Effects of Body Cross-sectional Shape on Flying Snake Aerodynamics

Despite their lack of appendages, flying snakes (genus Chrysopelea) exhibit aerodynamic performance that compares favorably to other animal gliders. We wished to determine which aspects of Chrysopelea's unique shape contributed to its aerodynamic performance by testing physical models of Chrysopelea in a wind tunnel. We varied the relative body volume, edge sharpness, and backbone protrusion of the models. Chrysopelea's gliding performance was surprisingly robust to most shape changes; the presence of a trailing-edge lip was the most significant factor in producing high lift forces. Lift to drag ratios of 2.7-2.9 were seen at angles of attack (alpha) from 10-30A degrees. Stall did not occur until alpha > 30A degrees and was gradual, with lift falling off slowly as drag increased. Chrysopelea actively undulates in an S-shape when gliding, such that posterior portions of the snake's body lie in the wake of the more anterior portions. When two Chrysopelea body segment models were tested in tandem to produce a two dimensional approximation to this situation, the downstream model exhibited an increased lift-to-drag ratio (as much as 50% increase over a solitary model) at all horizontal gaps tested (3-7 chords) when located slightly below the upstream model and at all vertical staggers tested (+/- 2 chords) at a gap of 7 chords.