Lu; You're going to love this.
Direct quote from 'Helicopter Flight Dynamics: The Theory and Application of Flying Qualities and Simulation Modeling', 1999, by Gareth D. Padfield, page 417.
"Pitch-roll and roll-pitch cross-couplings can be powerful and insidious. The natural source of both are the gyroscopic and aerodynamic moments developed by the main rotor and, in dynamic maneuvers with large amplitude excursions, the uncommanded and sometimes unpredictable off-axis motion can require continuous attention by the pilot."
Have a good day.