Forward Speed Disturbance
An increase in forward speed leads to incremental flapping, resulting in a change in nose-up disc tilt. The amount of change is reckoned to be about 1° per 10m/s speed increase, independently of the flight speed. The thrust vector is effectively inclined rearwards, supported by the nose-up pitching moment produced, providing a retarding force component and therefore static stability with respect to forward speed. This characteristic is present in the hover but nevertheless contributes to a dynamic instability there (see Section 8.4.2).
An increase in speed causes the airframe drag to rise and this contributes, more effectively with initial forward speed, to a positive speed-stability characteristic for the helicopter, except in the hover.
8.3.2 Angular Velocity (Pitch or Roll Rate) Disturbance
The effect of a disturbance in angular velocity (pitch or roll) is complex. In brief, a gyroscopic moment about the flapping hinge produces a phased flapping response and the disc tilt resulting from this generates a moment opposing the particular angular motion. Thus the rotor exhibits damping in both pitch and roll. Moments arising from non-uniform incidence over the disc lead to cross-coupling, that is rolling moment due to rate of pitch and vice versa.