THE PLT OF A LANCAIR MADE A FORCED LNDG AFTER THE ACFT'S PROP FRACTURED AND FAILED DURING CRUISE.

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LEFT BULLHEAD; AZ; CLBED TO 8500 FT MSL (CRUISE) FOR ABOUT 5 MINS. ABOUT 1/3 OF THE TRAILING EDGE OF THE PROP CAME OFF. SHOOK! I STOPPED THE WINDMILLING PROP AND DECLARED AN EMER. I WAS TOO FAR FROM BULLHEAD AND LUDLOW TO GLIDE; SO I HEADED FOR THE INTERSTATE ROUTE 40. I WAS ABLE TO LAND ON THE FREEWAY WITH NO HARM TO ANYONE OR MY PLANE. THE HIGHWAY PATROL WERE VERY COOPERATIVE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: A FRIEND OF THE RPTR (AN ENGINEER WHO PARTICIPATED IN THE DESIGN OF A FAMOUS EXPERIMENTAL ACFT) INSPECTED THE PROP AND TOLD THE RPTR THAT HE THOUGHT THAT THE FAILURE MAY HAVE BEEN A RESULT OF RESONANCE AIRFLOW WHICH WAS CAUSED BY THE PROP DESIGN. THE LEADING EDGE IS SIGNIFICANTLY CURVED BACK; WITH TOO MUCH MATERIAL AFT OF THE CENTER OF THRUST. THE BLADE DESIGN COUPLED WITH A FAIRLY HIGH ROTATION SPD OF 2850 RPM MAY HAVE LED TO FLUTTER. ALSO; CONTRIBUTING TO FRACTURE IS A STEEL WT INSERTED THROUGH A HOLE IN THE PROP TIP. PROP SHEDED AT THE EDGE OF THE STEEL AND THE STEEL LEFT THE PROP BEFORE ENG SHUT DOWN. THE EFFICIENCY OF THE PROP MAY BE AT THE COST OF SAFETY. RPTR RELATED THE PROB TO THE MANUFACTURER; AYMAR DEMUTH; WHO ASSERTED THAT THE PROP MUST HAVE BEEN DAMAGED. THE PROP WAS REPLACED WITH ANOTHER MANUFACTURER'S PROP AND RPTR IS ASSISTING IN DEVELOPMENT OF A MORE EFFICIENT PROP SUITED FOR THE LANCAIR. THE ACFT WAS FRESHLY BUILT WITH LESS THAN 170 FLT HRS TOTAL; PRIOR TO THE INCIDENT. RPTR ALSO ADDED THAT HE WOULD NEVER FLY GPS ROUTES AGAIN; UNLESS THERE WERE NUMEROUS LNDG AREAS ALONG THE ROUTE. NOTE: ENG IS A MODIFIED IO-320.