Falling with Style: Bats Perform Complex Aerial Rotations by Adjusting Wing Inertia

Abstract

The remarkable maneuverability of flying animals results from precise movements of their highly specialized wings. Bats have evolved an impressive capacity to control their flight, in large part due to their ability to modulate wing shape, area, and angle of attack through many independently controlled joints. Bat wings, however, also contain many bones and relatively large muscles, and thus the ratio of bats’ wing mass to their body mass is larger than it is for all other extant flyers. Although the inertia in bat wings would typically be associated with decreased aerial maneuverability, we show that bat maneuvers challenge this notion. We use a model-based tracking algorithm to measure the wing and body kinematics of bats performing complex aerial rotations. Using a minimal model of a bat with only six degrees of kinematic freedom, we show that bats can perform body rolls by selectively retracting one wing during the flapping cycle. We also show that this maneuver does not rely on aerodynamic forces, and furthermore that a fruit fly, with nearly massless wings, would not exhibit this effect. Similar results are shown for a pitching maneuver. Finally, we combine high-resolution kinematics of wing and body movements during landing and falling maneuvers with a 52-degree-of-freedom dynamical model of a bat to show that modulation of wing inertia plays the dominant role in reorienting the bat during landing and falling maneuvers, with minimal contribution from aerodynamic forces. Bats can, therefore, use their wings as multifunctional organs, capable of sophisticated aerodynamic and inertial dynamics not previously observed in other flying animals. This may also have implications for the control of aerial robotic vehicles.

Biomechanics of aerial righting in wingless nymphal stick insects

“…Active appendicular motions interacting with and using the surrounding flow field would facilitate righting and subsequent postural control for any jumping or falling arthropod. Similarly, in the origins of vertebrate flight, righting and control prior to the origins of wings may have influenced the evolution of aerodynamically functional wings, although inertial effects likely played more significant roles relative to the much smaller flying arthropods [1,10,34,35]. The kinematics and control features of aerial righting in small arthropods, as summarized in figure 7, are also potentially applicable to any small robotic device with projecting appendages (e.g.…”

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

“…On the other hand, upside-down landings require special manoeuvres. To perch on branches or in caves, bats perform a head-underheels manoeuvre, using their wings to change their moments of inertia to improve their flight control [73,101]. Similarly, some insects, including flies, ascend vertically when landing on overhung surfaces, and rotate the lower part of their body up upon impact [102].…”

“…Some animals capable of hovering, such as hummingbirds and swifts, rely more heavily on their wings rather than their legs for take-off or else drop from their perch [125]. Because bats roost upside-down, these animals are unusual in that they do not necessarily need to use additional leg forces to take off [101].…”

Inspiration for wing design: how forelimb specialization enables active flight in modern vertebrates

“…Hummingbirds are aided by their small mass, high wingbeat frequency and active upstrokes for performing these manoeuvres [104,143]. Bats are also thought to use active upstrokes [114,131,133,144], and further benefit from a combination of lower wing loading [114] and ability to actuate the numerous joints in their wings to adapt their shape [87,100,145]. Although lower mineralization reduces the weight of their wing bones [24], the combination of solid bones (figure 2c) and muscular membranes (figure 5a) makes bat wings proportionally heavier than those of all other extant flyers [145].…”

“…Design choices also depend on the intended application and actuation constraints of the flying robot. For instance, both inertial and aerodynamic control strategies enable rapid manoeuvres, but inertial control may be more advantageous for heavier wings and low-speed manoeuvring [145]. This underused method would be particularly useful for enabling larger robots with limited flapping frequencies to perform tight manoeuvres, such as turning in place.…”