The Art of the Helicopter John Watkinson - Karatunov.net

118 The Art of the Helicopter
4.2 Why articulated rotors are used
The variation in lift, lift distribution and drag as the blades turn, especially in forward
flight, produces alternating stresses which can fatigue materials. The adoption
of articulation was first suggested by Renard in 1904 and was essential in early rotorcraft
to reduce the stresses involved, particularly the alternating stresses in the blades
and the moments applied to the mast when rolling and pitching manoeuvres are performed.
Some texts claim that articulation is necessary to handle the lift asymmetry in
translational flight, but this is quite incorrect.
In addition to the feathering bearing, the fully articulated rotor head carries each
blade on freely turning bearings which allow flapping, or movement above or below
the plane of the rotor head, and dragging, a swinging movement in the plane of the
blades which is also called lead/lag. The presence of the flapping and dragging bearings
means that moments about their axes cannot be transferred from the rotor head to the
blades, and so bending stresses in the blade roots are dramatically reduced.
Figure 4.2(a) shows one arrangement which has been widely used by, for example,
Sikorsky and Enstrom. The feathering bearing is outboard of the flapping bearing.
This arrangement has the advantage that the loads fed into the control system when
the blades flap and drag are minimized. The flapping and dragging hinges can be
displaced or coincident in various designs and this is considered in section 4.7.
If the axes of the flapping hinges pass through the shaft axis, the result is called a
zero-offset rotor head and these will be considered in section 4.10. In a conventional
articulated head, the flapping bearings are horizontally displaced, or offset, typically
by a few per cent of the rotor diameter, and it will be seen from Figure 4.2(b) that
blade tension can produce a control moment on the rotor head if the shaft is not at
right angles to the tip path plane. Consequently the hull tends to follow the disc better
when offset is employed and so the machine becomes more manoeuvrable, although a
stronger mast is needed to withstand the moments.
4.3 Axes galore
Whether by flexing or by movement of a bearing, each blade can flap and feather as it
rotates. A consequence of these degrees of freedom is that three rotational axes need
to be considered when studying the behaviour of a turning rotor. The way in which
these axes interrelate is fascinating and allows an insight into the behaviour of a blade
in flight.
Figure 4.3(a) shows a helicopter with an articulated rotor in forward flight. The cyclic
stick will need to be trimmed forward to maintain airspeed, since this will reduce the
angle of attack of the advancing blade and increase that of the retreating blade, so that
they generate equal lift moments. The stick will also need to be trimmed towards the
retreating side to oppose the inflow roll. The tip path plane is tilted forward to obtain
a forward component of rotor thrust to balance drag.
4.3.1 The shaft axis
The most obvious, and least useful, of these axes is the shaft axis on which the rotor
head turns. An observer riding on the shaft axis could see flapping, dragging and
feathering taking place simultaneously, but some of these motions could disappear at
certain relationships between the shaft axis and the other two axes.