Unsteady aerodynamic force mechanisms of a hoverfly hovering with a short stroke-amplitude

Hovering insects require a rather large lift coefficient. Many insects hover with a large stroke amplitude (120°-170°), and it has been found that the high lift is mainly produced by the delayed-stall mechanism. However, some insects hover with a small stroke amplitude (e.g., 65°). The delayed-stall mechanism might not work for these insects because the wings travel only a very short distance in a stroke, and other aerodynamic mechanisms must be operating. Here we explore the aerodynamic mechanisms of a hoverfly hovering with an inclined stroke plane and a small stroke amplitude (65.6°). The Navier-Stokes equations are numerically solved to give the flows and forces and the theory of vorticity dynamics used to reveal the aerodynamic mechanisms. The majority of the weight-supporting vertical force is produced in the mid portion of the downstroke, a short period (about 26% of the stroke cycle) in which the vertical force coefficient is larger than 4. The force is produced using a new mechanism, the “paddlin...