The Bell 47G-3B with a Soloy conversion was being used to demonstrate an aerial application spraying technique for use along powerline right-of-ways. For about 15 to 20 minutes after takeoff, routine spray runs were made over several different powerlines while using water as a spray. Just prior to the accident, a spray run was made from west to east with a pull-up and turn-around over powerlines along the east side of the area. The pilot stated that following the turn-around, both the cyclic and collective failed to respond and the helicopter began to vibrate. He stated the engine was performing satisfactorily and the rotor RPM was normal. Witnesses stated the helicopter remained in a nose down attitude until impact. After the occupants were removed, the helicopter began burning. An examination of the wreckage revealed no preimpact malfunction or failure. Due to the extent of the fire damage, no useful information was found regarding the flight control, hydraulic, or other systems in the body frame area.