Effects of Planform Geometry and Pivot Axis Location on the Aerodynamics of Pitching Low Aspect Ratio Wings

We consider aerodynamic forces and flow development for several wing planforms with the same aspect ratio (AR = 4) and mean chord, undergoing a constant-rate pitch change between 0 and 45o. The wing planforms studied are rectangular, trapezoidal and triangular, which correspond to taper ratios 1, 0.5 and 0, respectively. Three pivot axes are considered: leading edge, mid chord and trailing edge. The reduced pitch rate based on chord and free stream speed is K = 0.39 and the Reynolds number is Re = 8.9k. We consider also the case K = ∞ with the same rotation rate as the other cases and zero free stream velocity. Noncirculatory effects are found for leading edge and trailing edge pivot axes, and absent for mid-chord pivot for all wing planforms. The lift and drag coefficients during the constant pitch rate part of the motion increase as the taper ratio decreases due to rotation rate effects, which is contrary to steady flow behavior in which lift and drag coefficients are only functions of aspect ratio, independent of wing planform geometry. Lifting-line theory including rotation rate effects gives reasonable estimation of lift. Flow visualization show primarily 2D flow during the pitch-up motion. The three-dimensional swirl flow in the wake due to tip vortices is observed after the end of the pitch motion. It develops faster for lower tapper ratio wings.

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