Spread wing-tips of the birds as a model for drag reduction
Black kite with spread outer primaries (photograph by Prof. Ingo Rechenberg)
Most of the birds soaring over land (e.g. kites, eagles, vultures and storks) show characteristically slotted wing-tips. There primaries (i.e. the feathers of the hand or, in technical terms, winglets) bend up and become staggered in height. The multiple-winglet configurations are thought to reduce the induced drag. Such effects are well known from other non planar lifting-systems, e.g. those described by Ludwig Prandtl. He has shown that in biplanes and multiple planes, the kinetic energy in the trailing vortex sheet, and hence the induced drag, can be reduced by the horizontal spreading of the vorticity in the wake.
Tip-Vortex
However, in multiple wings, this beneficial effect becomes somewhat counterbalanced by an increase of the friction drag. In this respect, the multi-winglet configuration considered here can be seen as a synthesis of a multiple plane and a planar wing. It reduces the induced drag by spreading of the vorticity in the tip region and keeps the friction drag low in the inner part of the lift generating system.
Michael Stache
Technische Universität Berlin
Bionik und Evolutionstechnik
