Ground effect isn't only a factor for fixed-wing aircraft, it also impacts rotary-wing aircraft including helicopters and the Kitty Hawk Flyer. When you fly a helicopter there is a sudden noticeable loss of lift as you climb out of the ground effect. I'll bet the Kitty Hawk is staying close to the surface at least partially to take advantage of ground effect lift, although the article doesn't make that clear.

I stand corrected, thanks.

Easy to understand summary of helicopter ground effect, with pictures:

http://www.copters.com/aero/ground_effect.html

I note that the rotary wing ground effect works to a height of about the rotor diameter, so it's not clear to me that this is a useful effect with these small rotors.