The effect of Gurney flaps on a NACA 0011 airfoil was investigated. Gurney flaps provide a substantial increase in lift while the penalty in drag is small. With the Gurney flap, the airfoil pressure distribution shows increased suction on the upper surface and higher pressure on the lower surface compared to the clean airfoil. This change in pressure is most profound on the lower surface just in front of the Gurney flap. Since separation occurs on the upper surface upon stall, this higher pressure condition on the lower surface continues into the post-stall regime. Thus, the NACA 0011 airfoil with Gurney flaps generates lift coefficients greater than one even under post-stall conditions.
The Post-Stall Effect of Gurney Flaps on a NACA-0011 Airfoil
Myose, R., Heron, I., and Papadakis, M., "The Post-Stall Effect of Gurney Flaps on a NACA-0011 Airfoil," SAE Technical Paper 961316, 1996, doi:10.4271/961316.