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material, and operates efficiently over the small variation of rotor speed about
the governed datum typical of the modern rotor speed control system. The rate
of force-cancelling mass to total installed mass is high, it responds to the total
bΩ frequency forcing, and requires no maintenance. Applications include the
Westland Lynx and W.30 helicopters.
Figure 8.21 indicates the effect of the Flexispring absorber on the cockpit
vibration levels of the Westland 30 as a function of forward speed.
It should be noted that, although both the above types of absorber can only
generate oscillatory forces in the plane of the rotor system, they will respond
beneficially to any forced response mode which involves in-plane motion of
the rotor head, resulting from the total applied rotor forcing system.
(b) A centrifugal pendulum type of absorber mounted on the rotor blade8. This
type of absorber has been used on the Bolkow Bo 105 and Hughes 500
helicopters. Figure 8.22 shows the Hughes installation which consists of
Lower
cover
Tuning
weights
Outer
ring
Rotor hub
Springs
Top cover
Spindle
308 Bramwell’s Helicopter Dynamics
Without absorber
With absorber
2
1
40 80 120
2
1
40 80 120
Forward speed, knots Forward speed, knots
Velocity, ins/sec
Velocity, ins/sec
Co-pilot’s seat, floor
vertical
Pilot’s seat, floor
vertical
Fig. 8.21 Effect of the vibration absorber on Westland 30 4R vibration
Fig. 8.22 Hughes blade mounted pendulum absorber
Rotor induced vibration 309
absorbers tuned to the 3Ω and 5Ω excitation frequencies for the four-bladed
rotor version, where their purpose is to reduce the response of the second and
third flapwise bending modes of the blade to the 3Ω and 5Ω frequency oscillatory
air loads in the rotating system.
Passive methods which fall into the second category are:
(a) The fuselage mounted classical mass-spring absorber.
This involves the mounting of a suitably heavy mass, usually situated in the
local region of the operating crew and passengers, tuned to the bΩ forcing
frequency. Use may be made of an existing heavy item such as the aircraft
battery, as in the Sea-King helicopter (Fig. 8.23 shows the installation), or the
mass may be parasitic, as in certain models of the Boeing Vertol Chinook
helicopter. In the former case, the natural frequency of the absorber is constant,
and to be effective the inherent damping of the moving mass must be minimal
so that the amplification at resonance is large. However, under such circumstances
the efficiency of the absorber will decline with excursions of rotor speed away
from the nominal governed value. This particular disadvantage is overcome in
the case of the Chinook helicopter by the use of an electrically actuated
system which changes the effective value of the sprung mass in accordance
with changes in rotor speed.
Fig. 8.23 Sea King battery vibration absorber
310 Bramwell’s Helicopter Dynamics
8.7 Active control of vibration
Recent developments in the reduction of helicopter vibration by the use of active
control systems have produced encouraging results. These developments have taken
place along two quite different approaches.
8.7.1 The application of higher harmonic pitch control to the rotor
blades (HHC)9
Since airframe vibration originates with the rotor blade oscillatory air loads, a potentially
attractive method of vibration control is the application of blade pitch at frequencies
greater than the rotor rotational frequency, in order to produce a forcing system
generating an airframe response which would oppose that arising from the basic rotor
forcing.
Figure 8.24 shows diagrammatically the concept of HHC. The rotor generates
oscillatory forces which cause the fuselage to vibrate. Transducers mounted at key
locations in the fuselage measure the vibration, and this data is analysed by an
onboard computer. Based upon this data, the computer generates, using optimal
control techniques, signals which are transmitted to a set of actuators which, typically,
vary the blade pitch at frequencies of (b – 1)Ω, bΩ, and (b + 1)Ω. Oscillatory aerodynamic
loads are thus produced which modify the total bΩ frequency response of the fuselage,
and the whole cycle of measurement of the modified vibration, data analysis and
resultant actuator response is repeated.
Using the appropriate control strategy, the process can be made to converge to
minimise the fuselage vibration. Key elements in the successful implementation of
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