Ground effect is a reduction in induced drag that causes the airplane to accelerate faster. It is due to the fact that, given the proximity of ground, the wingtip vortexes responsible for induced drag are disrupted and so is that kind of drag: hence the plane accelerates faster. IIRC, it should happen until a height of about 1 wingspan from the ground.
Biggles, IIRC it is modeled in AH2.