The aerial-aquatic locomotion of the flying fish
Nature’s Inspiration
The flying fish, which are fish of the taxonomical family Exocoetidae, is an animal unique for both being able both swim and fly. While there are other animals able to locomote through both air and water, the specific locomotion of the flying fish is unique and allows it to glide through the air for long distances while simultaneously possessing the speed and agility characteristics of fish while swimming [siddall]. The fish has been observed to glide for up to 400 meters in bursts of 40 at a time, skimming the ocean surface through a mode of locomotion called "taxiing" in between bursts [davenport]. Another notable characteristic of the fish is its ability to smoothly transition from swimming to flight. As shown in the figure below, after jumping out of the water, the fish spreads its notably large pectoral fins to act as wings. At this point it either takes off or begins to taxi in order to build speed faster than it can underwater until has generated enough of a lifting force to fly. This is not only biologically fascinating, but also inspiring from an engineering point of view gien how challenging it can be to design vehicles capable of locomoting through both air and water. The goal of the BAM Lab is to develop a fundamental understanding of the physics at play behind the fish’s trademark skills.
Stages of flying fish locomotion in chronological order from left to right. The flying fish can alternate between gliding and taxiing without submerging its entire body under water to extend its time above water, maybe to avoid being predated by a big fish, for example.
BAM Approach
We are unraveling the science of aerial-aquatic locomotion of the flying fish by developing a multibody dynamics model of the fish backed by experimental data. However, studying actual flying fish is difficult due to the environment they reside in and challenges involved with keeping them in captivity. We have developed a robotic model organism (RMO) [flammang] inspired by the flying fish (figure to the right) for aerodynamic and hydrodynamic experimentation. The RMO is not designed to look and behave exactly as the flying fish, but rather to allow us to experimentally study of different aspects of the fish's locomotion with a much larger parameter space than we could if we had a biological fish while still generating biologically relevant data. Our standard experimental approach is to first assess how relevant our data is to the actual flying fish by comparison with existing literature of the flying fish. Then we can vary parameters such as the shape and material properties of different fins, or the robot's orientation relative to the incoming fluid flow, to name a few. This approach not only provides insight into the mechanics of the flying fish, but also allows us to develop and study a large design space for flying fish inspired aerial-aquatic systems.
The BAM flying fish RMO.
Recent Results
Recently we evaluated aerodynamic performance of the RMO equipped with biologically inspiried pectoral and pelvic fins via wind tunnel experiments, demonstrating that it performs similarly to the flying fish as previously studied in [Park \& Choi] and [Deng]. After establishing this, we varied geometric aspects of the pelvic fin, which is not present in all species of flying fish, to study how it impacts the aerodynamics of the fish's gliding flight. Specifically, we varied pitch angle and placement along the length of the RMO and compared the impacts on aerodynamic efficiency, as measured by lift-to-drag ratio L/D, with the impacts on the longitudinal stability of the RMO as measured by pitching moment coefficient C . We found that the location and angle of the pelvic fin on flying fish, estimated by photographs, maximizes efficiency, but not longitudinal stability (figure below b). We cannot know yet whether this evolved specifically because of aerial locomotion pressures or if it was for some other biological reason, but the result is this hypothesis: that compared to those that only fly with pectoral fins, flying fish with enlarged pelvic fins can fly for longer distances due to the high L/D, and are more longitudinally agile due to the small slope of C , potentially making it easier for them to transition in and out of gliding.
This study has been published in the Journal for Integrative & Comparative Biology [saro-cortes].
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Photo contribution by Ryan Terrill showing a profile view of a flying fish in flight.
Engineering Impact
The flying fish is especially interesting to study in the context of designing of unmanned underwater-aerial vehicles (UUAVs), where juggling locomotion through two different fluid mediums often requires making design trade-offs. UUAVs are vehicles capable of swimming underwater, flying in the air, and independently transitioning between the two media. Vehicles like these can offer more mission flexibility for marine surveying and sampling, exploration of shores’ littoral zone, and surveillance. Like other vehicles that combine several modes of locomotion, UUAVs can be quite challenging to design since the requirement for the vehicles to move through multiple mediums – in this case, water and air – introduces a need to balance hardware and controls complexities. This is where the BAM Lab is stepping in with a bioinspired approach.
Publications
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Saro-Cortes, V., Cui, Y., Dufficy, T., Boctor, A., Flammang, B. E., & Wissa, A. (2022). An adaptable flying fish robotic model for aero-and hydrodynamic experimentation. Integrative and Comparative Biology, 62(5), 1202-1216.
Acknowledgments
