On May 13, 1998, about 1630 eastern daylight time, a homebuilt helicopter, a Revolution Mini-500, N355RM, registered to a private individual, operating as a 14 CFR Part 91 personal flight, crashed during a precautionary landing at Stockmar Airport, Villa Rica, Georgia. Visual meteorological conditions prevailed and no flight plan was filed. The helicopter received substantial damage, and the ATP-rated pilot was not injured. The flight originated from a private helipad at Cedartown, Georgia, about 45 minutes before the accident. The pilot stated that while in cruise flight, he noticed a "high frequency vibration/buzz sound," together with increasing coolant and exhaust gas temperatures, and elected to land. His intention was to perform a run-on, precautionary landing onto the single runway at Stockmar Airport, but the runway was in use, and he used a grassy area adjacent to the runway. The pilot further stated he chose the grassy area because it appeared to have been newly "bush-hogged"; however, while in the landing flare, he realized too late that the mowed level of the grass was high enough to conceal small obstructions. The left landing skid collided with a large, partially hidden rock and the helicopter rotated counterclockwise abruptly about its vertical axis approximately 240 degrees. During the ground gyration, the main rotor severed the tailboom and the tail rotor assembly separated from the helicopter. Postcrash inspection of the aircraft revealed that less than a full quantity of engine coolant was contained in the reservoir. The pilot stated the cooling system is a closed system, and he is at a loss to explain the leakage. He states, "It should be noted that prior to flight on this date, (accident date) the aircraft was parked inside a hangar on a concrete floor at 7GA9, (Whitesburg, Georgia) and no visible evidence of a leak was noted on the floor. Further, the aircraft was landed on a concrete pad at the private residence at Cedartown, GA and no visible evidence of a water leak was noted on the concrete pad."