The 6.8 seconds that a pedal-operated flying machine stayed aloft has been certified by the National Aeronautic Assn. in Washington as the first human-powered helicopter flight. The craft, dubbed Da Vinci III by the engineering students at Cal Poly San Luis Obispo who designed it, hovered about eight inches off the ground Sunday under the observation of an NAA consultant. The 97-pound device was powered by cyclist Greg McNeil. The students' ultimate goal is a $20,000 prize offered by the American Helicopter Society for the first flight lasting at least a minute and reaching a height of at least three meters.