Helicopter noise certification

Applied Acoustics 23 (1988) 213 230

Helicopter Noise Certification* A. C. Pike Helicopter Division, Westland plc, Yeovil, Somerset BA20 2YB (UK)

S UMMA R Y The object of noise certification is to reduce aircraft noise at source by incorporating internationally agreed limits into national civil airworthiness requirements. The operation of aircraft which fail to comply with these limits during type tests is prohibited, providing manufacturers with a powerful incentive to design quieter machines. In the case of helicopters, noise characteristics are 'designed in' at an early stage, making accurate noise predictions an essential prerequisite to the introduction of new civil helicopters. Noise certification will not only result in progressive reductions of helicopter noise, but will also change the character of the noise as the more intrusive sources are avoided, either by design or by the use of noise abatement techniques.

1 INTRODUCTION After many years of discussion within the International Civil Aviation Organisation (ICAO), helicopter noise certification is being introduced. This means that, in c o m m o n with fixed-wing subsonic aeroplanes, new civil helicopter designs (including certain modified or derived versions of existing types) must demonstrate compliance with maximum permitted noise levels. Failure to meet the appropriate noise limits precludes the issue of the Noise Certificate, without which the aircraft cannot be operated. The aim of noise certification is to reduce noise exposure, particularly in the vicinity of major airports, by encouraging the design of quiet aircraft. As such, it is potentially a very powerful method of limiting noise levels. Given such power, noise regulations must ensure that flight test results are * The views expressed in this paper are those of the author and do not necessarily represent the views of Westland Helicopters plc. 213 Applied Acoustics 0003-682X/88/$03-50 {~ Elsevier Applied Science Publishers Ltd, England, 1988. Printed in Great Britain

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repeatable, and that markedly different designs are assessed on an equitable basis, permitting direct and unbiased comparisons between aircraft types. The characteristics of helicopter noise, both objective and subjective, differ a great deal from type to type, being very strongly influenced not only by the choice of design parameters but also by the operating technique. Regulatory processes must be constructed to take due account of such differences in order that helicopter noise be controlled in a reasonable and systematic manner. Aircraft of all types are, of course, operated across national boundaries and must satisfy standards imposed by those countries in which they are to be used. Noise regulations have therefore been developed on an international scale so that an aircraft certificated in its country of origin is also acceptable to other states wherein noise limits apply. The development of noise flight testing procedures, units of measurement and maximum permissible noise levels is the responsibility of the ICAO Committee on Aviation Environmental Protection (CAEP)--formerly the Committee o n Aircraft Noise (CAN). International standards and recommended practices are published in Annex 16 of the Convention ~[ international civil aviation.

It should be noted that the annex does not itself constitute the actual standards that might be applied by individual Member States--these are given in national airworthiness requirements. It is intended, however, that the wording contained in the annex should be incorporated into national standards and it is structured to make this as easy as possible. Noise certification is granted on the basis that national requirements are at least equal to those given in Annex 16. Member States are required, in addition, to notify ICAO of any differences that may exist between national legislation and the wording of Annex 16 on a continuing basis.

2 HELICOPTER NOISE STANDARDS

2.1 Background Standards for noise certification of helicopters were introduced into Annex 16 at the sixth meeting of CAN (CAN 6) in June 1979. These standards had been developed during extensive studies of helicopter noise carried out by CAN Working Group B. The noise limits were derived from mean noise levels of several different types of helicopter measured during these studies, together with some increase in stringency in anticipation of improved technology that would permit the design of quieter helicopters. The standards were not actually applied because of concern over the

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economic impact on the helicopter industry, particularly when set against the reduction in noise that would have been achieved. In fact the rules were, with the benefit of hindsight, based on arguments that were unduly optimistic. The time required both to develop low noise technology and the problems of predicting helicopter noise during the design stage were seriously underestimated. Consequently, a Working Group B subgroup was established to examine the 'economic reasonableness' in consideration of 'stringency and extent of applicability of helicopter noise standards in the future'. The subgroup was handicapped by a shortage of time and unstable world economic conditions. In spite of this, it was able to conclude that the impact of the helicopter noise certification requirements given in Annex 16, Chapter 8, could adversely affect all categories of helicopters. 2 At CAN 7 (May 1983) it was decided to raise the limits for each of the three flight test conditions by 3 EPNdB for both derived versions and new designs. This situation remains today.

2.2 Structured repeatability test The Helicopter Noise Measurement Repeatability Programme (HNMRP) was initiated at a Working Group meeting in 1983, as a result of concern over the degree of variability in noise levels of the same helicopter type when measured by different test groups, both using CAN procedures. The objectives of the programme were twofold; first to establish the magnitude of differences in certification noise level (EPNL) obtained by the individual test groups, and secondly to discover the source(s) of those differences which were statistically significant. A very clear distinction had to be made between differences attributable to measurement and/or analysis techniques and those which result from changes in the source noise characteristics of the aircraft. The significance attached to the H N M R P may be gauged by the following details: Nations which acquired certification experience Australia Brazil Canada France Federal German Republic Italy Japan United Kingdom United States of America

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Noise measurement flight test programmes conducted by Australia Brazil Japan Canada-United States Federal German Republic-United Kingdom France-Italy-United States Aircraft and helicopter manufacturers participating directly Aerospatiale Agusta Bell Textron Kawasaki Sikorsky Westland Total number of flight test conditions carried out: 529 The aircraft used for these trials were the Bell 206L-1 or the acoustically equivalent Bell 206L-3 Long Ranger. This particular helicopter type was chosen simply because its wide availability enabled the greatest number of independent tests to be carried out. The flight tests began in the summer of 1984, with sufficient results being available for a programme evaluation meeting to be held in Washington in October 1985. A further meeting was held in Paris in April 1986 to finalise the H N M R P report in preparation for CAEP 1 in June of the same year. The aim of the H N M R P was, of course, to establish the repeatability of certification noise levels and to improve this where necessary by refining the test and analysis procedures, the ultimate goal being progressive and systematic reductions in the noise limits. While in the author's opinion much of this has yet to be realised, a vast quantity of high-quality data has been gathered, and it has been estimated that the effort expended has contributed the equivalent of ten years' normal experience to helicopter noise certification. A number of changes to the flight testing procedures based on analysis of H N M R P data were reported at CAEP 1. More detailed analysis, which is included in the helicopter future work programme, will undoubtedly focus attention on more fundamental causes of variability. Some of these are probably aerodynamic in origin and, therefore, aircraft specific. In some cases, blade/vortex interactions for example, more flexible definition of the flight condition will be required to significantly improve repeatability.

2.3 Current test procedures Test procedures and the noise units (Effective Perceived Noise Decibel, EPNdB) specified for helicopter certification are based entirely on those

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developed for large subsonic fixed-wing aircraft. Differences in the microphone layout and flight test conditions result from the different operating techniques employed in helicopter flying, and the generally lower intensity of the noise sources. The noise regulations set maximum permitted levels for three flight regimes; take-off, level flyover and approach along a six-degree glide path, as shown in Fig. 1. It should be noted that these flight conditions do not, and were not intended to, represent normal operating practice. The object was to develop technically reasonable criteria against which noise levels generated during each of the three main phases of flight can be assessed.

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Noise measurements are made at three locations; one directly below the flightpath and two others disposed symmetrically 150 m to either side of the flightpath monitoring point. Microphones are mounted 1-2m above the ground and are set for grazing incidence. The fundamental metric in the noise certification scheme is the Effective Perceived Noise Level, EPNL, in units of EPNdB. Essentially a single number describing the subjective effects of aircraft noise taking both frequency content and duration into account, it is calculated from the microphone signals for each event as follows: (1)

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One-third octave band spectra, including levels in the 24 bands from 50 Hz to 10 kHz, are obtained at intervals of 500 ms throughout the event by parallel filtering. For each spectrum, the sound pressure levels are converted into Noy values from which the instantaneous Perceived Noise Level, PNL, is calculated. A ~tone correction', C, to account for 'spectral irregularities', is determined for each spectrum using an algorithm which essentially examines the differences between sound pressure levels in adjacent bands. Tone Corrected Perceived Noise Level, PNLT, is given by PNLT = P N L + C. The maximum value ofPNLT, PNLTM, is determined, and all values within 10 TPNdB of this are summed logarithmically. The duration correction factor, D, is calculated using D = P N L T PN LTIM - 13. EPNL is then simply PNLTM + D.

The above measurements are then normalised to standard conditions by applying corrections to account for: (a)

Attenuation of the noise along its path as affected by 'inverse square law' and atmospheric absorption. (b) Duration of the noise as affected by distance and speed of the aircraft relative to the flightpath reference point. (c) Source noise changes during level flyover resulting from deviations from the reference value of a noise correlating parameter(s) agreed with the certificating authority due to: (i) airspeed deviations from reference; (ii) rotor speed deviations from reference; (iii} temperature deviations from reference. The source noise correction is intended to account for Mach N u m b e r effects and is potentially the most major revision to the standards in terms of test

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complexity to arise from the H N M R P . The reference to 'noise correlating parameter(s)' is necessarily somewhat vague because of the multitude of noise source mechanisms, each having its own Mach characteristics. The manufacturer is now faced with the problem of deriving a correction which properly accounts for (i), (ii) and (iii). 2.4 CAN 7 noise limits The limits are based on m a x i m u m certificated gross take-offweight and also allow for the fact that noise levels generated during the three flight regimes are different from one another. The lowest (most stringent) limit is set for flyover with those for take-off and approach being 1 and 2 EPNdB higher respectively, as shown in Fig. 2. The regulations also provide for a degree of 'trade-.off' enabling noise limits to be exceeded in one or two of the flight conditions if the excess is offset by reduced levels for the remaining condition(s). Specifically, the tradeoff procedure is given as follows: ~ (a) The sum of the excesses shall not be greater than 4 EPNdB. (b) Any excess at a single point shall not be greater than 3 EPNdB. (c) Any excess shall be offset by corresponding reductions at the other point or points. To demonstrate compliance with the limits, an average E P N L is calculated separately for flyover, take-off and approach from the corrected values for all valid runs. To be acceptable, each run must be conducted in wind speeds of less than 10 knots with a cross-wind c o m p o n e n t of less than 5 knots, temperature must be in the range 2-35°C and the relative humidity between 20 and 95%. A further restriction is that the corrections referred to in Section 2.3 above are limited to 2 EPNdB for flyover and approach and 4 EPNdB for take-off. For each test run, data from the three microphone positions are averaged as a single measurement. The minimum number of valid runs for each condition is six (18 in total) and the 90% confidence limits of the final average E P N L values must be no greater than _+1.5 EPNdB. 2.5 Requirement for Noise Certificate The noise standards are intended to apply to new designs of helicopter or modifications of existing (non-noise-certificated) types, where a change in type design is considered to significantly affect the noise characteristics of the aircraft. The reference to modifications is also noted to include military helicopters modified for civil use. The regulations do not apply to helicopters

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1990 must comply with the noise limits regardless of the date on which the C of A was applied for. It must be stressed that the date of applicability in contracting states is a matter of national legislation and may not, therefore, necessarily be the same as that given in the annex (see below).

2.6 Helicopter noise certification in the United Kingdom Requirements for helicopter noise certification were incorporated into British Civil Airworthiness Requirements (BCAR), Section N, by the Air Navigation (Noise Certification) Order 1986. The maximum permitted noise levels are as agreed at CAN 7 (see Fig. 2) and apply to the following categories of helicopters: (a)

New designs for which a successful application for a C of A for a prototype was made on or after 1 August 1986. (b) Modified versions of existing types for which a successful application to modify the C of A for a prototype (or an equivalent procedure) was made on or after 1 August 1986. Helicopters for which the type certificate was issued on or after 1 August 1991 are required to comply with the noise limits regardless of the date on which the application was made. Helicopters to which the order applies are not permitted to land or take off in the United Kingdom without a Noise Certificate issued either by the Civil Aviation Authority or the country in which the aircraft is registered. In the case of foreign-registered aircraft, the standards must be substantially equivalent to those required for UK certification. The order makes provision for the CAA to prevent operation of aircraft in contravention of the rules and makes it an offence (punishable by fines of up to £1000 levied on the operator) to so do.

2.7 EEC helicopter noise directive At the time of writing (September 1987) the EEC has not issued a helicopter noise directive independent of national legislation and is not planning to do so in the near future. The status of helicopter noise certification in several EEC Member States as reported at a CAEP Working Group meeting held in Monte Catini, Italy, in May 1987 is detailed below. In each case, the rule adopted (if any) is that proposed at the specified CAN/CAEP meeting. FRG

CAEP 1: any helicopter added to the Civil Register after 1 January 1987 must be type noise certificated.

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F r a n c e - - C A N 7: CAEP 1 has yet to be ratified. It is intended that the applicability date should remain 1 January 1980, i.e. as CAN 6. Italy--No rule at present. It is not yet clear whether CAN 7 or CAEP 1 will be adopted. The Netherlands--CAN 7: it is proposed that all new additions to the Civil Register be type noise certificated regardless of ICAO applicability dates. U K - - C A N 7: applicability date 1 August 1986. CAEP 1 will be adopted as soon as parliamentary processes allow. 3 IMPLICATIONS OF NOISE CONTROL If reductions in helicopter noise levels are to be achieved by the introduction of less noisy aircraft and not simply by severely restricting helicopter operations, the regulations must be realistic. First, noise limits must be a compromise between what is acceptable to the majority of those exposed to the noise and what can be achieved with the technology available. Secondly, noise measurements and the units on which the limits are based must in some way reflect subjective response if reductions in noise levels are to be noticeable. This latter point is true, particularly for helicopter noise, because of variations not only in absolute level but also in character. In order to appreciate the implications of statutory limits on helicopters it is necessary to understand clearly how noise control interrelates with other design considerations. As a prelude to such discussion, it is useful to examine what has happened to the noise of fixed-wing aeroplanes. Noise first became an issue in the arena of certification in 1969 70 because of environmental pressure resulting from the shatteringly high noise levels of turbojet aircraft. Fortunately, timely development of high bypass ratio turbojet and, more especially, turbofan engines, in which a large proportion of the thrust is produced by lower air velocity, enabled the introduction into service of much quieter civil transport aircraft. Significantly, these engines are also much more fuel efficient and, while developed originally to power aircraft of the Boeing 747 class, the technology is spreading to smaller machines as fuel costs make the production of smaller turbofans economically viable. The trend towards lower noise levels beginning with large, wide-body aircraft thus now includes machines with less than a quarter of the seating capacity of the Jumbo Jets. This in no way implies that engine developments that have given us less noise have been won easily the RB211, for example, brought Rolls-Royce to bankruptcy but the net result has been lower noise and lower operating costs.

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In contrast, the change from piston engines to gas turbines over the years, although enabling improvements in helicopter performance in terms of both speed and payload, has not resulted in noise reductions. The availability of higher installed power, particularly of multi-engine configurations, has allowed the designer to exploit the benefits of increased rotor tip speeds and blade loading, both of which raise noise levels (all things being equal). Noise from turbine-engined helicopters is, therefore, dominated by aerodynamic sources and is consequently far less amenable to solution by fitting quieter engines. Noise radiation arises not only from the rotor systems but also as a result of aerodynamic interactions and other effects resulting from the relative positioning of the rotors and supporting structure, fuselage, etc. This means that the noise characteristics of the helicopter are inherent in design parameters which themselves are highly interdependent. It is for this reason that significant reductions in noise level after the first flight of a new helicopter type are most unlikely without: (a)

an escalation in development costs which may ultimately include major redesign work; and/or (b) re-definition of the flight envelope to avoid 'noisy' regions. In today's financial climate, and bearing in mind the enormous costs associated with any new aircraft programme, the first of these options may prevent the introduction of civil variants, to the possible detriment of the whole project. The second alternative will, in all probability, be unacceptable to both military and civil customers alike if the helicopter is expected to meet operational requirements. Clearly, major new aircraft programmes cannot be put in jeopardy by a design approach that does not attempt to virtually guarantee compliance with the limits. It is therefore essential that the helicopter designer is able to estimate the expected noise characteristics of a new project with a reasonable degree of accuracy. Prediction methods must give noise levels comparable to those measured under certification test conditions and be sensitive enough to enable detailed parametric studies to be carried out. An added complication is that the dominant characteristics of the noise signature in subjectively weighted units (PNL, dB(A), etc.) may not be attributable directly to the rotors. It is essential to also account for any additional effects that result from putting together the rotor system and the remaining structure. Although the six-degree approach is essentially a lowpower operation, it is most often the noisiest of the three certification test conditions. The increment in external noise over that generated during either flyover or on take-offis due to aerodynamic interactions between main rotor blades and vortices shed from preceding blades. Interactions of this kind

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subject the interacting blade to transient changes in the local lift, producing a highly impulsive acoustic pressure signal. Blade/vortex interactions (BVI) will occur continuously when the shed vortices (the wake) remain in or very close to the rotor disc plane. The rate of descent at which this condition stabilises is a function of several design parameters and will not correspond (except by coincidence) to the ICAO approach procedure. Theoretical analysis of the interaction mechanism suggests further that the intensity of 'blade slap' is very sensitive to the vertical separation between the blade and vortex. This means that under the appropriate conditions, noise levels become highly variable. An example of another form of interaction is shown in Fig. 3, on which are plotted dB(A) time histories of the same Lynx helicopter fitted with both standard and the so-called 'quiet' tail rotors. At high forward speed, vortices from the main rotor pass through the advancing half of the tail rotor disc, giving rise to interactions which dominate the noise output of the aircraft. By reversing the direction of rotation of the tail rotor and reducing its rotational speed, the strength of the interactions is reduced markedly. The important point is that in both the case of BVI and of tail rotor interaction noise, predictions based on the noise level of the rotors operating in clean airflow, however accurate, will seriously underestimate the sound pressure levels actually generated by the helicopter in flight. It is perhaps not surprising that considerable efforts are being made to further understanding of interaction mechanisms. The use of relatively sophisticated theoretical methods, based on the transformation of fluctuating loads into acoustic signals using the dipole 98 A

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term of Lighthill's equation, has been restricted by the problem of obtaining aerodynamic loads of sufficient detail. Studies carried out at Westland Helicopters into the behaviour of BVI have resulted in the development of theoretical models capable of calculating airloads of high enough resolution to enable the prediction of impulsive noise signatures. 3'4 One outstanding feature is the moderate computing power required to perform the calculations--they are routine enough to use as design tools. PREDICTED AIRLOADS

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An example of the loads calculated for a 70-knot descent condition is shown in Fig. 4, together with the azimuthal derivative of these loads with respect to time which indicates areas on the rotor disc of more or less intense 'noise potential'. While initial results are encouraging, it should be understood that a very great deal o f work has yet to be done to validate the predictions by comparison with measured data. Until this has been completed, designers must resort to semi-empirical prediction techniques that are much less reliable. 4 DESIGN MARGINS It is normally accepted that any new design subject to noise certification should have at least a 90% probability of meeting the noise limits. Statistics are involved, of course, because noise prediction methods are as yet uncertain and are likely to remain so for some time. If noise levels could be established at the design stage with absolute precision, success or failure would be obvious immediately, and necessary parameter changes made. Unfortunately, margins must be incorporated into the design to allow for errors in the prediction methods. Assuming that EPNdB values are distributed normally, it is a fairly simple task to calculate probability, p, given the three predicted noise levels and the standard deviation, a, associated with each. Figure 5 shows curves for o- values of 1, 2, 3 and 4 EPNdB, ignoring values 10o O] H 2Z

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o f p below 50%. As might be expected, design margins need to be increased as the predicted values become more uncertain (~ increasing). Taking 90% as a minimum acceptable probability, it is necessary to design each of the three flight regimes to be between 0.8 and 4-5 EPNdB below their respective noise limits. This is a somewhat artificial situation, because it would be extremely difficult to actually design an aircraft to give three equal margins. A more realistic situation is to consider the noisiest regime and establish the design margins required for the other two. Figure 6 shows the effect of maximum levels both above (taking advantage of the trade-off allowance) and below the limit. It can be seen that for a given maximum noise level, a limiting value 100 u~

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Prediction methods must be as accurate as possible if design margin requirements are to be kept within realistic values. If any of the three predicted noise levels is close to its maximum permitted value, and especially if this value is exceeded, design margins for the remaining flight conditions sufficient to maintain p > 90% will become excessive.

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the flight conditions resulting in blade slap; the noise levels produced thereby; the design changes necessary to reduce impulsive noise generation during interaction; and (d) the design changes necessary to avoid interactions during a specific approach condition (i.e. 6 ~, I@).

Under these circumstances, if alternative procedures are not permitted during certification, there is no option but to assume that blade slap will occur, and to design the aircraft accordingly. Although the effect of severe blade slap on EPNL is a matter of some debate, an increase of 3 EPNdB-- which agrees well with measurements made on a Bell 206L-1 during the H N M R P 5 can be assumed. For a given aircraft, the approach level including the increase due to blade slap must be offset by reductions elsewhere to maintain an acceptable level of probability. As an example, Fig. 5 shows that, for predictions having a standard deviation of 3 EPNdB (which represents a reasonably attainable overall degree of accuracy), a design margin of 29 EPNdB per condition is necessary. If 3 EPNdB is added to the approach level to account for blade

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slap, all three noise levels must be reduced by a further 1.4 EPNdB. The most (acoustically) effective way of reducing noise levels during all phases of flight is to lower tip speeds. Figure 8 shows that the I'4EPNdB reduction necessary would involve lowering the main rotor tip speed from an already low value of 670 to 635 ft s- 1. To maintain the same gross all-up weight, and assuming that blade design has been frozen, the following additional changes would be required: (a) an increase in (b) an increase in (c) an increase in torque due to

the number of main rotor blades from 5 to 6; rotor hub complexity; transmission rating to cope with higher main rotor reduced rotational speed.

This may also necessitate a different type of transmission because of the higher gear ratios required: (a) an increase in tail rotor thrust capability to counteract increased torque, which may in turn require re-design of the rear fuselage; (b) a change in the main structure to handle increased loads, and changes in vibration excitation frequencies. All these modifications will increase the empty weight of the aircraft and will therefore be at the expense of payload. The extra main rotor blade and hub changes alone would, for example, account for almost 600lb. As an alternative to major design changes, the manufacturer may elect to certificate the helicopter at reduced operating weight, with the same affect on payload.

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A. C. Pike

In this respect, current test procedures specifying a single somewhat arbitrary approach angle constitute a hidden but nevertheless very real additional design requirement which will remain, irrespective of future changes in the noise limits. It is expected that the helicopter future work programme will include investigation into more satisfactory approach procedures. 5 CONCLUSIONS The primary objective of noise certification is progressive reduction in aircraft noise levels at source. Apart from the obvious result--the introduction into service of helicopters generating less noise--a change in the subjective character is to be expected as more intrusive sources of noise are avoided, either by design or by operational techniques. Although this paper has implied some criticism of current standards, noise certification is, in the author's opinion, essential if aircraft noise is to be regulated in a reasoned manner. Internationally agreed standards, acceptable to the public and manufacturers alike, will provide designers with a clear target at which to aim new designs and, hopefully, prevent the piecemeal introduction of local noise limits. The EH 101 (exemplifying the next generation of civil helicopters) has been designed specifically to ensure a high probability of meeting current noise limits (see Fig. 2). To achieve low noise levels, the aircraft features low rotor tip speeds and advanced technology composite blades and was, in fact, designed originally to meet C A N 6 limits.

REFERENCES 1. ICAO, Environmental Protection, Annex 16 to Convention on international civil aviation, Vol. 1, Aircraft noise, 1st Edition, 1981. 2. Subgroup Rapporteur, C A N 7 Report c)f Economic Subgroup Working Group B, CAN7 Background Information Paper 37, Montreal, 2 13 May 1983. 3. T. S. Beddoes, A wake model for high resolution airloads, US Army/AHS International Conference on Rotorcraft Basic Research. Raleigh, North Carolina, February 1985. 4. T. S. Beddoes, Unsteady aerodynamics: application to helicopter noise and vibration sources, AGARD Symposium on Unsteady Aerodynamics, G6ttingen, May 1985. 5. A. C. Pike and S. Owen, Helicopter noise measurement repeatability programme UK/FGR joint programme, UK Final Report, WHL Research Paper 680, March 1985.