Back to the future: Aeromechanics of Highly Maneuverable Bats

Researchers Sharon Swartz1 & Kenneth Breuer2
1 Department of Evolutionary Biology and Ecology, Brown University
2 Division of Engineering, Brown University
Period: 2003-2009

When Orville and Wilber Wainright imagined what it would be like to fly, it was only after exhaustive tests and measurements of the aerodynamic properties of different wing shapes that they hit upon the design that would change our world forever and lead to powered flight. In the same pioneering spirit, researchers Sharon Swartz and Kenneth Breuer are collaborating on a research program to explain the complex flight mechanics of bats which are known to fly with very high efficiency and with extreme maneuverability at high speeds. Powered, flapping flight is perhaps the most evolutionarily successful mode of animal locomotion and of high interest in recent years to engineers attempting to build “biomimetic” flight vehicles which might be preferred over fixed or rotating wing aircraft for particular classes of missions. The researchers are making wind tunnel measurements using both live bats (bats on loan to Harvard University’s Concord Field Station from Lubee, and bats at Lubee) and synthetic models to build models that can be used in design of artificial flight vehicles for extreme maneuverability. Bats are allowed to fly in a wind tunnel or flight lab with small reflective markers attached (every care is taken not to cause undue stress to the bats), and the detailed three-dimensional motion of the wing and body is tracked using stereoscopic phase-locked high speed cameras. Coupled to this, the wake behind the bat is measured simultaneously using techniques from which aerodynamic characteristics of unsteady flight can be extracted for a variety of flight conditions. By occasionally altering wind speed and adding in an obstacle for bats to fly around, their avoidance maneuvers and the flows induced by such behavior can be measured in order to understand how the extreme aerodynamic forces that are required to execute the their high-speed maneuvers are generated. The results obtained could be used to guide the future design of engineered vehicles which will achieve similar extreme aerodynamic performance.

Computer 3-D graphics visualizing air flow

Computer 3-D graphics Bat Flight Despite some superficial similarities, the mechanics of flight among bats, birds, and insect are quite different. Bats are more energy efficient, possess greater, partly due to the fact that bats feed on insects who present a moving target which the bat must track and attack. Anatomically, insect wings possess no internal joints, and bird wings only a few. In contrast, the three-dimensional conformation of bat wings (homologous to the hands of mammals), is determined by at least 18 intrinsic joints, and because the bat wing has internal musculature (unlike birds or insects) each of these joints is under at least partially independent control. This vastly increases the degrees of freedom in determining dynamic wing shape and structure. Moreover, bat wing bones have strong gradients in mineral density, resulting in aeroelastically tailored supportive ‘struts’ that undergo extreme deformations during flight. Bats have bendy bones. Lastly, unlike bird or insect wings, bat wing membrane skin is a unique biological material; it can elongate to as much as 400% of resting length, but is highly anisotropic, with greater than an order of magnitude difference in spanwise (low) vs. chordwise (high) elastic modulus. Bats have elastic wings.