On June 13, 1999, about 0930 Eastern Daylight Time, a homebuilt Rotorway Exec-90, a helicopter, N8235W, was substantially damaged while landing at a private helipad at Evington, Virginia. The certificated private pilot was not injured. Visual meteorological conditions prevailed for the local flight. No flight plan was filed for the personal flight conducted under 14 CFR Part 91. According to his written statement, the pilot was approaching to land, traveling through transnational lift. He was applying maximum power, but did not have sufficient rotor RPM. In an attempt to increase RPM, he lowered the collective, causing the helicopter to undershoot the intended landing point. The pilot could not touchdown at this point, because the field was at a 9 to 10 degree slope angle. Additionally, he did not have enough rotor RPM to hover up-slope. He made a 180 degree turn and flew down-slope. After the turn, the forward momentum plus the 1 to 2 knot headwind slightly increased rotor RPM. However, the pilot still did not have enough power to hover. He decided to perform a slide-on landing. During the landing, the right skid struck a mound of dirt, causing the helicopter to roll left. The main rotor blades struck the ground and tailboom. The pilot further stated that the engine may not have been developing full power. Additionally, his neighbors told him that they observed smoke emanating from the engine. On July 22, 1999, the engine was inspected and operated at the manufacturer's facility, in the presence of a Federal Aviation Administration Inspector. No interruptions, abnormalities, or discrepancies were found. The pilot reported a total flight experience of 110.5 hours, which 98.5 hours were in the accident helicopter. He received his private pilot license on December 4, 1998.