Mike Rapoport wrote:
A helicopter can maintain a higher altitude in "clean" air. When it hovers,
it pushes the air down and to maintain altitude it has to "climb" through
this decending air. This effect is strongest out of ground effect.

Same effect that a fixed wing airplane has: the tip
vortex is large enough to curl around and get into the top of the rotor
on a helicopter, and around the wingtips of a fixed-winger. This
decreases the angle of attack on the affected area and reduces lift.
The helicopter's hovering vortex is ring-shaped, like a doughnut. The
helicopter has another, smaller vortex on the inboard area, too,
curling up next to the mast and into the top.
The air movement extends out a long way, so that near the
ground that movement is hampered, the vortex gets smaller and drag
decreases, angle of attack increases, and lift increases.

Dan (a helicopter pilot wannabe)