DURING CRUISE FLIGHT, APPROXIMATELY 9.5 INCHES OF A PROPELLER BLADE SEPARATED FROM THE AIRPLANE'S PROPELLER, AND THE AIRPLANE BEGAN A SEVERE VIBRATION. THE PILOT CLOSED THE THROTTLE AND PULLED THE MIXTURE TO THE IDLE CUT OFF POSITION. HE THEN LANDED IN ALDER BUSHES, AND THE AIRPLANE NOSED OVER. RECORDS SHOWED THAT THE PROPELLER HAD SUSTAINED A PREVIOUS PROPELLER STRIKE AND THAT IT HAD BEEN STRAIGHTENED AND CUT TO 80 INCHES.