# A New Type Bionic Foldable Wing Design for High Maneuverable Unmanned Aerial Vehicle
## 3 Folding Wing Structure Design and Validation
### Aerodynamic Analysis of Folding Wing UAV
#### 3.3.2 Analysis of Longitudinal Aerodynamic Characteristics  
Figure 19 shows the influence of inlet velocity and morphing posture on lift characteristics of a bionic morphing UAV. Under the same morphing attitude, the stall angle of the UAV increases slightly with the increase in incoming flow velocity. Under the same flow velocity, the lift coefficient of the UAV is greatly improved due to the gradual increase in the wing area during the wing morphing process from fully folded to fully expanded. For example, when the inlet flow velocity is 10 m/s, the lift coefficient of the whole aircraft can be increased by at least 30% during the process of full expansion of the morphing wing.  
Figure 20 shows the influence of different morphing attitudes on the UAV drag coefficient under three different inlet velocities. The experimental results show that the drag coefficient of the UAV decreases with the increase in inlet velocity under the same morphing attitude. At the same flow velocity, when the morphing wing shrinks from fully expanded to fully folded, the drag coefficient of the UAV decreases due to the decrease in the wing area. The experimental results are consistent with the natural phenomenon of bird flight, birds in the process of gliding, if they encounter strong air, will quickly take the flight posture of folding wings to reduce resistance. Therefore, when flight conditions are met, the morphing wing UAV can properly fold its wings to reduce flight resistance and improve flight efficiency.  
Figure 21 shows the influence of different morphing wing attitudes on the UAV pole curves at three different incoming flow speeds. The experimental results show that the UAV with fully expanded wings has a larger maximum lift coefficient C\({}_{\text{Lmax}}\) at the same inflow velocity. At the same incoming flow velocity, in the low lift region, the UAV with fully folded wings has a smaller minimum drag coefficient C\({}_{\text{Dmin}}\), and the minimum drag coefficient C\({}_{\text{Dmin}}\) decreases with the increase in incoming flow velocity.  
Figure 15: Span change during structural movement.  
Figure 16: The impact force on the rotary joint. (**a**) Rotary joint 1; (**b**) rotary joint 6; (**c**) rotary joint 4.  
Figure 17: Low-speed flowback open wind tunnel.  
Figure 19: The lift coefficient varies with the angle of attack under different morphing attitude and inlet velocities.  
Figure 18: Wind tunnel test models of foldable wing UAV with different attitudes. (**a**) Wind tunnel test model with wing fully extended; (**b**) wind tunnel test model of left-wing expansion and right-wing contraction; (**c**) wind tunnel test model with wing fully folded.  
Figure 20: The drag coefficient varies with the angle of attack under different morphing attitude and inlet velocities.  
Figure 22 shows the influence of deformation attitude on lift and drag characteristics of the UAV at three different flow velocities. The experimental results show that when the inlet velocity is 10 m/s, the lift-drag ratio of the UAV with fully expanded wings is higher than that of the UAV with fully folded wings. When the inlet flow velocity is 15 m/s, the lift-drag curves of the UAV with two deformable attitudes are basically the same. When the inlet flow velocity is 20 m/s, the UAV with fully folded deformed wings on both sides has better lift-drag performance. According to the analysis, the UAV can flexibly change its flight attitude according to different flight speeds or flight environments, so that the UAV can always keep flying under the optimal lift-to-drag ratio. Therefore, the UAV has good environmental adaptability.