The Art of the Helicopter John Watkinson - Karatunov.net

110 The Art of the Helicopter
Although vibration may occur at a number of frequencies, often one of these will
dominate and cause a problem. Vibration absorbers are devices intended to reduce
vibration at a single specific frequency. The term is actually a misnomer because what
they actually do is to create an opposing or near-anti-phase vibration. These devices
work using mechanical resonance.
As was seen in section 2.9, a mass supported on a spring displays resonance. If the
damping is very low, the phase response just above the resonant frequency becomes
nearly 180 ◦ . If the resonant frequency is set just below the frequency of vibration,
the mass will automatically resonate in inverse phase and oppose the vibration. In
helicopters the battery may be used as a suitable mass and this is supported on carefully
calibrated springs. A mass absorber of this kind can only reduce vibration in the vicinity
of the device itself. Flexibility of the hull will allow vibration elsewhere; however, such
devices can oppose the hull vibration due to non-uniform downwash. Mass absorbers
only work at a fixed frequency and require the rotor to turn at a precisely controlled
RPM. The alternative is to use a system in which the spring stiffness can be modified
by a servo so that the absorber can be tuned to the actual RRPM in use. Such a system
is fitted in the Chinook. In some early helicopters vibrations fed into the control rods
were opposed in this way. Figure 3.40(a) shows a resonator consisting of a bob weight
on a leaf spring attached to a control bell crank.
A pendulum is a kind of resonator. The type used in a clock uses the earth’s gravitational
field to provide the restoring force. In helicopters, pendular vibration absorbers
fitted on rotors use the acceleration caused by rotation. Figure 3.40(b) shows that pendular
absorbers used to handle flapping vibration consist of weights that can swing on
the end of cranked shafts pivoted in the blade roots. When the rotor is turning, the
weights fly out horizontally and attempt to stay in the same plane. Vertical movement of
the blade excites the pendulum which is tuned to resonate in anti-phase. These devices
can be seen on the Kamov Ka-32 and on the Bo-105 and are known colloquially as
bull’s balls.
As these devices employ resonance, a single unit can only oppose vibration at a single
frequency. However, it is possible to fit more than one pendulum to the same mounting.
The Hughes 500 has a four-bladed rotor and so according to Figure 3.38(b) will be
prone to 4P rocking forces due to flapping at 3P and 5P. The two pendula are tuned to
the third and fifth harmonics of blade flapping.
Torsional vibration of the rotor shaft may be due to blade excitation in the dragging
plane. This is an application for the bifilar pendulum. A pendulum can be made to swing
in one axis only by attaching it with two strings, hence the name. Restoration forces are
much smaller for in-plane motion, suggesting that a pendulum would need a very short
arm in order to resonate at a harmonic of rotor frequency. Thus in practice the arms are
replaced by a pair of pins which operate in oversized holes as in Figure 3.40(c). As the
weight moves to one side, the geometry of the pins moves the weight inwards. As
the weight tends to fly out as the rotor turns, there will be a restoring force if the weight
is displaced. This is all that is necessary to create a resonant system.
The restoring force that determines the resonant frequency in pendulum and bifilar
absorbers comes from the acceleration due to rotation. This is a function of RRPM,
as is the frequency to be absorbed. Consequently these devices are self-tuning.
Another popular and obvious way of isolating vibration is to use flexible mountings
between the transmission and the hull. It is a fundamental concept of such mounts
that there will be a resonance whose frequency is determined by the sprung mass and
the stiffness of the mounts. The degree of isolation is proportional to the difference
between that resonant frequency and the frequency to be filtered out. Consequently
the lowest possible resonant frequency is required. This frequency may be lowered by