On December 11, 1998, at 1645 hours Pacific standard time, a Bell 47G2, N9021R, sustained substantial damage during a precautionary landing near Helendale, California, following the onset of airframe vibration. The student pilot/owner, the sole occupant, was not injured. The personal flight, conducted under the provisions of 14 CFR Part 91, originated at Kramer's Junction, California, about 1630, and was en route to Helendale. Visual meteorological conditions prevailed at the time and no flight plan was filed. The pilot reported that he was flying between 95 and 100 mph at 200 feet agl when he felt the helicopter begin to shake and slide to the right. He described the shaking as an "out of balance feeling, like when the washing machine gets unbalanced." He stated that the rotor rpm then began to decay. He reported that when he reduced power and stabilized the airspeed between 60 and 65 mph, the shaking stopped but it felt as though he had "lost the tail rotor." He stated that he was not able to "maintain lift," so he initiated an autorotation over hilly terrain. During touchdown, the tail rotor blades impacted an embankment and the main rotor blades severed the tail boom. The Bell 47G2 Flight Manual was reviewed and copies of the relevant portions are appended to this file. The manual indicated that the "never-exceed speed" (Vne) for the helicopter is 100 mph at any altitude from sea level to 1,400 feet msl. It further indicated that the airspeed indicator is marked with a red line at the 100 mph mark. According to the "Basic Helicopter Handbook," published by the Federal Aviation Administration, the tendency for the retreating rotor blade to stall in forward flight is a major factor in limiting the helicopter's forward airspeed. Blade stall occurs during powered flight at the tip of the retreating blade, spreading inboard as forward airspeed increases. The book indicated that the major warnings of approaching retreating blade stall conditions in the order in which they will generally be experienced are: an abnormal 2 per revolution vibration, pitch-up of the nose, and a tendency for the helicopter to roll. At the onset of blade stall vibration, the pilot should reduce the collective pitch, increase rotor rpm, reduce forward airspeed, and minimize maneuvering.