PLT OF C210 ON AN IFR FLT PLAN; DSNDED WITHOUT CLRNC TO GET OUT OF SEVERE TURB.

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IMC IN CLOUD AT 7000 FT CRUISE; FTW CTR CTL. SLIGHT TO MODERATE TURB. GENERAL CONDITIONS: SCATTERED WITH BASES +/-4700 FT. SCATTERED RAIN; LIGHT TO MODERATE; TOPS +/-10000 FT TO MUCH HIGHER WITH A FEW LARGER CELLS. THESE CONDITIONS DEVELOPING PROGRESSIVELY FROM MY DEP (MKN) AT NOON. I WAS IN AND OUT OF CLOUDS THEN LATER ALMOST ENTIRELY IN CLOUD. TURB INCREASING. I REQUESTED CLB TO 10000 FT; THINKING I WOULD TOP MOST OF THE BUILD-UPS AND BE ABLE TO CIRCUMNAV VISUALLY; AS ACFT HAS NO RADAR OR STORM SCOPE. CTR CLRED ME TO CLB TO 10000 FT. BEFORE REACHING 10K; TURB BECAME SEVERE. PENS AND CHARTS FLYING AROUND. MY HEAD BANGING TOP OF CABIN; ACFT VERY HARD TO CTL. TOLD CTR I HAD TO DSND. CTR ADVISED THEY HAD TO 'COORDINATE' WITH SOMEONE. I DID NOT UNDERSTAND; OR QUESTION. MY HANDS WERE FULL TRYING TO KEEP ACFT UNDER CTL. CLB CONTINUED EVEN AS I REDUCED THROTTLE TO 15 INCHES MANIFOLD PRESSURE. I SAID 'UNABLE; I'M DSNDING NOW' OR WORDS TO THAT EFFECT. DSNDED VERY QUICKLY; BROKE OUT; CLR +/-4500 FT AND LEVELED AT 4000 FT -- NO OBVIOUS DAMAGE TO ACFT (CONFIRMED LATER ON POSTFLT). CTR CTLR ADVISED CONTACT ZME CHIEF OF OPS WHEN ON THE GND; WHICH I DID. THE CTR CHIEF HAD LISTENED/REVIEWED TAPES WHICH CONFIRMED MY 'UNAUTH' DSCNT. HE THEN ASKED ME FOR MY SIDE OF THE STORY. AFTER LISTENING TO ME HE ASKED IF I HAD ADVISED THE CTLR OR TURB. I SAID THAT I DID NOT PRIOR TO DSCNT; BUT DID WHEN OUT OF TURB AT 4000 FT. CTR CHIEF THANKED ME AND SAID THAT WAS ALL HE NEEDED; AND CONCLUDED THE CALL. I THINK THE ENTIRE CALL TO ZME WAS RECORDED (I HEARD INTERMITTENT BEEPS). IN RETROSPECT; I SHOULD HAVE: 1) NOT TRIED TO CLB ABOVE THE GROWING CUMULUS; ESPECIALLY NOT WHILE IN THE CLOUD. 2) DECLARED AN EMER DSCNT; OR CLRLY STATED MY NEED TO DSND IMMEDIATELY. IN MY OPINION; THERE WAS DANGER OF ACFT DAMAGE OR COMPLETE LOSS OF CTL. I SHOULD HAVE MADE THAT CLR. NOTE: I WAS AWARE OF THE AIRMET/SIGMET FOR THE AREA AND WAS/AM KEENLY AWARE OF TSTM AVOIDANCE. I WAS NOT IN ANY TSTM PRIOR TO THE INCIDENT. I THINK I GOT INTO A BUILDING/ DEVELOPING STORM.