The aerodynamic effects of forelimb pose on the gliding flight of Draco lizards

Gliding arboreal lizards in the genus Draco possess a pair of patagia, which are thin wing membranes supported by highly elongated thoracic ribs and can be actively folded and unfolded. The uniqueness of Draco gliding flight is that the forelimbs of Draco can move freely independent of the patagia, which are the main lifting surfaces. During the main glide phase, the entire forelimbs are straightened, abducted from the body, and held very close to the patagial leading edges. The reasons for adopting this abducted pose have not been investigated before, especially from the perspective of fluid physics. In this study, wind tunnel experiments and computational simulations are conducted to compare the aerodynamic performances of the abducted pose with two other poses, which have the forelimbs held away from the patagial leading edges. The results show that the abducted pose leads to the highest maximum lift coefficient. This aerodynamic advantage is caused by the larger leading-edge radius due to the abducted forelimbs and small gaps between the abducted forelimbs and the patagial leading edges. Furthermore, it is found that the low aspect ratio of the patagium (0.985) allows the wingtip vortex to energize the flow over the top patagial surface at high angles of attack, which leads to a gentle stall characteristic. The current results also show the existence of distinct leading-edge vortices up to moderate angles of attack. Overall, this work deepens our understanding of the gliding flight aerodynamics of Draco lizards and is useful for future artificial flying machine applications.