The Art of the Helicopter John Watkinson - Karatunov.net

134 The Art of the Helicopter
Fig. 4.14 In a hingeless rotor, the same couple could be produced by a rotor with flapping hinges having an
equivalent offset.
to describe the rotor by its effective or equivalent offset. Figure 4.14 shows that this is
the radius of a virtual flapping hinge that would produce the same couple for the same
disc tilt.
In the hingeless rotor head the tip path plane will attempt to tilt as the result of a
cyclic control application, but the roll couple applied to the rotor head will depend
on the flapping stiffness. Hingeless rotor machines with stiff flapping flexures can be
highly manoeuvrable, as the Westland Lynx regularly demonstrates. In fact a hingeless
rotor helicopter can be rigged to hover upside down as many model helicopter pilots
have demonstrated.
Whilst the stiff hingeless rotor head leads to an agile machine, it will impose heavy
bending loads on the blades and possibly also on the feathering bearings, which must
be designed accordingly. Unless carefully designed, such rotors can also respond so
rapidly to cyclic inputs that control is difficult. A permanently operating gyrostabilizer
may be needed.
In hingeless rotors with stiff flapping flexures, the thrust vector offset with respect
to the CM is much less important, and the mast does not need to be so tall, although
machines with very short masts have suffered from powerful canopy vibration due to
the non-uniform rotor downwash. Stiff rotors can withstand a wider range of CM
travel because a rotor head couple can be used to keep the hull in the correct attitude,
being limited by stresses in the mast and its bearings.
The stiff hingeless rotor is the goal for the ultimate in manoeuvrability, at the cost of a
rougher ride and the need for a stronger mast and airframe. A more compliant hingeless
rotor head allows a more cost effective solution and a better ride. The bearings of the
articulated head are replaced by elements that can flex and thus need no lubrication
or maintenance. It is not inconceivable that flexural heads will be developed which
will last the life of the machine. Given the high maintenance costs of helicopters, any
development that tangibly reduces wear and maintenance is highly significant.
4.12 Feathering
Despite the enormous pull of the blades, it must be possible to rotate them on their
feathering axes in order to control the pitch angle. There are several approaches to the
problem of the feathering bearing and these are contrasted in Figure 4.15.
Raoul Hafner first used a torsion bar shown at (a) to eliminate the axial thrust in the
feathering bearing so that the remaining bearing became essentially a locating sleeve. It
is a characteristic of bending and torsion that the greatest stress is caused at the surface