Flight of a Samara_Alsomitra macrocarpa

The lift-to-drag ratio or the gliding ratio was about 3 ~ 4 and the rate of descent was 0.3 - 0.7 m/sec, which was smaller than those of other rotary seeds. The flight was so stable that samples were seen to take their optimal trimmed angle of attack with a value between the maximum gliding ratio and the minimum rate of descent. The aerodynamic function of the husk for the distribution of the seeds was also revealed by making wind tunnel tests of the husk.

If a horizontal wind, the speed of which is 10 m/sec, exists, then the horizontal
distance of flight is increased further by 10 m/sec x 24 sec = 240 m.

If wind is absent, the seed can get the highest distance by flying with a lift coefficient of CL = 0"27, at which the lift-to-drag ratio and thus the gliding ratio (the ratio of horizontal distance and the height loss) are maximum. However, if there is a wind, then the flight is strongly dependent on the time until the seed reaches the ground after the initiation of falling. This suggests that flight should occur at the lift coefficient giving the minimum rate of descent. Thus, it is an interesting fact that the actual lift coefficient of CL = 0.34 lies between two optimal lift coefficients,
the maximum lift-to-drag ratio (or the maximum gliding ratio) and the minimum rate of descent, but is closer to the former because the minimum rate of descent is almost constant beyond the selected value, CL > 0"34. Here, also, as seen in many other locomotions, the living creature acts to get the optimal performance.

Conclusion

The geometrical characteristics of the wing of Alsornitra macrocarpa, such as the slightly swept and twisted wing, the reflected trailing edge of the airfoil, the lightly loaded wing and adequately arranged CG position, are well fitted to assure the good performance and stability in gliding flight of the winged seed. The thin wing with a sharp leading edge and adequate aspect ratio (AR =3--4) produce the appropriate lift-to-drag ratio (L/D ~ 3- 4) for the flight in small Reynolds number (Re ~4 x 103). The low wing loading (mg/S ~ 0.5 N/m 2) also guarantees a smaller rate of descent (w=0.3-0,7 m/sec) than those of the rotary seeds. The flight is performed at a lift coefficient of CL =0.34, which not only gives the maximum gliding ratio but also guarantees approximately the minimum rate of descent. The above small lift coefficient is adopted because of the large drag coefficient at high angles of attack.
 By applying the local circulation method, the two-dimensional aerodynamic characteristics of the wing section is revealed. It is also made clear that the dispersal of the seeds is assisted by the wind surrounding the husk, and the resonant pendulum motion of the hung husk.