  Aerodynamic ground effect in flapping-wing insect flight is of importance to
comparative morphologies and of interest to the micro-air-vehicle (MAV)
community. Recent studies, however, show apparently contradictory results of
either some significant extra lift or power savings, or zero ground effect.
Here we present a numerical study of fruitfly sized insect takeoff with a
specific focus on the significance of leg thrust and wing kinematics.
Flapping-wing takeoff is studied using numerical modelling and high performance
computing. The aerodynamic forces are calculated using a three-dimensional
Navier--Stokes solver based on a pseudo-spectral method with volume
penalization. It is coupled with a flight dynamics solver that accounts for the
body weight, inertia and the leg thrust, while only having two degrees of
freedom: the vertical and the longitudinal horizontal displacement. The natural
voluntary takeoff of a fruitfly is considered as reference. The parameters of
the model are then varied to explore possible effects of interaction between
the flapping-wing model and the ground plane. These modified takeoffs include
cases with decreased leg thrust parameter, and/or with periodic wing
kinematics, constant body pitch angle. The results show that the ground effect
during natural voluntary takeoff is negligible. In the modified takeoffs, when
the rate of climb is slow, the difference in the aerodynamic forces due to the
interaction with the ground is up to 6%. Surprisingly, depending on the
kinematics, the difference is either positive or negative, in contrast to the
intuition based on the helicopter theory, which suggests positive excess lift.
This effect is attributed to unsteady wing-wake interactions. A similar effect
is found during hovering.
