Overview
Research on bio-inspired flight has primarily focused on developing bird-like wings that change span or use active aeroelastic wings as a means of control. In contrast, bats are highly maneuverable flying vertebrates capable of 180 degree turns, inverted flight, and high control authority at extremely low speeds. The bat, which is one of the most successful groups of mammal with its nearly 1000 species, is very similar to the primate in its bone structure, which mimics the human hand with greatly elongated fingers. (University of California Museum of Paleontology website)
Bats are able to change the camber of their wings at the fifth digit, functioning as a continuous flap that can be quickly adapted to suit flight demands. Their membranous wings are particularly well suited to low Reynolds number (around 100,000) flight, with maximum speeds of 10 meters per second.
Figure 1: At left, a generalized bat wing (hum= humerus, u= ulna, r= radius, c= carpus,
ca= calcar,
I-V= numbered digits). Adapted from Padian 1985. At right, bat skeleton
(Image by Ben Waggoner)
Research at Cornell has successfully reached two goals: studying evolved bat wing shape by ecological niche in terms of wing flight performance capabilities, and investigating the role of smart material actuators to vary the spanwise curvature (active camber change) while in flight. These two components are necessary components for development of an active morphing bat-like wing, which is currently in development.
Bat Wing Study and Optimization
Figure 2: Bat wing used in numerical Weissinger
lifting-line simulation based on N. leporinus, a
fishing bat
Bats have evolved with different wing shapes well suited to different needs. For example, the fishing bat Noctilio leporinus has slender wings and rounded wingtips for high flight efficiency with robust lateral stability when commuting over the water, whereas the insectivorous Nycteris hispida has wings with forward sweep to allow for post-stall maneuverability, as wingtip lift is preserved. In a recent study conducted by the LIMS group , the planform shapes of three distinct bat wings were collected from experimental biology literature and analyzed through a numerical Weissinger simulation of rigid wings by decomposing the wing into a series of spanwise airfoil stations. The results of this study allow for greater insight into development of man-made corollaries to the membranous bat wings on finite thickness wings. Specifically, indications as to the roles that tip shape, degree of camber change, and membrane area distribution along the span play in terms of lift-to-drag ratio and spanwise lift and drag distributions.
Figure 3: Planform wing shape and spanwise lift and drag results from numerical simulation
Numerical approximations have been compared to experimental results at the Cornell University 3’x4’ Environmental Wind Tunnel, which is able to replicate bat wing airflow conditions on a quasi-static plastic wings rapid prototyped to match bat geometry. From here, a heuristic optimization study employing the simulated annealing algorithm has determined the ideal ‘morphed’ configuration of each wing shape for camber change and wing twist distributions along the span in order to maximize such parameters as endurance, lift, and rolling moment capability.
Figure 4: Experimental setup at the Cornell Environemtnal Wind Tunnel Facility and comparison of experimental and computational results
Ongoing research is investigating optimal skeletal geometry required for a true membrane wing shape. At low flight speeds, the thickness of a rigid airfoil may not be as effective as a flexible skin, so a more biological structure is being developed. The structure will be outfitted with morphing actuators to morph the wing shape in flight, allowing for active flight parameter variation to suit mission requirements.
Smart Material Actuators
The Smart Joint is a composite beam element making use of shape memory polymer (SMP) along with strain actuation from shape memory alloy (SMA) to create camber change in a low-profile, lightweight package. This joint is thermally activated, such that it is passively rigid in any shape configuration and therefore doubles as a structural element. A model defining joint behavior has been created, and an optimization study defining the workspace of the Smart Joint in terms of force and deflection trade-offs has been proposed.
Figure 5: Smart Joint schematic showing heating process for shape transition between passively rigid states
Research is focusing now on the fabrication of these Smart Joints on the skeletal bat wing structure. Joints must be capable of carrying the aerodynamic loads predicted by aerodynamic simulation and confirmed by experiment, while still allowing for on-demand compliance and control such that various wing shapes can be achieved via embedded actuators. The functional morphing bat-like wing could be easily fabricated to function as a camber change mechanism by placing a combination of Smart Joints along the jointed fingers of a light-weight skeleton, which would then be skinned with a visco-elastic polymer to function as the aerodynamic surface.
Figure 6: Proposed morphing bat wing
Publications
- Leylek, E., Manzo, J. and Garcia, E. "A Bat-wing Aircraft Using the Smart Joint Mechanism", 3rd International Conference on Smart Materials, Structures, and Systems, June 8-13, Acireale, Sicily, 2008. (PDF)
- Manzo, J. and Garcia, E. "Methodology for a Tri-Phase Smart Joint Design", Journal of Intelligent Material Systems and Structures, 2008, doi:10.1177/1045389X08093826.
- Manzo, J. E. and Garcia, E. "Optimization and Implementation of the Smart Joint Actuator", Smart Structures and Materials 2008: Active and Passive Smart Structures and Integrated Systems II, March 10-13, San Diego, CA. published in: Proc. SPIE Vol. 6928, 2008. (PDF)