Spanwise Variation in Circulation and Drag of Wings at Moderate Reynolds Number

The measurement and prediction of the aerodynamic performance of airfoils and wings at chord Reynolds numbers below 105 are both difficult and increasingly important in the application to small-scale aircraft. Not only are the aerodynamics strongly affected by the dynamics of the unstable laminar boundary layer, but the flow is decreasingly likely to be two-dimensional as the Reynolds number decreases. The spanwise variation of the nominally two-dimensional flow along a two-dimensional geometry is often held to be responsible for the large variations in the measured profile drag coefficient cd at this scale. Here, local two-dimensional drag coefficients are measured along a finite wing using nonintrusive particle image velocimetry methods. Variations in cd(y) can be related to local flow variations on the wing itself. Integrated values can then be compared with direct force balance data, and the dynamical significance of spanwise variability will be reevaluated.