# Bio-Inspired Morphing Tail for Flapping-Wings Aerial Robots Using Macro Fiber Composites
### Macro Fiber Composites  
MFC were developed by NASA in the late 90s for controlling vibration, noise, and deflections in different structures due to their capability of sensing low displacement in their structure, which can be measured as a tension variation. Thus, the first works that can be found are related to structural control applications [25].  
Additionally, MFC started to be mentioned in piezoceramic surveys in the early 2000s [26]. In [27]; the properties of these piezoceramics for sensing, actuating and power generation were analyzed. Thus, the research about these MFC started to change from their use as a precise and low-cost sensor to a low weight actuator with high accuracy and  
Figure 1: Prototype of Ornithopter, Named ”Powerbird”, Developed for the GRIFFIN Project.  
blocking force. Even with their relatively low displacement, these actuators offer a very high force/weight ratio. Therefore, some new potential applications appeared.  
Some works started to look for potential applications in Unmanned Aerial Vehicles (UAV) [28]. However, one of the first practical applications as an actuator on robots was for a bio-mimetic fishtail [29], due to the lower frequency of the movement and the smaller amount of force needed to control the robot displacement in water.  
Similar publications developing underwater robots were also published at the same time [30; 31; 32]. All of them highlighted the potential of the MFC in the fields of low weight, high force, good displacement, precision, and flexibility. Therefore, they were able to mimic the movement behavior of different submarines creatures and reach not only forward motion, but also whirling and multiple direction displacement while using several MFC.  
The next step in MFC development was to try them outdoors in unmanned aerial robots. In small devices, wing actuation was obtained by using MFC in fixed-wings [33] or even in flapping-wings [34]. In the same way, [35] used MFC to perform morphing on a wing to change their profile. In the same year, other works were published, like [36], where the potential of the use of MFC in aerial systems was shown. In that work, they designed a fixed-wing aerial robot that was capable of using MFC for actuating the flaps of the wings and tail.  
The above publications pointed out the great potential of the MFC for actuating robotic parts that were normally actuated by mechanical systems. The high speed of the displacement and high blocking force were the main reasons. Therefore, some of the works began to use these actuators to drive the fins of robotic fishes [37].  
In aerial vehicles, the possibilities of these MFC for active flight control in Micro Aerial Vehicles (MAV) [38] were studied, validating the strength of the actuators and trying to maximize the effective roll and pitch. Composite materials were used, allowing passive load alleviation. Continuing with aerial systems, the suppression of tail buffeting using MFC has also been demonstrated [39], taking advantage of the high strain that these actuators can offer.  
The work presented in this paper aims to offer a novel solution, developing a new approach to a morphing tail using MFC. Recent works, like [40], have tried to perform morphing of bio-inspired tails made out of elastomers. They achieved up to 12 mm of displacement and validated the concept in a wind tunnel, with the idea of using it on bio-inspired fixed and flapping-wings designs.  
The rest of the paper is structured, as follows. Section 2 will focus on the model validation, explaining the MFC properties and theoretical analysis. Section 3 will present the tail concept and design, the attributes of the tail will be explained, the design and development will be described, and the final model will be presented. Section 4 will then present the experimental validation, showing the setup used, the tests performed, and the final results. Finally Section 5 will show the conclusions and future works.