The Art of the Helicopter John Watkinson - Karatunov.net

Rotors in practice 153
and lag dampers are in any case present to prevent ground resonance. Furthermore the
motion of the hull is decoupled to some extent by the articulation. In a hingeless rotor
the blades lag by flexing and the stiffness is higher, leading to a higher natural frequency
of lagging motion. The relatively stiff connection between the rotor and the hull means
that large rotor moments can excite hull flexing. If such hull flexing is resonant then an
unstable system could exist. A bending mode of the hull could result in a lateral motion
at the rotor head that is similar to the rocking experienced in ground resonance.
It should be appreciated that resonance may also be excited by the tail rotor. This
will be discussed in Chapter 5.
4.19 Dynamic rollover
Dynamic rollover is a hazard that can only occur when a helicopter is in contact with
the ground in some way. Normally the cyclic control has complete authority over the
attitude of the helicopter, but interference from the ground or from external loads can
reduce or even overcome the cyclic authority. Section 4.15 introduced the tilting hub
mechanism for cyclic control. If the hull of a helicopter is forcibly tilted, the rotor
will follow. Dynamic rollover occurs when external tilting forces overpower the cyclic
control. Teetering or zero-offset rotors are more prone than offset or hingeless rotors
because their cyclic authority is small to begin with.
Dynamic rollover becomes a possibility when the conditions on one side of the hull
are different from those on the other side. This may be due to an attempt to land or
take-off from a slope, an underslung load snagging a skid or a skid being stuck in mud
or the surface tension of wet sand. Most winches for personnel recovery are fitted at
one side of the machine and if a winch rope snags on the ground it can cause difficulty.
A side wind and the thrust of the tail rotor complicate the matter.
Figure 4.33(a) shows what can happen with a stuck skid. The pilot raises the collective
lever to lift off, but the stuck skid combines with the rotor thrust to produce a roll couple
that could exceed the power of the cyclic control. In many cases the machine will be on
its side before the pilot can react and the cyclic authority becomes academic. The roll
accelerates because as the hull tips it increases the cyclic input to the rotor and causes
further roll. A variation of this hazard is where the stuck skid suddenly comes free and
the machine ends up on its other side because the pilot does not remove the lateral
cyclic quickly enough.
Figure 4.33(b) shows a helicopter about to lift a slung load. Unknown to the pilot the
load strap has snagged a skid. As the machine rises, once more the rotor thrust and
the load strap tension create a couple on the hull and the dynamic rollover takes place.
Most helicopter winches are installed with a view to bringing loads alongside one of
the hull doors. The load is applied with a considerable moment arm from the machine
CM. Moments due to normal loads can be resisted by the rotor, but in the event that
the winch line became snagged an attempt to climb would result in dynamic rollover.
This is one of the reasons why the pilot is provided with means to jettison loads in an
emergency. Underslung loads can be jettisoned by releasing the load hook that may
be electrically operated. Winches are fitted with cable cutters that may be explosively
actuated.
Figure 4.33(c) shows an attempted landing on a slope. After the first skid has touched,
the machine rotates about that skid. As it comes down, the control axis must be kept
vertical by the application of cyclic control. The question is, what happens when the
cyclic reaches the end of its travel and the other skid still is not in contact? The rotor
disc cannot be kept horizontal and there is a danger of dynamic rollover. If the landing