Experimental investigation of wing geometrie for a better understanding of bird flight in ground effect.*

For more than a century, ground effect has been known for reducing flying cost. The commonly accepted explanation relies on two principles: the increase in lift by the ram effect and the decrease in lift-induced drag by downwash disruption. Large birds like pelicans and albatrosses are often seen gliding near the water surface. Although it is often claimed that they are taking advantage of ground effect, their wing arched geometry keeps most of the lower-wing surface at more than one chord-length from the water. Hence, other phenomena may be at play since birds are not that close to the ground as required with traditional ground effect theories.

To investigate potential new phenomena, three wing geometries are studied: rectangular flat wings, with and without end-plates, and a bird-inspired anhedral wing.

Wings are first placed in a wind tunnel. The ground is simulated with a fixed plate beneath the wings. The wing lift and drag coefficients are measured. Tip vortices and ground interaction are characterized with laser visualization and PIV. Since the plate imposes an inaccurate boundary condition, other tests are underway in a tow tank facility. The wings are towed above the water surface, which enables real flight boundary conditions.