Analytic Investigation of Advancing Blade Drag Reduction by Tip Modifications

Analytic techniques were applied to study the effect on the performance of the nonlifting advancing blade when the outboard 5% of the blade is modified to reduce drag. The tip modifications studied consisted of reducing airfoil thickness, sweepback, and planform taper. The reductions in instantaneous drag and torque were calculated for tip speed ratios from about 0.19 to 0.30, corresponding to advancing blade tip Mach numbers of 0.855 to 0.936, respectively. Approximations required in the analysis introduce uncertainties into the computed absolute values of drag and torque; however, the differences in the quantities should be a fairly reliable measure of the effect of changing tip geometry. For example, at the highest tip speed, instantaneous drag, and torque (at ψ = 90°) were reduced by 20Z and 24%, respectively, for tip sweep of 40° on a blade using an NACA 0010 airfoil and by comparable amounts for 30° sweep on a blade having an NACA 0012 airfoil section. The present method should prove to be a useful, inexpensive technique for identifying promising configurations for additional study and testing.