Uh-Oh, look up the old thread on Ground Effect here a few months ago. ground effect is available at all speeds to lesser effect above 25 knots or so. In airplanes it has effect up to 150 knots. Some doubt this, because they have an image of a ground cushion built up physically under the aircraft (could you please pass my ground cushion?). Made an interesting thread, never really resolved (meaning nobody ever changed their mind).
search for Autorotation and Ground-Effect and get lost in the 151 replies!