Large active wing deformation is a significant way to generate high aerodynamic forces required in bat's flapping flight. Besides the twisting, elementary morphing models of a bat wing are proposed, including wing-bending in the spanwise direction,wing-cambering in the chordwise direction, and wing area-changing. A plate of aspect ratio 3 is used to model a bat wing, and a three-dimensional unsteady panel method is used to predict the aerodynamic forces. It is found that the cambering model has great positive influence on the lift, followed by the area-changing model and then the bending model. Further study indicates that the vortex control is a main mechanism to produce high aerodynamic forces. The mechanisms of aerodynamic force enhancement are asymmetry of the cambered wing and amplification effects of wing area-changing and wing bending. Lift and thrust are generated mainly during downstroke, and they are almost negligible during upstroke by the integrated morphing model-wing.
Different from birds and insects,bats have complex wing-deformation capacity to generate high aerodynamic forces.In flight,the actively morphing of bat wing includes the twisting from wing root to wing tip,the cambering along the chordwise direction,the bending along the spanwise direction and the wing area-changing caused by the stretch and retraction of the wingspan.It was found that the high thrust and lift required in bat flight are dependent on the wing twisting and cambering respectively.Moreover,the integrated wing-morphing generates the aerodynamic lift and thrust mainly during the downstroke and almost negligible forces during the upstroke.The wing area-changing and bending can be used to amplify the positive forces in the downstroke duration and reduce the negative forces in the upstroke duration.
The aerodynamic mechanism of the bat wing membrane along the lateral border of its body is studied. The twist-morphing that alters the angle of attack(AOA)along the span-wise direction is observed widely during bat flapping flight. An assumption is made that the linearly distributed AOA is along the span-wise direction. The plate with the aspect ratio of 3 is used to model a bat wing. A three-dimensional(3D) unsteady panel method is used to predict the aerodynamic forces generated by the flapping plate with leading edge separation. It is found that, relative to the rigid wing flapping, twisting motion can increase the averaged lift by as much as 25% and produce thrust instead of drag. Furthermore, the aerodynamic forces(lift/drag) generated by a twisting plate-wing are similar to those of a pitching rigid-wing, meaning that the twisting in bat flight has the same function as the supination/pronation motion in insect flight.
