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Journal of Experimental Biology partnership with Dryad

Aerodynamic characteristics of flying fish in gliding flight
Hyungmin Park, Haecheon Choi

SUMMARY

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, the morphological 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 30∼35 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 drag components. 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.

FOOTNOTES

  • This work was supported by a National Research Laboratory Program (R0A-2006-000-10180-0), a World Class University Program (R31-2008-000-10083-0), and a Converging Research Center Program (2009-0082824) through the National Research Foundation of Korea funded by the Ministry of Education, Science, and Technology, Korea.

  • LIST OF SYMBOLS

    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/ν)
    S
    wing span of pectoral fins
    SL
    standard length
    uind
    wind-induced water velocity
    u
    freestream velocity
    xCG
    location of the center of gravity
    α
    angle of attack
    β1
    lateral dihedral angle of pectoral fins
    β2
    incidence angle of pectoral fins
    β3
    incidence angle of pelvic fins
    υ
    kinematic viscosity
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