THE ACFT SETTLED INTO TREES AFTER THE ENGINE LOST POWER DURING A SWATH RUN. THE FLT HAD BEEN IN PROGRESS FOR ABOUT 15 MINUTES. THE PLT STARTED THE 4TH RUN AND THE ACFT STARTED TO SETTLE. THE PLT DUMPED THE LOAD BUT DID NOT TOUCH THE THROTTLE OR MIXTURE. HE HAD SET THE THROTTLE AND MIXTURE FOR THE OPERATION AND TIGHTENED THE FRICTION ADJUSTMENT. THE MIXTURE HAD BEEN LEANED TO PEAK TEMPERATURE AND THEN ENRICHED SO THE CYL HEAD TEMP WAS PEAK TEMP MINUS 125 DEGREES. THE MANIFOLD PRESSURE WAS 32 INCHES AND THE PROP SET AT 2400 RPM THE PILOT WAS ASKED ABOUT HIS USE OR POWER AND MIXTURE AND HE SAID HE DID NOT MOVE OR ADJUST THEM FROM THE TIME HE SET THEM AFTER INITIAL LEVELOFF UNTIL GROUND IMPACT. HE SAIDTHAT AFTER THE ACCIDENT THESE CONTROLS WERE IN A NORMAL FLT POSITION. THE PILOT WAS WEARING A HELMET AND SAID IT WOULD BE POSSIBLE FOR THE ENGINE TO STOP AND HIM NOT BE ABLE TO HEAR IT. FROM THE CONDITION OF THE PROPELLER IT WAS DETERMINEDTHAT THE ENGINE WAS NOT PRODUCING POWER AT IMPACT. DURING INSPECTION, NOTHING WAS FOUND WRONG WITH THE POWERPLANT.