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27 This appendix discusses two distinct matters: the nature of helicopter noise emissions (Section A1) and the relationship among various measures of helicopter noise levels (Section A2). The former discussion provides insight into some of the constraints of on-site selection for subsequent field studies. The latter discussion, which presents the results of an analysis of the relationships among various helicopter noise measurements, can help with the design of field measurements. SECTION A1: CHARACTERISTICS OF HELICOPTER NOISE IN VARIOUS FLIGHT REGIMES Helicopter noise is an unavoidable by-product of creating the lift necessary to make helicopters and other vertical lift machines fly. When rotating and translating through the air, rotor blades displace the air because of their finite thickness. When these spatial disturbances of the fluid are added at a far-field observer loca- tion (keeping track of retarded time), they create harmonic âthickness noise.â The rotating and translating rotor also accelerate air to cause net forces (lift and drag) on the blades. This acceleration of the air, caused by the lift and drag forces, causes small compressible waves that, when added together at the correct retarded time, radiate harmonic noise to an observer far from the noise source. Heavier vehicles produce more noise, as shown in Figure A1, for a series of older military helicopters. Although there is some deviation about the trend line as a result of design characteristics unique to each model, the trend is readily apparent. Other unsteady aerodynamic sources dependent on design details of particular vehicles can add to the noise. The basic physics of these phenomena has been known for more than six decadesâand even longer for propellers. Major Helicopter Noise Sources Before addressing the origins and mechanisms of helicopter external noise it is useful identify the most noticeable, even if not necessarily the most annoying, sources. The order of importance for producing an acceptably quiet helicopter is shown in Figure A2 for a generic single rotor helicopter of the light to medium weight classâup to 10,000 lb. Impulsive harmonic noise sources generally dominate helicopter detectability, and are often thought to be the main source of annoyance, for both the main rotor and tail rotor. The tip region on the advancing side of the rotor near the 90-degree azimuth angle of the rotor disk produces most of the radiated harmonic noise. The thickness and loading noise sources on each blade element are amplified by the high advancing Mach numbers in this region. At high advancing-tip Mach numbers, thickness noise often becomes more dominant as Mach number increases. At very high advancing tip Mach numbers, High Speed Impulsive (HSI) noise develops. The local transonic flow around the rotor blade often couples with this radiating acoustic field causing acous- tic âdelocalizationâ that radiates local shock waves to an observer in the far field. When this occurs, the noise produced is nearly always highly annoying and dominates the acoustic signature of the helicopter. This type of noise tended to dominate the main rotor noise of the Huey helicopter of the Vietnam War era. When it occurs, HSI noise clearly dominates the acoustic radiation near the plane of the rotor. Most modern helicopters are designed so that delocalization does not occur in normal cruising operations. How- ever, thickness noise remains a main contributor to in-plane noise levels in cruising flight even for modern helicopters. It is also interesting to note that main rotor HSI noise cannot be heard in the helicopter cabin because the radiating waves originate near the tip of the rotor and radiate in the direction of forward flight. Most helicopters also produce a second impulsive noise caused by sudden, rapid pressure changes occurring on the lifting rotor blades. These pressure changes occur when the rotors pass in close proxim- ity to their previously shed or trailed tip vortices. They normally occur when the helicopter is operating in descending, turning, or decelerating flight, at times when the rotor blades are passing through or near their own wake system. A typical one-revolution period for this type of noise signature radiated from a single main rotor helicopter is shown in Figure A3. This âwop-wopâ sounding impulse stream, called Blade- Vortex-Interaction, BVI, is often the characteristic sound that distinguishes helicopter operational noise from other transportation noise sources in terminal operating areas. The noise produced by the anti-torque device of a single rotor helicopter can also be a major noise source. When tail rotors are used as the anti-torque device, the dominant sources are fundamentally the same as APPENDIX A1 Technical Discussion of Helicopter Noise
28 the main rotor. However, the higher operating rpms of the tail rotor make the lower and mid-frequency tail rotor harmonic noise more noticeable and objectionable to a far-field observer. Because the tail rotor is often unloaded in forward flight, tail rotor thickness noise can often be the first sound heard by a far-field observer. On some helicopters, the main rotor wake can pass in close proximity to the tail rotor disk in some operat- ing conditions and increase noise emission level. The problem is aggravated by helicopters that operate with âtop forward rotatingâ tail rotors. The problem has been minimized by more careful design and operation. AÃ©rospatiale introduced a lifting fan for directional control on many of its single rotor helicopters to mitigate tail rotor noise and reduce tail rotor drag in forward flight. The many-bladed fan (the âFenestronâ) FIGURE A1 Relationship between helicopter weight and Perceived Noise Level. FIGURE A2 Prioritized contributions of helicopter noise sources to overall emissions.
29 creates somewhat lower levels of harmonic noise, but at higher frequencies, and can be quite annoying. However, noise at these frequencies is reduced with distance from the source as a result of atmospheric absorption effects. Fenestron noise therefore contributes little to helicopter noise at long ranges. Lower frequency harmonic loading of the helicopter is next in order of acoustic importance. This sound is a direct result of the lift and drag (torque) produced by helicopters. It tends to be most important for civil helicopter operations directly underneath the helicopter. Although it is low frequency in character, it has substantial energy and is partially responsible for the excitation of ârattleâ in many instances. For military helicopters, however, the low- to mid-frequency radiated noise near the plane of the rotor is of prime concern, because it often sets the aural and electronically aided detection range of helicopters. This noise is determined by the in-plane drag time history of the rotor and by the thickness of the blades, as noted earlier. Engine noise can also be an important noise source. It is controlled by engine choice and on-board installed acoustic treatment. Transmission noise is important in close proximity to the helicopter or inter- nally, but unless excessive, is not usually an external noise problem. Last on the list of noise sources is Broadband noise. It is caused by changes in localized blade pressures caused by aperiodic and/or unsteady disturbances. It is normally of lower level on light to medium weight helicopters with normal operational tip speeds, but becomes more important on heavy helicopters as design tip speeds are lowered and the numbers of rotor blades are increased. It is also influenced to a great extent by the local inflow through the rotor system. Higher positive or negative inflow tends to reduce the noise by carrying the disturbed unsteady flow away from the rotor, thus avoiding additional unsteady blade loading and hence additional noise. Because of their ability to carry large loads and more easily handle the center of gravity issues associated with these large loads, tandem rotor helicopters have also become a workhorse helicopter for the military. The lack of conventional tail rotors on these machines reduces the noise to a degree; however, their large over- lapped rotor systems often create unsteady inflow to the rotors, making large harmonic noise levels common- place for such vehicles. Because of their high tip Mach numbers, tandem rotors also produce large amounts of thickness noise. Tandem rotors also produce large amounts of thickness noise. For a variety of reasons, most tandem rotor helicopters do not operate in commercial airspace in or around noise sensitive areas. The TiltRotor is another type of dual rotor rotorcraft that was also developed by the military. It is being proposed for civilian operations in a scaled down version for executive travel (Agusta 609) to combine a vertical lift capability with conventional turboprop airspeeds. In helicopter mode, the net inflow through the rotor can be controlled, thus controlling BVI noise in the terminal area. Thickness noise at cruise speeds is minimized by converting to aircraft mode at reduced rotor rpm. The reduced rpm in cruise decreases the noise level. Lower frequency noise is still present because the disturbance field of the wings induces periodic loading on the blades, creating far-field noise. FIGURE A3 A typical one-revolution period for âwop-wopâ of noise signature radiated from a single main two-bladed rotor helicopter. Dominant Acoustic Waveform Features, M ~ .85
30 Controlling BVI Noise in the Terminal Area As discussed previously, BVI impulsive noise occurs when the rotor operates near its own shed wake. Figure A4 shows that a vortex is shed from the tip of each rotor blade just as it does for a fixed-wing aircraft. The tip vortex trailed behind each blade interacts with the following blades to create sharp changes in local blade pressure (and thus lift.) The pressure changes push on the fluid and radiate BVI noise. Figure A5 is a sketch of the geometry of the BVI interaction process. The top view shows the geometry of the interaction process, whereas the side view illustrates the closeness of the shed tip- vortices to the top tip-path-plane. Figure A6 shows that this closeness can be controlled to some degree by the choice of the helicopter operating condition. In level flight, the helicopterâs shed tip vortices pass under the rotorâs tip-path-plane and radiate small to moderate amounts of BVI noise. However, as the helicopter descends, the rotorâs wake is forced to remain near the rotorâs tip path plane, causing the rotor to closely interact with the shed tip vortices of preceding blades. These strong changes in lift cause large levels of BVI noise radiation. Increasing the descent rates further causes most of the shed tip vortices to pass above the rotorâs tip-path- plane, which reduces BVI noise levels. Vehicle acceleration and deceleration and turning flight also can influence the location of the tip vortices with respect to the rotor tip-path-plane and hence dramatically change the radiated blade-vortex interaction noise. Figure A7 shows in-flight measurements of BVI noise, taken on a microphone about 30 degrees below the plane of the rotor. A rapid series of positive pressure pulses are seen to occur that reach FIGURE A4 Physical causes of helicopter blade-vortex interaction noise. FIGURE A5 Geometry of the BVI interaction process.
31 FIGURE A6 Effect of operating condition on blade slap. FIGURE A7 BVI noise as a function of descent rate and level flight.
32 a peak and then decrease with increasing rates of descent at approach air speeds. Because these pres sure pulses are very narrow they radiate most, but not all, of their energy in the mid- to high- frequency range and can easily annoy and disturb a far-field observer. A narrow band FFT of the pulse time histories illustrates the moderate to high frequency nature of the resulting BVI noise (Figure A8). Because radiated BVI noise levels can be controlled by changing the helicopter flight path has not gone unnoticed by the rotorcraft operational community. Helicopter International Association (HAI) has devel- oped a âFly Neighborly Programâ to make pilots aware that helicopters can be flown quietly near high density and/or sensitive population zones. Research has also shown that X-Force control (acceleration/ deceleration and drag/thrust control) can also be effective at minimizing BVI noise; a 0.1 g deceleration is equivalent to a 5.7 degree change in descent angle. A sketch of the use of such techniques is shown in Figure A9. Use of operational parameters to minimize noise exposure is well documented. One such example is shown in Figure A10, in which a Sikorsky S-76 helicopter was flown to minimize ground noise exposure. High rates of descent and deceleration were both used to substantially reduce radiated BVI noise levels. Source noise reductions depicted in Figures A9 and A10 are not always achievable in normal opera- tions. Weather, winds, other flight traffic, and maneuvering flight can substantially change BVI noise levels. In addition, the BVI noise may become intermittentâoccurring for a few seconds (seemingly dis- appearing) and then reappearing randomly. This often happens in near level flight operations in âbumpyâ air, creating intermittent BVI. FIGURE A8 Sound frequency as function of climb rate and level flight.
FIGURE A9 S-76 noise abatement approach. FIGURE A10 Reduced ground noise with modified approach procedure.
34 References for Appendix A1 Gervais, M. and F.H. Schmitz, âTiltrotor BVI Noise Reduction Through Flight Trajectory Management & Configuration Control,â Journal of the American Helicopter Society, Oct. 2004. Gopalan, G., F.H. Schmitz, and B.W. Sim, âFlight Path Control Methodology of Helicopter Blade-Vortex Interaction (BVI) Noise in the Presence of Wind,â Journal of the American Helicopter Society, Jan. 2006. Gopalan, G. and F.H. Schmitz, âFar-Field Near-In-Plane Harmonic Main Rotor Helicopter Impulsive Noise Reduction Possibilities,â presented at the AHS Annual Forum and Technology Display, Montreal, QC, Canada, April 29âMay 1, 2008. Greenwood, E., et al., âHelicopter External Noise Radiation in Turning Flight: Theory and Experiment,â presented at the American Helicopter Society Annual Forum, Virginia Beach, Va., May 1â3, 2007. Greenwood, E. and F.H. Schmitz, âThe Effects of Ambient Conditions on Helicopter Rotor Source Noise Modeling,â presented at the American Helicopter Society 67th Annual Forum, Virginia Beach, Va., May 3â5, 2011. Sargent, D.C., F.H. Schmitz, and B.W. Sim, âIn-Flight Array Measurements of Tail Rotor Harmonic Noise,â presented at the American Helicopter Society 64th Annual Forum, MontrÃ©al, QC, Canada, April 29âMay 1, 2008. Schmitz, F.H., âRotor Noise,â book chapter title from Aeroacoustics of Flight Vehicles: Theory & Practice, re-published by the Acoustical Society of America, 1995. Schmitz, F.H., âReduction of Blade-Vortex Interaction (BVI) Noise Through X-Force Control,â Journal of the American Helicopter Society, Vol. 43, No. 1, Jan. 1998. Schmitz, F.H. and B.W. Sim, âAcoustic Phasing and Amplification Effects of Single Rotor Helicopter Blade-Vortex Interactions,â Journal of the American Helicopter Society, Vol. 46, No. 4, Oct. 2001. Schmitz, F.H. and B.W. Sim, âRadiation and Directionality Characteristics of Advancing Side Blade- Vortex Interaction (BVI) Noise,â Journal of the American Helicopter Society, Oct. 2003. Schmitz, F.H., G. Gopalan, and B.W. Sim, âFlight Path Management and Control Methodology to Reduce Helicopter Blade-Vortex Interaction (BVI) Noise,â Journal of Aircraft, Vol. 39, No. 2, Marchâ April 2002. Schmitz, F.H. et al., âMeasurement and Characterization of Helicopter Noise in Steady-State and Maneu- vering Flight,â presented at the American Helicopter Society Annual Forum, Virginia Beach, Va., May 1â3, 2007. Sutton, C., S. Koushik, and F.H. Schmitz, âHigh Quality Acoustic Time History Measurements of Rotor Harmonic Noise in Confined Spaces,â presented at the AHS 69th Annual Forum, Phoenix, Ariz., May 21â23, 2013.