The pilot advised that during the approach he did not notice a piece of blue plastic tarpaulin in long grass adjacent to the landing area. As he lowered the collective control after touchdown, there was a loud bang and the aircraft began to shake violently. Main rotor RPM had virtually ceased before the pilot was able to shut down the engine. As the aircraft became stationary the pilot noticed a piece of the tarpaulin was attached to one of the main rotor blades. The pilot had not previously landed at this property, and was unaware that the tarpaulin had been left on a loading ramp. It had evidently been lifted into the air by rotor slipstream and had then fallen into the rotor disc. This caused one blade to become partially detached, with resulting imbalance of the rotor system and a short period of ground resonance.
