The pilot stated he advanced the throttle to about two-thirds and the parachute went from left to right. He decreased the throttle and the parachute stopped oscillating and kitted up correctly. He increased power and they became airborne in a slow climb at about 30 mph. He observed a tree line to his front. The aircraft was not achieving any lift so he maneuvered the aircraft towards a cut in the trees. The nose of the airframe collided with the trees. He added power to level the airframe and they collided with the ground hard. The aircraft sustained minor damage. Both the pilot and the passenger sustained serious injuries. The pilot stated he may have encountered a downdraft. The nearest airport to the crash site was located 15 miles to the northeast. The winds at the airport were reported as calm. The pilot reported no pre-crash mechanical failures or malfunctions that would have precluded normal operations before the accident. The reported loss of lift was not determined.