A.L. Sivkov, E.V. Belov REDUCTION OF NOISE OF A HELICOPTER ENGINE BASED ON RESEARCHES OF ACOUSTIC FIELDS OF LIGHT AND MEDIUM HELICOPTERS

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1 A.L. Sivkov, E.V. Belov REDUCTION OF NOISE OF A HELICOPTER ENGINE BASED ON RESEARCHES OF ACOUSTIC FIELDS OF LIGHT AND MEDIUM HELICOPTERS The Kazan State Power Engineering Institute The Kazan Branch of Military Artillery University Russia, Kazan, Krasnoselskaya street, 51 Russia, Kazan, Octyabrsy cantonment - 25 Ph.: (8432) ; the fax: (8432) ; Ph.: (8432) sivkov@dionis.kfti.kcn.ru The increasing use of helicopters for patrolling, search and rescue tasks and sanitary maintenance results in raising ecological requirements to helicopters relative to the noise level made in the locality and raising sanitary admissable standards for acoustic comfort in cabins. The absence of relible information about the acoustic characteristics of basic sources of noise does not allow us to conduct effectively measures for sound insulation and sound absorbtion in cabins of helicopters. The use of airplane soundproof materials gives low effect. The A.S. Figurov acoustic laboratory of Kazan branch of the military artillery university has conducted ground and flight researches of levels of sound pressure and amplitude and frequency spectrum of noise in various points of the acousting field of closed cabins volumes of Russia made light and medium wlight helicopters of one-screw Mi-2, Mi-8T, Mi-8MTV-1 type of the layout Interior and Hospital as well as of a new helicopter Ansat designed on Kazan the helicopter plant. The analysis of more than 800 values of levels of sound pressure and research of more than 7000 amplitude-frequency spectra have determined energy carrying frequences of the main sources of noise, which characterize the formation of the acoustic field in various frequency bands. The dependence of sound field intensity on the layout of power units and cabins has been determined. If has been found that in the region of low and medium frequences the noise in the cabin of a light helicopter is made by the noise of the jet gases ejected by power units engines. Model investigations of the noise of gases jet ejected by the engines have conducted for the selection of acoustically effective variant of noise making installations of turboejet engines of the Ansat helicopter. Recently a great interest is shown for the use of helicopters of light and medium weight due to their profitability as compared with airplanes to solve the tasks of transportation of freights and passengers, for patrolling, sanitary and saving activities. The production of new machines of this class and modernization of helicopters which are now in exploitation is planned and worked out. The large part of light helicopters of consists of Mi-2 helicopters, which belonged to the defense ministry. In the process of re-equipment of these helicopters into various civil variants implying the presence of passengers on board observance of rigid sanitary requirements is necessary as to permissible effect of noise on man in a closed cabins volumes of helicopters. It is known, that one of the basic sources of high levels of sound fluctuations in cabins and in the vicinity of the acoustic field of helicopters are engines of power units. At the same time it is considered, that the noise of jet gases of engines does not exert determining influence on the formation of the acoustic field in the cabins of the helicopter. The conclusion was made in the end of 70-th and early 80-th [1] on the grounds of inverstigations of acoustic characteristics of Mi-8, Mi-6 and Mi-26 types of medium and heavy helicopters. At the same time the acoustic fields of light helicopters were not considered, and degree of noise influence of power units engines on their formation was not determined. Among the results of investigations obtained by researches of acoustic laboratory the acoustic fields of Mi-2 and Ansat type light helicopters deserve special attention. This analysis is given in the paper. Helicopter Mi-2 is used as an example. The levels of sound pressure in octave band of frequences in the cabin of Mi-2 helicopter are represented in Fig.1. Geometrical mean frequences of octave band f, Hz In an process of inverstigation it has boen found that the acoustic field of a light helicopter in frequency band from Hz is determined by a noise made by blades of the lifting screw and the noise of gases ejected by power unit engines. The value of amplitudes of the sound pressure level in frequences Hz correspond to the noise of ejected gases of engines. The frequency aerodynamic noise of blades of the lifting screw depends on the rotation rate of the screw. The

2 110 Levels of sound pressure L, db The standard Front part of cabin Medium part of cabin Back part of cabin 50 31, Fig.1. Levels of sound pressure in octave frequency band in the cabin of Mi-2 helicopter maximum amplitude of the noise is in infrasonic band and is characteristic of the first harmonic and it corresponds to the peak of amplitude in the frequency about 12 Hz. In the range of frequencies up to 500 Hz the character of the acoustic field in the helicopter cabin in mainly determined by the noise of ejected gases of power unit engines. The noise has the maximum value of sound pressure levels in the frequences of 350 Hz. The internal mechanical noise of engines formed at rotation of driving wheels of rotor and a compressor turbine as well as the noise generated by the free turbine depend on the frequency of blades movement and is in the range of spectrum of more than 350 Hz. Combustion chambers are also the sources of internal noise of engines. The internal sources are shielded by suspended aggregates, panels and plating of a glider of the helicopter. Therefore they do not influence essentially on the formation of the near acoustic field. At geometrical mean frequency of 2000 Hz the noise in the helicopter cabin is determined by mechanical noise, made by the main reduction gearbox with maximum value of amplitude at the frequency of 2300 Hz and the level of sound pressure by 34 db higher than the admissable values [2]. The frequencies of discrete components of the mechanical noise of the main gearbox correspond to the frequency of a contact of elements of the gearbox. If has been found that level value of the sound pressure of a mechanical noise of the main gearbox in cabins of light helicopters exceed the analogous indices in cabins of medium helicopters and reach 110 db and the influence zones of the gearbox on formation of the acoustic field in the helicopter cabin are extended. The comparison of sound pressure levels on working places of the crew members and on locations of passengers in cabins of light and medium helicopters are shown in fig Excess of level L,dBA Medium helicopters Light helicopters - level of sound pressure in a crew cabin - level of sound pressure in a passenger cabin Fig.2. Levels of sound pressure in crew cabins and passenger interiors relatively to sanitary permissible level (80 dba) under the standard

3 Taking into account the degree of influence of basic sources of noise on formation of a sound field in the cabin of the helicopter, it is possible to note, that the levels of sound pressure on places of the crew members of light and medium helicopters depend on the layout of engines of the power unit and availability of partitions between crew and passengers. The partitions separating a crew cabin of medium helicopters from passenger interiors, considerably reduce the influence of basic sources of noise on the members of the crew. In light helicopters the crew and passenger places are in an integrated cabin and if the engines of the power unit of the helicopter are located in a medium part of a fuselage, the levels of sound pressure on the passengers and on the crew differ little from each other. The integrated cabin by light helicopters results in increasing the noise on working places of the crew members located even in 2 meters from a zone of arrangement of the main gearbox, the levels of sound pressure reach 108 db, and this considerably exceed a permissible level. The continuous influence of noise of such high level is drastically reflected on health, reduces functionability of the crew and a as result threatens the safety of flights [3]. The application of the reverse layout of the power unit when the engines are located behind the main gearbox, essentially reduces the intensity of a sound field on working places of the crew members in comparison with the intensity of a sound field on the locations of passengers. The analysis of the acoustic characteristics in cabins of medium and light helicopters has shown, that in all checkpoints of a closed volume of the cabin the levels of sound pressure measured on the scale A by a noise meters, do not correspond to the requirements of state standard [2]. The noise in octave frequency bands source of which are jet gases of engines (in octaves with geometrical mean frequences of 125, 250 and 500 Hz), exceeds the sanitary standards by db, which is much higher than permissible values. The levels of sound pressure in octaves with the boundaries of frequencies in ranges from 90 up to 355 Hz and from 1400 up to 2800 Hz are especially high. In this area the acoustic field is formed by the noise the of which is the jet of engine gases and the main gearbox. And only in an the octave with frequency of 8000 Hz the levels of sound pressure correspond to the standards. Thus, characteristics of an acoustic field formation in cabins of light helicopters is the increased influence of the aerodynamic noise of blades of the lifting screw in the range of frequences up to 100 Hz and jet gases of engines in the range of frequencies up to 500 Hz. The researches have also shown the significant influence of a mechanical noise of the main reduction gearbox to formation of the acoustic field in cabins of light helicopters in frequences up to 2500 Hz. The influence of the noise of the main reduction gearbox on the formation of the acoustic field in cabins of light helicopters is lower. The main gearbox, closed by panels and aggregates does not influence essentialy on the formation of the near acoustic field of light and medium helicopters. The levels of sound pressure measured on a parking place of medium and light helicopters during the test of power units exceed from 4 up to 20 times the levels permitted by the Air rules ÀP-36 [4] and recommended ICAO [5]. The high intensity of noise in the near acoustic field of light and medium helicopters limits essentially permissible operating time of the ground attendants according to the operational sanitary norms. The ground attendants, working in such conditions, should necessarily use individual means of noise protection. The intensity of noise on parking place depends in many respects on layout of the power unit of helicopters. The levels of sound pressure measured in the aft hemisphere of helicopters have the greatest value. It is the obvious that an optimum requirement for creation of a low-noise helicopter is taking into account the acoustic characteristics of basic sources of noise and expected noise at the stage of designing and construction study. The reduction of noise on Mi-8MTV helicopter which are in a serial production is more complex and expensive. difficult and more expensive. The reduction of noise a helicopter during the re-equipment will also require certain material expenditures. It was suggested that the intensity of a sound field for a designed Ansat helicopter should be reduced at the expense of reduction of noise of jet gases power unit engines. The analysis of results of experimental researches of acoustic fields in cabins of medium and light helicopters has shown, that an increased source of the aerodynamic noise of their power unit installations are the jets of turbojet engines, which introduce the appreciable contribution to the common acoustic field, influencing thereby on the character of its formation. The influence of noise of jet gases of engines on formation of the acoustic field of light helicopters especially is especially distinctly exhibited in the range

4 of low and mean frequencies. The authors suggest a variant of the device for reduction of jet gases of engines of Ansat helicopter. Engines of internal combustion as well as turbojet engines are used as power unit in modern light helicopters. The power unit of the Ansat helicopter consists of two engines of the production of Canadian firm Pratt and Witney. The engines have the capacity of 600 HP. Each engine is located at the top of the fuselage of a helicopter behind the main gearbox. The cause of the aerodynamic noise of engines forming an acoustic field in the cabin and beyond its limits are, as it was mentioned above, the jet gases of the engine. Generally the jet of the engine is a flow with a very high level of a turbulence. A turbulent flow consists of elements with random values of momentum, turbulization and pressure. The inter connection between these components is described by basic equations of motion and continuity. Any of these three variables can be used for calculation of noise generation. The absolute value of power of acoustic pressure oscillations generated by the jet flow, depends on a number of parameters of which the exhaust velocity of a jet is determining. The dependence of acoustic power of model and full-scale jets to exhaust velocity is known as the Lighthill law of the eighth degree, was based on data of turbulence in the zone of mixing of the jet at small subsonic velocities of exhaustion. This fact was observed in experimental investigations [6]: 2 8 ρcν W = k c D 2 1 (1) 5 2 ρ T , 6 + 0, 4 T c On the basis of a well-known propositions it is possible to determine the directions of activity for reduction of noise of jet engines of helicopters, and namely: reduction of the speed of jet gases; reduction of length of the initial segment of a jet; acoustic shielding of the site of the first 8-10 calibres of jet. At the same time, while selecting the design concept and designed exhaust devices reducing the engines noise, it is necessary to take into account gasdynamic requirements dictated by reasons of effectiveness and reliability of the engine, and namely: the removal of a working substance from the engine should be in a specific direction with minimum aerodynamic losses; the output equipment should have identical hydraulic resistance in all directions removing the working substance to output section, and the field of velocities should be uniform at the output; at all operational modes of the engine the output equipment should provide stationary stable character of flow; the output equipment should have overall dimensions appropriate for layout of the helicopter and have high rigidity of construction with a minimum weight. To create an output equipment satisfying to all requirements, is rather complex, and the problem consists in presence of the acceptable conciliatory proposal. In an outcome of equipment satisfying all these requirements is a rather complex task, and the problem consists in finding the accepfable compromise proposal. As a result of our investigation we have determined energy carrying frequences of basic sources of noise in cabins of light and medium helicopters and this allows us to select the most effective sound-absorbing and sound-proof materials. A noise muffler has been developed for a turbojet engine of a new Ansat helicopter which reduces total noise in the cabin by 6-9 dba and when using sound absorbing construction by 9-12 dba. The zones of maximum levels of sound pressure in cabins of light and medium helicopters have been determined. The accommodation of passengers is not of recommended in these zones. We have proposed the change of internal layout of cabins of light helicopters by separation of an integrated cabin into a crew cabin and a passenger interior. The use of the experimental installation for research of noise of the jet of the engine allows us to determine acoustic efficiency of designed variants of the output equipment of engines with various expenditore of air, the exhaust velocity of the jet and the vortex of a flow.

5 The results of the investigation have been used at the Kazan research and production enterprise Ìi - Helicopters while selecting the measures for sound-proof and sound absorbtion in cabins of medium Ìi-8MTV-1 helicopters, as well as at the Kazan helicopter plant in designing the new light Ansat helicopter. R E F E R E N C E S 1. Moonin À.J., Samokhin V.F., Shipov R.A. et al. Aviation acoustics. In two parts. Part.1. Noise in interiors of passenger aerplanes. - Moscow: Engineering, pp. (In Russian). 2. State standard A system of the standards of industrial safety measures. Noise. The common requirements for safety. - Moscow: Publishing of standards, pp. (In Russian). 3. Akhmetzyanov I.M., Zinkin V.N. et al. The analysis of the data on condition of the problem of low-frequency noise contamination and development of the proposals for the program of activities for perfection methods of normalization, control and reduction of low-frequency noise. The report of research. Institute of military medicine. Sant. Petersburg, pp. (In Russian). 4. Aviation rules, part 36. Certification of aircraft noise in locality. Interstate aviation committee. - Moscow, pp. (In Russian). 5. ICAO. Protection of the environment. Appendix 16 to the Convention on international organization of civil aviation. Vol 1. Aviation noise. Montreal. The 3-d iss., (In Russian). 6. Karpov U.V., Dvoryantseva L.A. Protection against noise at the enterprises of chemical industry. - Moscow: Chemistry, pp. (In Russian).