UCLA

Wings encountering rapid changes in flow conditions, such as gusts, or those operating on small unmanned and rotary-wing aircraft, routinely experience unsteady separated flow in which the loads bear little resemblance to steady predictions. Much of the lift produced under these conditions comes from the leading-edge vortex, whose growth and eventual departure govern the transient force history. Leading-edge sweep is known to introduce spanwise flow along the vortex axis, but its influence on the transient loads and leading-edge vortex structure of a translating wing remains unsettled. This work presents force measurements and dye flow visualization for a finite wing surging from rest, with sweep as the only geometric variable. A rectangular, untapered NACA 0012 planform of aspect ratio 4 was accelerated over one chord of travel and towed at constant velocity at a fixed angle of attack of 20 degrees, for leading-edge sweep angles of 0, 30 and 45 degrees. Lift, drag and pitching moment were measured at a Reynolds number of 100,000, and the flow was visualized at 5,000 in both chordwise and spanwise views. The measurements are interpreted alongside a low-order model in which the lift is decomposed into added mass, vortex growth and vortex motion contributions. Lift peaks at approximately one chord of travel regardless of sweep, while its magnitude falls by nearly half between the unswept and most highly swept planforms. The peak is shown to exceed what added mass and attached-flow circulation together can supply, and is there fore attributed to circulation held in the leading-edge vortex. Normalizing by the velocity component perpendicular to the leading edge collapses the three peaks, consistent with the rate of convective vorticity flux normal to the freestream. The visualization indicates that sweep neither delays separation nor alters when three-dimensional structures first appear, but instead redirects the separated flow outboard towards the wing-tips rather than inboard, allowing the vortex to remain coherent near the surface for longer. This is reflected in the force histories, where the unswept wing retains roughly half of its peak lift after the vortex departs while the 45 degrees swept wing retains close to ninety percent.