The Art of the Helicopter John Watkinson - Karatunov.net

Introduction to helicopter dynamics 69
upwards, there is no resultant and the machine stays still. In the still-air hover, the
inflow through the rotor is axial, and if, for simplicity, it is assumed that the centre of
gravity of the machine is at the mast, the forces on the blades will not vary as they turn.
The collective control can be used to make the rotor thrust greater than or less than
the weight of the machine. The force imbalance causes the machine to accelerate up or
down. However, this acceleration does not continue indefinitely. Figure 3.7(a) shows
that as the machine rises, the angle of relative airflow experienced by the blades is
Fig. 3.6 (a) With the collective lever lowered, the rotor thrust is less than the weight and the machine remains
on the ground. (b) With an appropriate setting of the collective lever, the rotor thrust can be made exactly equal
to the weight.
Fig. 3.7 (a) With increased collective, the machine rises, but in doing so changes the angle at which the air
approaches the blades and reduces the angle of attack. (b) With reduced collective, the opposite happens. Thus
the collective lever primarily controls vertical velocity.