Glide performance and aerodynamics of non-equilibrium glides in northern flying squirrels (Glaucomys sabrinus)

Abstract

Gliding is an efficient form of travel found in every major group of terrestrial vertebrates. Gliding is often modelled in equilibrium, where aerodynamic forces exactly balance body weight resulting in constant velocity. Although the equilibrium model is relevant for long-distance gliding, such as soaring by birds, it may not be realistic for shorter distances between trees. To understand the aerodynamics of inter-tree gliding, we used direct observation and mathematical modelling. We used videography (60 -125 fps) to track and reconstruct the three-dimensional trajectories of northern flying squirrels (Glaucomys sabrinus) in nature. From their trajectories, we calculated velocities, aerodynamic forces and force coefficients. We determined that flying squirrels do not glide at equilibrium, and instead demonstrate continuously changing velocities, forces and force coefficients, and generate more lift than needed to balance body weight. We compared observed glide performance with mathematical simulations that use constant force coefficients, a characteristic of equilibrium glides. Simulations with varying force coefficients, such as those of live squirrels, demonstrated better wholeglide performance compared with the theoretical equilibrium state. Using results from both the observed glides and the simulation, we describe the mechanics and execution of inter-tree glides, and then discuss how gliding behaviour may relate to the evolution of flapping flight.

Aerodynamic performance of the feathered dinosaur Microraptor and the evolution of feathered flight

“…Glide trajectories were determined by solving for the equations of motion in time to determine trajectories, using C L and C D combinations obtained from the measurements. Measurements of the gliding stage for real gliding animals 20 support that assuming constant C L and C D values are valid. The numerical approach is apparently similar to ANFLTSIM 15,33 , as far as we are able to replicate.…”

Touchdown to take-off: at the interface of flight and surface locomotion

“…Exploiting the principles of claws, van der Waals forces and wet adhesion, animals have evolved to generate the required attachment forces (figure 2) [60][61][62][63][64][65][66][67][68]. This enables animals to negotiate and exploit complex surfaces with a combination of effective aerial approaches, contact strategies, surface locomotion techniques and take-off manoeuvres of which the dynamics are not well understood [1,[69][70][71][72][73][74][75][76][77][78][79][80][81][82][83][84][85]. By contrast, aerial robots are just starting to implement some of these successful perching and locomotion strategies.…”

“…Gliding vertebrates, including flying squirrels, colugos, snakes, lizards and frogs, use their extended aerodynamic surfaces to navigate in the air [71,[74][75][76][77][78][79]. Flying squirrels and colugos employ their patagium, the skin connecting the feet to the hands, for manoeuvring [71,74,75]. Snakes, as well as lizards of the genus Draco, extend their ribs to achieve a higher glide ratio [76,79].…”

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

“…In comparison, there have been far fewer studies on the aerodynamics of animals that can only glide (Emerson and Koehl, 1990;McCay, 2001;Bishop, 2007;Alexander et al, 2010;Miklasz et al, 2010;Park and Choi, 2010;Bahlman et al, 2013), despite their large morphological and taxonomic diversity and the far greater number of independent evolutionary origins of gliding flight (Dudley et al, 2007;Dudley and Yanoviak, 2011). In particular, little is known about the physical mechanisms of gliding in snakes, with previous studies focusing on kinematics (Socha, 2002;Socha, 2006;Miklasz et al, 2010) and only one study to date that considered aerodynamics .…”