Behavior of pop up feathers during flow separation.
Birds are products of millions of years of evolution. Thus, many characteristics of their bodies and wings are designed to make bird flight easier and more effective. One such interesting characteristic of many birds is the presence of a passive appendage on the wing in the form of feathers that pop up during landing or turbulent circumstances. In this paper an attempt is made to describe the behavior of these popped up feathers and its effect on the lift during flow separation with a simple inverted pendulum model (1). This is done by applying the model to a two dimensional airfoil with a wing-flap attached to its upper surface to reproduce the relevant geometry of the bird wing with pop up feathers. Two cases for how the wing-flap is attached to the airfoil are investigated, one where the wing-flap is hinged to the airfoil and one where the wing-flap is attached to the airfoil with a torsion spring. First the theory behind the model is explained followed by an analysis of how this model is applied to the airfoil and wing flap configuration. The optimal wing-flap configurations are then calculated numerically and compared to lift data of the pure airfoil. Since the model is a simplification of the complex interaction between the wing-flap and the fluid flow, the results are expected to be modest. It was found that the optimal wing-flap configurations increased lift with about 4.4% and 5% for the hinge-joint and torsion spring case respectively.
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