Citation Excel (C560XL)
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Critical
Flight Crew Diverted; Flight Crew Landed in Emergency Condition; Flight Crew Overcame Equipment Problem; Flight Crew Regained Aircraft Control
Aircraft
During the climb we briefly received a Master Caution and the annunciation; AP (Autopilot) PITCH MISTRIM.  Several seconds later the airplane abruptly pitched down about 5 degrees.  The pilot monitoring (PIC) then retrieved the emergency checklist and we began running the procedure.  Before disengaging the autopilot; the airplane again abruptly pitched down about 3-5 degrees.  After the autopilot was disengaged; I hand flew the airplane briefly and noticed the pitch portion of the flight controls were partially stuck.  I then asked the Captain to see if he could feel the same thing.  We exchanged controls; and he confirmed that the controls were sticky about the pitch axis.  We then exchanged control of the airplane back; and I resumed flying the airplane.  The Captain then requested immediate vector to the nearest airport.  ATC then gave us a vector to ZZZ and a descent altitude.  We then engaged the yaw-damp; but left the autopilot off.  During the descent; we had at least four more events where the flight controls appeared to be stuck.  Each time I was able to over-power the control.  I also noticed that once I freed the controls; they appeared to be freed for 20-30 seconds.  During the descent; I was monitoring the outside temperature to detect when we got to warmer air; as I was concerned that all the rain that we had at ZZZ may have frozen and caused the controls to momentarily freeze.  After we passed the freezing level; I did not notice any more problems with the pitch control.  After landing at ZZZ; we de-planed the passengers at [the] FBO.  At that time; we observed water draining out of the bottom of the airplane.  We estimated that about a quart of water drained out of the airplane.  We then re-positioned the airplane to ZZZ1.  After parking; we again observed water coming out of the airplane.
Citation 560XL First Officer reported flight control anomalies in flight.