Scale Effects in Wind Tunnel Testing: Extrapolation of the Drag Coefficient

Abstract

Subscale wind tunnel data extrapolation is both a necessary and challenging undertaking. Most readily available wind tunnel facilities can achieve Reynolds numbers that are typically an order of magnitude lower than those of the full-scale flight article. Consequently, the wind tunnel data need to be scaled, with the Reynolds number disparity impacting force and moment measurements. Although computational fluid dynamics is ascending as a valuable tool for enabling scaling, the most common extrapolation methodologies are analytic and semi-empirical in nature. Consequently, a new approach is presented to scale both the minimum drag coefficient and the sectional pressure drag coefficient that uses the subscale data to define extrapolation exponents. Implementation requires a minimum of two subscale tests to enable specification of the exponents determined through coalescence of the extrapolated subscale tests. The approach is simple to implement and can be applied to both airfoils and wings. Numerous cases are presented for method validation.