# A New Type Bionic Foldable Wing Design for High Maneuverable Unmanned Aerial Vehicle
## 2 Preliminary Design of the Folding Wing Structure
### Bionic Folding Wing Layout Design  
#### 2.3.1 Research on Bionic Folding Wing Layout  
Birds have excellent flight ability. The study of the flight structure of birds can provide guidance for the design of folding wing structures. As shown in Figure 4, the wings of birds are composed of bones, muscles, and feathers. Feathers are connected to bones, and bones are connected to each other through different joints [23]. These feathers are strong enough to maintain lift, yet smooth enough to fold into the attached feathers. Primary flight feathers are distributed on the lateral side and secondary flight feathers on the medial side, with the former mainly at the phalanx and metacarpal bone and the latter at the ulna [24]. Birds have better aerodynamic characteristics when they glide with their wings fully open, and wings fold to reduce wind resistance and have higher maneuverability when diving. Therefore, birds can make active use of bones and muscles to fold their feathers to complete wing expansion and folding according to the environment and requirements, so as to change the shape of their wings and adjust to different flight attitudes to adapt to different environments [25]. [PERSON] observed pigeons in the wind tunnel in flight condition, recorded the wing profile and wing area of the wings at the optimal glide angle, and found that with the increase in speed, the wing span, wing area, and aspect ratio of pigeons decreased significantly [26].  
Figure 5 shows a group of aerial photos of pigeons gliding steadily at different speeds. At the lowest speed, about 8.6 m/s, the elbow and wrist joints are fully extended, the flight  
Figure 4: Wing structure of birds.  
features are spread to the maximum area, and the leading edge of the wing is swept slightly forward to the wrist. As speed increases, there is some flexion in the wrist, which leads to increased overlap of the flight feathers and thus a decrease in wing area. As the speed increases further, the elbow also gradually flexes, bringing the wrist closer to the body. At speeds above 20 m/s, the wings sweep so far back that their \"leading edge\" is parallel to the incoming flow.  
Because of the complex control of birds' muscles, bones, and flight feathers, it is difficult to design an aircraft control system if the same control method is adopted. Therefore, we simplify the birds with multiple degrees of freedom as a whole and extract the main bionic features to arrange the wings. The simplified configuration of birds can be described by inner wings B and C with secondary flight feathers and outer wings D and E with primary flight feathers, as shown in Figure 6.  
Among them, inner wing C and B with secondary flight feathers, outer wing E and outer wing D with primary flight feathers are the core features. Inner wings C and B rotate with part A of the body, while outer wings E and D rotate around inner wings C and D, respectively. In addition to rotating joints that mimic joints in bones, the drive mechanism mimics muscles, allowing the wing to perform its corresponding motion functions. Based on the mechanism of bird wing expansion, shape, and bone movement, we designed a new foldable morphing wing structure that can change wing area, wing span, and sweep angle simultaneously.