The Art of the Helicopter John Watkinson - Karatunov.net

130 The Art of the Helicopter
(a)
(b)
Fig. 4.11 (a) A teetering rotor has a single flapping hinge. The blades are rigidly connected together in the
flapping and dragging axes and may be mounted at a typical coning angle to reduce stress. (b) In an underslung
teetering head, the flapping hinge is elevated so that it approximately coincides with the CM of the rotor.
bearing is then raised above the feathering axes at the hub so that it is essentially at the
rotor CM. There will then be less vibration when the tip path axis is not parallel with
the shaft axis. The result is called an underslung rotor shown in Figure 4.11(b). As the
rotor head itself tilts to stay in the plane of the rotor disc, the effect of Figure 4.5 cannot
occur and so dragging hinges are not needed to overcome it. Forces due to varying drag
in translational flight are resisted by blade stiffness.
The rigid connection between the blades means that their dragging frequency will be
very high. It will be seen in section 4.17 that this means no drag dampers are required.
The undercarriage needs no damping to prevent ground resonance and can be a simple
tubular skid arrangement. This further reduces the weight of the machine.
Two-bladed rotors have a disadvantage that both of the blades simultaneously enter
a lift trough in forward flight at 90 ◦ and 270 ◦ of rotation. This causes a twice-perrevolution
hop (two-per) at the hull. Later teetering rotors incorporated a degree of
flapping flexibility in the rotor head to decouple the hop from the hull. This is shown
in section 4.20.
At speed the hull of a zero-offset machine tends to blow back because the drag D
is applied a moment arm c below the suspension point as shown in Figure 4.12. At
speed, the tail plane will have to apply a download T at moment arm b to counter the
blowback. As a result the shaft and tip path axes nearly coincide. In a machine with an
articulated or flexural head the hinge offset will generate a restoring couple if the hull
is blown back so a less powerful tail plane is sufficient.
4.10 Dangers of zero-offset heads – negative g
The main advantage of the zero-offset rotor head is that no moments can be transferred
to the mast from the blades and so the mast can be very light. The absence of dragging