Helicopter fuselage vibrations are of significant levels and produce a unique vibration problem.
Reducing the vibration levels increases passenger comfort, reduces crew fatigue, allows higher
cruise speeds to be achieved and improves equipment reliability. Vibration Reduction
Techniques can be divided into two distinct categories, either passive or active techniques.
Active control techniques offer the potential for good vibration reduction performance over
significant areas of the fuselage.
This study describes strategies for the Active Control of Structural Response. The technique
aims to minimise the structural vibration of a helicopter. Accelerometers measure the vibration
at several key points on the fuselage. A multivariable control algorithm processes this
information and calculates a set of control forces for a set of hydraulic actuators, located at
strategic points in the structure. Vibration reductions result from the superposition of the
actuator induced vibrations forces with those induced in the fuselage by the rotor. This research presents a number of different active control strategies for the reduction of
helicopter fuselage vibration. Two distinct active vibration control approaches are a frequency
domain controller and a time domain controller, and the Thesis establishes the advantages and
disadvantages of each of the control strategies. The time domain option is based upon direct
feedback of vibration through constant gain matrices. The subsequent vibration waveform
contains information over a wide spectrum of frequencies and consequently control is possible
over a range of frequencies. Alternatively the frequency algorithms are specifically concerned
with the control of discrete frequencies, the blade-passing frequency being dominant in the
case of a helicopter. This thesis describes a third novel approach to the design of an adaptive
controller for the reduction < of Helicopter vibration. This new technique is a hybrid
time/frequency domain solution combining the advantages from both the time domain linear
quadratic feedback controller and the frequency domain quasi-static controller. Both fixed gain
and adaptive control designs have been implemented, and comparisons of the performance of
the various control approaches to the problem of minimising vibration in helicopter structures
are made. An estimator provides control system adaptability that permits the periodic update
of the fuselage model and produces robustness to changes in the structural dynamics.
A simulation study presents results for the performance and robustness of the control
strategies. Experimental investigations considered the effects of linear and nonlinear actuator
dynamics, the performance of the strategies during aircraft manoeuvres and the robustness the
strategies to changes in the structural response. Experimental validation of the strategies was
achieved by testing on a helicopter airframe test rig at the premises of Westland Helicopter Ltd,
who collaborated in the SERC funded project within which this work was carried out.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.