SINGLE PLT IFR HAS SPATIAL DISORIENTATION.

Narrative

CHAIN OF EVENTS: I WAS FLYING S ON V115 AT 6000 FT. I WAS BELOW THE TOPS OF NUMEROUS BUILDING CUMULUS CLOUDS; GOING RAPIDLY IN AND OUT. EACH TIME I ENTERED A CLOUD; I GOT VERTIGO; FEELING AS IF I WAS IN A STEEP DIVE. UPWARD THERMALS WERE CAUSING ME TO SLOWLY GAIN ALT; BUT BECAUSE OF THE VERTIGO; MY NATURAL INCLINATION WAS TO CONCENTRATE ON THE ATTITUDE INDICATOR; KEEPING THE NOSE LEVEL. I WAS AWARE THAT I GAINED APPROX 500 FT AND WAS TRYING; BUT NOT ADEQUATELY; TO CORRECT FOR IT. AT ABOUT THIS TIME; ZTL REQUESTED I CHK MY ALT. I RESPONDED AND FORCED MYSELF TO IGNORE THE VERTIGO AND DSNDED BACK TO MY PROPER ALT. I WAS NOT USING THE AUTOPLT; WHICH DOES NOT HAVE PITCH CTL. HUMAN PERFORMANCE CONSIDERATIONS: MOST OF MY FLYING TIME IS FROM HELIS WHILE IN THE ARMY (NOW RETIRED). I HAVE ROUGHLY 300 HRS FIXED WING TIME AND ONLY 30 HRS ACTUAL INSTS; MOSTLY HELIS. THIS WAS AN IFR TRAINING FLT; AND MY FIRST EVER IFR FLT WITHOUT A COPLT. I WAS APPREHENSIVE ABOUT THE FLT BECAUSE A CONVECTIVE SIGMET HAD BEEN EARLIER ISSUED FOR THE VICINITY OF MY DEST. AT THE TIME OF THE INCIDENT; IT APPEARED THAT EMBEDDED TSTMS WERE POSSIBLE. I HAD FLOWN PRACTICE IFR (HOOD) ON 2 FLT WITHIN THE WK PRECEDING THIS FLT AND HAD EXTENSIVELY PLANNED THE FLT. HOWEVER; MY OVERALL RECENT EXPERIENCE WAS RELATIVELY LOW AND I BELIEVE THAT THIS PLUS MY WORRY ABOUT THE WX MADE ME MORE SUSCEPTIBLE TO THE EFFECTS OF VERTIGO. OBVIOUSLY; THERE IS NO SUBSTITUTE FOR ACTUAL IMC EXPERIENCE. I NOW KNOW I NEED TO DIRECT MY TRAINING TOWARDS INCREASED IMC TIME.