On July 26, 1997, about 1710 eastern daylight time, a Bell 47G-3B-1, N35134, was substantially damaged as it impacted the ground during a forced landing at the Knox County Fairground, Mt. Vernon, Ohio. The certificated commercial pilot and two passengers were not injured. Four people on the ground received minor injuries when struck by flying debris. Visual meteorological conditions prevailed and no flight plan was filed for the sightseeing flight conducted under 14 CFR Part 91. The pilot was taking passengers for 5 minute flights from the County Fairgrounds. The pilot stated that with two passengers onboard, he reached approximately 75 to 100 feet AGL, when the helicopter experienced "an unexplained loss of RPM." Unable to clear obstacles in his departure path, the pilot initiated a 180 degree right pedal turn back to his point of departure. The pilot stated that obstacles near the landing pad required him to stay high and "a tailwind caused the rotor RPM to further decay." Witnesses stated that the helicopter impacted the ground hard, and the main rotor blades severed the tail boom. On July 28, 1997, a Federal Aviation Administration (FAA) Inspector examined the wreckage. He stated that the fuel sumps were drained and no signs of contamination was found, and the fuel tank vent passages were unobstructed. The helicopter company's chief pilot stated to the FAA Inspector that the helicopter lost rotor blade power shortly after lift off and out of ground effect, but the pilot did not report any engine malfunction. The Inspector monitored two engine operations, but without rotor engagement due to rotor blade damage sustained during the accident. According to the Inspector, the engine was started twice, and magneto functional checks were performed with a "normal" RPM drop observed each time.