Shaft Tilted in Air in Hover

If the shaft is tilted while the rotor is hovering in air, aerodynamic forces will be generated that will force the tip path plane to align itself perpendicular to the shaft whether the rotor has hinge offset or not. The sequence of steps leading to this is shown in Figure 7.2.

No Hinge Offset

With Hinge Offset

FIGURE 7.1 Effects of Shaft Tilt in Vacuum

Right Side

First, there is the tilt of the shaft alone as the rotor disc acts as a gyroscope and remains in its original plane. Since the blade feathering is referenced to the shaft, however, the angle of attack of the right-hand blade is increased and that of the left-hand blade decreased by the same amount. This causes the rotor to flap until it is perpendicular to the shaft, where it will again be in equilibrium with a constant angle of attack around the azimuth and the moments will be balanced. This alignment is very rapid, usually taking less than one rotor revolution following a sudden tilt. Because of this, the flapping motion in hover has practically no effect on the stability of the helicopter in terms of holding a given attitude.