Assessing Community Annoyance of Helicopter Noise (2017)

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102 This Appendix discusses two distinct matters: the nature of helicopter noise emissions (Sec- tion A.1) and the relationship among various measures of helicopter noise levels (Section A.2). The former discussion provides insight into some of the constraints on site selection for sub- sequent 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. A.1 Characteristics of Helicopter Noise in Various Flight Regimes Helicopter noise is an unavoidable by-product of creating the lift necessary to make helicop- ters and other vertical lift machines fly. When rotating and translating through the air, rotor blades displace the air due to their finite thickness. When these spatial disturbances of the fluid are added at a far-field observer location (keeping track of retarded time), they create harmonic “thickness noise.” The rotating and translating rotor also accelerates 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 har- monic noise to an observer far from the noise source. Heavier vehicles produce more noise, as shown in Figure A-1 for a series of older military helicopters. While there is some deviation about the trend line due to design characteristics unique to each model, the trend is readily apparent. Other unsteady aerodynamic sources dependent on design details of particular vehi- cles can add to the noise. The basic physics of these phenomena has been known for more than six decades—and even longer for propellers. A.1.1 Major Helicopter Noise Sources Before addressing the origins and mechanisms of helicopter external noise, it is useful to 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 A-2 for a generic single rotor helicopter of the light to medium weight class—up to 10,000 lbs. 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 A P P E N D I X A Technical Discussion of Helicopter Noise

Technical Discussion of Helicopter Noise 103 (Source: Old Army Report—Circa 1974) Figure A-1. Relationship between helicopter weight and perceived noise level. (Source: Schmitz—Sketch from student’s University of Maryland PhD thesis.) Helicopter Noise Sources Figure A-2. Prioritized contributions of helicopter noise sources to overall emissions.

104 Assessing Community Annoyance of Helicopter Noise develops. The local transonic flow around the rotor blade often couples with this radiating acous- tic field causing acoustic “delocalization” 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. However, 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 proximity 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 A-3. This “wop-wop” 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 (Source: Schmitz, F. H.; Boxwell, D. A.; and Vause,C. R.:High-Speed Helicopter Impulsive Noise. J. American Helicopter Soc., vol. 22, no. 4, Oct. 1977, pp.28-36.) Dominant Acoustic Waveform Features, M ~ .85 Figure A-3. A typical one-revolution period for “wop-wop” of noise signature radiated from a single main 2-bladed rotor helicopter.

Technical Discussion of Helicopter Noise 105 fundamentally the same as 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 objection- able 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 operating conditions and increase noise emission level. The problem is aggravated by heli- copters that operate with “top forward rotating” tail rotors. The problem has been minimized by more careful design and operation. Aérospatiale introduced a lifting fan for directional control on many of their single rotor heli- copters to mitigate tail rotor noise and reduce tail rotor drag in forward flight. The many-bladed fan (the “Fenestron”) 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 due to 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 excita- tion of “rattle” 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 above. 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 internally, 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 heli- copter for the military. The lack of conventional tail rotors on these machines reduces the noise to a degree, but their large overlapped rotor systems often create unsteady inflow to the rotors, making large harmonic noise levels commonplace for such vehicles. Because of their high-tip Mach numbers, 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 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

106 Assessing Community Annoyance of Helicopter Noise present because the disturbance field of the wings induces periodic loading on the blades, creating far-field noise. A.1.2 Controlling BVI Noise in the Terminal Area As discussed above, BVI impulsive noise occurs when the rotor operates near its own shed wake. Figure A-4 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 A-5 shows a sketch of the geometry of the BVI interaction process. The top view shows the geometry of the interaction process, while the side view illus- trates the closeness of the shed tip vortices to the top tip-path-plane. (Source: Boxwell, D. A.; and Schmitz, F. H.: Full-Scale Measurements of Blade-Vortex Interaction Noise. J. American Helicopter Soc., vol. 27, no. 4, Oct. 1982, pp. 11–27.) Figure A-4. Physical causes of helicopter blade-vortex interaction noise. (Source: Schmitz, F. H. and Sim. B., Sketch from HAI briefing, Los Angeles, CA 2005.) Figure A-5. Geometry of the BVI interaction process.

Technical Discussion of Helicopter Noise 107 Figure A-6 shows that this closeness can be controlled to some degree by the choice of the heli- copter operating condition. In level flight, the helicopter’s shed tip vortices pass under the rotor’s tip-path plane and radiate small to moderate amounts of BVI noise. However, as the helicopter descends, the rotor’s wake is forced to remain near the rotor’s 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 rotor’s tip-path plane, which reduces BVI noise levels. Vehicle acceleration/deceleration and turning in flight can also influence the location of the tip vortices with respect to the rotor tip-path plane and hence dramatically change the radiated BVI noise. Figure A-7 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 is seen to occur that reach a peak and then decrease with increasing rates of descent at approach airspeeds. Because these pressure 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 A-8). The fact that the radiated BVI noise levels can be controlled by changing the helicopter flight path has not gone unnoticed by the rotorcraft operational community. The Helicopter Inter- national Association (HAI) has developed a “Fly Neighborly Program” 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. In fact, a 0.1g deceleration is equivalent to a 5.7-degree change in descent angle. A sketch of the use of such techniques is shown in Figure A-9. Use of operational parameters to minimize noise exposure is well documented. One such example is shown in Figure A-10, 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: Schmitz, F. H. and Sim. B., -Sketch from HAI briefing, Los Angeles, CA 2005.) BVI Noise – Operational Factors Figure A-6. Effect of operating condition on blade slap.

108 Assessing Community Annoyance of Helicopter Noise Source noise reductions depicted in Figures A-9 and A-10 are not always achievable in normal operations. Weather, winds, other flight traffic, and maneuvering flight can substantially change BVI noise levels. In addition, the BVI noise may become intermittent—occurring for a few seconds (seemingly disappearing) and then reappearing randomly. This often happens in near level flight operations in “bumpy” air—creating intermittent BVI. A.2 Correlational Analysis of Helicopter Noise Metrics Version 7.0d19 of FAA’s Integrated Noise Model (INM) permits users to predict helicopter noise exposure in a range of units (noise metrics). INM’s databases contain information for a variety of helicopter types that include physical descriptions of aircraft, noise-power-distance (NPD) curves, standard arrival, departure, and level flight profiles, and for some helicopters, hover-in-ground-effect profiles, directivity profiles for each operating mode, and spectral class data for some helicopters. The NPD curves include A-weighted metrics maximum noise level (Lmax or LAmax) and sound exposure level (SEL), and for some aircraft, tone-corrected perceived noise level [PNL(T)] and effective perceived noise level (EPNL). INM uses spectral class data to compute C-weighted metrics: C-weighted maximum noise level (LCmax) and C-weighted SEL (CEXP) and time above C-weighted threshold. (Source: Schmitz, F. H. and Sim. B., Sketch from HAI briefing, Los Angeles, CA 2005.) BVI NOISE Figure A-7. BVI noise as a function of descent rate and level flight.

(Source: Schmitz, F. H. and Gapolan, G. – Sketch from HAI briefing, Las Vegas, NV 2004.) Figure A-8. Sound frequency as function of climb rate and level flight. (Source: Schmitz, F. H., et. al., Measurement and Characterization of Helicopter Noise in Steady-State and Maneuvering Flight, presented at the AHS Annual Forum, 2007.) Figure A-9. S-76 noise abatement approach.

110 Assessing Community Annoyance of Helicopter Noise Table A-1 lists the helicopters that are currently included in the INM database. Note that FAA has published a long list of substitutions for helicopters not included in the database and a recommended helicopter from the database to use as a surrogate for that helicopter. A.2.1 Helicopter Spectral Classes INM helicopter spectral classes are representations of average spectra for groups of helicopters with common characteristics. Figure A-11 and Figure A-12 show two of INM’s spectral class charts for the B212, BO150, and S70 helicopters (Figure A-11) and the SA335, S65, and H500D helicopters (Figure A-12). Note that the spectral class data are unavailable for frequencies lower than the one-third octave band centered at 50 Hz. The database structure allows for lower frequency information, but none is currently available. A.2.2 Correlations Among Helicopter Noise Metrics A hypothetical helicopter exposure case was constructed to examine the relationships among the noise metrics that INM computes. The purpose of the exercise was to inform the selection of noise metrics for the field measurements of this research project. The numbers and types of measurements required for the social survey and subsequent analyses can directly affect the cost and design of the research. The hypothetical case modeled noise exposure for a generic heliport with a large number of operations. The first case studied featured simple straight-in and straight-out departure flight paths, using the standard profiles built into INM for the nine helicopters that have both A-weighted and PNL based NPD data. One hundred arrivals and one hundred departures were evaluated using an equal distribution of the following helicopter types: B206B3, B407, B427, B429, B430, EC130, R22, R44, and SC300C. (Contour values 75 DNL to 55 DNL, Grid point spacing 0.1 nm.) (Source: Schmitz, F. H. and Gapolan, G. Sketch from HAI briefing, Las Vegas, NV 2004.) DECELERATING MANEUVER REDUCED GROUND NOISE Lappos, Erway, 2000 Figure A-10. Reduced ground noise with modified approach procedure.

HELICOPTER INM NAME DESCRIPTION A109 Agusta A-109 B206L Bell 206L Long Ranger B212 Bell 212 Huey (UH-1N) (CH-135) B222 Bell 222 B206B3 Bell 206B-3 B407 Bell 407 B427 Bell 427 B429 Bell 429 B430 Bell 430 BO105 Bölkow BO-105 CH47D Boeing Vertol 234 (CH-47D) EC130 Eurocopter EC-130 w/Arriel 2B1 H500D Hughes 500D MD600N McDonnell Douglas MD-600N w/ RR 250-C47M R22 Robinson R22B w/Lycoming 0320 S61 Sikorsky S-61 (CH-3A) S65 Sikorsky S-65 (CH-53) S70 Sikorsky S-70 Blackhawk (UH-60A) S76 Sikorsky S-76 Spirit SA330J Aérospatiale SA-330J Puma SA341G Aérospatiale SA-341G/342 Gazelle SA350D Aérospatiale SA-350D AStar (AS-350) SA355F Aérospatiale SA-355F Twin Star (AS-355) R44 Robinson R44 Raven / Lycoming O-540-F1B5 SC300C Schweizer 300C / Lycoming HIO-360-D1A SA365N Aérospatiale SA-365N Dauphin (AS-365N) Table A-1. Helicopters included in INM v7.0d database. Figure A-11. Spectral class example 1.

112 Assessing Community Annoyance of Helicopter Noise Figure A-13 shows the 55 through 75 DNL contours for this generic helicopter test case. The grid points shown are 0.1 nautical miles apart (approximately 608 feet). The resulting DNL contours are relatively small, even with 200 daily helicopter operations. Figure A-14 and Figure A-15 compare the noise metrics that INM can compute relative to the DNL value at each of the grid points within a 4 nautical mile square grid with 0.1 nautical mile spacing. Figure A-14 shows the traditional level based metrics, while Figure A-15 shows the Time Above metrics. Figure A-12. Spectral class example 2. (Contour values 75 DNL to 55 DNL, Grid point spacing 0.1 nm.) Figure A-13. DNL contours for test case operations.

Technical Discussion of Helicopter Noise 113 Figure A-14. Relationship of traditional level based noise metrics to DNL for an example heliport. Figure A-15. Correlation of Time Above Metrics to DNL for an example heliport (threshold 65 dB for TALA and TALC and 95 dB TAPNL) (TALA – time above A-weighted SEL, TAPNL = time above PNL-weighted SEL, TALC = time above C-weighted SEL.)

114 Assessing Community Annoyance of Helicopter Noise Table A-2 shows the variance accounted for (coefficients of determination) for each of the noise metrics with DNL. All of the metrics other than the Time Above metrics are highly cor- related with DNL. For all practical purposes, if one of the equivalent energy metrics is known, all of the other equal energy metrics are also known (except for constants and scale factors.) These results are similar to the results for fixed-wing aircraft (Mestre et al. 2011). The R2 values between DNL and individual metrics displayed in Table A-2 demonstrate that essentially all of the metrics modeled by INM are highly correlated with DNL. Note that in each case in Table A-2 the correlation of determination was based on a linear fit except for the Time Above metrics. For the Time Above metrics, a 2nd order polynomial fit was used. The choice of linear or 2nd order fit of DNL to the individual metrics was based on the shape of the data plot and the method that provided the best correlation. TAPNL is the metric most independent from DNL, albeit in a not particularly useful manner. Figure A-15 shows that the TAPNL data have a very narrow dynamic range, with a nearly vertical slope between DNL 75 and DNL 80. Time Above 95 PNL goes from nearly 0 to 1400 minutes within a range of only Ldn = 5 dB. Note that none of the metrics, the traditional level based metrics nor Time Above, include any corrections or adjustments for impulse type noise that occurs as part of some helicopter operating modes. Note also that the spectral data used by INM to compute C-weighted and PNL metrics do not contain any information below the one-third octave band centered at 50 Hz. NOISE METRIC R2 RELATIVE TO DNL CNEL 0.99997 LAEQ 1 LAEQD 0.99997 LAEQN 0.99997 SEL 0.99998 LAMAX 0.95152 NEF 0.92129 WECPNL 0.92128 EPNL 0.92126 PNLTM 0.92887 CEXP 0.99538 LCMAX 0.95927 TALA 0.86722 TALC 0.86848 TAPNL 0.6641 Table A-2. Coefficients of determination (R2) of noise metrics with DNL.