Wingtip Vortex Behavior in the Vicinity of the Maximum Lift to Drag Ratio Lift Condition

Adverse effects of lift induced drag on the aerodynamic efficiency of aircraft are well known. Lift induced drag is generated as a byproduct of downwash from the wingtip vortices. The flow physics associated with wingtip vortex core axial flow transformation from wake-like (velocity less-than the freestream) to jetlike (velocity greater-than the freestream) behavior in the vicinity of the maximum lift to drag ratio (L/D) lift condition is explored. Particle Image Velocimetry (PIV) experiments were performed in the UD Low Speed Wind Tunnel in the near wake of an AR 6 wing with a Clark-Y airfoil to investigate the characteristics of the wingtip vortex at angles of attack ranging from 2 and 8 degrees. Results showed changes in the velocity distributions in the vortex inner and outer cores. Vorticity and exergy distributions indicated the existence of the wake-like to jet-like transformation in the range of 4° to 6° angle of attack. This range corresponds with the maximum L/D angle of attack of the Clark-Y tested. A relationship between the vortex core axial velocity profile changeover and the angle of attack at maximum L/D was identified. Improved understanding of this relationship could be extended not only to improve aircraft performance through the reduction of lift induced drag, but also to air vehicle performance in off-design cruise conditions.