On July 1, 1996, at 0930 central daylight time, a Hiller UH12E helicopter, N14637, was substantially damaged while executing a precautionary landing following a partial loss of engine power near Franklin, Louisiana. The helicopter, owned and operated by a private owner, was being operated under Title 14 CFR Part 91. Visual meteorological conditions prevailed and a flight plan was not filed. The commercial pilot, sole occupant of the aircraft, was not injured. The local business flight originated from Patterson, Louisiana, at 0815. During personal interviews, conducted by the FAA inspector, the pilot reported that he heard a "bang" and the helicopter engine started "running rough" while the helicopter was operating at about 100 feet AGL en route between Patterson and Franklin. The pilot further stated that he elected to execute a precautionary landing to "investigate the source of the engine problems." The pilot further stated that during the precautionary landing the "rotor RPM started to bleed and sufficient RPM could not be maintained for a successful landing." During the low rotor RPM landing touchdown to a muddy grass area, the front cross tube collapsed and the aft skid settled into the mud. Examination of the airframe revealed that the tailboom, tailrotor gearbox, and a portion of the vertical stabilizer were severed from the helicopter as result of impact with the main rotor blades. The reason for the partial loss of engine power could not be determined. The pilot did not complete any of the several Pilot/Operator Reports (NTSB Form 6120.1/2) that were mailed to him.