Ground effect is a
phenomenon experienced by
aircraft as they approach the surface (it can be
water or
land). Below a certain
altitude the air being redirected
downward by the
wings (aircraft are usually in
slow flight and thus
pitched up when this close to the ground) cannot move out of the way fast enough. As a result, there is an
artificial high-pressure zone formed beneath the
wings. To a
pilot, ground effect is annoying because it means during the landing process your
stall speed suddenly drops dramatically as you enter it. Thus, right before you
touch down, your airplane seems to '
float' and won't stall down onto the
runway, meaning you might end up a lot further down the runway than you thought. As a passenger in a modern jetliner, you will experience ground effect as those few seconds between when the pilot cuts engine thrust to zero and the airplane finally thumps onto the runway. The faster you are going when you approach, the more noticeable the effect.
This has been the source of some interesting near-aircraft designs. Since the ground effect means that wings are much more efficient, there have been vehicles which fly, but only in ground effect; these can be much heavier than ordinary aircraft. The Soviet Air Force had a prototype ocean transport as well as several inland sea operating versions, called ekranoplan.