After completing a swath run, the pilot pulled up, made a turn, then started a let down for another pass. As he increased power, the engine RPM went from 3000 to 3100. At the same time, the pilot heard a whining noise and noted that the rotor RPM was decreasing. The pilot was unable to stop the descent before the helicopter struck the ground. After impacting the ground, the helicopter bounced, then rolled over. An exam of the centrifugal clutch assembly, PN 47-620-609-1, revealed it had an estimated 60% contact between the shoe linings and the drum face. A minimum of 85% contact was required. The clutch did not conform to the toe-in requirements and none of the heels of the clutch shoes were making contact with the drum. The helicopter had flown 220 hours since the last annual inspection when the clutch shoes were deglazed and the clutch drum was checked, and it had flown approximately 13 hours since the last 100 hour inspection.