Aero Acoustics of Fixed Wing and Rotary Wing Aircraft

Aero Acoustics of Fixed Wing and Rotary Wing Aircraft Manjunath.T.G*, Fantin Marius .R*

Abstract Aero acoustic the emerging field in aerospace research as it deals with reduction of noise generated in the aircraft. This paper deals with aero acoustics which can be achieved by active methods and passive methods. Active methods mean using aids and equipments to directly control the noise. Passive method means using aids to control the noise indirectly. In the fixed wing aircraft the noise can be reduced by improving the mixing characteristics the nozzle which is passive method. Whereas in the rotary wing aircraft the noise is mainly from the rotary blades and its interaction with air, so reducing noise here is an active method. In the current age the defense department is looking for stealthier aircraft this is one such method to achieve it.

Introduction Acoustics is the science (physics) concerned with the production, control, transmission, reception, and effects of sound. Its origins began with the study of mechanical vibrations and the radiation of these vibrations through mechanical waves, and still continues today. Acousticians has done Research work to look into the many aspects of the fundamental physical processes involved in waves and sound and into possible applications of these processes in modern life. Aero acoustics is a branch of acoustics that studies noise generation via either turbulent fluid motion or aerodynamic forces interacting with surfaces. Noise generation can also be associated with periodically varying flows. Although no complete scientific theory of the generation of noise by aerodynamic flows has been established, most practical aero acoustic analysis relies upon the socalled Acoustic Analogy, whereby the governing equations of motion of the fluid are coerced into a form reminiscent of the wave equation of "classical" (i.e. linear) Acoustics.

* Manjunath.T.G, Fantin Marius .R Dept. of Aeronautical Engineering, Sathyabama University

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When noise generation associated with the jet engine was beginning to be placed under scientific scrutiny. Computational Aero acoustics (CAA) is the application of numerical methods and computers to find approximate solutions of the governing equations for specific (and likely complicated) aero acoustic problems.

Fixed Wing Aircrafts The most widly used type of aircraft and most noise pollution caused by the same, since commercial airlines now cover every point on earth for transportation and cargo. Even in the military sector the aircraft that go fast are not quite and again cause distrubence to people and is itself a threat to its combat life. In this section we shall see about the various ways noise is generated and its control method.

Origin of noise The there main noise producing areas connected with gas turbine engine: 1. Fan or Compressor 2. Turbine 3. Exhaust Exhaust noise has received the most attention from research workers because it increases more rapidly as airflow velocities increases. Exhaust noise originates in the zones of high turbulence, which arise from the shear action at the boundaries where the high-speed jets and the atmosphere meet. High frequency noises comes from small eddies near the exhaust duct and the lower frequency noises occur further downstream where there are larger eddies. In addition, a regular pattern of shock waves forms within the jet exhaust core when its velocity exceeds local M 1.0. The shock wave pattern produces a single frequency tone and some application of particular frequencies in the noise-mixing region. The fig (1) shows how a noise generated from jet exhaust.

Figure 1

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Noise generated as the exhaust gases leave the engine is much less than that generated by turbojet. This is principally because the turbofan will generally employ more turbine wheels to drive the compressor and the fan. This, in turn, causes the hot exhaust velocity and noise level to be lessened. Most fully ducted turbofan engines are designed with what is termed exhaust mixing to blend the fan and hot airstreams more effectively and lessen the sound emission coming from the exhaust duct. On these engines the sound inlet is likely to be louder than from the tailpipe. This is also the case today with the high bypass fan engines which draw so much energy from the hot gases to drive the fan, compressor and accessories that the fan emits the greatest noise.

Reduction of noise In the older days aircraft were powered by piston engine and also people did not care about reducing the noise but it is not the case so now. The increase in environmental awareness and requirements of stealth aircrafts has to lead to this field of study. The first device introduced for noise reduction is hush kit. It reduces noise emissions from lowbypass turbofan engines, as fitted to older commercial aircraft. The most important factor in reducing the noise is to increase the mixing of exhaust gases.

How to measure noise In fig, observe the way in which the FAA measures noise levels in reference to aircraft taking off, landing and sideline noise. Because of the location of the microphones used to measure takeoff noise, it is evident that an aircraft that climbs out more steeply could be sensed as being quieter. Fig (2) shows how a few example aircraft compare in reference to the FAR 36 noise limits. The airbus A300, for example, has noise level of 91 Eqndb on takeoff and 101 Eqndb on approach. The reason the measured noise and the noise limit are higher on approach is because of the relatively shallow angle the aircraft would be at, versus the steep that is typical of climb out.

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Figure 2

Exhaust Nozzle The purpose of exhaust nozzle is to collect and straighten the gas flow and to increase the velocity of the exhaust gas before discharge from the nozzle. For large values of specific thrust, the kinetic energy of the exhaust gas must be high, which requires a high velocity exhaust (exit pressure Pe equals the ambient pressure Po). The two types of nozzles used in jet engines are the convergent and convergent-divergent (C-D) nozzles. The functions of an exhaust nozzle may be summarized as follows: 1. Accelerate the flow to a high velocity with minimum total pressure loss 2. Match exit and atmospheric pressures as closely as desired. 3. Permit afterburner operation without affecting main engine operation –this function requires a variable-are nozzle. 4. Allow for cooling of walls 5. Mix core and bypass streams for turbo fans if necessary. 6. Allow for thrust reversing if desired. 7. Suppress jet noise and infrared radiation if desired. 8. Thrust vector control if desired.

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It should be borne in mind that all the above functions should be obtained with minimal cost, weight, and boat tail drag while meeting life and reliability goals.

Nozzle types based on nozzle shapes Circular nozzle: This type of nozzle is found everywhere in the aviation industry. The most basic nozzles used earlier are circular nozzles. One disadvantage of the circular nozzle is its poor mixing characteristics. Due to this poor nature more noise is produced. Fig shows that as distance increase mixing characteristics for circular nozzle is poor compared to other types of nozzle.

Non-circular nozzles: These nozzles arcs found only in the recent years. They are found to reduce the noise, (i.e.) it has good mixing characteristics hence the noise produced is very low. It has been found to mix fluid streams even more efficiently. Mixers have been used to reduce takeoff jet noise. Mixers have also been used to enhance the mixing process of the high temperature and high-speed gas plume from air-engine with ambient cold air. It has a modest reduction in radiated noise.

Lobed nozzles: A lobed nozzle, which consists of a splitter plate with corrugated trailing edge, has been given great attention by many researchers in recent years. It has also been applied widely in turbofan engine exhausts and ejectors. Lobed nozzles have emerged as an attractive approach for enhancing mixing between fuel and air in combustion chambers to improve the efficiency of combustion and reduce the formation of pollutants.

Rectangular nozzle: These types nozzle are not yet been fully deployed as it is under research. So far rectangular nozzles are present F-22 Raptor and NASA’s X plane (scram jet engine). In a rectangular nozzle the mixing property is more than any other nozzle type also it is well suited for thrust vectoring.

Comparative study Nozzle configuration The existing data shows that various kinds of nozzle have various mixing property. Amongst it lobed nozzle has the best mixing property, it is so due to its design only as exhaust gases is deliberately sent in between the flow of ambient air when the engine 5

running. This in between flow enables the mixing of both at a faster rate and thus reduces the noise level. Also the fig 5 shows different nozzle and its mixing as the distance increases. The fig shows clearly that mixing is better for Non-circular nozzle than circular nozzle. The fig 6 shows mixing of a lobed nozzle.

Figure 5 In the fig topmost diagram shows an exhaust from circular nozzle, followed by lobed and then square.

Figure 6

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It is clear from the figures that circular nozzle will have long jet stream behind them and longer mixing time. It is not case with non-circular nozzle. The fig 5, 6 was obtained from CFD analysis and. Thus for fixed wing aircrafts the noise can be reduced by passive method as seen above.

Rotary Wing Aircraft Rotary wing aircraft means helicopter, we all know that it is one that is most noisy than fixed wing aircraft. Why is it so and how can it reduced is what we are going to see in this section.

Sources of helicopter noise 1. Rotor noise 2. Engine noise 3. Transmission noise The noise from a rotor can be divided into several distinct sources, which will be described as follows:

Thickness noise Thickness noise is dependent only on the shape and motion of the blade, and can be thought of as being caused by the displacement of the air by the rotor blades. It is primarily directed in the plane of the rotor.

Loading noise Loading noise is an aerodynamic adverse effect due to the acceleration of the force distribution on the air around the rotor blade due to the blade passing through it, and is directed primarily below the rotor. In general, loading noise can include numerous types of blade loading: some special sources of loading noise are identified separately.

Blade-vortex interaction (BVI) noise BVI occurs when a rotor blade passes within a close proximity of the shed tip vortices from a previous blade. This causes a rapid, impulsive change in the loading on the blade resulting in the generation of highly directional impulsive loading noise. BVI noise can occur on either the advancing or retreating side of the rotor disk and its directivity is characterized by the precise orientation of the interaction. In general, advancing side BVI noise is directed down and forward while retreating-side BVIs cause noise that is directed down and rearward. It has been shown that the main parameters governing the strength of a BVI are the distance between the blade and the vortex, the vortex strength at the time of the interaction, and how parallel or oblique the interaction is (Hardin 1987, Malovrh 2005).

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Broadband noise Another form of loading noise, broadband noise consists of various stochastic noise sources. Turbulence ingestion through the rotor, the rotor wake itself, and blade selfnoise are each sources of broadband noise.

High-speed impulsive (HSI) noise HSI noise is caused by transonic flow shock formation on the advancing rotor blade, and is distinct from loading noise. The source of HSI noise is the flow volume around the advancing blade tip, hence it cannot be captured by examining only the acoustic sources on the surface of the blade, HSI noise is typically directed in the rotor plane forward of the helicopter, like thickness noise.

Tail rotor noise While most noise from a helicopter is generated by the main rotor, the tail rotor is a significant source of noise for observers relatively close to the helicopter, where the higher-frequency noise of the tail rotor has not yet been attenuated by the atmosphere. Tail rotor noise is particularly annoying to the human listener due to its higher frequency (as compared to the main rotor) which places it directly in the band in which the human ear is most sensitive.

Shrouded tail rotor directs noise sideways

Methods of noise reduction Almost all helicopter engines are located above the aircraft, which tends to direct much of the engine-noise upwards. In addition, with the advent of the turbine engine, noise from the engine plays a much smaller role than it once did. Most research is now directed towards reducing the noise from the main and tail rotors. A tail-rotor which is recessed into the fairing of the tail (a fenestron) reduces the noise level directly below the aircraft, which is useful in urban areas. In addition, this type of rotor typically has anywhere from 8 to 12 blades (as compared to 2 or 4 blades on a conventional tail rotor), increasing the frequency of the noise and thus its attenuation by the atmosphere. This type of rotor is in general much quieter than its conventional 8

counterpart: the price paid is a substantial increase in the weight of the aircraft, and the weight that must be supported by the tail boom. For example, the Eurocopter EC-135 has such a design. For smaller helicopters it may be advantageous to use a NOTAR (from NO TAil Rotor) system. In this yaw-control method air is blown out of vents along the tail boom, producing thrust via the Coandă effect. Some designs have been done to reduce the rotor noise itself, for example the Comanche military helicopter attempted many stealth mechanisms, including attempts to quiet the rotor. Helicopter pilots can select operating modes which limits the engine torque and other parameters to ensure legal limits are respected to reduce noise. Pilots can disable the restrictions in an emergency to get extra power.

No Tail Rotors NOTAR, an acronym for NO TAil Rotor, is a relatively new helicopter anti-torque system (see the helicopter article for more details) developed by McDonnell Douglas Helicopter Systems which eliminates the use of the tail rotor on a helicopter, yielding quieter and safer operation.

Concept

Diagram showing the movement of air through the NOTAR system.

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Although the concept, which uses the Coandă effect, took some time to refine, the NOTAR system is simple in theory and works to provide directional control the same way a wing develops lift. A variable pitch fan is enclosed in the aft fuselage section immediately forward of the tail boom and driven by the main rotor transmission. This fan forces low pressure air through two slots on the right side of the tail boom, causing the downwash from the main rotor to hug the tail boom, producing lift, and thus a measure of directional control. This is augmented by a direct jet thruster and vertical stabilizers.

Advantages 

Reduced noise Benefits of the NOTAR system include greatly reduced external noise (NOTARequipped helicopters are among the quietest certified helicopters). This is because up to 60% of the noise from conventional helicopters is produced by the interaction of the tip vortices of the main and tail rotor.

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Increased safety and reliability Helicopter accidents may be caused by the tail rotor striking tree branches, power lines, the ground or other obstructions. Eliminating the tail rotor removes this hazard and enables NOTAR helicopters to go where tail rotor layout helicopters cannot i.e. close to trees or buildings They are also safer for ground crews to work near as there is no danger posed from a spinning tail rotor.

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Reduced vibration Since there is no interaction between tip vortices of the main and tail rotor, the operational vibration is reduced.

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Reduced Pilot Workload The thrust force of the coandă effect caters to the need of antitorque force. As the torque effect requires more antitorque, the coandă effect provides more lift to provide that antitorque.

Disadvantages 

Efficiency The NOTAR system is not as efficient as the tail rotor, and NOTAR helicopters sacrifice some power as a result.

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Maneuverability

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Although generally agile and stable, at speed the properties of the airflow over the tail boom change, and the Coandă effect fails. The 'H'-shaped tail characteristic of NOTAR helicopters is used to provide anti-torque at speed using conventional moving control surfaces. As a result, the helicopter can be difficult to turn when traveling at speed, and the large control surfaces of the tail inhibit maximum sideways velocity. 

Aerodynamics The translating tendency and the tail rotor roll forces continue to exist.

Coaxial rotors Coaxial rotors are a pair of rotors turning in opposite directions, but mounted on a mast, with the same axis of rotation, one above the other. This configuration is a noted feature of helicopters produced by the Russian Kamov helicopter design bureau.

Theoretical and practical considerations Angular momentum One of the problems with any single set of rotor blades is the tendency of the helicopter body to begin spinning in the opposite sense to that of the rotors once airborne. This is described by the principle of conservation of angular momentum: initially, the helicopter

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possesses zero total angular momentum (i.e., is not spinning about the rotor axis). The engines of the helicopter, by turning the rotor blades, input a sizeable amount of angular momentum into the rotor blades. Since the helicopter as a system (treating the rotor blades and the body as two components of that system) remains near zero total angular momentum, the body begins to pivot about the rotor axis in the opposite direction to the rotors. In other words the torque exerted by the engine, as well as turning the blades as intended, also turns the helicopter body in relation to the rotor. This phenomenon is catastrophic from the point of view of the pilot, who wishes to maintain stable flight. To counteract the effect, the tail rotor was introduced to provide a constant input of angular momentum to the body in the opposite direction to that from the engine. Since angular momentum is a directional quantity, the two components of the helicopter system, while possessing equal magnitudes of angular momentum, possess it in opposite directions, which cancel each other out. Thus, the condition of zero total angular momentum is maintained, but the helicopter's fuselage remains stationary and stable level flight becomes possible. Varying the torque exerted by the tail rotor upon the helicopter's tail boom (which controls the magnitude of the angular momentum input) facilitates controlled turning, and contributes to the helicopter's extreme maneuverability, due to the fact that in the hover condition (no lateral movement relative to the ground) the helicopter can be pivoted about the rotor axis independently of other flight controls. Control of rotational motion with the other two designs is achieved by the simple expedient of ensuring that the two sets of rotor blades rotate in opposite directions, canceling each other out in terms of angular momentum. Rotational maneuvering is a more complex topic with respect to these designs, however, and involves engineering features that are beyond the scope of this article. Coaxial rotors solve the problem of angular momentum by turning each set of rotors in opposite directions, allowing the fuselage to maintain zero angular momentum until the pilot varies the angular momentum inputs in a controlled fashion to facilitate turning.

Dissymmetry of lift Once a single-rotor helicopter is in forward flight, a second phenomenon manifests itself, called dissymmetry of lift, which possesses the potential to disrupt stable flight at speed. Dissymmetry of lift imposes an upper speed limit (known as the Never-Exceed Speed or VNE) upon single-rotor helicopters, by virtue of the fact that during one rotation of the rotor disc, a rotor blade experiences, in extreme parts of the flight envelope, two widely contrasting unstable conditions. On one side (the advancing side) of the rotor disc, rotor blades travel through the air sufficiently quickly for the airflow over them to become transonic or even supersonic, which causes fundamental changes in the airflow over the rotor blades, while on the other (retreating) side of the rotor disc, the rotors travel through the air much more slowly, possibly slowly enough to enter the stall condition, thus failing to produce lift. Both aerodynamic régimes result in (frequently catastrophic) flight instability.

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Coaxial rotors solve the problem of dissymmetry of lift because one set of rotors is cancelled by the corresponding increased lift on the same side of the other set of rotors, and vice versa, resulting in a helicopter that can fly, theoretically at least, faster than a single-rotor design, and more stably in extreme parts of the flight envelope. Coaxial-rotor helicopters still possess a never-exceed speed, however, because the problems arising from rotor tips entering the supersonic aerodynamic régime still apply, and typically, even a coaxial-rotor helicopter is designated not to fly at any speed which would result in the rotor tips reaching an airspeed in excess of Mach 0.8. Practice coaxial-rotor helicopters are slower than conventional helicopters for a given power simply because the twin rotors have higher drag

Other benefits One other benefit arising from a coaxial design include increased payload for the same engine power - a tail rotor typically wastes some of the power that would otherwise be devoted to lift and thrust, whereas with a coaxial rotor design, all of the available engine power is devoted to lift and thrust. Reduced noise is a second advantage of the configuration - part of the loud 'slapping' noise associated with conventional helicopters arises from interaction between the airflows from the main and tail rotors, which in the case of some designs can be severe (the UH-1 Iroquois or 'Huey' is a particularly loud example). Also, helicopters using coaxial rotors tend to be more compact (occupying a smaller 'footprint' on the ground) and consequently have uses in areas where space is at a premium - several Kamov designs are used in naval roles, being capable of operating from confined spaces on the decks of ships, including ships other than aircraft carriers (an example being the Kara Class cruisers of the Russian navy, which carry a Ka-25 'Hormone' helicopter as part of their standard fitment).

Disadvantages A principal disadvantage of the coaxial rotor design is the increased mechanical complexity of the rotor hub - linkages and swash plates for two rotor discs need to be assembled around the rotor shaft, which itself is more complex because of the need to drive two rotor discs in opposite directions. In an elementary engineering sense, the coaxial rotor system is more prone to failure because of the greater number of moving parts and complexity, though the engineering tolerances in aerospace are usually sufficiently precise to mitigate this somewhat. Additionally, while the resulting design has the capacity to be even more maneuverable than a conventional helicopter, achieving this in practice requires some ingenuity. As an example, the Kamov Ka-50 Werewolf (NATO reporting name 'Hokum') took a long time for Kamov to develop from prototype to operational status (though part of this long development time was because of additional complexities, such as the unique K-37-800 ejector seat mechanism on the Werewolf).

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Conclusion From this we conclude that in case fixed wing aircraft in order to reduce the noise generated it is better to change nozzle configuration and improve the mixing characteristics. As less noise is generated the aircraft will be producing less noise and lower IR signature which means higher stealth characteristics. As jet stream is reduced thus increasing the mixing characteristics. So when heat-seeking missile is launched against aircraft as IR traces is low it will be hard for it attack. In case of rotary wing aircraft the vibration of the rotary wings should be reduced and tail rotor must either be closed or eliminated, giving us once again higher stealth characteristics in its own class.

Reference The contents of the paper like figures and pictures were gathered from following books and research papers. 1. Hay, J. A. & Rose, E. G. 1970 in flight shock cell noise. J. Sound & Vibe., vol. 11, 411-420. 2. Cain, A. B., Bower, W. W., Walker, S. H. & Lockwood, M. K. 1995 Modeling supersonic jet screech Part 1: vortical instability wave modeling. AIAA paper 950506. 3. Panda, J. June 1995a Measurement of shock oscillation in under expanded supersonic jets. AIAA paper 95-2145. 4. Panda, J. 1996 An Experimental Investigation of Screech Noise Generation. AIAA paper 96-1718, presented at the2nd AIAA/CEAS Aero acoustics Conf. 5. Powell, A. 1953b On The mechanism of choked jet noise. Proc. phys. Soc. (London), Vol. 66, pt. 12, No. 408B,1039-1056. 6. Powell, A. 1953a On the noise emanating from a two-dimensional jet above the critical pressure, The Aeronautical Quarterly, Vol. IV, 103-122. 7. Tam, C. K. W. 1988 The shock-cell structures and screech tone frequencies of rectangular and non-axisymmetric supersonic jets. J. Sound & Vib., vol. 121, no. 1, 135-147. 8. Morris, P. J., Bhat, T. R. S. and Chen, G. 1989 A linear shock cell model for jets of arbitrary exit geometry. J. Sound& Vib., vol. 132, no. 2, 199-211. 9. Z.J.wang,2005 Evaluation of high order spectral volume method for benchmark computational aero acoustic problems. AIAA paper, vol-43 no.2, pg 337-348 10. Mehul P. Patel, Reed craver and Alan. B .Cain CFD studies on flow through nozzles

using wind at low mach numbers, AIAA paper, 42nd Aerospace Sciences Meeting and Exhibit Reno, Nevada.

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11. Van der Wall, B., “Simulation of HHC on Helicopter Rotor BVI Noise Emission using a Prescribed Wake Method”, 26th European Rotorcraft Forum, Den Haag, The Netherlands, September, 2000. 12. Kobiki, N., Murashige, A., “A Study on BladeTorsion Characteristics -Comparison andEvaluation of Analysis with DNW Test Results-“, Heli Japan 2002, Tochigi, Japan, November 11-13, 2002.

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