The pilot reported that prior to departure he attached a three-foot by four-foot American flag to the landing skids of the helicopter. Following an uneventful takeoff, he climbed to an altitude of about 200 feet above ground level. Shortly thereafter, the flag disconnected from the skids and impacted the tail rotor. The pilot initiated an autorotation to a nearby field. Examination of the helicopter by the pilot revealed that about half of the tail boom and tail rotor assembly was separated. The pilot reported that prior to the flag separating, no preimpact mechanical malfunctions or failures existed with the airplane that would have precluded normal operation.