Aerodynamic Behavior of Compliant Membranes as Related to Bat Flight

We present computations of membrane airfoil behavior subject to aerodynamic loading and compare them with in vivo measurements of membrane wings of bats during flight. The computational method assumes an inviscid potential flow (with net circulation determined by a Kutta condition), is computed using XFOIL and iteratively coupled with a finite element model describing the membrane behavior. We find that a simple model assuming uniform loading is largely confirmed, particularly for very compliant membranes in which the pressure loading is focused at the center of the airfoil. Stiffer wings transition to the more traditional pressure distribution predicted by thin airfoil theory for rigid wings. Comparisons with sail theories are also made, illustrating the effect of compliance. Additionally, the in vivo measurements of membrane deformation during bat flight are acquired from detailed kinematics recorded from Cynopterus brachyotis, flying in a wind tunnel. We demonstrate that the expansion of the wing area during the downstroke of the flight cycle exhibits area increases of up to 100% during the downstroke. In addition, comparisons with the computational theory show good qualitative agreement.