BEFORE DEPARTING, THE PLT RECEIVED A WX BRIEFING & WAS ADVISED THAT VFR FLT WAS NOT RECOMMENDED THRU WESTERN NEBRASKA & EASTERN WYOMING. A FRONTAL WX SYS WAS FORCASTED TO PRODUCE THUNDERSTORM ACTIVITY WITH ASSOCIATED TURBULENCE & RAIN. REPORTEDLY, THE PLT WAS ANXIOUS TO MAKE THE FLT SO HE COULD DELIVER NEEDED PARTS FOR A MACHINE THAT WAS TO BE USED IN A PUBLICLY ADVERTISED DEMONSTRATION. ADDITIONALLY, HE FELT HE COULD HELP HIS PARTNER COMPLETE THE PROJECT IN TIME FOR THE DEMONSTRATION. THE PLT ELECTED TO FILE A VFR FLT & THEN DEPARTED AS PLANNED. WHILE EN ROUTE, THE ACFT ENCOUNTERED WX & SUBSEQUENTLY CRASHED. JUST BEFORE IMPACT, THE ACFT WAS SEEN SPIRALING AFTER IT CAME OUT OF A LOW OVERCAST WITH NO RIGHT WING OR TAIL SECTION. AFTER THE PLANE CRASHED, A WITNESS SAW THE RIGHT WING 'DRIFTING DOWN.' AN EXAM OF THE WRECKAGE REVEALED EVIDENCE THAT BOTH HORIZONTAL STABILATORS FAILED FROM POSITIVE AIRLOADS & THE RIGHT OUTBOARD WING SECTION FAILED FROM NEGATIVE AIRLOADS. NO EVIDENCE FOUND OF A MALFUNCTION/FAILURE BEFORE INFLT BREAKUP.