Noise, or unwanted sound, is generated whenever the passage of air over the aircraft structure or through its power-plants causes fluctuating pressure disturbances that propagate to an observer in the aircraft or on the ground below. Since the flight condition cannot be maintained unless these air- and gasflows are controlled efficiently, there are ample opportunities for sound to be produced. Fluctuating pressure disturbances result from inefficiencies in the total system and occur whenever there is a discontinuity in the airflow-handling process, particularly in the engines, where power generation involves large changes in pressure and temperature. This is not to say that the airframe itself is devoid of sound-producing opportunities, for it has a large surface area and, in the configuration that is adopted for take-off and approach, both the landing gear and high-lift devices (slats and flaps) create significant amounts of turbulence.

To the community beneath the aircraft, the self-generated noise from the airframe is normally significant only during the approach phase of operation, where the sources shown in Figure 3.1 can combine to exceed the level of each major noise source in the power-plant. For this reason, airframe noise has been thought of as the ultimate aircraft noise “barrier”. Perhaps we should briefly consider this and other airframe-associated sources before moving into the more complex subject of the aircraft power-plant.

Over the past thirty to forty years, a vast knowledge of aircraft noise-control techniques has been acquired. Some of the findings have contributed to the improved airport-noise climate of today; the less successful and the failures have taught both useful and bitter lessons. This chapter concentrates on success – success in the field of the jet engine, for it was this propulsion system that really started the serious noise problem.

For a broader understanding of the problem, the specific means of controlling aircraft power-plant noise should be discussed in relation to design philosophy, which, in turn, is related to either date of concept or aircraft mission requirements. The following sections deal with the range of power-plants in service in two convenient categories, low- and high-bypass-ratio types.

Suppression of the early jets

As explained in Chapter 3, it is the basic cycle and the maximum thrust level of the engine that determines the level of jet noise produced. The first generation of pure jets and, to a large extent, the low-bypass-ratio engines that succeeded them, all had extremely high exhaust velocities, which caused high levels of jet noise. At full power, supersonic exhaust flows with velocities of up to 600 m/sec were not uncommon and the characteristic crackling and tearing sounds of the mixing noise were augmented by the presence of shock-associated noise.

Without today's turbine cooling technology, there was no opportunity whatsoever to reduce the jet noise in these engines by modifying the engine cycle.

The aircraft noise problem is an environmental “minus” that was born alongside the commercial gas turbine aeroengine more than thirty years ago. The issue came to a head in the 1960s, around the time when the number of jet-powered aircraft in the commercial fleet first exceeded the number of propeller-powered aircraft. During that decade, significant sums of money were expended in research and development exercises directed at quietening the exhaust noise of the subsonic fleet and in researching the noise of even higher-velocity jets that would have been a major problem if the supersonic transport had become a commercial success. Later in the 1960s, with the advent of the bypass-engine cycle, the emphasis was more on the noise generated inside the engine, with the large proportion of the extensive research funds necessary being provided by the taxpayers in those nations with aircraft- and engine-manufacturing capabilities.

Government action was not only limited to supplying the funds for research contracts, but major nations cooperated on a political front to develop noise certification requirements that were demanded of the manufacturers of all new aircraft produced from 1970 onwards. Coincidentally, technology moved forward and produced the high-bypass or turbofan-engine cycle, which, in reaping the benefits of the accelerated noise programmes of the 1960s, was only about one-quarter as noisy as the engines it replaced. Initially, however, the turbofan cycle was limited in its application to the new breed of larger wide-body jets, which had at least twice the passenger-carrying capacity of their forebears.

Aircraft noise prediction involves two types of activities: predicting the noise of an individual aircraft and assessing the cumulative effect of the complex pattern of operations in and out of a specific airport. The latter depends on the former for a wide range of aircraft types.

Aircraft noise

Without available direct measurements, the only method of assessing the impact of a completely new aircraft or power-plant design is to utilise a reliable prediction procedure. Such a procedure may be able to make use of a limited amount of directly relevant data, for example, engine test data where a development programme is under way, or it may have to rely entirely upon empirical component prediction procedures. The latter situation arises at the advanced project stage of any new aircraft design – a current example could be an aircraft with propfans or a second-generation supersonic transport with a novel variable-cycle propulsion system concept.

To be successful, aircraft noise prediction must be based on a reliable definition of aircraft performance and a confident prediction of the noise characteristics of the power-plant (as a function of power setting, altitude and flight speed). Where there are substantial and related measured data to support a new concept, their projection to the new aircraft situation can follow fairly well-defined routes. For example, if the new aircraft incorporates power-plants that are not much different from versions already in service, flight test data can be transposed fairly simply to a new situation. Where the measured information is obtained from static engine tests during the development programme, methods are being established for transposing these data to the flight situation.

Over the years, much has been said about the future. Usually, environmentalists predict that worse is to come, the industry expects a rosy future, whilst governments try to present a “balanced” view.

The future always depends on the starting point; and no two observers, or victims of aircraft noise, have the same viewpoint. Some live near well-developed and possibly operationally saturated airports and therefore experience the benefits of advancing technology in the form of lower individual aircraft noise levels and a generally improving environment; others are adjacent to new or expanding fields, where the growth in the number of operations is the overriding factor; a small percentage of people would complain in any case – if aircraft were silent there would be some who would perceive a “stealth” factor.

The importance of aircraft noise in society in the future will be a function of several factors. Uppermost are whether the manufacturing industry will be successful in advancing noise control technology, whether air transportation is going to expand to any significant degree and how sensibly government and airport operators apply controls. All these factors bear examination in trying to build a picture of the future, as does the question of whether the available noise-forecasting tools are right for the job. This is most important; the weighting applied to individual elements of forecasting methods must be examined, since they can have a tremendous influence on predicted trends in noise “exposure” and, hence, any decisions based on those predictions.

Noise contours, or statements of the noise level heard at various positions on the ground around an airport, are computed rather than measured because of the large areas of ground covered and the length of time over which noise data have to be averaged.

The noise at any point on the ground in the vicinity of an aircraft operation will depend upon a number of factors. Uppermost amongst these are the types of aircraft and their power-plant; the power, flap setting and airspeed conditions throughout the operation; the distances from the points on the ground to the aircraft; and the effect of local topography and weather on propagation of the noise. Most airport operations will include a variety of aircraft and flight procedures and a wide range of operational weights. Because of the large quantity of data (which can be regarded as aircraft or airport specific) required to compute the noise of each individual operation, it is normal to make certain simplifications to reflect average noise exposure over long time periods. These time periods may range from as little as one day to several months.

The normal process of computation embraces three main steps:

1. the calculation of noise level from individual aircraft movements at a matrix of observation points around the airport;

2. the integration of all the individual noise levels over a defined period of time; and

3. the interpolation and plotting of the information in the form of noise contours.

The simplifying assumptions that are most frequently made include the noise levels of “groups” of similar aircraft types, average climatic conditions and the average operational pattern over the time period in question.

Most technological advances are accompanied by some degree of apprehension about their potentially catastrophic effect on safety and the environment. History provides countless examples. Notable in the field of transportation are the applications of the steam engine to the locomotive, the internal combustion engine to the horseless carriage, and the whole array of engines used in the flying machine. Immediate public safety is usually the primary concern, but pollution and noise are also cited as possible sources of long-term harm to the human species. Although the passage of time has seen some of these concerns justified, humanity has learned to live with them, whilst exploiting to the full the benefits of a wide variety of transportation modes. Indeed, in developed nations, the quality of life would be downgraded considerably without the benefit of fast, reliable and diverse modes of transport. As a result, the cost of environmental protection is considered a necessary burden.

Noise, as an environmental issue, has had a major impact on both the conceptual and detailed design of only one source of power in the field of transportation, arguably the one that has had great impact on international relationships, the modern aircraft engine. It was in the early 1960s, following the successful introduction of the jet engine into commercial airline service, that aircraft noise became an issue of substance. Hitherto, it had been regarded merely as an aggravation, and grievances were settled either privately, by the parties concerned, or in the civil courts. Interestingly, in the United Kingdom this latter facility was regarded as unnecessary, and Parliament virtually prevented U.K. citizens from having recourse to it as far back as 1920.

Aircraft noise and related data are acquired for a multitude of purposes but, broadly speaking, they fall into two categories – those for public presentation and those for research experiments. The method of measurement and the systems involved in processing data for public consumption are normally closely controlled, with a range of specifications determining the instrumentation system standards and the way in which data are acquired and analysed. The most rigid form of measurement control occurs in noise certification. Since the certifying authority does not itself conduct aircraft noise tests, it is the task of the aircraft manufacturer to comply with all the requirements and present the information in a standard form. The equipment and provisions made by the manufacturer are closely scrutinised by the authority for compliance before and during the tests.

The formulation of certification specifications has been a monumental and evolving task that has been going on for the best part of two decades and it is still unfinished owing to the continually changing state of the art. Many thousands of man-hours have been consumed in the task of refining international standards, with the important objective of reducing the variability of results obtained by different organisations that use different equipment and operate in different climatic conditions around the world. The necessity for such stringent specifications is now being amply justified as aircraft noise requirements at the national and local level creep ever closer to industry's capabilities.

Noise, litter, housing and road development, local lighting, crime and a miscellany of other factors all produce mixed emotional reactions in Homo sapiens. Noise has often been cited as the most undesirable feature of life in the urban community. Aircraft noise is second only to traffic noise in the city in its unsociable levels, frequency and time of occurrence, and is often at the top of the list in rural areas. The rapid spread of civil aviation, from the early “jet set” élite of the 1960s to the tourists of today, has only served to intensify the problem, in that the increased number of flights in and out of airports has given rise to much greater intrusion on community life and, hence, to “noise exposure”.

The growth of the jet-powered fleet is outlined in Figure 1.1. Following the initial surge of purchases in the late 1950s and throughout the 1960s, fleet expansion steadied at about 5% per annum in the 1970s. Because aircraft have grown considerably in carrying capacity, the expansion rate since 1980 has declined to about 2–3%, but this rate is expected to be maintained to the end of this century. Although, in general, aircraft have become progressively quieter since the introduction of the bypass and turbofan engines in the 1960s and 1970s, the reduction in the noise level has not sufficiently offset the increase in operations or their psychological impact to bring an end to the problem.

Although levels of noise “exposure” can be quantified in terms of physical variables, each person's reaction to noise depends on his or her tolerance level.

Conceived as the transport of the future, in an era when fuel accounted for less than 10% of airline costs, the Concorde (Fig. 5.1) was a special case by any yardstick. Capable of flying at Mach 2, or over 2000 kph, for distances of about 6500 km without in-flight refuelling, it outpaces most and outsustains all the world's military aircraft. In fact, in mid-1987, by which time the Concorde had carried its millionth passenger, it had also operated at supersonic speeds for more than twice the time of the whole of the Western world's military fleet put together.

Its commercial failure – only sixteen production-standard aircraft were manufactured – was entirely a function of the change in the world economy in the 1970s. With the price of aviation fuel rising almost tenfold in a decade, there was a sustained drive to reduce the fuel consumption of all aircraft engines. Accordingly, concerted efforts to alter the design of subsonic engine components, allied to the emergence of the turbofan engine, meant that the Concorde's subsonic contemporaries were achieving up to 40% better fuel consumption than their forebears. The Middle East war of the early 1970s, which sparked the alarming increases in the price of oil shown in Figure 5.2, only served to emphasise the Concorde's uncompetitiveness, for the 40% improvement in fuel consumption only softened the blow to the airlines, which still found fuel to be 30% of their overall operating costs and would have found the Concorde even more consumptive. Over half the Concorde's weight is taken up by fuel. Hence, it is hardly surprising that the production run was limited.