Take a flying fish out of water, and it will glide like a bird.
That's the word from two South Korean scientists who decided to test the fish's performance in a wind tunnel.
Their findings — the fruit of what they say is the first direct investigation of flying fish aerodynamics — showed that the fish could stay airborne just as well as small-to-midsize birds.
The news was published online Thursday evening in the Journal of Experimental Biology.
Flying fish are equipped with wing-like fins that they keep close to the body as they swim but spread out when they launch into the air, usually in attempts to escape predators. The fish stay aloft by periodically dipping their tail fins in the water and propelling themselves onward, similar to stones skipping across water.
Senior author of the study, Haecheon Choi, a professor of fluid mechanics at Seoul National University in South Korea, said he first got the idea for the experiment while reading a children's science book to his daughter. According to the book, flying fish could stay airborne for about 1,300 feet, or nearly a quarter of a mile — a distance that took Choi by surprise.
Examining the available research, Choi saw that experiments to date had merely estimated the flying abilities of the fish based on their body proportions — weight and wingspan, for example — or from watching them in action in the wild. They were "best guesses," said Frank E. Fish, a zoologist at West Chester University in Pennsylvania, one of the scientists to peer-review the paper.
Choi and lead author Hyungmin Park, also of Seoul National University, set up an experiment that would directly show how well the fish performed.
The fish species they chose, Cypselurus hirai, has four wing-like fins — two large and two small — in a "biplane" formation. The scientists selected five fish of similar size, injected them with urethane foam, arranged the two pairs of fins in different positions and tested the movement of air around them in a wind tunnel to assess their flightworthiness. They placed a tank of water underneath the fish to see how a liquid surface affected their flight.
After assessing the fish's lift-to-drag ratio and other aerodynamic measures, the researchers concluded that flying fish glide low over the water because air friction on the fish's body is the least when the creature is close to the surface. This allows the fish to travel farther before it has to plunge back into the water.
Perhaps one of the most fascinating findings was how well the fish could glide in comparison with other winged creatures, commented Fish. The lift-to-drag ratio, which essentially measures how much horizontal distance is traveled during descent, suggested the fish can glide better than insects such as the dragonfly, and just as well as birds such as the petrel and wood duck.
Choi said the study's findings have practical application as well. "We want to apply the concept to existing airplanes which fly very near the water surface," he said.