Aerodynamics of aerofoil sections measured on a free-flying bird

Abstract

Birds are adapted to a wide range of flight conditions, from steady fixed-wing glides to high angle of attack manoeuvres involving unsteady separated flows. They naturally control and exploit the transitional Reynolds number regime of Re≈ 105 that is currently of interest in unmanned air vehicle technologies. This article presents a reconstruction of the inner portion of a wing of an eagle in free flight, during a rapid pitch-up manoeuvre at the end of a shallow glide to an elevated perch. Photogrammetric techniques were used to map the identified points on the wing and these were used to fit a mathematical model of the upper and lower surface topography using polynomial regression techniques. The surface model accounts for spanwise twist, spanwise bending, and varying chord distribution, as well as for the shape of the aerofoil. The aerodynamics of the two-dimensional aerofoil sections were analysed using XFOIL and were compared against two technical aerofoils, namely the Selig S1223 and Clark Y aerofoils, at 1×105≤ Re≤2×105. The bird aerofoil maintains a robust, near-constant drag coefficient over a wide lift coefficient range.