Stall hysteresis of an airfoil with slotted flap

Stall hysteresis discovered in the wind-tunnel performance of a GA(W)-2 airfoil with a 25% chord slotted flap is examined further by using the data obtained for lift, pitching moment, surface pressure distribution, and the hot-film velocity vector. Test cases include 30- and 40-deg flap deflections, each having an optimum and narrow gap at a chord Reynolds number of 2.2 x 10(6) and a Mach number of 0.13. The flap optimized to produce the highest C-Imax for each flap angle apparently did not have a proper contour and nose location for the slot flow to function effectively at off-design conditions. It is shown that suction pressures over the flap, suppressed by a thickening wing wake at stall, are not reversible to their prestall values within the decreasing alpha side of loop. It is suggested that the flap design include the use of a new flap parameter called the slot flow angle to describe the slot flow orientation and a pressure recovery factor to select a proper contour for the flap upper surface.