Even unstable Dutch roll can be suppressed by a pilot who has good visual references. He acts as a damper using instinctive pedal and lateral stick inputs. I know of two unstable designs in which the pilot was not aware of the instability until asked to hold the controls fixed and refrain from making corrections. However, in the case of the EC-135 prototype, the pilot was apparently aware of the problem and bothered by the increased workload. There were a few options available for solving the problem, but some of them impacted other characteristics, such as gust response and precise low-speed maneuvering.
Several parameters affect the Dutch roll characteristics.
The effect is similar to what happens on fixed-wing aircraft that have substantial dihedral, or upward slant, on the wings and tail. In the case of a helicopter, most of the dihedral effect comes from the flapping of the rotor, as explained in Chapter 30 of my book, Even More Helicopter Aerodynamics. However, some of the effect comes from the airframe components. Any vertical surface located above the center of gravity increases the dihedral effect, while any low surface reduces it. Yaw damping Yaw damping is effective in stabilizing the Dutch roll mode.