The pilot had just washed the helicopter, and carried out a circuit to dry the machine. At the completion of the circuit he intended to carry out a practice autorotation. The pilot reported that after turning onto final he noticed that the wind direction had altered so he decided to extend the final leg and selected a different touchdown point. As he flared the aircraft he noticed that the ground sloped and decided to apply some collective and touchdown on a flatter surface. The aircraft touched gently but the pilot reports that the main rotor RPM had decayed and a violent resonance occurred. The subsequent inspection of the tailboom found that the skin had been rippled near the forward end.
