A STUDY ON THE AERODYNAMIC CHARACTERISTICS OF VERY SMALL INSECTS WING FLAPPING PATTERN

Unlike larger insects (such as bumblebees, fruit flies and hawkmoth, Reynolds number Re > 100), very-small insects (such as encarsia formosa (EF), frankliniella occidenalis (FO) and anbremia sp. (AS), etc., Re < 100) have much complex patterns of wing flapping, with three degrees-of-freedom (DOF) being involved and corresponding to different under-lying mechanisms of high-lift generation. Meanwhile, micro aerial vehicles (MAV) are associated with relatively high Re, i.e., Re = 10³ ~ 10⁵, and their mission environment is complex and changing. When MAVs are faced with overload or high maneuverability mission, flying with very small insect wings flapping patterns may be a new method. To better understand the aerodynamic characteristics of wing flapping pattern of very small insects at high Re, flapping wing model-based experiments are conducted at a range of Reynolds number from 3.9 × 10³ to 1.0 × 10⁴, taking into account wing planforms and flapping kinematics of three typical very-small insects (i.e., EF, FO and AS). It is found that the vertical lift on the wing increases dramatically at the beginning "rowing" phase of the upstroke. It is further noticed, however, the time-mean vertical lift coefficient ( \bar \boldsymbol C_V^\text Upstroke ) on the wing of very small insects is significantly increased during the upstroke in the Re range considered in this work, e.g., \bar \boldsymbol C_V^\text Upstroke > 3.1 for EF, which is noticeably greater than those (1.5 ~ 2.0) of their larger counterparts. Moreover, it is observed that a pair of counter-rotating vortices are produced at the leading and trailing edges of the wing at the starting of the "rowing" phase. With the "rowing" advancing, the trailing-edge vortex sheds while the leading-edge vortex keeps attached, indicating that the high-lift generation during the upstroke is attributed to the delayed stall at the MAV Re range, instead of the "rowing" mechanism identified at the very-low Re range. In addition, it is noticed that the "fling" mechanism still plays a key role in generating the high lift during the "fling" phase, although Re effects and wing-wing interactions may strongly influence the force behavior.