The Art of the Helicopter John Watkinson - Karatunov.net

Fig. 8.13 The origin of pitchup instability. See text.
Helicopter performance 343
of cyclic pitch opposing the pitching. In the case of a hingeless rotor head, couples can
be passed across the rotor head and these can cause the hull to follow the rotor. Thus
a helicopter with a hingeless rotor head will tend to be less stable in pitch. We expect
that because such rotor heads offer greater manoeuvrability.
Helicopters with a conventional tail rotor have good directional stability. As was seen
in Chapter 5, the tail rotor acts like a fin and has good yaw damping.
In aeroplanes, lateral stability is obtained by the use of dihedral or sweepback. If
the aeroplane is rolled by a disturbance, it will slip in the direction of the roll and
the relative airflow will have a sideways component. In the case of dihedral, the wing
on the low side will see an increased angle of attack whereas the other wing will see
a reduced angle of attack resulting in a righting couple. In the case of sweepback, the
aspect ratio of the wing on the low side will improve and that of the other wing will
worsen, achieving the same result.
The mechanism in the helicopter is quite different. To find out what happens it is
necessary to consider all moments acting about the roll axis which passes through the
CM. In fact moments may be analysed about any axis, including one passing through
the rotor head as Figure 8.14 shows. Essentially if a helicopter is disturbed in its roll
axis it will slip and the main rotor will see a lateral component of airspeed. This results
in sideflap that tends to oppose the roll and return the helicopter to a level attitude.
Thus the rotor alone acts as if it had dihedral. However, the hull is suspended below
the rotor and the displaced CM acts like a pendulum to create a righting moment. In
addition, the lateral component of airspeed will cause a side thrust that tilts the hull
in the same direction as the roll. Figure 8.14 shows that although the rotor is laterally
stable, the hull is aerodynamically unstable. The outcome depends upon the vertical
position of the CM, the aerodynamics of the hull and on the type of rotor head, but
in most cases the stability of the rotor dominates and helicopters tend to be highly
stable, possibly too stable, in roll. Dynamic stability is good because the rotor attitude
has aerodynamic damping. The zero-offset rotor head is the least stable because there