Some UK-government establishment research towards quieter aircraft

Journul

ofSound and

SOME

Vibration (1976) 47(2), 207-236

UK-GOVERNMENT TOWARDS

ESTABLISHMENT QUIETER

RESEARCH

AIRCRAFT

F. W. ARMSTROK National Gas Turbine Establishment, Pyestock, Farnborough CC’1 4 OLS, England AUD

J. WILLIAMS Royal Aircraft Establishment, Farnborough, England (Received 19 June 1975, and in revised form 16 February 1976)

This paper aims to give an impression of the scope of research programmes and development of national experimental facilities at NGTE and RAE, directed specifically towards the evolution of economic quieter aircraft. By way of introduction, the particular nature of these two Government Establishments and their widely ranging roles within the aeronautical scene are indicated. As further background to the subject matter of the paper, the interactions between aircraft noise. technical design factors, operational techniques and economics are briefly reviewed from a research standpoint. The paper then discusses major problem areas associated with the development of quieter aircraft, the particular research work required, and the related development of special facilities. While activities at the Establishments properly cover a large variety of relevant technical disciplines, attention has to be restricted here to a few specific examples. They include engine exhaust noise research. the use of absorbent liners in engines, the development of noise-testing facilities including flight simulation, and research on airframe effects such as noise-shielding and self-noise. .4n outline is also given of some multi-variate design synthesis studies involving flexible matching of powerplant and airframe design parameters, directed towards achieving a better balance of aircraft noise, performance, and economic characteristics.

1. INTRODUCTION 1.1. PREAMBLE Noise and operating economics are vital factors affecting the future evolution of civil aircraft. While the development of civil aviation has always been moulded by a variety of influences, ranging from technical opportunities to operational constraints, we face today a particularly acute situation. The very success of air transport has produced a serious noise problem around airports, provoking legitimate public pressure for its alleviation. At the same time, trends in world economic conditions, and particularly the increased price of fuel, emphasize the vital importance of maintaining the best possible economics in civil aircraft operation. The above requirements tend now to be somewhat conflicting, in contrast to the situation of a few years ago when a move to the high bypass-ratio turbo-fan engine offered simultaneous and significant improvements in both economy and noise. Although some gains may be available from further engine evolution in the same direction, these will now be harder to win. The challenge is therefore one of developing the technology of engine, airframe and systems in such a way that noise nuisance can be lessened without impairing the development of air transport in a rather unfavourable economic climate. This presents a many-sided problem, 207

208

F. W. ARMSTRONG

AND J. WILLIAMS

calling for well-balanced programmes of research and development covering a large number of technical areas. In the UK, although much of the work is naturally done within the aeronautical industry, a significant part is also played by the Government Research and Development Establishments. The main purpose of this paper is to convey an impression of their contribution in terms of research programmes and the development of national experimental facilities for noise studies. 1.2.

THE R&D ESTABLISHMENTS

AND THEIR FUNCTIONS

The Establishments principally concerned with this problem are the Royal Aircraft Establishment (RAE) and the National Gas Turbine Establishment (NGTE). Both are Research and Development Establishments of the Ministry of Defence (Procurement Executive) and are involved in work relating to both military and civil aviation. The civil work is aimed at meeting objectives agreed with the Department of Industry, which is the UK Ministry responsible for policy and funding of civil aircraft development. The RAE, which is the largest R&D establishment in Europe, has been concerned for the past 60 years with research and development relating to aviation generally. This covers the aircraft and most associated flight systems, with certain exceptions which include engine research. The NGTE is the corresponding establishment for gas turbines and jet propulsion, originating some 30 years ago from the combination of the teams which had been working on jet propulsion at RAE and under Whittle at Power Jets Limited. The activities of these major government establishments have evolved over the years in response to changing policies and requirements. Their main current functions can be summarized as follows. (1) Research. While this covers a wide spectrum which ranges from fundamental scientific study of future aeronautical possibilities to highly specific problems arising on current projects, the work can be described as essentially “aimed” research. (2) The provision of central national facilities and an appropriate testing service. In addition to the obvious need that large and expensive facilities should be utilized fully and be available to the whole industry, economy of construction and operation is also served by central siting when power and air supplies, etc., can be common. (3) Overseeing and monitoring of UK government-sponsored aeronautical research programmes. This involves general co-ordination of the intramural and extramural work to ensure that available resources are used to best effect, participation in the technical formulation and assessment of contractor research proposals, and monitoring of the research as it proceeds. The research staffs of the Establishments thus maintain close contact with their counterparts in both Industry and Universities. (4) The provision of technical advice, based on the pool of expertise available, to departments of government concerned with aviation and to British Industry. There is also extensive liaison with other research establishments at home and abroad. 1.3.

DESIGN PROBLEMS

AND THE SCOPE OF RELEVANT

RESEARCH

To combine social acceptability with attractive economic and operational characteristics, future civil aircraft will require both a high standard of technology and a well-judged synthesis of design variables. The interactions between noise, performance and economics are strong and complex, involving important aspects of airframe and propulsion system design. The situation is illustrated in a simplified way by Figure 1. This emphasizes the need for continuing and detailed study of these design interactions as a complementary activity to the broadly based research programmes needed to ensure the continued advance of technology in airframes, powerplants and other systems.

SOME RESEARCH

TOWARDS

ENGINE-AIRFRAME

I

700

AIRCRAFT

INTERFERENCE

AIRFRAME -SURFACE AIRFRAME

QUIETER

SHIELDING

- FLOW REFRACTION

INTERACTION

NOISE

I

AIRFRAME

DESIGN

COMPONENT NOISE ACOUSTIC TREATMENT THRUST

LIMITS

I

AIRCRAFT

PERFORMANC

TOL

THRUST

TOL

FLIGHT

AIRFIELD MISSION

PATHS

LENGTH

II

REOUIREHENTS

Figure I. Aircraft design factors influencing noise.

The jet engine continues to be the subject of intensive and competitive development. Research ranges over a wide field, aimed at improving performance, weight, cost and environmental factors. While a substantial effort is concentrated directly on research into engine noise-as befits the essential cause of aircraft noise nuisance-it should be emphasized that advances in other technical disciplines can yield significant benefits in noise reduction on succeeding generations of engines. For example, developments in high temperature technology, aerodynamics and mechanical design paved the way for the introduction of the modern high bypass-ratio turbo-fan with its fundamentally more favourable noise characteristics. Engine design may well evolve further in the same direction, making use at the same time of improved knowledge from research into the mechanisms of noise generation and absorption. Research related to airframes is likewise wide-ranging. In addition to continuous advance in the fields of aerodynamics, structures and materials, which from the resulting improvements in airframe efficiency can lead to noise reduction, certain aspects related more directly to noise generation and propagation are under active investigation. The latter include shielding of engine noise sources by airframe surfaces or favourable refraction by vortex flows, and possibilities for reducing the airframe self-noise which is emerging as a significant phenomenon for aircraft with quiet engines during low-altitude flight with undercarriage and high-lift devices extended. Considerable attention is also now being paid to aero-acoustic interference effects between powerplant airflows and neighbouring airframe surfaces. As regards aircraft operation, research is proceeding on the practicability of reducing noise nuisance and on the more effective use of terminal airspace by the adoption of steeper flight gradients, particularly for landing. The assessment of whether this technique could be used widely for routine airline operations requires detailed and complementary investigations of such aspects as crew workload, ATC implications, aircraft handling qualities, safety margins, and guidance aids. The scope of relevant research is thus very wide, with many inter-related aspects [l]. RAE and NGTE are involved throughout, both with co-ordination and monitoring of the government-sponsored extramural programme and as research centres in their own right. Clearly however, the limitations of a single paper require concentration on selected areas, so the topics discussed here are associated directly with the noise problem. In section 2 the technical development of experimental facilities for noise research is discussed. Section 3 deals with some examples of recent NGTE and RAE research aimed at the clarification and

210

F. W. ARMSTRONG

AND J. WILLIAMS

prediction of noise generation and propagation, and at evolving practical methods of aircraft noise reduction. Section 4 illustrates some joint RAE/NGTE studies of airframe/ engine design interactions and possible cost implications of noise reduction, using the RAE multi-variate design synthesis and optimization techniques.

2. DEVELOPMENT 2.1.

BASIC POWERPLANT

OF EXPERIMENTAL COMPONENT

AND

MODEL

FACILITIES

FOR NOISE

RESEARCH

FACILITIES

The history of gas turbine research and development has always been characterized by an extensive use of experiments on individual engine components-both model and full-scale. Work aimed at noise reduction is no exception. Although tests on complete engines occupy an obvious and important place, component research allows some of the major noise sources to be separated and studied individually. Furthermore, many aspects can be covered by the use of reduced-scale models with considerable benefit in economy, flexibility and rate of testing. Powerplant noise research thus involves a variety of experimental facilities, the requirements for which have, ironically, grown more demanding and complex as a result of the progress made in developing quieter and more economic engines! For while the noise of the early jet engines arose mainly from the propulsive jet itself, today’s efficient high bypass turbo-fan engine represents a relatively “balanced” assembly of noise sources. Jet noise, though much reduced, is still significant, and other important sources are the large transonic fan and the engine exhaust system including the turbines. Figure 2 illustrates the

SIMPLE JET ENGINE

MDEIW TURBO-FAN ENGINE Figure

2. Ingredients

of engine

noise.

position, in comparison with the noise characteristics of a simple jet engine. To progress further, the powerplant noise engineer therefore has to work for reductions all round, the situation being further complicated by the design variables introduced by the use of sound absorbent treatment within the powerplant ducting. The evolution of the noise research

SOME RESEARCH

TOWARDS

QUIETER AIRCRAFT

“i 1

facilities at NGTE, and indeed of the research programme itself, has reflected this broadening of the scope of the engine noise problem [2]. In recent years several important facilities have been built and put into operation. These are briefly outlined below; taken together with others available at Rolls-Royce and RAE, they confer on the UK a first-class experimental capability for research and development work in aid of powerplant noise reduction. The use of anechoic chambers has long been common in many branches of acoustics and noise control engineering. They provide a controlled and sheltered environment in which the characteristics of noise sources can be studied in the absence of confusing reflections and other spurious effects. If sufficiently large in relation to the frequency and spatial extent of the source, they allow accurate measurement of its far-field directional properties. Houcver. the design of anechoic chamber facilities for aircraft propulsion noise research can pose significant problems due to the special requirements which arise in this class of work. For instance, large and high-velocity airflows are inevitably associated with gas turbine powerplants; and component rigs, even at model scale, therefore involve substantial How, 01 air within the anechoic chamber. This necessitates carefully designed air entry and exit sqstcms which provide for the required airflows without undue compromise of acoustic quality. The study of exhaust system and jet noise requires a capability for producing jets of both high velocity and high temperature, which themselves entrain large quantities of air through turbulent mixing. The rig needs a high-pressure air supply incorporating a means of heating which must not introduce extraneous noise of a level which would intrude upon the noise of the model exhaust system or jet itself. Turbo-component rigs involve both substantial airflows and high-speed rotating machinery transmitting considerable power. A fan 01 compressor rig requires a shaft power supply, while a turbine rig entails a dynamometer. The presence of high-speed machinery, and/or combustion systems for testing with hot flows, requires that the design of the facility should take account of fire hazard. If sma!l-scale model work is envisaged, the acoustic data acquisition and analysis systems need to be capable of handling signals far above the audible frequency range, because of the inverse variation of frequency with model scale. Finally, cost considerations are always signiticant in the design of such facilities, leading often to a need for carefully judged design compromises. At NGTE, three special-purpose anechoic chamber facilities have been designed. constructed and put into service during the last decade. These provide for a very- lvide range r-11‘ testing, from work on small-scale model jets ofa few inches diameter to installations iris olv ing high temperature turbines and exhaust systems. While each chamber is required to handle :I high airflow rate in relation to its own volume, the design solution adopted varies according to the particular duty intended. In each case, model testing was used to prove and optimise the concept prior to full-scale construction. The main features of these facilities are \unimarized in tabular form in Figure 3. First to be constructed, in 1965/66, was the facility intended for small-scale model experiments on jets and exhaust systems. This provides a capability for continuous running of high-temperature jets; the jet is directed vertically upwards and exhausts through an aperture in the roof, while entrained air enters around all four sides. Since its commissioning. this facility has been operated for thousands of hours on a variety of programmes ranging l’ram basic research to tests on nozzle designs for specific aircraft projects. In 1971, the NGTE fan noise anechoic facility was commissioned. Although considerabl> smaller than the large facility [3] operated at Ansty by Rolls-Royce (1971) Limited. it complements that facility by offering a different arrangement of the test rig in the anechoic chamber. Whereas at Ansty the test machine is mounted in a wall, central mounting ib u\ed in the NGTE chamber. This allows exploration of both the forward and rearward noise fields without changing the installation, and renders the facility particularly convenient ior experiments involving successive changes to the intake system which might affect rearward.

F. W. ARMSTRONG AND J. WILLIAMS

212

MODEL

FACILITY

MAIN

5.2

DIMENSIONS

X (WITHIN

WEDGE

JET

TIPS

m

SQUARE

4.6m

OR

TRAVERSE

ARRAY

APPROX.

2 m

FORWARD

X

HIGH

APPROX.

25m

X 14 m

HIGH

SQUARE

LOWER

250

AND

ARC

TRAVERSES

3 m

RADIUS

REAR

RADIUS

1

AT 6m

OR OR

ARRAY 12 m

RADIUS

400

Hz

, 12m

TRAVERSE

SIMULTANEOUS

APPROX

100

Hz

HZ

LIMIT

EXHAUST

SYSTEMS

APPLICATION

7-6m

ANECHOIC

FACILITY

)

MODEL

TYPICAL

m

LARGE

CHAMBER

X

6.1

RADIUS

FREQUENCY

9-5m

HIGH

TRAVERSE, MICROPHONE

FAN

CHAMBER

TEST

JETS OR

AND -

COLD

TRANSONIC

FANS,

0.38m

kW

HOT

LIMITED

MODEL

APPROX. DRIVE

EXHAUST DIAM.

WITH

1000

TURBINES

POWER

FLIGHT

OR

SYSTEMS

OR WITHOUT -

COLD

HOT

LIMITEO

SIMULATION

SIMULATION

CAPABILITY

CAPABILITY

FLIGHT

Figure 3. NGTE anechoic chamber facilities.

as well as forward, noise. The axis of the drive shaft and test machine is horizontal, and air induced by the compressor is admitted via vertical slots formed by off-setting the wall panels. Exhaust air is collected by a large duct surrounding the drive shaft whence it is discharged from the building. Work to date has concentrated on single-stage transonic fans. Studies have been made of various aspects of noise generation and propagation, including the “blockage” effect of the rotor on the forward propagation of tone noise arising from rotor/stator interaction [4]. The third of these facilities is by far the largest. This is known as the Anechoic Facility and is intended for the testing, largely on behalf of industry, of turbines, exhaust systems and jets at temperatures representative of engine operation. The building was completed in late 1973. Driving-air for the test rigs is piped to the facility from the compressors which serve the large NGTE Engine Test Facility. The presence on site of such supplies allowed a considerable saving in capital cost. Moreover, the very large flows available allow the possibility of producing a measure of flight simulation, by surrounding a test jet with a co-flowing stream moving at flight speed. Atmospheric air entrained by the system under test enters the chamber via banks of silencing splitters situated on each side of the rig cubicle. The flows are drawn from the chamber through a large acoustically-treated collector duct, suction being provided by groups of low-head exhauster fans mounted downstream of silencing splitters in the adjoining exhaust house. For tests at elevated temperature, butane is used as the fuel to minimize the risks of fire in the chamber, and of damage by wetting of the absorbent wedges, which could occur with liquid kerosene. The contrast in size between the small model jet chamber and the Anechoic Facility is illustrated by the two photographs which together form Figure 4. Fuller details and illustrations of the three facilities discussed above are given in references [2] and [4]. An entirely difTerent form of facility, also important in aircraft propulsion noise work, is the flow duct type for testing the sound absorbent treatment used in powerplant ducting. The principle used is to measure the attenuation of sound as it passes through a duct which is lined with the absorbent material under examination. To simulate the powerplant environ-

SOME RESEARCH

TOV’JARDS

QUIEIFK

AlRCR.At‘l

(a)

(b) Figure

4.

Small and large anechoic chamber facililies at hGTE. (a) Model jet chamber; (b) large anechoic

facility.

ment, high intensity sound is used and a flow of air is passed through the test duct. A reverberation chamber is normally placed at each end of the duct to facilitate the measurement of overall sound power and to minimize sound losses at the generating end. Both small and large examples have been built at NGTE in recent years. The large one, known as the Absorber Facility, incorporates some novel features and was completed in 1972. This is shown in Figure 5, the airflow direction being from right to left. To allow testing to cover the two

F. W. ARMSTRONG AND J. WILLIAMS

214 DOI4 hSTKtAM RtVtKBtRATlON

I

TtS-r

StCTlOh

UPSTREAM

58-63

SPLITTERS

BLOWtR

m

I

I-

(4

(b) Figure 5. Absorber test facility. (a) General arrangement;

(b) interior, showing the test duct.

important cases of sound propagating with the airflow (engine exhaust ducts), and against the airflow (engine intakes), a noise generating system can be placed either upstream or downstream of the duct test section, The splitters installed between the blower and the upstream reverberation chamber serve to prevent fan noise from intruding upon acoustic measurements in this chamber when it is acting as a receiver-i.e., in tests when the noise generator is mounted downstream of the test duct and measurements are being made of the sound propagated upstream. The standard test duct is rectangular in cross-section, of nominal dimensions 0.76 m (30 in) x 0.41 m (16 in). Test panels can be mounted in each of its four walls. Main airflow Mach numbers up to about 0.7 can be achieved. Alternative circular test ducts are also available. The production of the very high sound powers required for a large flow duct facility constitutes a significant problem. In view of the high pressure air supplies available, the chosen means for meeting this requirement at NGTE is the Hartmann generator. This device consists basically of a simple convergent nozzle producing a jet which impinges on a resonant cup mounted a short distance downstream. The arrangement produces an intense noise, particularly at supercritical nozzle pressure ratios. It is also robust, inexpensive to manufacture, and is readily controllable by varying the supply pressure. A programme of development was undertaken at the Establishment [5] to optimize the characteristics of a system of Hartmann generators for the Absorber Facility, the design objective being to achieve a

SOME RESEARCH

TOWARDS

QUIETER

AIRCRAFT

11s

sound pressure level of 160 dB in the test duct. Although initially an array of 32 Hartmann units set around the entry or exit section of the test duct was used, this has recently been superseded by a much simpler system of larger units mounted in either reverberation chamber. The new system comprises three 10 cm (4 in) diameter units and one 5 cm (2 in) diameter unit. Compared with the earlier system, it offers the following advantages : (1) greater flexibility of operation, moving the massive 32 unit assembly from one end of the duct to the other being a time-consuming job ; (2) a first harmonic at the lower frequency of about 400 Hz (instead of 1 kHz), useful for tests on liners designed to reduce low-frequency engine noise; (3) a readily tunable spectrum, by differential setting of the generators.

Figure 6. Hartmann

generator

characteristics.

Figure 6 shows the sound output characteristics of the new system when tuned to provide a spectrum of largely broadband character. For comparison, the much more tonal spectrum given by a single unit is shown. 2.2.

GROUND-BASED

FACILITIES

WITH

FLIGHT

SIMULATION

The prediction of aircraft noise for flight conditions is much more complex than under static conditions. Apart from conventional Doppler-shift and flight-path considerations, relative mainstream flow past the aircraft can have significant effects on the generation and propagation of the source noise from the engine itself, from the airframe, and from the engine-airframe interactions. Forward-speed can affect the perceived noise appreciably through changes not only in the overall sound pressure level but also in the noise spectrum and directivity characteristics. Our fundamental understanding of these effects is still poor. Their practical importance has already been forcibly illustrated by the difficulties of reconciling static-rig and in-flight measurements on engines-as regards both fan noise and exhaust noise (section 3.1), by the recognition of important airflow refraction effects (section 3.3)

F. W. ARMSTRONG AND J. WILLIAMS

216

and by the significant airframe noise measured in flight with quietened engines (section 3.4). Ground-based facilities [6] providing adequate forward-speed simulation for model experiments are essential to complement full-scale investigations under static and flight conditions (Figure 7). They can be especially profitable not only for research, but also for detailed and

El

FLIGHT

THEORETICAL

VEHICLES

FRAMEWORKS _.

. .

‘.

LINEAR

ROTATING

TRACKS

ARMS

AERIAL

ROAD VEHICLES

Figure 7. Ground-based flight tests.

WIND-TUNNELS

facilities for forward-speed

CABLES

simulation,

to complement

full-scale static and

safe studies of critical components and special problem areas; thereby expediting comparison of novel project concepts, exploratory development, and guarantees for in-service applications. Naturally, to ensure meaningful evaluation of the effects of forward-speed on aircraft noise characteristics, reliable measurements of related aerodynamic flow conditions as well as noise must be possible, so that correlation of the observed noise changes with variations in aerodynamic behaviour can be achieved. Acoustic wind-tunnels, using “fixed” models in an anechoic working-chamber enclosing a quiet airstream, are now required to ensure a thorough approach to noise-model research work (basic and applied) and ultimately for the support of specific aircraft projects; analogous to the extensive aerodynamic testing that is general practice nowadays. The special advantages of tunnels compared with other facilities in this respect are well known, including capability of continuous operation, repeatable test conditions, high productivity, good measurement accuracy, testing flexibility, and the precise alleviation of reflections from neighbouring surfaces. From analytical and experimental studies [7,8], most of the problem

FLOW RETURN

0

5

IO

TREATMENT

7-3m

METRES

Figure 8. Present 24 ft tunnel circuit.

DIAM.

SOME RESEARCH TOWARDS QUIETER AIRCRAFT

‘17 _.

areas of noise-testing in conventional wind-tunnels have now been identified, their magmtudes critically assessed, and the special treatments or limitations involved have become quantifiable and tractable in many respects. In particular, various parasitic noise fields which could mask the actual model noise measurements can be avoided by acoustic lining of the working-chamber to minimize reverberation effects, by acoustic treatment of the tunnel circuit to substantially reduce the intrinsic background noise associated with the tunnel drive, and by using special microphone and model-rig arrangements to minimize their self-noise and local interference in an airstream. Moreover, apart from the desirability of good quality mainstream flow past the model in the test-section, a large tunnel size is usually required from acoustic as well as aerodynamic interference considerations.

Figure 9. HP1 15 model installed in the RAE 24 ft diameter wind-tunnel.

218

F. W. ARMSTRONG

Such demands led us to develop further along with new experimental techniques and foam sheet now lining the boundaries anechoic conditions inside the test-section a lower limit of less than 50 Hz at full-scale the test-section airstream is large enough

AND J. WILLIAMS

the RAE 24 ft-diameter open-jet tunnel (Figure 8), for noise-model testing. Sound absorbing wedges of the tunnel working chamber broadly ensure for frequencies above 200 Hz, corresponding to if the test model is smaller than +-scale. Moreover, to permit far-field noise conditions to be attained

within the airstream and measurement locations free from boundary interference effects for frequencies again above 200 Hz, at least with a compact noise source. More generally, this lower limit in frequency tends to rise with increasing spatial extent of the noise model source distribution. Over the past three years, the tunnel has been successfully employed for noise research studies on single and coaxial jets, jet/surface interaction and airframe shielding of simulated engine noise (Figure 9); but usually at airspeeds only up to 30 m/s to avoid excessive tunnel background noise. The tunnel capability could be much improved [8] at a fraction of the cost required for a new equivalent acoustic tunnel, by the provision of a new low-noise fan of improved efficiency, located in the back leg of the return-circuit (instead of adjacent to the test-section), thus facilitating adequate acoustic treatments and aerodynamic improvement of the circuit inside the existing concrete shell. This would enable noise model experiments to be then carried out successfully up to about 50 m/s (I 00 kn) still retaining the 7.3 m diameter test-section, or up to about 70 m/s (140 kn) if a reduction of the test-section diameter to 5.5 m were accepted. As part of the RAE aero-acoustic studies towards practical acoustic tunnel designs, the existing l/5-scale model of the 24 ft open-jet tunnel will be aerodynamically modified and acoustically treated (step-by-step). The feasibility of supplementary noise-model testing in the larger and faster of the RAE tunnels with closed test-sections (Figure 10) will also be NEW

I

I

CLOSED-SECTIONS

CLOSED-SECTION

w4m

Sm x 4.2m:130mls

I

RAE

x 2.h

: IOOrnlS I

LOW-SPEED

I SMALL 0.4m

FREE-JET DIAM:‘3Om/s

(ANECHOIC

CHAMBER)

LARGE -

7.3m

OPEN-JET DIAM:

(ANECHOIC

SOm/s CHAMBER)

SMALL

OPEN

-JET

I.Sm

DIAM:

70mls

STATIC-TEST

Figure 10. Acoustic wind-tunnel and “free-jet” tunnel facilities. explored, taking into account promising new developments in directional acoustic receivers and noise discrimination techniques, in an attempt to counterbalance inherent limitations on acoustic treatment of the test-section walls (without aerodynamic interference) and to avoid costly acoustic treatment of the tunnel circuits. Anechoic chambers with the capability of static noise testing on jets have also been adapted at NGTE and RAE to provide relatively simple small “free-jet” tunnels, with the primary jet efflux of largest available diameter used as the mainstream flow about a much smaller “model-jet” co-axially centred. Experience to date has implied that the ratio of the mainstream diameter to model-jet diameter should

SOME RESEARCH

TOWARDS

‘19

QUIETER AIRCRAFT

exceed 30 unless reliable aero-acoustic correction factors can be estimated for lower diameter ratios. Outdoor mobile facilities of the tracked-vehicle type can be more usefully exploited (instead of flight vehicles) for noise testing of large-scale models or even full-scale investigations, using representative real engines and with simulation of the local airframe surfaces. and in principle with “in-flight” motion of the noise generator through a true atmosphere and past a stationary observer. Naturally, a variety of special testing problems have to be clarified and quantified, such as measurement/analysis requirements under non-stationary conditions, parasitic noise and aerodynamic interference from the vehicle and model rig, and environmental effects. Although feasible developments of the Earith Tracked Hovercraft Facility for aircraft noise research in the UK had to be terminated in 1974, much valuable experience was accrued for the critical appreciation and more reliable application of mobile facilities elsewhere and of allied flight testing techniques. Other types of ground-based facilities (Figure 7) have also warranted serious examination for particular applications, each facility with its individual merits and problems. For example, Rolls-Royce have successfully developed a “spinning rig” (rotating arm), self-driven by a tip-jet, for model testing on the noise characteristics of high speed jet nozzles and silencer arrangements [93. Recently, devices for static-rig fan testing have also been developed in attempts to produce desired aerodynamic conditions (distortion free) at the fan inlet and yet not impede radiation of the acoustic signals [lo]. Carefully controlled and detailed flight experiments with research-orientated moditications of small aircraft have played a worthwhile complementary role to model testing in groundbased facilities. The RAE in-flight work with the HP I1 5 slender-delta research aircraft directed to fundamental noise shielding and vortex refraction studies (section 3.3) is a good example. This is now being followed up by HSA research on a modified “Miles Student” aircraft, under MOD(PE) contract and with RAE/NGTE technical collaboration. In some extreme cases-typified by the airframe noise investigations at RAE (section 3.4) flight testing of available aircraft may prove the only technique readily applicable, at least to gain some practical experience rapidly. Moreover, noise measurements with representative new aircraft or engines will ultimately be required for confirmation of project estimates, project development, demonstration and certification. For most noise R&D, however, dependence on a flight testing approach alone would be unduly limiting and costly.

3. SOME CURRENT

RESEARCH

AND

DESIGN

TOPICS

3.1. ENGINE EXHAUST NOISE Although forward radiated noise makes an important contribution to the annoyance caused by aircraft, the maximum noise level in most cases occurs behind the aircraft and is due to noise emanating from the powerplant exhaust. In a turbo-fan engine, exhaust noise consists of the sum total of several components. Specifically identifiable sources are jet mixing, the turbine system, and fan noise propagated rearward down the bypass duct. Additional to these is “excess” or tailpipe noise, whose origins and propagation characteristics are not yet properly understood. The balance between these ingredients of exhaust noise depends upon the engine thermodynamic cycle, the design of certain components such as turbine, fan, and exhaust system, and upon the degree to which absorbent treatment and “silencer” nozzles are used. Moreover, this balance varies with the engine power setting-e.g., between take-off and approach. With this rather complex situation, research aimed at the reduction of engine exhaust noise is now a many-sided activity, covering the noise generation mechanisms and propagation characteristics of the various sources mentioned above.

220

F. W. ARMSTRONG AND J. WILLIAMS

Although NGTE is providing a contract testing service for turbine noise research in the large Anechoic Facility referred to earlier, and is itself conducting research on fans and absorbent liners which has a bearing on rearward propagated fan noise, these topics will not be dealt with further here because of space limitations. The discussion which follows is rather concentrated on jet noise and excess noise. Figure 11 summarizes the scope of NGTE involvement in these closely related research topics. .

CO-ORDINATION

.

JET

NOISE:

OF

STATIC

INTRAMURAL

/ EXTRAMURAL

SINGLE

JETS

RESEARCH

-COLD

AND

PROGRAMMES

HOT, SMALL

CO -AXIAL

JETS - HOT

ANECHOIC

JET

PRIMARY, CHAMBER

‘SILENCER‘

0

JET

NOISE

:

‘FLIGHT’

SINGLE

NOZZLES

JETS -COLD

CO-AXIAL

AND

JETS-HOT

‘SILENCER’

HOT, R.A.E.

PRIMARY,

24tt

TUNNEL

NOZZLES,

FORMULATION

OF

PREDICTION

METHODS.

b

EXCESS

NOISE

MODEL JETS

EXHAUST WITH

SYSTEMS,

HIGH

ANECHOIC

TURBULENCE

AND

1

CHAMBER TUNNEL

Figure 11. Jet and excess noise: work at NGTE.

Jet noise continues after many years as an important subject for research, not only in its own right as a significant engine noise source but now also in relation to clarifying the true extent of the excess noise problem. Experimental work on the noise of low and medium velocity subsonic jets began in the small anechoic chamber in 1969/70. This investigation was prompted both by an observed lack of detailed consistency in existing results in this regime, and by a growing appreciation that the noise of typical engines running at low power was greater than expected. Figure 12 illustrates the latter situation in a simplified presentation

TREhP

OF

CLEh

FvlO3EL

JETS J

-3.4 LOG ,o

-0.2 ,ET

VE,‘?!.

3 :‘r

S33:

0.2 dI!LCSIT”:

Figure 12. Noise of engines and model jets-static.

which compares ground static noise measurements covering various engines, with the general trend of model jet data. Such comparisons gave rise to the term “excess noise” to denote the extra noise component remaining when the known jet and turbine sources had been accounted

SOME RESEARCH

TOWARDS

‘?I ..-

QUIETER AIRCRAFT

for. The widespread speculation regarding the origins of this noise is reflected by the variety of terms which have been used to describe it.-tailpipe noise, extra-jet noise, core noise, etc. Following the development of a suitable plenum silencer to ensure that the test rig would produce an aerodynamically and acoustically “clean” jet over a wide range of temperature and velocity, the jet noise investigation yielded good quality data and revealed temperature effects [I l] not expected on the basis of existing theory. Complementary results were also obtained at about the same time in France, and collaboration resulted in a joint publication [12]. The NGTE work was then extended to measure the noise of co-axial jet systems. with the aim of producing a better basis for estimating the jet noise of future high-bypass ratio turbo-fan engines. Using the same technique of plenum chamber silencing to eliminate extraneous rig noise, a wide range of primary jet temperature, primary and secondary jet velocity, primary/secondary area ratio and other parameters was covered. The resulting data [13] has since been used in the formulation of improved prediction methods. C.‘oncurrently with these basic jet noise studies, many tests were done in the early 1970s on models of “silencer” nozzles for possible application to specific aircraft such as Concorde. Exploratory experiments on the excess noise problem were also commenced in the anechoic chamber in this period. The latter work featured a model engine jet-pipe system in which the turbine exhaust cone and its cambered struts were accurately represented. By mounting adjustable swirler vanes in the approach ducting of this model, and using a series of interchangeable final nozzles of various sizes, a wide range of internal aerodynamic conditions could be obtained. The “excess” noise of the system, relative to the basic jet noise, was correlated with velocity and swirl angle at the struts. Additional information was obtained in separate tests when hot-wire surveys of turbulence were made at various positions in the model. The work has been reported recently in reference [ 141. The persisting difficulty of gaining a full understanding of the noise of engines --and especially of the changes which occur between static conditions and flight-has more recently led to research under simulated flight conditions using the RAE 24 ft wind-tunnel .A test programme with cold model jets was conducted in late 1973/early 1974. As with the anechoic chamber experiments, a plenum type silencer was used to prevent contamination of the jet noise measurements by extraneous rig noise. To avoid the risk of unsteady flow arising in the tunnel airstream as a result of its passage around the bulky silencer, the latter was placed just outside the main tunnel flow at the base of the vertical supply pipe to the jet. Although

+10

c

DERIVED FQOM MODEL JET /TESTS IN 2LFT TUNNEL

TYPICAL

3o”

ENGlhE

5o”

IN FLIGHT

Figure

9o”

7o” ANGLE

13. Noise of engines

TO

JET

ilO

AXIS

and model jets-“flight”.

I

222

F. W. ARMSTRONG

AND I. WILLIAMS

the background noise of the tunnel limited the useful noise measurements to a “flight” speed at 30 m/s (100 ft/s), the results obtained were notable for their consistency and gave a convincing indication of how the noise of relatively “clean” jets varies with forward speed [ 151. Following this encouraging initial experience, a hydrogen jet heating system was developed, together with appropriate safety systems required by the closed-circuit nature of the tunnel facility. A major series of experiments using heated jets was then conducted late in 1974. This covered single jets, co-axial jets with a hot primary stream, and a limited number of tests on “silencer” nozzles. The results are currently being analysed in detail. However, it is already clear that for “clean” model jets under both cold and hot conditions, the effect of simulated flight on a jet of given velocity is to produce noise reductions in the whole of the rearward arc and as far into the forward arc as the test environment permitted. This result from model jet testing contrasts strongly with the measured changes in engine noise between static and flight conditions. The situation is illustrated by Figure 13, where measurements for an engine are compared with a corresponding curve derived from model jet tests in the tunnel. The attenuation in the engine case falls rapidly as the angle to the jet axis increases, and passes through zero to give an increase of noise in the forward arc. Although the curve shown is for a particular engine, similar characteristics have been observed for a variety of engine types-pure jet, low bypass and high bypass ratio turbofansover a range of jet and flight velocities [16]. The above comparison indicates that the problem of engine excess noise (i.e., noise not accounted for by “clean” jet and turbomachinery) is more far-reaching than would be supposed by considering static measurements alone (see, e.g., Figure 12). In particular, the loss of attenuation at high angles to the jet axis is not confined to low power conditions. It follows that the alleviation of this effect would yield reductions not only of approach noise, but also of noise following take-off,. Work aimed at the understanding and control of engine exhaust noise under flight conditions must therefore rate as a particularly important area of engine noise research. Progress has been made by workers in the industries, research establishments and universities on both sides of the Atlantic, but much remains to be done. For instance, a question still unresolved is the degree to which excess noise actually exists as an acoustic signal within the jet-pipe, as opposed to being generated as a result of unsteady aerodynamic phenomena originating internally, passing down the jet-pipe, and then interacting with, say, the nozzle or the jet mixing region to create noise. Further advance on this and other aspects of the problem will require a continuing interplay between theoretical studies of possible source and propagation mechanisms, model experiments aimed at isolating and studying particular effects under controlled conditions, and carefully designed “diagnostic” tests on engines. 3.2. POWERPLANT SOUND ABSORPTION Acoustically absorbent treatment is already widely used in engine installations. It was incorporated from the outset in the current generation of high bypass ratio turbo-fans, and has more recently formed a crucial feature of the .“hushkits” developed for older engine types. With its acoustic effectiveness thus proven, and a steadily accumulating service experience without major maintenance or operational problems, the attractions of seeking further exploitation of this noise reduction technique for future powerplants are powerful. In current powerplants, the absorbent treatment is restricted in most cases to wall lining of intake and exhaust ducts whose lengths are not greatly influenced by acoustic considerations. One route for further exploitation could therefore be to introduce duct splitters and increase the length of the ducting. This would increase the surface area of absorbent material and also produce a more favourable geometric arrangement for sound absorption. Figure 14 shows a possible layout in schematic form, in comparison with a current installation. Large

SOME RESEARCH

TOWARDS

QUIETER

AIRCRAFT

(b) Figure 14. Powerplant extensive treatment.

layouts with acoustic treatment.

(a) Typical current powerplant:

(b) layout with

noise reductions have already been demonstrated in tests of powerplants with extensive acoustic treatment of this type. For example, ground tests by NASA of a heavily treated turbo-fan engine, with STOL applications in mind, showed over 20 PNdB reduction. The choice of such a route for future development would however involve significant problems and disadvantages. Intake splitters may be difficult to de-ice, and they tend to produce both a reduction in fan efficiency and an increase in fan noise at source. Circumferential bypass duct splitters can make thrust reverser design difficult unless their length is kept short. Large increases in intake and exhaust system length would have powerful repercussions on installation weight, and to some extent also on drag. Splitters add weight and produce aerodynamic drag in the duct. The fuel economy of a low specific thrust powerplant, z-

J

r

0

I

I

100

200

CRUISE

Figure 15. Variation of performance

SPECIFIC

I

300 THRUST

I

400

1

500

iN kg-‘s-l1

and installation weight sensitivities with specific thrust.

224

F. W. ARMSTRONG AND J. WILLIAMS

as typified by the modern turbo-fan, is particularly sensitive to duct aerodynamic losses. This is illustrated in a simplified way in Figure 15, where the effect on cruise specific fuel consumption of a 1 % loss of intake total pressure is plotted against cycle specific thrust and bypass ratio. A similar trend occurs with nacelle weigh-also plotted on Figure 15though for the future some alleviation should be available from the application in nacelles of more advanced materials such as composites. The attendant economic penalties therefore place limits on the use of more extensive acoustic treatment in powerplants. This constraint is now accentuated by the recently magnified importance of fuel consumption as a component of direct operating cost. The situation calls for research emphasis to be placed on improving the acoustic effectiveness of treatment configurations which offer low penalties. Progress will be aided if better understanding can be gained-in the following areas : (1) the basic mechanisms by which liners absorb sound in the presence of an airflow; (2) the interaction between various types of noise sources, duct shapes and liner designs; (3) as a special case of item (2), the relationship between liner behaviour in typical flow duct test facilities and in actual engine environments; (4) the relationship between static ground testing of engines and the actual flight situation, in regard to the effectiveness of absorbent treatment. The further development of techniques for detailed measurements close to and within liner elements is particularly relevant to items (1) and (2) above. Concurrently with a search for generally improved understanding as outlined above, new designs of liner need to be carefully evaluated under appropriate test conditions. These include various multi-layer types, and bulk-filled liners designed to withstand engine environments. .

CO-ORDINATION

.

CONTRACT

.

RESEARCH

ON

.

DEVELOPMENT ( TWO

OF LOCAL MICROPHONE

.

DUCT

OF

INTRAMURAL

TESTING

ATTENUATION

LINER

/ EXTRAMURAL

-

SERVICE

LARGE

PERFORMANCE

IMPEDANCE METHOD 1

STUDIES

:

RESEARCH

ABSORBER

-

PROGRAMMES

FACILITY

ABSoRBEA

FAC’L’TY

MEASUREMENT

/

-

(a)

DEVELOPMENT

(b)

INTERPRETATION

Cc)

STUDIES

OF

Figure 16. Powerplant absorbers:

FAN

SMALL

OF

NOISE

IIUCT

COMPUTATION OF

TEST

TREATMENT

FACILITY

R’GS

/

AEISORBER

FACILITY

METHODS

DATA CONFIGURATIONS

work at NGTE.

NGTE is heavily involved in this important research field. Figure 16 indicates, in tabular form, the nature of current activities on various aspects of absorbent treatment. Considering these briefly in turn, reference has already been made to the responsibility for co-ordination of government-funded research, and to the fairly recent construction of the large Absorber Facility. Since it entered service in late 1972, this facility has been operated for several hundred hours on extensive programmes of testing for both engine and airframe industries. Smaller experimental programmes have been concerned with NGTE in-house research work and with development of the facility and associated measurement techniques. Under the latter heading, particular attention is being paid to the application of the “two-microphone” method of local impedance measurement. The resistive and reactive components of the

SOME RESEARCH

TOWARDS

QUrETER AIRCRAFT

l-l_->

impedance of an absorber cell can in principle be determined from measurements of the amplitudes and phase differences of the acoustic signals at the face of the liner and at its rear wall. However, the technique is by no means easy to apply. The achievement of satisfactory accuracy requires the development of special microphone probes, calibration techniques and data handling methods. The potential outcome is a means for readily measuring the local acoustic performance at chosen points in an absorbent liner during the same test in which an “average” value for liner impedance is obtained from overall measurements of duct attenuation between the reverberation chambers. Looking further ahead still. the two-microphone method holds out the possibility of making irl .sif~ measurements of :mpedance at chosen points in actual engine ducting. Taking advantage of the availability at NGTE of absorbent duct flow facilities and ot‘an anechoic chamber in which model fans can be tested, another current research programme aitns to establish whether the attenuation of a lined duct is altered when a turbomachine fortns the noise source. There is evidence from various quarters (e.g., reference [17]) to suggest that in the case of sound propagation against the airflow, attenuations arc ot’ten higher when a liner is installed in an engine intake. Controlled comparative tests are therefore being performed in which given sets of liners are mounted first in the intake duct o!‘a model trsnsonic fan, and then within a circular test duct in the .4bsorber Facility. In parallel with the experimental programmes, work is proceeding on the development of computational techniques for estimating the acoustic performance of absorbent ducts. These cater for ducts of both circular and rectangular cross-section, the latter case bemg treated as an approximation to a sector of an annular engine duct. Such techniques form an essential basis for the scientific interpretation of experimental data and for assessing the noise reductions offered by various powerplant treatment schemes. Finally, although the main objective of research on acoustic treatment is to pave the way for further reductions of powerplant noise, the potential benefits in two other directions deserve mention. First, improved understanding should allow more accurate predictions ot treatment effectiveness at the design stage, thereby lessening the technical risk and devrlopment cost associated with new projects. Secondly, there exists an interaction between the available standard of acoustic treatment and the constraints on engine design. An improvement in treatment standard might, for example, allow a particular new engine desrgn to bc accomplished with one less turbine stage by using a fan having increased tip speed and therefore higher source noise. The benefit in such a case would be that a lighter and less expensive engine could be designed to meet a given installed noise requirement. 3.3. AIRFRAME SHIELDING

OF ENGINE NOISE

Shielding of engine noise sources, by the airframe surface (acting as an acoustic barrier) and including edge diffraction effects, can give substantial attenuation of noise at observers within the acoustic shadow on the ground, in particular directly beneath the aircraft flight path for engines mounted above the airframe structure as in Figure 17(a) [ 18, 191. Theoretical treatments are equivalent in their most tractable form to Keller’s theory for diffraction in postulating that the diffracted field is determined by the ray propagating along the shortest path from source to observer via the diffracting edge, The intensity of such an incident ray can be derived directly from the polar diagram of the particular noise source with the behaviour of the ray determined from the known exact solution for diffraction by a sharpedged half-plane. This exact theory itself agrees well with RAE experimental results (for a point source and half-plane) which exhibit increasing attenuation with decreasing wavelength and reach over 20 dB attenuation well within the geometrical shadow, falling to about S dB attenuation at the geometrical shadow boundary, and then reducing to zero attenuation outside the transition zone. For a finite practical shield, the diffracted field associated wirh

226

F. W. ARMSTRONG

AND J. WILLIAMS

/ ;AILPLANE REARWARD

WING SHIELDS FORWARD NOISE

I

sHm_Ds NOISE

(a)

--___-(b) Figure 17. (a) Airframe surface shielding engine noise; (b) ray paths through a vortex.

each edge is calculated and these intensities are then superimposed, neglecting the phase differences between rays from different edges in the case of broadband noise. Strictly speaking, the ray theory treatment is valid only for wave-lengths 1 which are not large compared with a typical path length I; or more formally the non-dimensional wave number 27rl/i. 9 1. Additional shielding of the engine noise sources, by the airframe flow field, can arise from the refraction of sound through intense velocity gradients (or temperature gradients), in particular to the side of the flight path when the engines are located inboard of the strong trailing vortices associated with wing high-lift conditions. The vortex flow field can cause not only simple deflection of the ray paths, but also ray spreading and focusing so that the sound energy is redistributed to give regions of sound attenuation and augmentation respectively as in Figure 17(b). Both acoustic ray theory and experimental results [20], for propagation through a conical vortex shed from the sharp swept leading-edge of a delta wing at incidence, have exhibited regions containing up to nearly 10 dB attenuation and augmentation, with overall noise reduction benefits to the side-line areas from which the noise source is seen through the vortex rather than from over the top; though quantitative agreement was not achieved between the theory and experiment. Recent research on airframe shielding under in-flight conditions [18] includes complementary experiments at both full-scale and model-scale on a small delta-wing research aircraft, the Handley Page HP 115, which offered simultaneously a large wing area for surface shielding, a simple planform amenable to diffraction calculations, and strong conical vortices providing substantial refraction effects. A Hartmann-type generator was mounted centrally above or below the wing, generating a strong fundamental tone (3 kHz full-scale) which was easily discernible above the aircraft engine noise in flight and above the 24 ft tunnel background noise in the +-scale model experiments. From analysis of the model results, the significant attenuation inside the airframe shadow by surface shielding could be

SOME RESEARCH

.

TOWARDS

VORTEX REFRACTION

AIRCRAFT

721

TOTAL

SHADOW +

QUIETER

BOUNDARY

4.0

Figure 18. Noise reduction below HP1 15 model at IO” incidence.

clearly separated from that to the sideline by vortex refraction (Figure 18). Encouraging agreement was found between the levels of wing surface shielding measured inside the shadow under comparable flight and wind-tunnel conditions (Figure 19). The theoretical prediction shown here represents shielding by a simple half-plane (single edge), which can be regarded as an upper limit to that achievable, the difference from the experimentally measured values being broadly commensurate with diffraction effects from only one instead of all three edges of the wing,

NOISE REDUCTION (dB)

1

/--”

,/

-

b

I

200

100

0

100

200

0 OUT OF SHADOW -

Figure 19. Comparison

400

ANGLE +5

plane); ---,

JO0

--.

500

RELATIVE SHADOW

INTO

(B-25

dB)

I

60’ TO

70”

GEOMETRIC

BOUNDARY

SHADOW

between noise reduction measured in flight and wind-tunnel. tunnel: 0, flight.

-----,

Theory (half-

Other static-rig experiments have broadly confirmed expectations that the surface shielding effectiveness for fan intake noise is commensurate with sources located sensibly in the plane of the intake-entry, while the shielding for jet exhaust noise corresponds to sources extending several nozzle diameters downstream unless rapid-mixing devices are incorporated. Recent wind-tunnel experiments also imply that the shielding effectiveness for jet exhaust noise tends to reduce with increase in tunnel airspeed. Computer programs have been developed at RAE for calculating the noise signature of

228

F. W. ARMSTRONG

AND J. WILLIAMS

general aircraft configurations featuring shielding of engine noise by the airframe surface, including estimation of the PNL time-histories and EPNL values. These have already been used in aircraft project studies of possible designs involving shielding by the wing, tail-unit, and fuselage. Nevertheless, the reliable application of such computer programs still involves difficulties associated with ensuring adequate input data as well as adequate methodology, for the complex types of noise source distributions, airframe geometry, and flow-field effects associated with practical aircraft designs under flight conditions. 3.4. AIRFRAME SELF-NOISE The significance of airframe self-noise, generated in flight by the relative airflow over the airframe, has become of increasing practical concern particularly in respect of landing approach noise with engines at low power settings [21, 221. Some fundamental concepts of the basic aero-acoustic mechanisms have been postulated but, even if the acoustic output could be realistically formulated as specific functions of the appropriate aerodynamic inputs, there are practical difficulties in predicting the latter in absolute terms. Hence, the available techniques for the practical estimation of airframe noise are still largely crude and empirical, envisaging simple acoustic sources (usually dipoles) as the noise generators, and incorporating simple correlation parameters such as airspeed, lift coefficient, drag coefficient and wing loading. Wind-tunnel experiments have proved difficult because the background noise from the tunnel in operation and from the model-rig tends to be comparable with airframe noise levels from the model itself, usually increasing together with increasing tunnel airspeed. Thus, investigations have so far been mainly associated with full-scale flight tests on a variety of aircraft; the work in the USA since about 1968 deserves special mention (e.g., reference [22]). RAE flight experiments [21] were started some two years ago on jet-engined aircraft, including the HP 115 delta-wing research aircraft, the HS 125 executive transport, the BAC 111 short/medium range transport and the BAC VC 10 long-range transport. Special flight and measurement techniques were developed, with the aircraft flown over a large number of microphones at as near constant speed as possible and at nominally constant altitude, keeping all the engines at flight idle (the lowest rpm permitted). Typically, third-octave band spectra measured immediately beneath the VC 10 aircraft (Figure 20) exhibit large increases

60

I

I

40

63

I 100

I

I,

160 250 400

I,

I,

630

Ik

CENTRE-FREQUENCY Figure

20.

VClO noise spectra: comparison

of “landing”

/

1.6k 2.5k

4k

IOk

(Hz)

and “clean” configurations:

at ISOm. OASPL (dB) (40 Hz-l .6 kHz) 3 161 knots landing configuration n 161 knots “clean” aircraft x static engine noise

/

e3k

94.5 85.5 81.4

aircraft overhead

SOME RESEARCH AOASPL 0

I

dB

4

OASPL 6

5

7%) __.

QUIETER AIRCRAFT

(OASPL’DIRTY’-

3

2

TOWARDS

‘CLEAN’)

7

I

9 1

8

I

IO I

II I

SLATS

FLAPS

20’

FLAPS

35’

FLAPS

45’

GEAR

DOWN,

GEAR

DOWN,

FLAPS

45’=,

I

I

DOORS

SHUT

DOORS

SLATS

OPEN

OUT,

I

GEAR

DOORS

DOWN,

SHUT

(

Figure 21. VClO noise increases due to changes from the “clean” aircraft configuration noise extracted).

(residual engine

in noise levels at low frequencies, below about 1 kHz, due to the operation of the landing devices. The OASPL values for the airframe noise with the deployment of individual devices confirm the significance of flap angle, slats, U/C gear and U/C doors, when compared with the clean aircraft levels at the same airspeed (Figure 21); overall there is 11 dB difference between the landing and clean aircraft configurations. Narrow band analysis of the noise signature has also enabled the predominant frequency particular sources to be largely identified. The marked rise in airframe noise level with increasing

components airspeed

too

attributable

is illustrated

to these

by Figure 22,

PREDICTED (CLEAN) .’ .’

m

.’ /=

2

2

MEASURED

/’ 90

::

/

0 :

80

(CLEAN)

.’ /’

,,;Q

I

I

I

I

140

160

180

200

Al RSPEED

I 220

I 240

I

260

( KNOTS)

Figure 22. VCIO: Measured and predicted OASPL at 180 m altitude (residual engine noise extracted). x, “Clean” aircraft: ! , undercarriage down; O, landing configuration.

for the VC 10 in both the clean and landing configurations. The measurements for all four aircraft imply variations of overall sound pressure levels with a somewhat lower power of airspeed than the V6 relation appropriate to dipole sources. The measured OASPL levels in the clean configurations are appreciably lower than the “clean” estimates from Gibson’s empirical formula [22] : OASPL (dB) = 10 log [( V6//?) +pc2] + 8.4, for altitude h (ft), mean chord c (ft), airspeed V (kn), and air-density p (slugs/ft3). Those measured for the landing configuration are naturally higher; see Figure 23. The sideline measurements also give OASPL levels appreciably higher than would be expected from

230

F. W. ARMSTRONGAND J. WILLIAMS

vc IO 1 ”

105 HP 115 ,, )’

WL

VW

AIRCRAFT

._

DIMENSIONAL

PREDICTED (CLEAN)

__

PARAMETER

Figure 23. Measured and predicted OASPL for four aircraft at 160 knots airspeed and 45 m altitude. x, “Clean” aircraft; n, undercarriage down; 0, landing configuration. representation by simple vertical dipoles at the aircraft position. Thus, although such simple empirical concepts and formulae may be helpful, available frameworks for the prediction of airframe noise levels of new aircraft configurations still seem far from adequate. In this connection it should be noted that excess nnise can also arise from mutual aerodynamic interference between the aircraft components, such as with turbulent unsteady flows from upstream components interacting on downstream surfaces, and by the influence on the airframe of even small flows out of the engine (nozzle efflux or intake spillage). Overall, analysis of experimental results at RAE and elsewhere does imply that airframe self-noise levels for aircraft with conventional landing configurations are likely to be only about 10 EPNdB below the current certification levels of FAR Part 36 and ICAO Annex 16. Further research and technical appraisal seems justified on practical techniques for reducing the airframe noise of future aircraft designs, possibly including better airflow control over the airframe in the landing configuration, modification of airframe acoustic impedances, and steeper/slower approach or low-drag approach.

4. AIRCRAFT

DESIGN

SYNTHESIS

AND OPTIMIZATION

The extensive advances of digital computer technology and numerical optimization techniques over the past two decades have made possible the development of large computer programs for design prediction and optimization in the individual aeronautical disciplines, together with interdisciplinary programs to explore and optimize design synthesis of the whole aircraft (or system). By allowing flexible matching of declared major design parameters, for the airframe and powerplant in particular, the application of such programs can provide early guidance towards a better balance of aircraft flight performance, airworthiness, operational economics and noise characteristics. Naturally the outcome will depend on the compound optimization function (a,nd relative weightings) preferred by the user, on the standards of technological advances (and costs) assumed, and on the particular missions chosen, all of which can be related to the market and time-scale envisaged for the production aircraft.

‘31

SOME RESEARCH TOWARDS QUIETER AIRCRAFT

The RAE multivariate design synthesis and optimization computer program [23] has been developed and used mainly for preliminary studies of new subsonic swept-wing transport aircraft, the analytic framework of the design synthesis method being subjected continually to revision with experience [24]. The present set of up to 20 Design Variables (to be optimized)

MISSION WING*

TAIL* SIZE

[CRUISE*

CONSTRAINTS ’

AIRFIELD

1

GEOMETRY. S,ZE DESIGN C,.M

*

SIGNIFIES

TYPICAL

DESIGN

VARIABLES

Figure 24. Schematic illustration of some design variables parameters and constraints, and mission requirements and constraints, defining an airframe/engine for multivariate design synthesis and optimization.

essentially define the principal characteristics of the airframe and engine in absolute terms as in Figure 24. A complementary set of about 60 Design Parameters (e.g., numerical coefficients) are given prescribed values according to the assumed standards of aerodynamic, structural and engine technology. These design variables and design parameters are used in specific design relationships to define the performance capabilities, mass breakdown, direct operating cost (DOC), noise footprint, etc., of the aircraft. Particular Mission Requirements (e.g., range) are specified which the performance of the aircraft must satisfy, without violating other prescribed Mission Constraints (e.g., airspeed limits) and other prescribed Design Constraints (e.g., stability limits); see Figure 24. This computer program can first ad-just trial values of the design variables to obtain a “Feasible Solution” for an aircraft design which would perform the specified mission, while satisfying the mission and design constraints, keeping also within the acceptable practical limits allowed for the various design variables. This result provides a starting point for optimization of the design variables by search/ gradient techniques, to achieve a minimum or maximum value of a selected Optimization Function (e.g., DOC), still meeting the same requirements and constraints. Here, we can illustrate only briefly some relevant applications to the design of economic quieter aircraft for short-medium range operation at subsonic speeds, and to related R&D planning. Investigations have been carried out on alternative methods of exploiting advanced technology, representing what might be achieved in a “next generation design” (in-service early 198Os), on the assumption that R&D in the appropriate areas continues and attains the degree of success currently predicted. Figure 25 compares results [24] for the feasible alternative improvements in DOC, cruise Mach number and field-length, when a CTOL aircraft design based on current technology standards is completely re-optimized to exploit the advances in wing aerodynamic design at high subsonic speeds without attempting substantial reduction of the noise footprint area. It should be noted that the feasible DOC reduction of nearly 10% would be halved if the aerodynamic advances were used only to re-size the aircraft, allowing an improvement in aerofoil section shape but keeping the wing planform geometry and thickness-chord ratio unchanged. Another example concerns the effects of anticipated technological advances, increased fuel costs, and changes in mission specification on feasible optimum designs (min DOC) of next

/

Figure 25. Illustration CTOL aircraft design.

of comparative

may

incorporate

= 0.34

swept-wing

9

performance

\

d.o.c.

design synthesis

and optimization

computer

program

for a

d.o.c.

AR = +3000 km

ran*

Longer

Datum

\

Datum

or

either

Ad=-6OOm

\

field,

to attain

in cost, speed, range and field

h..

technology

=28

A =8-O

some improvements

using advanced

results from the multivariate

aircraft

d.o.c.

Datum

performance

Datum

J

+O-06

speed

High/

AM=

The best reoptimised

cost

reoptimisad

Ad.o.c. = -9%

/

Lower

operating

Aircraft

ratio

Wing sweep

Aspect

W/S = 530 T/w

Thrust/weight

ratio

Wing loading

km

deg

d =2000m

max.

I’? = 4000

payload

M=@88

Range with

spead

AIRCRAFT

Take off distance

Cruise

DATUM

SOME RESEARCH

TOWARDS

QUIETER AIRCRAFT

23.3

generation CTOL subsonic transports for short-medium range operation. Typical assumed improvements over current design standards included an advanced aerofoil section, a new engine of slightly lower specific thrust and higher turbine temperatures, partial use oftitanium instead of aluminium, and also reductions in furnishing/systems/electronics weights. Figure 26 implies that these envisaged advances of technology together could reduce the DOC of

LOW \

SF’ THRUST ENGINE

\ \ \

d

,”

\ lo-

\ \

z

M=O9

\

w CFRP

LONGER FIELD

TECHNOLOGY

0

20

IO %

FUEL

30 SAVED

Figure 26. Effect of design standards on DOC and fuel saving. the optimized design some 15 % compared with current standards, but that this could be counterbalanced by a doubling of fuel price despite the resulting fuel mass saving of nearly 20 %. The area of the 80 PNdB footprint is reduced to about one-third, largely because of the quieter advanced engines. The further trends in DOC and fuel saving are also shown from altering in turn the permitted cruise-speed or field-length, from using engines of much lower specific thrust, or from incorporating composite materials (CFRP) extensively in the aircraft structure. As regards reduction of noise annoyance with minimum cost penaltieb, some feasible technical choices in engine and airframe design have been assessed (together with their interactions) by optimization studies carried out jointly at RAE and NGTE particularly for CTOL and STOL short-range transports [24, 251. Here, the optimization function is taken first as minimum DOC. However, by adopting a noise-biased optimization function [(DOC) + k x (NFA)], with a coefficient k regarded as a “surcharge rate” (&/km2) for the noise footprint area NFA, the minimum cost penalty for some reduction of footprint area can be assessed from optimizations with various values assigned to k. For expediency, the noise annoyance index is taken as the nett area contained by the 80 PNdB footprint, corresponding to a combination of take-off ground-run and climb-out, plus a landing approach and ground-run. The choice of EPNdB or an even more elaborate index would complicate the parametric analysis unreasonably, without much clarification of the major factors of interest in these research studies. The CTOL and STOL studies illustrated by Figure 27, for field lengths of 1500 m and 750 m respectively and a cruise Mach number of about 0.8, assume an orthodox layout with a high aspect-ratio wing of low sweepback, advanced mechanical high-lift devices, and two high bypass-ratio engines mounted conventionally; i.e., using modern design standards, but without attempting to incorporate airframe shielding of engine noise or powered lift

234

F. W. ARMSTRONG AND J. WILLIAMS

” 5

I.4

STEEP

STOL -GRADIENT

t it5 k

I.2

-

MIN

DOC

-

__------L it 5

1

I.0

?‘ CTOL DATUM

STEEP-GRADIENT

0

S

80

Figure 27. Economic engine characteristics.

IS

IO PN dB

I 20 FOOTPRINT

25 AREA

penalties for reduced noise exposure. --,

30

35

40

% Km’

Aircraft design optimized with varied

augmentation. The datum CTOL aircraft design optimized for minimum DOC then has engine characteristics corresponding closely to the latest existing types, with specific thrust about 300 N/kg/s (bypass-ratio z 5) and with acoustic lining of the ducts to lower the noise levels by about 6 PNdB compared with the untreated engine. The scope for reducing the noise footprint area of the CTOL aircraft further by taking advantage of steep-gradient techniques appears somewhat limited if re-optimization of the airframe design variables and associated engine size (thrust) alone is allowed, without radical changes in design standards. Possibly reduction of the 80 PNdB footprint area down to three-quarters of the CTOL datum value, equivalent to an assumption of about 2 PNdB quieter engine, could be achieved at a modest economic penalty; between 1% and 5 % DOC increase, depending primarily on the steep-gradient techniques employed and the minimum safe thrust levels permitted on the approach. Aircraft re-optimization biased towards much greater reductions in noise footprint area, at least cost but accepting an appreciable increase over minimum DOC, can usefully be explored by allowing possible variations in other engine design parameters besides engine size. Here, these include more extensive and more complex acoustic treatments with resulting penalties in engine mass/sfc/cost, changes in specific thrust, and trade-offs between permissible maximum ratings at take-off and cruise [25]. For example, a reduction of the 80 PNdB noise footprint area down to one-quarter of that for the CTOL datum is possible for a CTOL steep-gradient aircraft incorporating about 12 PNdB of silencing instead of 6 PNdB, specific thrust of 250 N/kg/s instead of 300 N/kg/s, and a take-off rating factored by 0.9; though now with a penalty of about 10 % on DOC (Figure 27). The reduction of noise footprint area from employing lower thrusts and/or steeper gradients, for take-off and fully-flared landing at least, tends to require increased lift/drag ratios, leading to higher values of wing aspectratio and flap-chord ratio, with lower values of sweep, wing-loading, “installed thrust”, and approach speed. STOL aircraft designs using mechanical high-lift devices do not provide much further reduction in noise footprint area when compared with steep-gradient CTOL aircraft, while they have substantially higher DOC (Figure 27), though in some cases the decrease in footprint length at the expense of greater width may be attractive. Such simple attempts to decrease noise by capitalizing on a possible requirement for much shorter airfield distances

SOME RESEARCH

TOWARDS

QUIETER AIRCRAFT

23s

than CTOL, over and above the acceptance of steeper gradients, appear to be unprofitable largely because some airframe design variables become constrained by their practical limits (e.g., near zero sweepback, max flap-chord ratio, min safe approach speed) to ensure short landing distance, while take-off thrust needs to be higher to achieve short take-off distance. The alternative introduction of powered-lift augmentation brings in other and more novel considerations, at least for civil transport applications, introducing further assessment complexities as regards flight performance, noise characteristics and costs. Overall, specific thrust values below about 200 N/kg/s (cf 300 N/kg/s currently) do not seem attractive unless cruising Mach numbers below 0.8 are acceptable to accommodate the then much reduced value of cruise thrust to take-off thrust. With such low specific thrusts, engines incorporating variable-pitch fans might offer significant benefits, at least for aircraft designs intended to take advantage of very steep landing-approach techniques. For most new designs, a careful blend of airframe/engine re-optimization together with some engine silencing can be significantly more economic than relying on engine silencing alone. Worthwhile reductions in noise could result from improving airframe components; e.g., by reducing the drag of high-lift devices, and from the use of advanced materials in wing construction leading to a higher aspect-ratio. Also, airframe shielding of the engine noise sources could possibly provide noise reductions of the order of IO PNdB, likewise reducing noise-footprint areas by as much as three-quarters. Naturally, more detailed assessment of aircraft design implications and possible cost penalties is needed to quantify such arguments. 5. CONCLUDING

REMARKS

In this paper we have attempted to illustrate the nature and scope of some current work at the Establishments which is aimed at the achievement of reduced noise nuisance together with good operating economics. A measure of selectivity has been essential, so that some important items have scarcely been mentioned. Among the latter are research on fan noise generation and propagation, the study of powered-lift systems, and research on the use of steep-gradient and/or low-drag approach techniques for noise reduction, all of which are the subject of active programmes. We have likewise refrained from attempting to discuss all aspects of the aircraft noise reduction problem. While an obvious and primary aim is that future transport aircraft should have still better noise characteristics than the latest “wide-body” designs, two further aspects deserve at least a mention. First, the next decade will see further development of the current large wide-body aircraft which will be “stretched” in the classic manner to provide versions offering greater range and/or payload. There will be increases of take-off weight and a requirement for extra engine thrust. As these developments are put in hand, economic considerations will certainly dictate that major design changes be minimized. Thus an important task will be to resist adverse trends in noise, by detailed powerplant and airframe improvements. Secondly, it should be noted that military aviation also has its noise problems. These appear in a variety of forms, ranging from nuisance to civilian communities arising from peace-time training operations, to the need to lessen the vulnerability of low-flying aircraft by reducing the aural warning of their approach to enemy-occupied areas. The understanding gained from research on the noise of both fixed-wing aircraft and rotorcraft, stemming essentially from civil aviation needs, is already proving of significant value’in assessing and alleviating such problems. In conclusion then, it seems clear that continuing and integrated effort from research and development workers will be needed if community pressures for an improved noise environment are to be reconciled with the economic realities which apply to air transport. The activities outlined here represent one aspect of the total UK response to this important problem facing the international aviation fraternity.

236

F. W. ARMSTRONGAND J. WILLIAMS ACKNOWLEDGMENTS

This paper is based on a lecture to a Royal Aeronautical Society symposium concerning “The impact of economics on the design and operation of quieter aircraft”, April 1975. The authors much appreciate helpful technical discussion with their colleagues at NGTE and RAE towards the preparation of this paper. Acknowledgment is due to the Ministry of Defence (PE) and the Department of Industry for permission to publish the paper. However, the opinions expressed therein are personal and do not necessarily represent official views. REFERENCES 1. J. SEDDON1973 Aeronautical Journal 77, 459-464. Research and development for future air transports. 2. F. W. ARMSTRONG1975 Aeronautical Journal 79, 15-27. Recent developments in noise research at the National Gas Turbine Establishment. design, 3. J. L. FLINTOFF 1971 Aeronautical Journal 75, 397-406. The Ansty Noise Facility-its instrumentation and future commitments. 4. M. G. PHILPOT 1975 American Institute of Aeronautics and Astronautics Paper 75-477. The role of rotor blade blockage in the propagation of fan noise interaction tones. 5. D. L. MARTLEW 1975 American Institute qf Aeronautics and Astronautics Paper 75-529. The use of Hartmann generators as sources of high intensity sound in a large absorption flow duct facility. 6. J. WILLIAMS1975 AGARD Advisory Report 83, Appendix 4. Problems of noise measurement in ground-based facilities with forward speed simulation. 7. T. A. HOLBECHEand J. WILLIAMS1972 AGARD Report 601, Paper 8 (RAE Technical Report 72155). Acoustic considerations for noise experiments at model scale in subsonic wind tunnels. 8. J. WILLIAMSand T. B. OWEN (to appear). Some acoustic and aerodynamic analysis for possible improvement of the RAE 24 ft-diameter open-jet low-speed wind-tunnel. 9. W. SMITH1974 2nd International Symposium on Air-Breathing Engines. The use of a rotating-arm facility to study flight effects on jet noise. 10. B. W. LOWRIE1975 American Institute of Aeronautics and Astronautics Paper 75-463. Simulation of flight effects on aero-engine fan noise. 11. B. J. COCKING 1974 NGTE Report No. R331. The effect of temperature on subsonic jet noise. 12. R. G. HOCH, J. P. DUPONCHEL,B. J. COCKING and W. D. BRYCE 1973 Journal of Sound and Vibration 28, 649-668. Studies of the influence of density on jet noise. 13. B. J. COCKING (to appear) NGTE Report. An experimental study of co-axial jet noise. 14. W. D. BRYCEand R. C. K. STEVENS1975 American Institute of Aeronautics and Astronautics Paper 75-459. An investigation of the noise from a scale model of an engine exhaust system. 15. B. J. COCKINGand W. D. BRYCE1975 American Znstitute of Aeronautics and Astronautics Paper 75462. Subsonic jet noise in flight based on some recent wind-tunnel results. 16. K. W. BUSHELL1975 American Institute of Aeronautics and Astronautics Paper 75-461. Measurement and prediction of jet noise in flight. 17. J. H. DITTMARand J. G. GROENEWEG1974 NASA TN D-7826. Effect of treated length on performance of full-scale turbofan inlet noise suppressors. 18. R. W. JEFFERYand T. A. HOLBECHE1975 American Institute of Aeronautics and Astronautics Paper 75-513. An experimental investigation of noise-shielding effects for a delta-winged aircraft in flight, wind-tunnel and anechoic room. 19. G. HELLSTR~~M 1974 ZCAS Paper 74-58. Noise shielding aircraft configurations: a comparison between predicted and experimental results. 20. G. F. BUTLER,T. A. HOLBECHEand P. FETHNEY1974 AGAR D CP 131, Paper 9. Some experimental observations of the refraction of sound by a rotating flow. 21. P. FETHNEY1975 American Institute of Aeronautics and Astronautics Paper 75-511. An experimental study of airframe self noise. 22. J. S. GIBSON1974 ZCAS Paper 74-59. Recent developments at the ultimate noise barrier. 23. D. H. PECKHAM(Unpublished.) Multi-variate analysis applied to aircraft optimisation-a first progress report. 24. D. L. KIRKPATRICKand M. J. LARCOMBE1973 AGARD Conference Proceedings 147, Paper 19. Initial-design optimisation on civil and military aircraft. 25. D. R. HIGTONand T. A. COOK1973 AGARD Conference Proceedings 135, Paper 22. Some engine and aircraft considerations affecting noise.