Some of my recent work, carried out in collaboration with Philip Watts, a fluid dynamicist currently studying at the California Institute of Technology, is taking us a step closer to determining the consequences of wing architecture and mechanical properties for flight performance. We have worked to develop, and now to refine, an aerodynamically realistic computer model of bat flight based on gray-headed flying foxes, the species for whom we have the most detailed biomechanical information. Our prototype model incorporates detailed information concerning three-dimensional wing kinematics, distribution of wing mass, and mechanical properties of wing tissues, and models the response of each wing segment to an elliptical force distribution under the assumption of constant circulation.
To date, we have been able to validate that our model predicts humeral longitudinal and shear stresses to within 1.5 to 4.5 times the empirically measured values. We have used sensitivity analysis to show that the longitudinal stresses are very sensitive to bone geometry and extremely robust to variations in body mass, flapping frequency, location of center of lift and wing posture, while shear stresses are insensitive to bone geometry, body mass, and flapping frequency, but extremely sensitive to wing posture and location of center of lift. We have employed the model to make preliminary estimates of joint moments during flight and predictions concerning stresses developed in the wing membrane skin during natural flight behaviors; our results suggest that we may be able to explain long-recognized hallmarks of bat structural specialization such as extremely robust clavicles and the development of a scapulohumeral locking mechanism in direct relation to the shoulder forces and moments predicted by our model.
