GYROSCOPIC ROLLING MOMENTS
"Gyroscopic precession is a phenomenon occurring in rotating bodies in which an applied force is manifested 90 degrees later in the direction of rotation from where the force was applied. Although precession is not a dominant force in rotary-wing aerodynamics, it must be reckoned with because turning rotor systems exhibit some of the characteristics of a gyro. The diagram to the right shows how precession affects a rotor disk when force is applied at a given point.
A downward force applied to the disk at point "A" results in a downward change in disk attitude at point "B". And upward force applied at Point "C" results in an upward change in disk attitude at point "D".
This behavior explains some of the fundamental effects occurring during various helicopter maneuvers. For example, the helicopter behaves differently when rolling into a right turn than when rolling into a left turn. During roll into a left turn, the pilot will have to correct for a nose down tendency in order to maintain altitude. This correction is required because precession causes a nose down tendency and because the tilted disk produces less vertical lift to counteract gravity. Conversely, during a roll into a right turn, precession will cause a nose up tendency while the tilted disk will produce less vertical lift. Pilot input required to maintain altitude is significantly different during a right turn than during a left turn, because gyroscopic precession acts in opposite directions for each."
Source: Mr. Paul Cantrell, veteran helicopter flight instructor; http://www.copters.com