Fish in a wind tunnel?   How else will you learn how they fly?

It turns out flying fish can remain airborne for over 40 seconds and cover distances of up to a quarter mile hitting a top speed up to 40 miles an hours, says Haecheon Choi, a mechanical engineer from Seoul National University, Korea.

Choi said a children's science book inspired him to look into the aerodynamics of flying fish, and a paper of his results appear in The Journal of Experimental Biology.  Choi and colleague Hyungmin Park posted similar results in a poster for the American Physical Society meeting in 2006.

To do the study, Choi went fishing in the East Korean Sea, caught five darkedged-wing flying fish (Cypselurus Hirai), and he and Park took them to the Korean Research Centre of Maritime Animals, where they were dry-mounted; four with their fins extended at varying angles and one with its fins held back against the body. They fitted 6-axis force sensors to the fish's wings and tilted the fish's bodies at various angles and then measured the forces on the fins and body as they simulated flights.   By varying the attack angle, measuring the lift, drag and pitching moment in a free-stream velocity of 12m/s for each case, they say they were able to learn how the flying fish optimize their gliding.

When they calculated the flying fish's lift-to-drag ratios and horizontal distance traveled versus decreased elevation, they found that the flying fish glided better than insects and as well as birds like petrels and wood ducks.

When they varied the tilt angle, they found that the lift-to-drag ratio was highest and fish glided farthest when they parallel to the surface - what they do above the ocean. Measuring the airborne fish's pitching moment in the wind tunnel, they also say that the fish were very stable as they glided but when they analyzed the stability of the fish with its fins swept back in the swimming position it was unstable in the air, though obviously exactly what you need for aquatic life.   Flying fish are optimized for life in both environments. 

But fish always stay close to the surface of water so Choi and Park decided to see if the could note a benefit or aerodynamic effect from that.   They found that the lift-to-drag ratio increased as their fish flew near the floor of the wind tunnel and when Park replaced the solid surface with a tank of water, the lift to drag ratio rose even more, and the fish were able to glide even further. 

They also visualized the air currents passing around the flying fish's wings and body and noted duo saw jets of air accelerating back along the fish's body. Park says that the tandem arrangement of the large pectoral fin at the front and smaller pelvic fin at the back of the fish's body accelerates the air flow towards the tail like a jet, increasing the fish's lift-to-drag ratio further and improving its flying performance even more.

Maybe a flying fish airplane?   Choi and Park say they want to try that next.

Citation: Park, H. and Choi, H. (2010), Aerodynamic characteristics of flying fish in gliding flight, J. Exp. Biol. 213, 3269-3279.