Aerodynamics of the flying snake Chrysopelea paradisi: how a bluff body cross-sectional shape contributes to gliding performance

Abstract

A prominent feature of gliding flight in snakes of the genus Chrysopelea is the unique cross-sectional shape of the body, which acts as the lifting surface in the absence of wings. When gliding, the flying snake Chrysopelea paradisi morphs its circular cross-section into a triangular shape by splaying its ribs and flattening its body in the dorsoventral axis, forming a geometry with fore-aft symmetry and a thick profile. Here, we aimed to understand the aerodynamic properties of the snake's cross-sectional shape to determine its contribution to gliding at low Reynolds numbers. We used a straight physical model in a water tunnel to isolate the effects of 2D shape, analogously to studying the profile of an airfoil of a more typical flyer. Force measurements and time-resolved (TR) digital particle image velocimetry (DPIV) were used to determine lift and drag coefficients, wake dynamics and vortexshedding characteristics of the shape across a behaviorally relevant range of Reynolds numbers and angles of attack. The snake's crosssectional shape produced a maximum lift coefficient of 1.9 and maximum lift-to-drag ratio of 2.7, maintained increases in lift up to 35 deg, and exhibited two distinctly different vortex-shedding modes. Within the measured Reynolds number regime (Re=3000-15,000), this geometry generated significantly larger maximum lift coefficients than many other shapes including bluff bodies, thick airfoils, symmetric airfoils and circular arc airfoils. In addition, the snake's shape exhibited a gentle stall region that maintained relatively high lift production even up to the highest angle of attack tested (60 deg). Overall, the crosssectional geometry of the flying snake demonstrated robust aerodynamic behavior by maintaining significant lift production and near-maximum lift-to-drag ratios over a wide range of parameters. These aerodynamic characteristics help to explain how the snake can glide at steep angles and over a wide range of angles of attack, but more complex models that account for 3D effects and the dynamic movements of aerial undulation are required to fully understand the gliding performance of flying snakes.

Lift enhancement on spanwise oscillating flat-plates in low-Reynolds-number flows

“…Recently, some wind-tunnel experiments have been conducted to uncover the mystery of high-lift generation in the flying snakes. 12,13 It was found that the counter-intuitively high lift coefficients observed were attributable to the suction by the vortices on the dorsal side of the airfoil which has a snake-like cross-sectional shape. In a subsequent computational study, the enhanced lift was linked with the fact that the primary dorsal vortices were induced to remain close to the surface as a consequence of the interaction between the separated shear layer and the secondary vorticity.…”

Autorotation

“…One being the flying snake [47] the other being the question of whether some flights of birds invoke this principle? If one observes carefully the snake, on the co-published video [47], it seems to us that the snake is alternately letting its tail go over and under itself. Thereby it provides a 4π rotation which in principle could be continued indefinitely.…”

“…The lift is depending on the inclination of the relatively flat side of the snake as discussed in ref. [47]. The motion of the wings of birds is another candidate to be further investigated [48,49,50].…”

Why arboreal snakes should not be cylindrical: body shape, incline and surface roughness have interactive effects on locomotion

“…Many different lineages of arboreal snakes have also convergently evolved variable abilities to form a ventrolateral keel (Figs 2, 3) (Smith, 1943;Wright and Wright, 1957;Pitman, 1974), which seems well suited for catching irregularities in tree bark and preventing slipping. However, unlike the consequences of cross-sectional shape for swimming (Graham et al, 1987;Aubret and Shine, 2008) and gliding (Holden et al, 2014), the consequences of variable crosssectional shapes for the locomotion of snakes on solid surfaces are poorly understood.…”