Bill Daniels wrote:

The rule I learned was that ground effect became measurable at one wingspan
above the ground and near the ground could double the L/D. Running in
ground effect is a lot of fun but you'd better be very smooth on the
elevator since the pitch control gets 'twitchy'.

(I strongly suspect that unanticipated elevator sensitivity in ground effect
is a secondary cause of some of the G103 "PIO" accidents.)

Interesting observation. Flying in ground effect places the center of
pressure of the wing at about mid-chord, while out of ground effect the
center of pressure is about 1/4 chord. So in short, ground effect
usually has a stabilizing effect by essentially shifting the CG forward
with respect to the center of lift.