Flying Fish Glide as Well as Birds, Researchers Find
ScienceDaily (Sep. 11, 2010) — We're all familiar with birds that are as comfortable diving as they are flying but only one family of fish has made the reverse journey. Flying fish can remain airborne for over 40s, covering distances of up to 400m at speeds of 70km/h. Haecheon Choi, a mechanical engineer from Seoul National University, Korea, became fascinated by flying fish when reading a science book to his children. Realising that flying fish really do fly, he and his colleague, Hyungmin Park, decided to find out how these unexpected fliers stay aloft.
A flying fish above the water. (Credit: iStockphoto/Mark Durstewitz)
Their discovery that flying fish glide as well as birds is published in TheJournal of Experimental Biology.
But getting hold of flying fish to test in a wind tunnel turned out to be easier said than done. After travelling to Japan to try to buy fish from the world famous Tsukiji fish market, the duo eventually struck up a collaboration with the National Federation of Fisheries Cooperatives of Korea. Park went fishing in the East Korean Sea, successfully landing 40 darkedged-wing flying fish. Selecting five similarly sized fish, Park took them to the Korean Research Centre of Maritime Animals, where they were dried and stuffed, some with their fins extended (as in flight) and one with its fins held back against the body, ready to test their aerodynamics in the wind tunnel. Fitting 6-axis force sensors to the fish's wings and tilting the fish's body at angles ranging from -15 degrees to 45 degrees, Park and Choi measured the forces on the flying fish's fins and body as they simulated flights.
Calculating the flying fish's lift-to-drag ratios -- a measure of the horizontal distance travelled relative to the descent in height during a glide -- Choi and Park found that the flying fish performed remarkably well: gliding better than insects and as well as birds such as petrels and wood ducks. And when they analysed how the fish's lift-to-drag ratio changed as they varied the tilt angle, the duo found that the ratio was highest and the fish glided furthest when they were parallel to the surface, which is exactly what they do above the ocean. Measuring the airborne fish's pitching moment, the duo also found that the fish were very stable as they glided. However, when they analysed the stability of the fish with its fins swept back in the swimming position it was unstable, which is exactly what you need for aquatic manoeuvrability. So flying fish are superbly adapted for life in both environments.
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First published online September 10, 2010
Journal of Experimental Biology 213, 3269-3279 (2010)
Published by The Company of Biologists 2010
doi: 10.1242/jeb.046052
Aerodynamic characteristics of flying fish in gliding flight
Hyungmin Park and Haecheon Choi*
School of Mechanical and Aerospace Engineering, Seoul National University, Seoul, 151-744, Korea and Institute of Advanced Machinery and Design, Seoul National University, Seoul, 151-744, Korea
* Author for correspondence (choi@snu.ac.kr)
Accepted 17 June 2010
The flying fish (family Exocoetidae) is an exceptional marine flying vertebrate, utilizing the advantages of moving in two different media, i.e. swimming in water and flying in air. Despite some physical limitations by moving in both water and air, the flying fish has evolved to have good aerodynamic designs (such as the hypertrophied fins and cylindrical body with a ventrally flattened surface) for proficient gliding flight. Hence, themorphological and behavioral adaptations of flying fish to aerial locomotion have attracted great interest from various fields including biology and aerodynamics. Several aspects of the flight of flying fish have been determined or conjectured from previous field observations and measurements of morphometric parameters.However, the detailed measurement of wing performance associated with its morphometry for identifying the characteristics of flight in flying fish has not been performed yet. Therefore, in the present study, we directly measure the aerodynamic forces and moment on darkedged-wing flying fish (Cypselurus hiraii) models and correlated them with morphological characteristics of wing (fin). The model configurations considered are: (1) both the pectoral and pelvic fins spread out, (2) only the pectoral fins spread with the pelvic fins folded, and (3) both fins folded. The role of the pelvic fins was found to increase the lift force and lift-to-drag ratio, which is confirmed by the jet-like flow structure existing between the pectoral and pelvic fins. With both the pectoral and pelvic fins spread, the longitudinal static stability is also more enhanced than that with the pelvic fins folded. For cases 1 and 2, the lift-to-drag ratio was maximum at attack angles of around 0 deg, where the attack angle is the angle between the longitudinal body axis and the flying direction. The lift coefficient is largest at attack angles around 3035 deg, at which the flying fish is observed to emerge from the sea surface. From glide polar, we find that the gliding performance of flying fish is comparable to those of bird wings such as the hawk, petrel and wood duck. However, the induced drag by strong wing-tip vortices is one of the dominant dragcomponents. Finally, we examine ground effect on the aerodynamic forces of the gliding flying fish and find that the flying fish achieves the reduction of drag and increase of lift-to-drag ratio by flying close to the sea surface.
Key words: flying fish, gliding, wing morphology, lift, drag, stability, ground effect
Abbreviations: A, total planform area (=A1 + A2 + A3) • A1, planform area of pectoral fins • A2, planform area of pelvic fins • A3, planform area of body • AR, aspect ratio (=S2/A1) • b, half wing span (=S/2) • c, average chord length of pectoral fins (=A1/S) • CD, drag coefficient • CL, lift coefficient • CM, pitching moment coefficient • D, drag force • h, flight height (distance between the ground and the lower surface of the body) • L, lift force • L/D, lift-to-drag ratio • M, pitching moment • r, ratio of flight height to half wing span (=h/b) • Re, Reynolds number (=uc/) • S, wing span of pectoral fins • SL, standard length • uind, wind-induced water velocity • u, freestream velocity • xCG, location of the center of gravity • , angle of attack • β1, lateral dihedral angle of pectoral fins • β2, incidence angle of pectoral fins • β3, incidence angle of pelvic fins • , kinematic viscosity
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