The Art of the Helicopter John Watkinson - Karatunov.net

Rotors in practice 159
hull and a very uncomfortable ride would result. Flapping blades act much like the
suspension of an automobile and decouple the effects of gusts and alternating components
of lift in forward flight. Another problem is that a perfectly rigid rotor would
respond extremely quickly to cyclic inputs, possibly too quickly for a human pilot.
As a result the extremely stiff rotor is little used. Instead the goal is to make hingeless
or bearingless rotors where vibration decoupling and stress relief are achieved not by
an identifiable hinge, but by a low maintenance structure which is designed to flex.
The term virtual hinge will be met to describe the existence of an axis about which
flexing effectively takes place. Virtual flapping hinges decouple the non-constant lift
in forward flight and virtual dragging hinges allow the blades to conserve momentum.
The presence of the flexibility reduces bending stress in the blades and reduces the cyclic
response rate of the rotor. It is also possible to incorporate the feathering hinge into
the flexible structure.
The designer can choose an appropriate degree of flexibility depending on the role
of the machine. For some military purposes, such as nap-of-the-earth flying, extreme
manoeuvrability is required along with the requirement to sustain zero or negative g
when cresting a hill at speed. This would suggest a rotor that is relatively stiff in flapping,
which would also be able to start in almost any wind. Conversely a civil machine would
benefit from a softer flapping flexure.
Dragging flexures are also subject to variations. If the dragging virtual hinge is made
very stiff, the rotor can be made supercritical so that dragging dampers are not needed.
However, stiff-in-plane rotors tend to produce more lateral vibration because the blades
transmit the drag changes as they move between the advancing and retreating sides. As
an alternative, the soft-in-plane rotor reduces vibration by allowing the blades to drag,
but requiring drag damping to avoid ground resonance. The use of elastomeric damping
will still allow a low maintenance structure. Clearly the dragging and flapping stiffness
can advantageously be different and some ingenuity is required to provide suitable
geometry.
Figure 4.35(a) shows the hingeless rotor head of the Bo-105. The only real bearings
are for blade feathering. Flapping and dragging movements are accommodated through
flexing of the blade shank. The dragging stiffness is supercritical and no dampers are
needed. This is also a relatively stiff head in flapping, and pendulum vibration dampers
are needed.
Figure 4.35(b) shows another stiff-in-plane head, that of the Lockheed AH-56
Cheyenne. This has door-hinge feathering bearings for low drag and a virtual flapping
hinge. The flapping hinge is very stiff indeed and the following rate of the rotor
is very high, requiring full-time gyro stabilization. This will be considered in detail in
Chapter 7.
Figure 4.35(c) shows a soft-in-plane hingeless rotor from the Westland Lynx. Here
the flapping flexure is a massive piece of titanium that is only flexible in the context of
the enormous forces set up in a rotor head. The flexures are relatively stiff in flapping.
Since the rotor can exert large moments on the hull, the Lynx does not need a tall mast;
in fact it was a design goal to keep the height down to allow the machine to fit into
transport aircraft, and a special low profile gearbox was designed to go with the rotor.
With such a low rotor, the Lynx needs the tail boom to be angled down to give blade
clearance before turning up to mount the tail rotor.
Figure 4.35(d) shows the head of the Bell 412. This has outboard dragging hinges
and the drag damping is obtained by an elastomeric block acting on a inward extension
of the blade grip which also mounts the pitch arms. The dragging hinges are mounted
on flex beams that can twist to act as feathering bearings and bend to allow flapping.
Two such flex beams are stacked to make a four-blade rotor.