The pilot flew locally about 40 minutes on an aerial observation flight. He reported that the helicopter was positioned upwind when he began flying crosswind over a tree line and it descended to between 10 to 15 feet above the ground. The pilot stopped the helicopter's descent and forward speed to make a left pedal turn. While in a stabilized and "stationary" downwind hover for about two to three seconds, the helicopter suddenly pitched forward without cyclic input and it started to move forward and descend. For about one to two seconds the helicopter continued to accelerate forward while losing more altitude. The pilot was unable to stop the helicopter's forward movement with neither full aft cyclic nor the descent by adding power to the engine's red line. He decided to make a right turn into the wind with right pedal and cyclic where further altitude was lost. The front of the right skid came in contact with the ground and the helicopter settled onto its right side after the main rotor struck the ground. The helicopter sustained substantial tail boom damage. The pilot indicated that the helicopter was within weight and balance limits and that there was no mechanical malfunctions during the accident flight.