A LEFT ROLLING MOMENT WAS INDUCED WHEN LANDING FLAPS WERE SELECTED ON A CRJ200. THE FLT CREW CONDUCTED A GAR; DECLARED AN EMER AND RETURNED FOR LNDG.

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ON ARR; I ELECTED TO FLY A MISSED APCH AND DECLARE AN EMER DUE TO BEING VIRTUALLY UNABLE TO MAKE THE ACFT TURN L SAFELY. THE FLT HAD PROCEEDED NORMALLY FROM INITIAL INSPECTION THROUGH TAXI-OUT; TKOF; CLB; CRUISE; AND DURING THE ARR. NO CTL ISSUES WERE DETECTED. THIS INCLUDED SEVERAL ATC ASSIGNED TURNS; BOTH WITH THE AUTOPLT ENGAGED AND DURING MANUAL FLT. ON THE DOWNWIND PORTION OF THE VISUAL APCH; ATC INSTRUCTED US TO SLOW TO FINAL APCH SPD TO ALLOW FOR DEPARTING TFC. WE WERE AT ABOUT 2000 FT; FLAPS 30 DEGS; 160 KTS. VREF SPD AT OUR WT WAS ABOUT 125 KTS. I CALLED FOR FLAPS 45 DEGS; BUG VREF; AND INSTRUCTED THE FO TO KEEP AN EYE ON AIRSPD AND TO INFORM ME IF I WENT BELOW 145 KTS (I WANTED A SAFETY MARGIN UNTIL WE WERE ALIGNED WITH THE RWY). AS THE FLAPS CAME DOWN; I FELT AN INCREASING L WING-DOWN ROLLING MOMENT. I APPLIED R WING-DOWN AILERON TRIM UNTIL THE ACFT BECAME MORE STABLE. I COMMENTED TO THE FO THAT 'THIS IS HARD' REFERRING TO CTLING THE ACFT. AT THIS POINT WE WERE IN PLACE TO BEGIN THE BASE-TO-FINAL TURN. I ATTEMPTED 2 OR 3 TIMES TO MAKE THE TURN. EACH TIME AS L AILERON WAS APPLIED; THE ACFT FELT LIKE IT WOULD SNAP-ROLL L IF ANYTHING MORE THAN THE SMALLEST BANK WAS APPLIED. I LEVELED THE ACFT; CALLED FOR A MISSED APCH; AND PERFORMED A MOSTLY NORMAL MISSED APCH. ONCE THE FLAPS CAME UP (INITIALLY TO 8 DEGS; THEN UP) THE AILERON CTL RETURNED TO NORMAL. INITIALLY; TWR ATC TOLD US TO TURN TO FLY THE RWY HDG AND TO CLB AND MAINTAIN 2000 FT. I WAS UNABLE TO MAKE THE L TURN DUE TO THE CTL ISSUES; AND WAS OCCUPIED WITH MAINTAINING ACFT CTL; SO I WAS UNABLE TO ARREST THE CLB UNTIL WE WERE ALMOST AT 3000 FT. WE NOTIFIED TWR ATC OF THIS; THAT WE WERE ON A HDG OF ABOUT 360 DEGS AND AT ABOUT 3000 FT. WE COMMUNICATED WITH ATC; DECLARING AN EMER. WE EXPLAINED OUR CTL ISSUE AND ASKED FOR A R TFC STRAIGHT-IN APCH. WE WERE GIVEN THIS BY ATC AND PERFORMED A MORE OR LESS NORMAL LNDG. ON THE SECOND APCH; FLAPS 45 DEGS WAS DELAYED UNTIL ABOUT 2 MI OUT WHEN ALIGNED WITH THE RWY. AS THEY CAME DOWN; THE L ROLL TENDENCY AGAIN OCCURRED BUT; AS WE DID NOT NEED ANY LARGE TURNS AND HAD A R XWIND; THE ACFT WAS CTLABLE. ON THE MISSED APCH WE WERE INITIALLY UNABLE TO COMPLY WITH THE ATC ASSIGNED HDG AND ALT DUE TO THE CTL ISSUES. ASAP WE INFORMED ATC OF THIS; DECLARED THE EMER; AND REQUESTED AND RECEIVED AN AMENDED CLRNC. WE WERE ABLE TO COMPLY WITH ALL FURTHER CLRNCS. ALL THE ATC CTLRS WERE EXTREMELY HELPFUL DURING THE EVENT. DURING THE EVENT CRM WORKED VERY WELL. THE FO DID AN EXCELLENT JOB OF HANDLING THE PNF TASKS AND PRIMARY COMS. ALL COMPANY PROCS WERE COMPLIED WITHIN A FAIRLY TIMELY MANNER. WE WERE VERY CAREFUL TO MAKE SURE TO RUN ALL APPROPRIATE CHKLISTS ON THE MISSED APCH AND SECOND APCH AND LNDG. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THAT THE ROLLING TENDENCY ONLY OCCURRED WITH FLAPS 45 SELECTED. NO FLAP ASYMMETRY WAS DETECTED BY THE FLAP SYSTEM OR FROM CREW OBSERVATION. THE CAUSE OF THE MALFUNCTION WAS UNKNOWN; AND THE REPORTER IS NOT AWARE OF ANY SIMILAR OCCURRENCES IN THE FLEET.