AN IFR C182 WAS UNABLE TO MAINTAIN ASSIGNED ALT AFTER ENCOUNTERING A CAT 5 TSTM WHILE IN RADAR CONTACT WITH ATC.

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I WAS ON AN IFR FLT PLAN ORIGINATING AT JZI AND ENRTE TO GHP WHEN THE SIT OCCURRED. I HAD BEEN IN CONTACT WITH COLUMBUS APCH FOR AT LEAST 20 MINS WITHOUT ENCOUNTERING ANY IMC AND WITHOUT BEING ALERTED OF THE TSTM I WAS ON A DIRECT PATH TO ENCOUNTER. I PROCEEDED ON MY FLT PLAN WITHOUT CONCERN AND HEARD THE CTLR OFFERING VECTORS TO OTHER AIRPLANES THAT WERE APCHING SEVERE TSTMS AND ASSUMED THE CTLR WOULD OFFER ME THE SAME PROTECTION. I CONTINUED TO ENTER IN AND OUT OF IMC WITH NO WORRIES AND FAILED TO QUESTION THE CTLR OF POTENTIAL STORMS IN MY PATH. PROCEEDING ON COURSE I ENTERED INTO MORE AND MORE IMC AND FOUND MYSELF IN THE MIDDLE OF A SEVERE TSTM. I CONTINUED INTO THE IMC NOT KNOWING THAT A SEVERE TSTM WAS IMBEDDED. UPON ENTERING THE STORM MY ALT BECAME UNCTLABLE AND I CUT MY PWR TO KEEP MY AIRSPD BELOW THE YELLOW ARC. ATC CONTINUED TO PROVIDE NO ASSISTANCE. I CONTINUED TO TRY TO MAINTAIN CTL OF THE ACFT AND ALERTED ATC THAT I NEEDED VECTORS OUT OF THE STORM I WAS IN. ATC ALERTED ME I WAS IN THE MIDDLE OF A CLASS 5 TSTM AND THEY WOULD ATTEMPT TO GET ME OUT OF THE STORM. I WAS TOLD TO TURN 90 DEGS TO THE L AND FOLLOWED THAT INSTRUCTION. I EVENTUALLY MADE IT OUT OF THE STORM AND QUESTIONED EACH CTLR ABOUT THE POTENTIAL STORMS I WAS GOING TO ENCOUNTER.