Effect of ground on a perching maneuver.

An experimental investigation is carried out to understand the effect of ground proximity on a perching maneuver. Different pitch rates are executed during the decelerating motion at different ground heights to test the influence of ground proximity on the aerodynamic performance of a perching wing. We also extended this study by introducing the time delay between decelerating and pitching motion to analyze its effect on the generation of unsteady forces. Here, the unsteady forces were measured by using the force sensor, and the evolution of the unsteady flow around the perching wings was captured by using a two-dimensional particle image velocimetry (PIV). This study shows that for the given pitch rate the unsteady lift force increase with the increase of ground proximity. This indicates that the perching birds can generate the same amount of lift force at slower pitch rates when the perching motion is carried out close to the ground, which explains the reason why natural flayers like birds, i.e., eagle, glide close to the ground while executing the perching maneuver during landing flight.