The relationship between 3-D kinematics and gliding performance in the southern flying squirrel, Glaucomys volans

Abstract

SUMMARY Gliding is the simplest form of flight, yet relatively little is known about its mechanics in animals. The goal of this study was to describe the body position and performance of a gliding mammal and to identify correlates between kinematics and aerodynamic performance. To do this, I used a pair of high-speed digital cameras to record a portion of the middle of glides by southern flying squirrels, Glaucomys volans. The squirrels launched from a height of 4 m and landed on a vertical pole. Reflective markers were applied to anatomical landmarks and the 3-D coordinates of these points were computed to describe the kinematics of the glides. From these data I estimated the lift and drag generated during the glide, and correlated these variables with gliding performance as measured by glide angle, glide speed and stability. In the majority of the glide sequences the squirrels accelerated in the downward direction and accelerated horizontally forward as they moved through the calibrated volume in the middle of the glide trajectory, rather than exhibiting a steady glide in which the body weight is balanced by the resultant aerodynamic force. Compared to human engineered airfoils, the angles of attack used by the squirrels were unexpectedly high, ranging from 35.4°to 53.5°, far above the angle of attack at which an aircraft wing would typically stall. As expected based on aerodynamic theory, there was a negative correlation between angle of attack and lift coefficient, indicating that the wings are stalled, and a positive correlation between angle of attack and drag coefficient. Also as expected, there was a negative correlation between lift-to-drag ratio and angle of attack, as increasing angle of attack produced both less lift and more drag. Within glides, there was a strong correlation between nose-down pitching rotations and limb movements that tended to increase the angle of attack of the wing membrane, suggesting that the animals actively control their pitch by moving their limbs. The squirrels used much steeper glide angles than those reported for other gliding animals, ranging from 40.4° to 57.4°. It is likely that this is because they did not launch from a great enough height to reach their minimum glide angle. In some trials the glide angle increased over the captured portion of the glide,whereas in others it decreased, and the magnitude of the changes varied substantially, rendering it difficult to ascertain which portion of the glide trajectory was represented. Decreases in glide angle were strongly correlated with increases in lift coefficient, but were uncorrelated with drag coefficient.