On September 2, 2003, about 1040 eastern daylight time, an amateur built Lancair IVP, N789HS, was substantially damaged during takeoff from Manassas Regional Airport (HEF), Manassas, Virginia. The certificated private pilot and passenger were not injured. Visual meteorological conditions prevailed for the planned flight to Destin-Fort Walton Beach Airport (DTS), Destin, Florida. An instrument flight rules (IFR) flight plan was filed for the personal flight conducted under 14 CFR Part 91. The pilot stated that prior to the flight, he received an IFR clearance with complex changes to the route on his flight plan. Subsequently, the pilot felt rushed and distracted during the pre-takeoff checklist, and forgot to extend the flaps to the takeoff position. He initiated a takeoff on runway 16R, a 3,702-foot long, 100-foot wide, asphalt runway. During the takeoff roll, the airplane initially became airborne, but settled back on the runway. It then traveled off the right side of the runway, struck a runway sign, and came to rest in a grass area. Examination of the airplane by a Federal Aviation Administration inspector revealed that the left main landing gear collapsed, the nose gear sheared off, and both wings sustained substantial damage. The inspector noted that the airplane departed off the right side of the runway about 1,800 feet from the start of the takeoff roll. The reported winds at the accident site, at 1055, were from 140 degrees at 5 knots.