The Art of the Helicopter John Watkinson - Karatunov.net

128 The Art of the Helicopter
OP is an arbitrary hinge axis and could be a flapping or a lagging hinge. In the literature,
the flapping hinge is called the δ hinge and the dragging hinge is called the α hinge.
These hinges can be turned away from the mutually orthogonal in three axes, 1, 2 and
3. Only two of these axes are shown in Figure 4.9(a); axis 2 is redundant. If OP is a
flapping hinge and is projected into the plane of the rotor, OXY, the angle between the
projection and OY is called δ3 (delta-three). If OP is projected onto the plane OZY, the
angle between the projection and OY is δ1 (delta-one). In an orthogonal head these
angles are both zero.
Figure 4.9(b) shows that to avoid coupling between flapping and feathering, the
spherical bearing between the pitch control rod and the pitch control arm must be on
the axis of the flapping hinge which must also be at 90 ◦ to the feathering axis.
The pitch control arm is generally on the leading edge of the blade. If the pitch control
arm is made slightly shorter as in Figure 4.9(c), there will be an interaction between
flapping and feathering. Figure 4.9(d) reveals that upward flapping will reduce the pitch
of the blade, tending to lower the blade, whereas downward flapping would have the
reverse effect. Effectively a positive δ3 hinge has been created. This is a stable condition,
whereas the reverse relationship between the control rod joint and the flapping axis to
give a negative δ3 hinge would be unstable.
In some autogyros the dragging hinge was inclined to produce a δ1 hinge. This had the
effect of coupling the dragging and the blade pitch to allow jump take-off. The blades
would stay in fine pitch whilst being driven, but upon the drive being disconnected the
blades would swing forward, increasing the pitch.
In some toy free-flying helicopters a reverse δ1 hinge is used which sets the machine
automatically into autorotation when the fuel runs out.
4.8 Types of rotor head
For many years the designer was faced with a choice between a teetering two-bladed
head and an articulated multi-bladed head. These techniques were developed because
they greatly reduce bending loads on the blades. The teetering head imposes the least
stress on the mast, but has some drawbacks as will be seen in section 4.10. In the
early years of helicopter design, available blade materials and designs precluded the
use of other types of rotor head and the drawbacks had to be accepted. Although
it is less demanding on the blades and the feathering bearings, the articulated rotor
head contains a mass of bearings subject to oscillating motion that causes wear. These
bearings require frequent replacement. Large machines may require continuous oil feed
to the bearings, whereas smaller machines require the periodic application of a grease
gun. The hingeless rotor head is a desirable goal if only because it reduces the amount
of maintenance required.
The effect of blade flapping in articulated and teetering heads is also used to prevent
excessively rapid response to cyclic control inputs. This will be made clear in
section 4.11.
The stress due to coning can be relieved by fitting the blades at a preset coning angle.
In practice, the disc and shaft axes will be slightly different since the hull blowback will
never be perfectly balanced by the tail plane, particularly if the CM is displaced. As
the hinges are only resolving a small geometric problem, it is clear that a rotor strong
enough or flexible enough to accommodate the geometric conflict can dispense with
actual flapping and dragging hinges. The result is a hingeless rotor. The term ‘rigid
rotor’ is sometimes used to describe such a system, but clearly it is a misnomer and the
term hingeless is to be preferred.