The design concept of high-frequency sound modulators for acoustic tests for the strength and flying life of an aircraft

Abstract

Modern aircraft are exposed to intense pulsations of sound pressure arising both from engine noise, and from turbulent effects of the environment during flight. The pulsations cause vibrations of the structure leading to fatigue damage of the fuselage skin and primary elements of structure, as well as to the equipment failure. The operating experience shows the necessity of taking into account the acoustic fatigue starting with loads of 130 – 135 dB. At a load value above 160 dB, the acoustic fatigue becomes one of the main factors determining the strength of aircraft and space structures. Despite the development of computational methods for estimating durability under acoustic loads, the most reliable methods are still based on experimental studies. The frequency spectrum is the main characteristic of acoustic loading. Reproducing of acoustic loads in a controlled frequency range 150 – 600 Hz with the intensity of pulsations specified above appeared enough for testing most aircraft structures. However, space apparatus and equipment must be tested for the effect of acoustic noise within the frequency range from 150 to 1200 – 2000 Hz. Domestic low- and medium-frequency generators operating in a frequency range of 20 – 600 Hz are currently used in reverberation chambers in Russia. To generate higher frequencies of acoustic vibrations, we used an ERT-200 electropneumatic converter (LING (USA), the maximum frequency 1000 – 1250 Hz) and a most advanced SEPMOD modulator [SEREME (France), the maximum frequency is 1000 – 2000 Hz]. Information about the design of those sound modulators is not available to consumers. Further development of aerospace technology necessitates developing of domestic high-frequency sound generators for acoustic tests. We propose a method which ensures operation of the sound generator in a controlled frequency range of 150 – 1200 Hz. The essence of the method consists in designing a sound modulator (being part of the generator) which provide that an increase in the electromotive force, driving the movable element of the valve assembly into reciprocating motion, will not entail an increase in heat generation in the electromagnetic propulsion device. For this, we propose to perform the electric propulsion device in the form of synchronously operating two parts located at opposite ends of the movable element of the valve. Supplying each part with the maximum permissible current, we managed to increase the total excitation current by half. The calculations showed that the use of more energy-intensive permanent magnets of neodymium type NdFeB and considered principle of forming a propulsion unit provide a range of sound frequency control of 150 – 1200 Hz at a pulse acoustic load up to 160 dB.