The Art of the Helicopter John Watkinson - Karatunov.net

Helicopter performance 345
moment from the main rotor. The helicopter may enter a spiral dive if disturbed in roll.
Spiral instability may be remedied by lowering the CM, reducing fin area or by moving
it upwards or forwards or by adding side area to the hull high up ahead of the mast.
Upward relocation of the tail rotor and/or increasing the main rotor flapping offset
will also be beneficial.
On the other hand if the fin and tail rotor are high mounted, they may hardly oppose
the dihedral effect of the main rotor at all and the helicopter may have too much lateral
stability such that if disturbed it suffers a phenomenon called a dutch roll.
Dutch roll describes a flight path like a corkscrew. Following a slip, excessive lateral
stability creates a restoring roll couple so powerful that there is insufficient momentum
for the helicopter to overshoot the correct attitude. The helicopter then oscillates about
its path near-sinusoidally in pitch, roll and yaw but with phase differences between the
motions. The motion may be damped heavily, so that it dies out or it may stabilize with
certain amplitude. In some cases the amplitude will grow indefinitely. In most cases
pilot intervention prevents such a divergence. However, for IFR operation, Dutch roll
is considered as detrimental as spiral instability. Dutch roll may be remedied by raising
the CM, by lowering the tail rotor, by reducing the main rotor flapping offset, by
increasing fin area, by moving the fin back and/or down or by adding side area to the
hull low down. Fences may be seen on the bottom of the hull for this purpose.
Hover stability is marginal in most helicopters because the hull mass is located
below the rotor. It might be thought that this would act like a pendulum and give
stability but this is not what happens. The hull has mass and is more or less flexibly
suspended from the rotor according to the type of rotor head. As a result the attitude
of the hull will always lag the attitude of the rotor. Figure 8.16 shows that if a helicopter
is trimmed to stationary hover and then the controls are locked, it will begin
a divergent oscillation. The rotor on its own is stable in the hover because if it moves
in any horizontal direction, flapback will tend to arrest the motion. However, it can be
seen from the figure that the hull swings from side to side under the rotor, but because
the swashplate is referenced to the hull, that swinging causes cyclic inputs. These are
not phased to damp the motion, but in fact augment the motion.
Fig. 8.16 Helicopters are unstable in hover and, without pilot intervention, will wander to and fro as
shown here.