On February 29, 2004, about 1300 eastern standard time, an amateur built Rotorway Exec 162, N198BG, was substantially damaged during a forced landing in Medina, Ohio. The certificated private pilot and a passenger were seriously injured. Visual meteorological conditions prevailed and no flight plan was filed for the personal flight conducted under 14 CFR Part 91. The pilot reported that he was flying at an altitude of 1,100 feet above the ground, and an airspeed of 105 mph, when he heard a pop and loud bang. He immediately initiated an autorotation and felt that he had the helicopter under control, until it began to pitch down. Due to strong vibrations, the pilot was not able to read any of the instruments, including rotor RPM. The pilot stated he tried everything he could think of to get control of the helicopter without using any large control inputs. The helicopter struck trees and came to rest in a residential area. The pilot of a Robinson helicopter who was flying behind, and to the left of the Rotorway, stated he observed the helicopter begin a rapid, steep decent. He did not observe anything separate from the helicopter. The helicopter was examined by a Federal Aviation Administration inspector, with assistance of a representative from Rotorway. During the impact, a main rotor blade struck the tail boom in the area of the horizontal stabilizer. The tail boom was severed, the tail rotor drive pulley was split in half, the tail rotor drive belts were broken, and the third bulkhead was fractured into several pieces. All of the main rotor drive belts and pulleys remained attached. Continuity of the cyclic and collective controls was confirmed; and no engine abnormalities were noted. One tail rotor blade was severed at the pitch pin. Both tail rotor blades exhibited dents; however, they did not display the rearward bending of spar and skin that would be consistent with blade rotation at impact. Due to the impact damage, the pre-impact condition of the tail rotor drive system could not be determined. According to a Rotorway representative, the helicopter was originally equipped with a ground adjustable horizontal stabilizer. When adjusted properly, the nose of the helicopter should rise when entering an autorotation, regardless of the helicopter's forward speed. The accident helicopter was equipped with an after market horizontal stabilizer trim system, which allowed up to 40 degrees of trim. Due to impact damage, the position of the horizontal stabilizer could not be determined.