How does pressure distribution change when a wing enters ground effect?

Question

How does ground effect change the pressure coefficient distribution over the top and bottom sides of a wing?

I want to compare the diagram of pressure coefficients vs percent chord length for wing in ground effect vs same wing out of ground effect.

Answer 1

Ground effect is the increase of lift coefficient than can be seen when an airfoil or a wing is at a distance from the ground less than its chord or span. Generally it can be seen that $C_L$ increases the closer to ground the airfoil or wing is, being the angle of incidence kept constant. For a wing this can also be seen as a reduction in its induced drag.

This report reviews several experimental investigations of wings in ground effect. The plot you are looking for is the following one:

Here the pressure coefficient is plotted along the chord (x/c) for several distances above ground (h/c; distances are measured from the trailing edge). AoA is the same (6°) for all the plots. We should look at the plots with solid signs because in those experiments the ground of the wind tunnel was moving at the same speed of the freestream and this gives more realistic results since eliminates the boundary layer on the ground, as it is in realty.

As can be expected, the ground modifies above all the pressure distribution on the belly of the airfoil, creating a bigger $C_p$, i.e. slower speeds, the closer to the ground the airfoil is. Also on top of the leading edge the $C_p$ increases a bit (more negative).

All these can be explained as a "blocking" effect of the ground which "diverts" more and more airflow over the airfoil giving:

1. an increased velocity over the top of the leading edge;
2. and a reduced velocity on its bottom.

Due to Bernoulli this in turn generates:

1. more underpressure (suction) on top of the leading edge;
2. more overpressure on the bottom.

The total effect of 1. and 2. is therefore more lift, a bigger (more negative) pitching moment and more sensitivity to stall.

Other interesting plots can be found in the report.