THE GYROPLANE WAS ON A LOCAL DEMONSTRATION FLT. THE PASSENGER (PAX) STATED THE PLT TOOK OFF, CLIMBED TO APRX 500 FT AGL,DESCENDED TO APRX 20 FT AGL & MANEUVERED NEAR OBSERVERS. HE SAID THE PLT THEN ADDED POWER & INITIATED A 30 DEG CLIMB ATTITUDE. THE PAX REMEMBERED EXPERIENCING A NEGATIVE 'G' SENSATION & SAID THE PLT WAS MOVING THE STICK AROUND, TRYING TO FIND OUT 'WHERE THEY WERE.' THE GYROPLANE THEN DESCENDED & STRUCK THE GROUND. AN EXAM OF THE ACFT REVEALED NO EVIDENCE OF FAILURE OR MALFUNCTION OF THE POWERPLANT, AIRFRAME OR FLT CONTROLS. THE PLT DID NOT HAVE THE REQUIRED ROTORCRAFT CATEGORY & GYROPLANE CLASS RATING. HIS MEDICAL CERTIFICATE REQUIRED HIM TO WEAR CORRECTING LENSES WHILE EXERCISING THE PRIVILEGES OF HIS AIRMAN CERTIFICATE. ACCORDING TO A WITNESS, THE PLT WAS NOT WEARING GLASSES. THE GYROPLANE HAD NOT BEEN ISSUED AN AIRWORTHINESS CERTIFICATE BY THE FAA.