The Art of the Helicopter John Watkinson - Karatunov.net

342 The Art of the Helicopter
involve the addition of passive structures or changes to the shape of the existing
structure.
Stability in helicopters is a complex issue in comparison to aeroplanes because helicopters
are more asymmetrical, have more coupling between their degrees of freedom
and greater structural elasticity owing to the use of hinges and/or flexible blades. The
inherent flexibility between the rotor and the hull is responsible for many peculiarities
of helicopter behaviour. In many cases the rotor tries to do one thing and the hull tries
to do the opposite and it is unclear which one will dominate. It also follows from this
that the stability characteristics of helicopters having zero-offset rotor heads will differ
from those having real or effective offset.
In all aircraft, stability can be subdivided into specific areas. Speed stability is the
ability of an aircraft to return to trimmed airspeed after a disturbance. Pitch stability
is the ability to return to the correct pitch attitude after a disturbance. Directional
stability is the ability to keep the nose at the front and lateral stability is the ability to
return the wings/rotor to a level attitude after a disturbance. If all of these are acceptable
the machine may still have a problem due to the interaction of the lateral and
directional stability mechanisms. This is known as spiral stability. Helicopters may
further be assessed by their stability in the hover.
Speed stability of a main rotor is extremely good owing to the phenomenon of flapback
described in Chapter 3. If forward speed increases for any reason, the asymmetry
of lift between the advancing and retreating blades results in a rolling couple which
the main rotor will precess into rearward pitch. This tilts back the rotor thrust vector
tending to reduce the airspeed. Thus the rotor itself has good speed stability. However,
the hull is suspended below the rotor and the hull drag effectively acts at some
distance below the rotor head, causing a downward pitching moment on the hull. Thus
an increase in airspeed causes the hull to pitch down. Depending on the type of rotor
head, this downward pitching moment will result in different hull attitude changes.
As the swashplate is controlled from the hull, hull pitchdown will cause the rotor to
pitch down.
Thus the helicopter itself may not display speed stability if the effect of the hull
dominates the effect of the rotor. In practice the helicopter may need a tail plane to
provide speed stability. If set at a suitable negative angle of incidence, the tail plane
produces a downthrust that increases with airspeed and compensates for the hull drag.
In practice the rotor on its own has excess speed stability and large amounts of
forward cyclic would be needed to increase speed. Thus some of the pitchdown due to
hull drag can be used beneficially to reduce the cyclic travel needed. The tail plane area
and incidence can be selected in such a way that the speed stability is just on the positive
side of neutral. This means that airspeed increases will always result in the cyclic stick
being trimmed forwards.
If the helicopter is disturbed on its pitch axis in forward flight, the RAF seen by the
blades will change, but the advancing and retreating blades will see different changes. If
a nose-up pitch disturbance is experienced, Figure 8.13 shows that the angle of attack
of both blades will increase, but the increase seen by the advancing blade is greater.
This produces a rolling couple to the retreating side that is precessed into a further
nose-up pitching moment by the gyroscopic action of the rotor. Thus it will be seen
that a rotor in forward flight is unstable in pitch. This is a further reason for the use of
a tail plane in the helicopter. Dynamic stability here can be good because the tail plane
has aerodynamic damping.
With a zero-offset rotor head, any tilt of the rotor disc in pitch is opposed because
no couples are passed across the head and the hull and the swashplate attitude are little
changed. If the rotor disc pitches but the swashplate doesn’t there will be an application