Prediction and experimental evaluation of planar wing spanloads for minimum drag

An evaluation and analysis of optimum wing spanloads was conducted by comparing wing performance values obtained from theoretical models and experimental wind-tunnel measurements. Three wing spanloads were designed for a prescribed lift and wing root bending moment using a Lagrange multiplier optimization method, with and without incorporating a representative viscous effect. Wind-tunnel models were constructed for each spanload, and experiments were conducted to determine the relative difference in drag measured as changes were made to the design constraints. The results indicate that, for a fixed lift and wing root bending moment, significant decreases in drag from an elliptically loaded wing are achievable by using a higher aspect ratio with a nonelliptic spanload. Under the fixed design constraints, a minimum drag wing was produced by balancing increases in profile drag and decreases in induced drag associated with a planar wing-tip extension. Experimental five-hole probe wake survey measurements were used to determine the spanwise lift distributions of the wing models, which were used to verify the conformity of the experimental spanloads to the predicted values.