Lyman...
He's saying that the AoA in ground effect, according to the article, is lower. Example: out of ground effect the critical AoA is 17 degrees, in ground effect the critical AoA is 14 degrees.
hawk37...
There is research that actually says ground effect will increase the stall angle and research that says it will decrease stall angle.
It initially makes sense that the stall angle will decrease in ground effect as the airflow is restricted from following the curvature of the airfoil and, as a result will experience separation at a lower angle of attack. This explains it a bit:
Ground Effect
There are a lot of effects that happen when a wing is in ground effect. Boundary layer effects in ground effect are very complicated. Sometimes lift actually increases as the wing leaves ground effect. If a wing has a bit of a curved lower surface, this effect can be attributed partly to the venturi effect created by air flowing between the ground and the lower airfoil surface. That would create suction that would decrease the lift of the airfoil - lift would then increase as you got further from the ground, with the venturi effect being lessened.
This research paper produces evidence that stall angle increases in ground effect: researchbank.rmit.edu.au/eserv/rmit:6319/Walter.pdf
A couple quotes from the paper:
"In all aerofoils, the stall tends to occur between 10o – 30o AoA. The effect on the stall characteristic is clearly seen in the curve at 0.167 chord compared to the curve at 3 chord lengths.
All aerofoils show the stall occurring later when the aerofoil is in close ground proximity compared to when outside ground effect."
"In the case of the Clark Y and 6-Series aerofoils, the lift force did not vary as expected prior to stall, with force decreasing with decreased ground clearance. The aerofoil with the Gurney tab displayed the typical characteristic of increased lift force with decreased ground clearance. All aerofoils displayed a decrease in drag with decreased ground clearance.
The stall of the three aerofoils occurred at a higher AoA at close ground clearance than outside ground effect. Post-stall, the three aerofoils showed a decrease in lift and drag force with decreased ground clearance. Further study is required to determine how the ground simulation has affected these results. It is possible that the trends in the lift and drag data are a result of interaction with the ground boundary layer, and further study conducted in the field or with more accurate ground simulation may produce alternate findings."