Post stall aerodynamic response of a wing in a tandem arrangement – effect of spacing and aspect ratio of downstream wing

The effect of streamwise spacing and aspect ratio of the downstream wing on the post-stall aerodynamic response of the upstream wing was experimentally investigated at a chord-based Reynolds number of 100,000. Wings consisting of a NACA0012 profile having a chord length of 4 inches were utilized. The aspect ratio of the rear wing varied from 0.5 to 5. The streamwise spacing between the trailing edge of the front wing and the leading edge of the rear wing varied from 0.5c to 4.5c. The angle of attack α of the upstream wing was changed statically between 0^o to 80^o while the downstream wing remained at α = 0^o. It was found that the presence of the downstream wing did not change the pre-stall lift curve slope, maximum lift coefficient and stall angle of the upstream wing, but the post-stall behaviour was significantly altered. The increase in lift due to the increase in the angle of attack beyond stall-angle was much less than an isolated wing when the spacing between the wings was minimum. For streamwise spacing >1.0c, the post-stall lift curve slope values followed an isolated wing curve up to a critical angle of attack, after which a sudden secondary stall was observed. Unlike a conventional stall, where the decrease in the lift is accompanied by an increase in drag, the secondary stall caused a significant reduction in both lift and drag. The critical angle of attack increased with the increasing streamwise spacing but only when the projected wing area in the flow direction was approximately equal to or greater than 25% of the spacing between the wings. The secondary stall remained insensitive to the changes in the aspect ratio of the downstream wing. Flow visualization indicated that the rear wing suppressed the wake of the upstream wing in the post-stall angles of attack range, and the vortices originating from the trailing edge and shear layer from the leading edge of the front wing did not cross the wake centerline.