On August 21, 2015, about 1100 central daylight time, an unregistered amateur-built Mini 500 helicopter, received substantial damage during a forced landing near Butler, Missouri, following a reported mechanical failure while in cruise flight. The pilot was not injured. During the mishap, the helicopter rolled onto its right side resulting in separation of the tail boom, and one of the main rotor blades. The aircraft was not registered to and was operated by the pilot under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed for the flight, which was not operated on a flight plan. The flight originated from the Butler Memorial Airport (BUM), Butler, Missouri, about 1050.The pilot reported that he departed from runway 18 at BUM and remained in the traffic pattern. He said that on his second circuit he turned west from the downwind leg of the traffic pattern, crossed the runway and then turned north (right downwind for runway 18). He said that he was straight and level and the helicopter was operating normally when it suddenly shuddered and made a noise he described as sounding like a tire blowing out on a car. He performed an autorotation to a soybean field. He said that he leveled the helicopter about 5 feet above the ground, and about 2 feet above the soybean crop. He stated that he believed that the tail rotor contacted the crop causing some instability. The rotor speed bled off and the helicopter settled onto its right skid and rolled onto its side. Postaccident examination of the helicopter failed to reveal any preimpact anomalies, however, complete examination of the power transmission system was not possible. In a telephone conversation with the pilot, he stated that when he initially experienced the problem, he thought the power transmission belts had failed, but upon examination after the accident he found they were still intact. When asked what he believed happened, he surmised that the one-way clutch that is part of the power transmission system had failed. He said that he had not taken anything apart on the helicopter and he did not think that he would.