Even when nature doesn’t seem to make sense, it’s probably just because it’s 100 steps ahead of even our most brilliant engineers. Case in point – the knifefish.
This long, thin, slender aquatic acrobat has a long dorsal fin that runs almost the entire length of its body, extending into the water like a giant curtain. But it’s not just there to act as a runner; it undulates back and forth to help propel the fish forward and backward.
While studying the dynamics of the fin to understand how it maintains such balanced control during quick darts across the ocean floor, researchers at Northwestern University came to a startling discovery. While half of the fin was waving in one direction, the other half was pushing back in the opposite direction.
Intuitively, this doesn’t make a whole lot of sense. After all, why would you put a propeller at the front of an airplane only to offset it with a second on the back pointing the opposite direction? The secret, it seems, is in the way the fin creates forward motion.
Unlike a traditional paddle-like fin that pushes forward or backward against the water, this giant dorsal fin acts more like a wave – like somebody shaking out a carpet. While the movements do create thrust, it’s hardly the most efficient system. The sideways motions of the wave’s propagation is basically wasted movement.
Or is it?
After mimicking the fish’s fin on a robotic submarine, the researchers confirmed what their observations told them had to be true, even though it goes against decades of engineering textbooks. The small sideways motions and the dorsal fin creating thrust in opposite directions increases both stability and maneuverability.
Anyone who has built a model airplane or has gone kayaking knows that these two characteristics typically go against one another. Airliners are giant pieces of machinery with huge tails and wide wingspans. Why? It makes the ride stable and smooth. In contrast, a fighter plane has swept back wings and almost no tail. The result is an inherently unstable system, but a plane that can fly circles around any Airbus. Similarly, sea kayaks are long and difficult to maneuver, while it’s nearly impossible to stop a white water kayak from tipping over, even in calm water.
“We are far from duplicating the agility of animals with our most advanced robots,” said MacIver, a co-author of the paper. “One exciting implication of this work is that we might be held back in making more agile machines by our assumption that it’s wasteful or useless to have forces in directions other than the one we are trying to move in. It turns out to be key to improved agility and stability.”
The paper, “Mutually opposing forces during locomotion can eliminate the tradeoff between maneuverability and stability,” was published in the Proceedings of the National Academy of Sciences by Malcolm MacIver, associate professor of mechanical engineering and biomedical engineering at Northwestern; Izaak D. Neveln and James B. Snyder, both Northwestern doctoral students in MacIver’s Neuroscience and Robotics Laboratory; Eric Fortune, a professor of biological sciences at the New Jersey Institute of Technology; Noah Cowan, a Johns Hopkins associate professor of mechanical engineering; Eatai Roth, a former Johns Hopkins doctoral student now at the University of Washington; and Terence Mitchell; a former Johns Hopkins postdoctoral fellow now at the Campbell University School of Osteopathic Medicine.