THE HOMEBUILT ACFT CRASHED WHILE ATTEMPTING AN EMERGENCY LNDG IN A FARMERS FIELD. THE FINAL TURN TO LAND WAS STEEPENED TO AVOID A HOUSE. THE ACFT FLIPPED OVER AND CONTACTED THE GROUND IN AN INVERTED, LEFT WING & NOSE DWN ATTITUDE WHICH RESULTED IN A 180 DEGREE REVERSAL BEFORE FINAL IMPACT. THE PLT/OWNER/BUILDER HAD BEEN IN THE PROCESS OF COMPLETING THE ACFT FOR 20 YRS. HE HAD COME TO THE ARPT ON THE ACC DAY TO WORK ON THE ACFT AGAIN BUT INSTEAD DECIDED TO TRY A MAIDEN FLT. THE FLT LASTED 40 MINUTES. WITNESSES NOTICED THE ACFT FLYING ERRATICALLY AND LOSING ALT AS IT FLEW WEST ALONG THE HIWAY RETURNING TOWARD THE ARPT OF DEPARTURE. IT CRASHED INTO A CORNFIELD ABOUT 3 MILES FROM THE ARPT. INVESTIGATION OF THE ACFT WRECKAGE SHOWED SEVERAL IRREGULARITIES IN CONSTRUCTION METHODS & ASSEMBLY STANDARDS. MAINTENANCE PERFORMED WAS ALSO SUBSTANDARD. NO PROBLEMS OF APHYSICAL NATURE FOR THE 61 YR OLD PLT WERE DISCOVERED DURING AUTOPSY.