a surface implies an increment of weight that should be minimized for a low weight application as the one studied in this paper, as flapping-wings have strict payload limitations. Related to that, as the MFC should be glued to the surface, preferably with epoxy glue, any error in the design of the tail that implies detaching the MFC will lead to weakening the MFC and the appearance of breaks. That will make them useless, as it will create short circuits in the internal piezoelectric structure. Finally, MFC only allow movement in one specific direction, contracting or expanding, depending on the piezoelectric configuration. In this work, MFC is used for bending a surface when they elongate. Therefore, one MFC can bend one surface only in one specific direction. If one surface shall be bent in both directions, then more MFC should be used in both sides of the surface to allow it. The MFC used in this design was the P1-M8514, offering a maximal blocking force of 152 Newtons with a total size of 101 \(\times\) 20 mm and a weight of approximately 2 g. The decision for using these instead of others was based on obtaining the highest displacement with the lowest weight, also offering a good blocking force. Because the priority of this application is low weight and significant displacement, it was considered the best choice. Therefore, it was decided to place the fiberglass sheet only up to the end of the first MFC. To maximize the deformation, it was discussed setting two or three MFC in series. However, morphing against gravity is the most restricted movement. It is also the most important one, because it allows the system to gain height, while the opposite direction moves the ornithopter in the ground direction. At last, the decision was to set three MFC in series in the direction against the gravity force and only two in the opposite direction, as the movement in this direction is not as critical. By doing this, the displacement in the critical direction was maximized while maintaining enough displacement in the other direction. A resistant nylon fiber fabric was the material used as a surface for the tail. MFC were directly attached to the fabric by using epoxy. Therefore, the epoxy, the MFC, the fiberglass, and the nylon fiber fabric formed the complete structure of the tail. It could be directly actuated, performing the morphing and the form of the tail could be kept. As each row of MFC should be actuated simultaneously, the MFC of the same row were welded in parallel with silicon cables. Therefore, it is possible to use these MFC as if they were one long MFC.