Flying fish aerodynamics

Put flying fish in a wind tunnel, and they’re as aerodynamically polished as most birds.

Earlier analyses of their bodies suggested as much, but the calculations were hypothetical.

“We directly measure the aerodynamic forces,” wrote aerospace engineers Hyungmin Park and Haecheon Choi of Seoul National University in a Sept. 10 Journal of Experimental Biology paper. “The gliding performance of flying fish is comparable to those of bird wings such as the hawk, petrel and wood duck.”

Park and Choi’s specimens were caught in the Sea of Japan and formally belonged to Cypselurus, a flying fish genus with a cylindrical body, exceptionally broad pectoral fins and unusually developed pelvic fins near its tail. Another genus, Exocoetus, has narrower pectoral and smaller pelvic fins; researchers liken them to biplanes and monoplanes (.pdf).

Propelled by a tail-motor action on the surface of waves, the fish regularly make gliding flights of more than a hundred feet, at speeds above 30 mph. (In a widely circulated video shot from a Japanese ferry, a fish stayed aloft for 45 seconds.) Flying helps the plankton-eating fish escape from predators.

With the fish dry-mounted and filled with urethane to maintain shape (.pdf) in wind tunnel tests, Park and Choi were able to measure the fishes’ lift-to-drag ratios, how far they moved horizontally per unit of vertical fall, and other aerodynamical measures.

At their precise angle of exit from the water, the fish achieve their greatest lift. The fin arrangement pushes air down the fishes’ bodies towards their tails, like a jet. And when the fish are just above the water’s surface, they benefit from a ground effect, in which air pressure underneath their fins creates lift.

The researchers are now curious about the potential role of texture differences between the top and bottom of flying fish pectoral fins.

The pair are also designing a flying fish-inspired plane. In an e-mail, Choi gave few details, but it would appear to be quite low-flying.

“The ground effect for reducing the drag force is very important in the design,”