What is wig craft , what is surface effect action

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Principle
A wing generates lift, in part, due to the difference in air pressure gradients on the wing surfaces: both upper and lower. During normal flight, the upper wing surface experiences reduced static air pressure and the lower surface comparatively higher static pressure. In normal flight, these air pressure differences also accelerate the mass of air downwards balancing momentum. However kinetic energy is proportional to the square of the velocity so designers aim to minimize the accelerated air velocities to reduce wasted energy in both the downward mass acceleration and wingtip vortices. Flying close to a surface increases air pressure on the lower wing surface (the ram or cushion effect) and decreases air acceleration so the ground effect improves the aircraft lift to drag ratios in two ways. Momentum is still balanced because the air pressure beneath the wing is pressing on the underlying surface—the water or flat land.