The Art of the Helicopter John Watkinson - Karatunov.net

Rotors in practice 129
Pitch change or feathering is, of course, still necessary, and this may be achieved by an
actual bearing or by a further degree of flexural freedom. The feathering bearing must
be able to withstand the flapping and dragging moments in addition to the axial pull.
Although the elimination of hinges reduces maintenance, it is not necessarily a
goal to reduce flapping. An articulated rotor can flap to decouple rapid variations
in thrust from the hull. There is thus a good argument for a rotor head that combines
the mechanical simplicity of the hingeless head with the ride quality of the
articulated head. As will be seen in section 4.11, the designer can make different compromises
between ride quality and manoeuvrability by changing the stiffness of the
flexures.
4.9 Zero-offset heads
The main characteristic of the zero-offset head is that the blades cannot transfer
moments to the mast, and the helicopter hull hangs from the rotor like a pendulum.
The mast and gearbox do not have to be as strong and so can be lighter, an important
consideration in early machines where power was limited. The Sikorsky R-4 had a
three-blade head with zero-offset as shown in Figure 4.10.
Figure 2.9 showed the result in the hover if the centre of gravity of the hull does not
coincide with the mast. The tip path plane remains horizontal, as the lift vector must
be vertical, but the articulation must act like a universal joint so that the mast can tilt
with respect to the tip path plane until the centre of gravity is below the centre of the
rotor head.
The zero offset rotor head needs to be mounted on top of a relatively tall mast to
obtain a reasonable range of CM positions without undue changes in hull attitude. It
may be necessary to shift balance weights if the number of passengers is changed, and
the fuel tanks will have to be located on the CM.
Figure 4.11(a) shows that the teetering rotor is a special case of a two-bladed zerooffset
articulated rotor where the two blades are rigidly attached to one another (except
for the feathering bearings) and share a common articulation bearing. Both Bell and
Hiller have produced a number of machines using teetering rotors. Bending stresses due
to coning are reduced in the teetering rotor by attaching the blades at the typical coning
angle. As the coning of the blades raises the centre of mass of the rotor, the teetering
Fig. 4.10 A zero-offset head. The axes of the flapping hinges are coincident with the shaft axis.