In a sideslip disturbance, the rotor ‘sees’ a wind unchanged in velocity but coming from a different direction. As a result the direction of maximum flapping is rotated through the angle of sideslip change and this causes a sideways tilt of the rotor away from the wind. There is therefore a rolling moment opposing the sideslip, corresponding effectively to the dihedral action of a fixed-wing aircraft. In addition the sideslip produces a change in incidence of the tail rotor blades, so that the tail rotor acts like a vertical fin providing ‘weathercock’ stability.
8.3.3 Yawing Disturbance
A disturbance in yaw causes a change of incidence at the tail rotor and so again produces a fin damping effect, additional to that of the actual aircraft fin. Overall, however, basic directional stability tends to be poor because of degradation by upstream flow separations and wake effects.
8.3.4 General Conclusion
It is seen from the above descriptions that longitudinal static stability characteristics are significantly different from, and more complex than, those of a fixed-wing aircraft, while lateral characteristics of the two types of aircraft are similar, although the forces and moments arise in different ways.