For example, it is known that the separated wingtip slots reduce the induced drag ( Tucker, 1995), and the small feather called alula near the leading-edge delays stall at a high angle of attack ( Álvarez et al., 2001 Lee et al., 2015). Previous studies showed that these characteristics contribute to the improvement of bird flight performance. It is known that avian wings have various features in their structural design ( Figure 1A) and flight techniques that make them seem different from aerial vehicles with rigid wings: for example, flexible muscles, feather transmissibility, and flexibility (e.g., Brown and Fedde, 1993 Müller and Patone, 1998 Reynolds et al., 2014). Besides, biomimetic robots have also been used as a model to study living organisms ( Romano et al., 2019a), contributing to developing a mixed field of engineering and biology ( Romano et al., 2019b).īirds are frequently selected as sources of inspiration because birds are similar in size (i.e., Reynolds numbers) to drones and have excellent flight capabilities. It is expected that biomimetics will play an essential role in the development of new technologies that have social significance in the future ( Lepora et al., 2013). This research approach, called biomimetics, has an impact not only on drone-related research topics but also on a variety of research fields such as robotics and bioengineering ( Lepora et al., 2013). Several strategies have been proposed to improve the capabilities of current drones, inspired by animal flight control studies on flying insects, birds, bats, and other animals ( Franceschini et al., 2007 Lentink, 2014).

In order to deal with these challenges, engineers have often been inspired by the functions of flying animals in nature ( Bechert et al., 2000 Chin et al., 2017 Luca et al., 2017). The perturbations in the attitude must be fixed as quickly as possible in order to stay airborne even though the disturbances are difficult to predict. The drones tend to become unstable under the unpredictable wind that is commonly observed in natural environments. The results point out that the simple attachment of the flexible flaps on the upper surface of the wing is an effective method, providing a novel biomimetic design to improve the aerodynamic robustness of small-scale drones with fixed-wings operating in unpredictable aerial environments.Īs unmanned aerial vehicles, called drones, have been used for various tasks recently ( Floreano and Wood, 2015 Liu et al., 2016), it has been increasingly more important to improve their flight performance, such as stability and efficiency especially when they fly in urban areas. Our results demonstrate that the stiffness of the flaps strongly affects the aerodynamic performance, and the force fluctuations are observed to be reduced when the deformation synchronizes with the strong vortex generation. Through force measurements and flow visualization in a low-speed wind tunnel, it is found that the flexible flaps can suppress the large-scale vortex shedding and hence reduce the fluctuations of aerodynamic forces in a disturbed flow behind an oscillating plate. In this study, we conduct an experimental study on the effects of the flexible flaps inspired by the covert of bird wings on aerodynamic characteristics of fixed-wings in disturbances. Unlike rigid rotors of drones, bird wings are composed of flexible feathers that can passively deform while achieving remarkable aerodynamic robustness in response to wind gusts. 2Graduate School of Engineering, Chiba University, Chiba, Japan.1Graduate School of Science and Engineering, Chiba University, Chiba, Japan.Yuta Murayama 1, Toshiyuki Nakata 2* and Hao Liu 2*

Alula feather