ABSTRACT
Despite the best efforts of engineers over the last four decades, the problem of noise disturbance around airports continues to grow in importance. Restrictions are now regularly imposed on aircraft movements into and out of major airports, with obvious economic consequences. An understanding of the mechanisms of noise generation is clearly fundamental to the development and implementation of effective control measures. The principal aims of this chapter are to provide brief descriptions and explanations of the various mechanisms of aircraft noise generation, together with the associated controlling parameters, and to cite technical literature that may be consulted for greater detail. The dominant contributor to overall aircraft noise is the power plant,
although, at approach, a significant component of the noise comes from the airframe. The propulsion systems of the majority of commercial passenger aircraft comprise a number of high bypass ratio turbofan engines, each with many distinct noise sources. Each engine source has a particular dependence on operating conditions and a characteristic acoustic far-field frequency spectrum and directivity. Airframe noise is also generated by numerous sources, particularly on the wings (with high lift devices deployed) and the landing gear (Figure 7.1). Achieving cost-effective and energy-efficient reductions in aircraft noise
requires evaluation of the relative contributions to total radiated noise from the many engine and airframe sources. A typical component breakdown for an aircraft with acoustically treated engines was predicted by Owens (1979); the data are re-plotted in Figure 7.2, expressed in terms of perceived noise loudness. For the generation of aircraft to which Figure 7.2 applies, the combined
fan noise from the inlet and bypass duct dominates the overall noise of the aircraft at both takeoff and approach conditions. Next in importance is jet noise at takeoff, and airframe noise at approach. Since the time when this analysis was performed there has been considerable progress in
the reduction of engine noise; airframe noise is now comparatively more important. This chapter presents a brief, mostly qualitative, description of the dom-
inant noise sources present on a modern turbofan-powered aircraft, within a framework of the theory of aerodynamic noise generation. The issue of aircraft noise assessment is not addressed in this chapter; information on this subject can be found in specialist texts such as that of Smith (1989). The chapter is divided into four main sections: Section 7.2 outlines the fundamental theory of aerodynamic noise generation, Section 7.3 presents a qualitative description of the basic noise sources on an engine, Section 7.4 presents an overview of the noise sources on an airframe and Section 7.5 concludes the chapter with a brief overview of numerical methods for aerodynamic noise prediction.
