Ground effect

From RC Helicopter Wiki

Ground effect is the need for less collective pitch (and therefore less power) when hovering within approximately one rotor diameter of the ground. It is caused at least partially by the downdraft of the helicopter being slowed by the ground.

Hover performance of full sized aircraft is often stated both 'in ground effect' (IGE) and 'out of ground effect' (OGE). Once out of ground effect, the height of the helicopter over the ground has no bearing on the amount of power needed to hover, it being determined solely by the density altitude. For model aircraft especially, the ground effect "cushion" can make it difficult to land on small high areas, as the ground effect will tend to push the helicopter away.