Undulating their bodies keeps flying snakes from tumbling out of control

Flying snakes can glide as far as 78 feet (24 meters) without tumbling out of control because they undulate their bodies mid-flight, as if they were swimming through the air. This seems to be a specialized strategy to stabilize their flight rather than an evolutionary remnant of general snake behavior, according to a new paper in the journal Nature Physics. The work could eventually lead to a new, improved control template for dynamic flying robots.

Co-author Jake Socha of Virginia Tech has been studying these fascinating creatures for about 20 years. The peculiar gliding ability of these snakes—there are five known species, including Chrysopelea pelias and Chrysopelea paradisi—was first noted by a British scientist in the late 1800s, who observed one gliding through his tea garden in southeast Asia one day. But scientists had paid little attention to determining the precise physics and biomechanics at play until Socha published a 2002 paper outlining his preliminary findings on the fundamental aerodynamics.

Socha found that the snake will push its ridge scales against the tree trunk, using the rough surface to maneuver up to a branch. Then it dangles its body off the end of the branch and contracts sharply like a spring to launch itself into the air. The initial angle of inclination as the snake is hanging determines the flight path. To ensure maximum gliding distance, the snake will suck in its stomach and flatten its body, curving inward like a Frisbee to create lift, undulating its body in an S-shaped motion, which serves to increase the air pressure underneath.

Three years later, Socha reported on the key variables involved: the glide angle and horizontal speed, as well as a snake’s body mass, body length, and the amplitude and frequency of its body undulations as it glides. Specifically, he determined that size (body mass and length) does matter—the smaller the snake, the farther it can horizontally glide. While undulation frequency wasn’t especially important in terms of predicting flight behavior, the amplitude of the undulation did play a key role in stabilizing the snake mid-glide.

By 2010, Socha’s lab was letting test snakes leap off artificial branches attached to a tall tower in the lab, capturing the movement as they glided to the ground with four cameras. That enabled him to create 3D models of the body positions mid-flight and analyze the forces acting on the bodies and the basic gliding dynamics. He found the snakes could glide as far as 24 meters (about 78 feet) from the launch branch.

The next step was to take a 3D printed model of a flying snake in flattened gliding mode and study the patterns of fluid flow by submerging the model in a tank of flowing water. He also collaborated with George Washington University aeronautical engineer Lorena Barba, among others, to perform computer simulations of the air flow patterns of the 3D model. They found that suction seems to play an important role: when the snake flattens its body, it makes itself more aerodynamic. Vortices of air form above it, “sucking” it upward like the low-pressure region in the eye of a tornado—an effect that is particularly well-suited to the snake’s angle of attack (usually between 20 to 40 degrees).

And that brings us to Socha’s latest flying snake research, which employs motion capture to look a bit more closely at the precise kinematics of aerial undulation and its effect on glide performance. He and his co-authors were specifically interested in determining whether the undulating behavior was a specific flight control strategy for the animals or just a “behavioral remnant of snake locomotion” since, as the authors note, “all snakes are capable of lateral undulation, an evolutionary ancient motor pattern produced by waves of muscle contraction propagating down the body.”

For these experiments, Socha et al. used live flying snakes (Chrysopelea paradisi, in this case), placing infrared reflective markers along the body’s dorsal surface. They recorded those marker positions as snakes glided through a large indoor arena in Virginia Tech’s Moss Arts Center. The trials were conducted inside a four-story black box theater known as the Cube, modified for the snakes’ safety.

Socha’s team hung black plastic sheeting along the sides and back wall and pulled curtains over the walkway railing. They also covered the floor in soft foam padding so the snakes wouldn’t be injured upon landing. They placed an artificial tree covered in fake leaves and vines in the center of the glide arena, with a branch from an actual oak tree serving as a suitable launch branch. They then just let the snakes jump and glide to their hearts’ content over nine days, using a 23-camera motion capture system to record the infrared markers.

Based on that data, the team developed an “anatomically accurate” 3D mathematical model of snake flight and conducted simulations of glides, both with and without undulation, incorporating inertial and aerodynamic effects. They found that while simulated glides without undulation could achieve some distance, such glides ultimately failed because of the destabilizing effects of pitching and rolling. By contrast, simulated glides that incorporated undulation stabilized the rotational motion, enabling a much longer glide. “This work demonstrates that aerial undulation in snakes serves a different function than known uses of undulation in other animals,” the authors concluded.

There are caveats, of course. “Interpreting why animals have evolved certain shapes and behaviors, especially in relation to stability, must be treated with caution,” Jim Underwood of University of London wrote in an accompanying essay. “Merely showing that a certain snake gliding strategy is unstable is not quite sufficient to show that it cannot be a good one: were you to freeze an owl in an exactly lifelike gliding pose, there is still no way the bird can be thrown to achieve a stable glide.”

The work nonetheless should prove useful for the improved design of robots inspired by the movements of flying snakes. “The newly discovered kinematic components of aerial undulation… provide the theoretical basis for design of a bio-inspired flying snake robot that glides using aerial undulation as a control template,” the authors wrote, adding that this “should markedly simplify the control of a flying snakelike robot.”

Bonus: the same theoretical model will benefit Socha et al.’s further research on flying snakes.