Wing kinematics and aerodynamic forces in miniature insect Encarsia formosa in forward flight

Abstract

Miniature insects fly at very low Reynolds numbers, and the effect of air viscosity is large. Previous studies in this area are on hover flight. Here, we study the forward flight, by measuring the wing kinematics and analyzing the flows of a typical miniature insect (Encarsia formosa, wing length of about 0.5 mm). In the beginning of the upstroke, the wings quickly accelerate backward at a very large angle of attack and smash on the air (“impulsive rowing”), generating a large thrust; in the rest of the upstroke, the wings come together and move upward, slicing through the air and generating a small negative vertical force and negative thrust. In the beginning of the downstroke, the wings fling open and produce a leading-edge vortex (LEV) on each wing; in the rest of the downstroke, the wings move downward and forward with the LEV staying attached, generating a large vertical force and some negative thrust. The large thrust produced by the “impulsive rowing” overcomes the body drag and the negative thrust produced by the wings in the other parts of the flapping cycle; the vertical forces, produced by the “flinging” and by the downward/forward motion of the wings carrying the LEVs created at the fling, provide the weight supporting force. That is, the tiny insect overcomes the strong viscous effect by fast smashing the wings on the air, by fast flinging open the wings, and by using the LEVs created at the fling.