Analogue Computation of the Impedance of a Powered Flying Control System

Abstract

This thesis describes an investigation to determine the impedance of a powered flying control system by analogue simulation. The first stage of this study is concerned with the impedance of a hydraulic servomechanism excited at its outputend by a sinusoidal force whilst the input valve was locked in a neutral position. The first part of the analogue simulation investigated the impedance in the presence of the coulomb friction force. The effect on impedance of changes in the bulk modulus of the hydraulic fluid, the leakage across the jack piston and out of the jack, the magnitude of the coulomb friction, the supply pressure and the static valve opening has been determined. The results from this simulation are compared with the results obtained from laboratory tests on the hydraulic servomechanism and a good agreement is shown to exist between the two results. The second part of the analogue simulation investigated the impedance of the hydraulic servo in the absence of the coulomb friction force. The results from this simulation are compared with the theoretical results obtained by linearising the valve flow characteristics using the small perturbations technique. A qualitative agreement between these results has been shown to exist. The second stage of this study investigated a technique for obtaining the impedance of the control system, consisting of the hydraulic servomechanism and the aircraft control surface, from measurements made separately on the hydraulic servomechanism and the aircraft control surface. It is shown that measurements of the direct response or mobility of the servomechanism can be combined with the direct and cross responses or mobilities of the control surface to give the control system response, mobility or impedance by using the sub-system technique.