The pilot approached the helipad flying into a 15 knot wind. He overflew the pad and turned left through 180 degrees and overflew the pad again. The helicopter was then slowed to about zero knots IAS and placed in a tight left turn with the intention of approaching the helipad for a landing into wind. After the turn onto downwind was completed translational lift was lost when the pilot reduced airspeed to close to zero knots. The tight left turn was then initiated. The combination of the loss of translational lift and the turn probably required more power from the helicopter than it was capable of delivering. The rotor R.P.M. decayed significantly as the helicopter was overpitched and a high sink rate developed. A passenger, who was a more experienced pilot, took control and attempted to recover rotor RPM but he was unsuccessful. The helicopter impacted heavily with the ground and a pile of gravel and rolled onto its right side.