The efficiency of ground effect comes from replacing having to throw
air downwards, which costs energy (less energy the more air you throw
at a lesser speed, ie. long wings), with just hovering over a high
pressure area that you only have to set up once, instead of
continuously creating it.

You get the reduced induced drag of a longer-winged craft without
the parasitic drag of longer wings (the point of long wings being
to reduce the downward speed of thrown air).

The effectiveness of ground effect is more at lower speeds, where
induced drag dominates. At high speed, parasitic drag dominates
and the effect doesn't reduce that.

(Induced drag is from energy lost to downwards-thrown air, which
has to receive enough momentum per unit time to support the weight
of the airplane. Since energy goes as the square of this downward
velocity, you're better off throwing twice as much air half as fast,
which has the same momentum but half the energy, which still supports
your weight. Hence long wings.

Parasitic drag is from skin friction and turbulence produced and
pressure drag, that does not help in keeping you aloft; this is the
chief drag at high speed, where you're throwing vast quantities of
air downwards per unit time and so at very small downward velocity.
Ground effect doesn't help this.

If you want to capture a live bird in a closed garage, keep him
flying poking him when he lands with a long pole ; flying at low
speed for long is not possible, just a few minutes, and the bird
will exhaust himself. The same bird can fly fast hundreds of miles.)
--
Ron Hardin


On the internet, nobody knows you're a jerk.