In addition to what
speedrestriction and
barit says there is one
maybe more important thing.
Ground effect - or to be more specific -
Span Dominated Groundeffect.
When the aircraft is below about one span altitude, there is less
space for the vortex to build up below the wing. The leak of pressure
from the lower side of the wing gets less the closer to the ground you are.
This results in;
1. Less vortex buildup
2. Vortex being forced to the sides.
3. Less induced drag
X-axis is altitude in % of span, Y is Induced drag
Cheers,
M