  Compared with fixed-wing flight, flapping flight can generate a higher lift
and is also more maneuverable, largely resulting from the benefits of wing
rotation. By analyzing the real wing kinematics of fruit flies, we found that
the wing rotation is smart modulated and can be simplified into the advanced
$\Phi>0$, symmetrical $\Phi=0$, and delayed $\Phi<0$ modes, where $\Phi$ is the
phase between stroke reversal and wing rotation. Thus, an experiment of a
Robotic fly at Reynolds number of 240 was designed to address how insects
control the flow patterns to modulate the flight forces by the smart wing
rotation. Experimental results demonstrated that the flow pattern during a half
of stroke is featured by the formation of a spiral LEV, which has a convex
shape and connects to the trailing vortex via a tip vortex and a root vortex to
form a vortex loop. Through wing rotation, the formation of LEV is enhanced and
the pinch-off of LEV can be significantly delayed, resulting in the
stabilization of LEV and the generation of unconventional forces. In addition,
by modulating the modes of wing rotation, insects obtain distinct wing-vortex
patterns including positive, neutral, and negative camber patterns, by which
insects control the magnitude and direction of unconventional forces owing to
the wake capture mechanism. Therefore, the smart wing rotation not only
contributes to the generation of unconventional forces, but also accounts for
the control of flow pattern during insects steering maneuvers.
