Downwash velocities are decreased when the airplane is operating close to the ground. Since the ground is a solid boundary to which the normal component of velocity must vanish, its presence alters the streamline pattern that exists around the airplane out-of-ground effect. In order to determine ground effect, an image system representing the airplane’s vortex system is placed below the airplane a distance equal to twice the height of the airplane above the ground. The vortex strength of the image system is of opposite sign to the original vortex system. Thus, midway between the two systems, their induced velocities in the vertical direction will cancel, satisfying the boundary condition along the ground.
An additional graph to estimate the effect of the ground on downwash is not needed in light of this information. Instead, one can again make use of Figure 8.6a. For example, suppose an airplane is operating a height above the ground, h, equal to half of its span. Also, let the distance /(>c equal b and
h, lb = 0.05. Relative to b’,
ft = 0.637b’
/ac = 1.273ft’ ft, = 0.0637 ft’
Relative to the image system the tail is located at,
l, = 1.273ft’
ft, = (0.637 + 0.637 + .0637)ft’
Thus, from Figure 8.6a, for the image system
ea = 0.06^
This downwash is subtracted from that for the airplane out-of-ground effect which, in this case, is
e„ = 0.527Aa
Thus, the downwash in this case is decreased by 11% due to the ground effect. This decrease will, of course, improve the static stability and change the trim.