A CE560 pilot reported the rudder became immovable inflight after water in the tail section froze at FL300 in the climb. The aircraft diverted to an enroute airport where ice was discovered inside the tail section near the tail stand placard.

Narrative

Aircraft remained overnight outside in raining conditions. Aircraft was ferried to ZZZ1 flying through rain most of the way. After departing during rain; during climb out through approximately 30;000 FT I noticed that the airplane was not flying quite right on autopilot; (autopilot was having some difficulty holding a heading) and that the yaw damper was having trouble maintaining coordinated flight. I therefore disconnected the autopilot and discovered that the rudder was frozen. No movement of rudder was possible in either direction. No other control problems were noted. PIC called company while I continued to monitor and fly aircraft. The decision was made to divert the aircraft to an enroute airport. Due to the likelihood of ice buildup in tail as a known problem; it was agreed that this was probably the issue; and since the divert airport was above freezing; we continued toward it with the understanding that we would see if the rudder would unfreeze. During descent I hand flew the aircraft during descent to monitor the controls and at approximately 13;000 FT and +10C we both felt the rudder release suddenly with immediate and full control of the surface. At that point we both agreed that there was no longer a control issue and a normal landing and taxi in with caution was made to nearby airport. Upon landing company immediately wanted a mechanic to investigate aircraft while there may still be evidence on board of freezing. A mechanic was found; and he removed the access panel just aft of the tail stand placard on the bottom of the aircraft. I would estimate that there was possibly a half gallon of water ran out of the access panel and a piece of ice about the size of the palm of your hand remained under the panel which was melting quickly. Pictures were taken of ice and sent to maintenance control. It was approximately 60 degrees F at the airport after landing and the panel was pulled off about 30 minutes after landing. Using a flashlight I could see where water had been running down the bottom of the aircraft from the tail stinger area; and it seems likely to me that this area of the aircraft was not draining and therefore holding water; which froze.