# Bio-Inspired Morphing Tail for Flapping-Wings Aerial Robots Using Macro Fiber Composites
### UAS Actuation State of Art  
Miniaturization and weight optimization have been explored in multi-rotors. Approaches, like [10] or [11], show the interest of developing low weight actuators, optimizing payload, using low weight materials, and very efficient mechanical actuators. These approaches have given favorable results in terms of reducing weight and being capable of using a smaller and less dangerous platform. Even so, more miniaturization would reduce the capabilities of these systems. Therefore, a mechanical limit has been reached in these terms. Conventional servomotors, used in all previous applications, impose limitations in terms of weight and efficiency. Some of the authors involved in this work have researched various alternative materials for performing actuation, trying to obtain the highest force/weight ratio, and removing traditional components from the aerial systems. In [12; 13], some potential materials, which could be used as actuators, are presented and detailed. In [14; 15], the application of a soft landing system for multirotors using 3D printed soft materials is shown. There are various materials with high potential for being used in these types of applications. For example, Twisted and Coiled Polymers (TCP), as seen in [13], or shape memory alloys have the potential for being used as a closing mechanism in aerial manipulation. As pointed out above, flapping-wings appears to be an alternative to multirotors and fixed wings. However, payload limitations are very relevant. Hence, all of th weight reduction that can be achieved is positive. Additionally, flapping-wings devices opened the field of bio-inspired approaches taking inspiration in animals, like birds or bats. The idea is to optimize the behavior of these aerial systems mimicking animals, as nature tends to optimize their creatures in terms of energy consumption and efficiency. Flapping-wings properties have been studied over the years in terms of aerodynamics [16] or control algorithms, using even deep learning approaches [17]. They have been applied for inspection tasks, using them to stream video images [18]. Additionally, some approaches removing mechanical parts for piezoelectric actuators have been done. Some examples are [19; 20], where flight, and even takeoff, using these actuators are proven. Nevertheless, the above approaches used the micro aerial system of very low weight and size for their experiments and validation. Making these approaches scalable to medium-sized flapping-wings systems is a great challenge. Therefore, this work takes a medium-sized flapping-wings system analyzed in [21] as a model. This commercially available system is interesting due to its robustness and modularity. For designing the tail, biological inspiration is taken based on previous work of bird tail studies. Tails complexity comes to its greatest exponent in sexually dimorphic species, as can be seen in [22]. However, these designs do not improve the performance of these species in flight. They are conceived to improve the sexual selection of males with longer tails. On the other side, a more simple tails' geometrical design can improve several aspects of fly efficiency, like maneuverability. Focusing on these types of tails in [23], it is shown that the design that reduces the aerodynamic cost to the minimum is shallow and deep fork tails. Therefore, the design developed in this work should take advantage of these conclusions, as energy efficiency is one of the key aspects of this work. The idea is to develop the tail for the integration in an ornithopter especially designed for the GRIFFIN project and still on development [24]. Figure 1 presents a prototype of this design. The idea is to achieve a bio-inspired morphing of the developed tail using MFC. By doing so, it will be possible to get rid of the two servos and the mechanical components normally used to obtain this kind of movement. The weight will be reduced, and a more natural and smooth motion will be obtained.