Why do albatrosses and pelicans morph to an arch-shaped wing configuration when gliding near the water surface?*

Albatrosses and pelicans are thought to fly near water surfaces in order to take advantage of the so-called "ground effect'' and, thus, fly more efficiently. Traditional understanding of the "ground-effect'' phenomenon is based on two principles: 1. ram effect, and 2. downwash disruption. Pelicans and albatrosses, though, fly quite high above the water surface for any ram effect to occur. In addition, it is unlikely that any alteration of downwash has any pronounced effect on the effective angle of attack over the rest of the wing planform, especially for such high aspect ratios. We believe that water-surface gliding pelicans and albatrosses intentionally arch their wings and point their tips toward the surface in order to generate a uniform wing loading while reducing wake vorticity. The methodology to demonstrate the effectiveness of arched wings close to the ground is based on two distinct experimental approaches. Two-dimensional airfoils are tested with their axes perpendicular to or at an angle with the water surface. The ground clearance gap, as well as the airfoil velocity and angle of attack, is varied. Three-dimensional arched wings are tested in a high-speed hydrodynamic towing tank, as well as over a stationary ground.