THE PLT STATED THAT HE WAS CRUISING AT 2,500 FT MSL EN ROUTE SOUTHBOUND TO ENID, OKLAHOMA, WHEN HE ENCOUNTERED FOG BUT FLEW FOR 15 TO 20 MINUTES BEFORE EXECUTING A 180DEG NORTHERLY TURN BACK TO HUTCHINSON, KS. ACCORDING TO THE PLT, HE WAS IN THE CLOUDS TRYING TO FLY OUT OF IFR CONDITIONS WHEN THE ACFT FLEW INTO THE GROUND AND HE LOST CONCIOUSNESS. NATIONAL WEATHER SERVICE RECORDS SHOW FORECASTS, VALID FOR APRX THE TIME OF THE ACCIDENT, CALLING FOR IMC AND MARGINAL VMC THROUGHOUT OKLAHOMA AND SOUTHERN KANSAS. THE PLT FURTHER STATED THAT HE RECEIVED NO WEATHER BRIEFING AND FILED NO FLIGHT PLAN. POST ACCIDENT CALCULATIONS REVEAL THAT THE ACFT IMPACTED THE TERRAIN IN A SHALLOW DESCENT NEAR CRUISE POWER. IT PORPOISED AND WHEELBARROWED BEFORE DIGGING IN THE SOFT DIRT. THE NOSE GEAR SHEARED OFF AND THE ACFT FLIPPED INVERTED, SHEDDING IT'S RIGHT CABIN DOOR AND EJECTING THE PLT RATED PASSENGER.