Aerodynamic ground effect in fruitfly sized insect takeoff

TL;DR

This study uses numerical simulations to investigate ground effects on fruitfly-sized insect takeoff, revealing that natural takeoff experiences negligible ground effect, while modified conditions can produce up to 6% difference due to unsteady wing-wake interactions.

Contribution

It provides a detailed numerical analysis of ground effects in insect takeoff, highlighting the negligible impact during natural takeoff and complex effects under modified conditions.

Findings

Ground effect during natural takeoff is negligible.

Modified takeoffs show up to 6% difference in aerodynamic forces.

Unsteady wing-wake interactions cause variable effects contrary to helicopter theory.

Abstract

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.