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HHC are hydraulic actuators with an adequate high-frequency response characteristic
and a successful control algorithm.
Flight condition
Main rotor
Helicopter
structure
Airframe
vibration
Sensors
Measured
vibration
Adaptive
control
unit
Blade
control
actuators
Controlled
blade pitch
signals
Fig. 8.24 Concept of HHC
Rotor induced vibration 311
The oscillatory blade pitch motions can be applied either through a conventional
swashplate or spider control system, or by means of individual blade actuation in the
rotating system (Fig. 8.25). In the latter case, the input can be applied near the root
of the blade, or by the use of an outboard aerodynamic servo tab.
In defining the control algorithm, self-adaptive techniques are used in order to
cater for change in fuselage dynamics (due to loading changes, for example) and
rotor behaviour with flight condition.
The control algorithm depends on the assumption of a linear relationship between
measured vibration at bΩ frequency and the higher harmonic rotor forcing of the
form
Y = TX + B
where Y is a vector consisting of the bΩ sine and cosine Fourier components of the
measured vibration at a number of fuselage locations. The HHC input X is a vector
consisting of Fourier sine and cosine components of blade pitch at (b – 1)Ω, bΩ and
(b + 1)Ω frequencies. T denotes the rotor/fuselage transfer matrix, and B is the
background uncontrolled vibration. Since these parameters vary with flight condition,
a statistical estimator (Kalman filter) is used to track them during flight.
The final part of the control system is an optimal controller which uses the estimated
T and B parameters to minimise the index of performance J, where
J WY AX
i
m
i i j
n
j j = +
= 1
2
= 1
Σ Σ 2
where Wi are the relative weightings of the m vibration measurements Yi and Aj are
the relative weightings of the n HHC blade pitch inputs Xj. This index allows for the
weighting of the various vibration measuring positions according to helicopter role,
Fig. 8.25 HHC blade pitch actuation
312 Bramwell’s Helicopter Dynamics
and the ability to limit the authority of the HHC inputs when constrained by the
proximity of rotor aerodynamic limits.
The estimates of the T and B parameters are continuously updated, which allows
the system to adapt to changes in rotor aerodynamic and fuselage dynamic states.
Figure 8.26 indicates a typical blade pitch waveform with and without HHC. A
feature of some concern is the increase in blade pitch at the 270° retreating blade
azimuth position. This could, near the flight envelope boundary, have the effect of
introducing a premature onset of blade stall.
It is possible that additional blade area may have to be provided to prevent this.
8.7.2 The active control of structural response (ACSR)10
Subsequent to the development of HHC, an alternative approach to the active control
of helicopter vibration has evolved. This consists of connecting a number of actuators
between convenient points on the airframe to apply oscillatory forces to the structure.
The magnitude and phase of the loads generated by the actuators are determined by
a control algorithm which minimises the vibrational response of the fuselage at a
number of key positions. Figure 8.27 shows the basic concept of ACSR.
The basis of ACSR is that, if a force F is applied to a structure at a point P and an
equal and opposite force (the reaction) is applied at a point Q, then the effect will be
to excite all the modes of vibration of the structure which possess relative motion
between points P and Q. This requirement for relative motion in the modal response
between the points where the actuator forces are applied is an essential feature of
ACSR.
Experience to date has indicated that positioning actuators across the main rotor
Fig. 8.26 HHC blade pitch waveform
With HHC
90 180 270 360
Rotor azimuth degs
Normal trim-no HHC
8
4
0
–4
–8
Blade
cyclic
pitch
degs
Rotor induced vibration 313
Vibration
source
Structure
dynamics
Adaptive
control unit
Actuators Sensors
Cancellation forces
Actuator demand
Vibratory forcing
Measured vibration
Actuator control
manifold
Actuator Compliant element
Integral ACSR
actuator/struts
Airframe top structure
Torque struts
Main rotor gearbox
Fig. 8.28 ACSR gear box mounting struts with integral actuators
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Bramwell’s Helicopter Dynamics(154)
