On July 22, 2000, at 1535 eastern daylight time, an amateur built Lutke Rotoway Exec 90 helicopter, N1183M, piloted by a private pilot, was substantially damaged during a malfunction of the rotor system and the subsequent loss of power and hard landing in Kalamazoo, Michigan. The flight was operating under the provisions of 14 CFR Part 91, and was not operating on a flight plan. Visual meteorological conditions prevailed at the time of the accident. The pilot and his passenger reported no injuries. The flight had departed from Newman Airport (4NO), Kalamazoo, Michigan, at about 1500. The pilot reported he was flying his experimental helicopter about 700 feet above ground level in cruise flight when the top motor mount made of cast aluminum broke. He reported that the belt tension to the main rotor was lost and he was unable to maintain rotor rpm. He executed an autorotation to a small back yard that was surrounded by trees. The helicopter landed hard and the main rotor blades flexed down and chopped off the tailboom.