Flying Around Noise NASA and the US Army develop tools to predict noise and the acoustic footprint of maneuvering rotorcraft

By Frank Colucci oise threatens maps the ground noise footprint in operations, both civil and military. current conditions and shows a route AHS Community Noise NIn July, the New York City Council to minimize the disturbance. “The pilot Initiative banned sightseeing flights from all city knows he’s making noise. He doesn’t heliports to satisfy community noise know what direction to take to correct it. This article is the sixth in an protests. Forty-five years after Hughes We’re trying to get to that point where ongoing series on helicopter flew a Vietnam OH-6A Loach you have that information and give it in noise since 2013. In recognition of with Quiet One technologies, internet a very compact way to the pilot.” the increasing community noise video shows modern insurgents alerted The Flight Acoustics Group within complaints and successful efforts to Apaches by the noise of the attack the NASA Langley Aeroacoustics Branch to restrict helicopter usage, AHS helicopters. Researchers from NASA teams researchers from NASA and the International initiated an effort and the US Army are collaborating Army and Missile Research, with HAI to help the technical on tools to predict the source-noise Development and Engineering Center and regulatory community better of maneuvering rotorcraft and its (AMRDEC). A paper presented by NASA understand and address helicopter ground footprint. “We’re just starting to research engineer Dr. Eric Greenwood noise and noise complaints. Go to understand the maneuver acoustics,” at the AHS Forum in Virginia Beach www.vtol.org/noise to learn more explains Susan Gorton, project manager this May explained how FRAME – or to download past articles and for the NASA Revolutionary Vertical Lift Fundamental Rotorcraft Acoustics technical reports. Technology (RVLT) initiative at NASA Modeling from Experiments – uses AHS has been convening Langley Research Center in Hampton, collected flight data to predict the community noise workshops at Virginia. “We’re not talking about acoustic impact of helicopter operations. each Annual Forum, and has held violent maneuvers. We’re talking about “It’s the first time we’ve been able to presentations and discussions the turns and maneuvers helicopters show good correlation between the with FAA to educate the do every day in their jobs.” One far predictive maneuver noise and flight headquarters staff on the technical future application of the research test data,” says Ms. Gorton. FRAME underpinnings and the dramatic, may be a cockpit flight director that computer code generalizes steady-state immediate noise reductions possible through Fly Neighborly procedures. AHS is also working to support helicopter noise research of the FAA’s Aviation Sustainability Center (ASCENT) and other organizations, and encouraging dialogue between the different constituencies.

acoustic data to various maneuvers, density-altitudes, temperatures and aircraft configurations. The RVLT work falls under NASA’S broader Advanced Air Vehicle program and pursues rotary-wing investigations separate from fixed-wing noise studies. Civilian investigators and the Army share in the fundamental research looking for common analytical tools. Also at the AHS Forum, Army researcher Dr. James Stephenson described a wavelet extraction computer code to isolate The Bell 525 uses rotor blade refinements to meet current and anticipated FAA noise standards. helicopter Blade Vortex Interaction These increasingly stringent requirements to lower peak noise nevertheless do little to reduce (BVI) noise from other source-noise. flyover noise or community complaints. (Bell Helicopter) FRAME and the BVI extraction codes 32 VERTIFLITE September/October 2015 to address rotor noise issues, the AHS International focus on noise has been very helpful by highlighting issues, providing information from many parts of the community, fostering and encouraging interaction and promoting awareness of the issues.” Complicated Noisemakers elicopter noise comes from multiple sources and varies Hdramatically with aircraft type, weight and maneuvers. Main rotor impulsive noise in high-speed forward flight comes from air compressed by the advancing blade. BVI from successive blades generates distinctive slapping or chopping shock waves at lower speeds. Tail rotor blades make their own impulsive contribution, Commercial helicopters like the S-92 operating over noisy Manhattan must still adhere to Fly and complex rotor noise mixes with Neighborly procedures including defined noise avoidance routes. Current noise models do not engine and transmission noise. “We take into account ‘urban canyon’ acoustics. (Sikorsky Aircraft) don’t completely understand all the source-noise mechanisms and how they may help develop low-noise operations. with legislators and regulatory agencies. interact,” acknowledges Susan Gorton. Dr. Oliver Wong, Army Aviation None have the data to prove the “In the past, we did a lot of studies, Development Directorate experimental problem exists.” HAI member operators particularly on Blade Vortex Interaction aeromechanics lead at Langley explains, routinely practice noise abatement in approach. With the noise complaints “The acoustic issues are the same for procedures and follow noise avoidance now, we’re looking at Blade Vortex the Army and NASA – the generation of routes, he says. “Where we’ve had data, Interaction in level flight en route and the source-noise, the propagation of the we’ve found out there’s an extremely hovering – those are different kinds source-noise to the ground. What you high level of compliance with the of noise. We’ve also concentrated a do with the noise when it gets there is routing.” lot on main rotor noise. The tail rotor, where it splits . . . . The characteristics you NASA researchers also make the engine, depending on where the look for are different.” their plans and findings available to observer is, those noises can also be very Army noise data and its users are regulators. “We do coordinate with significant.” largely classified, but one possible the FAA noise division,” adds Ms. Helicopter noise also varies with military application is ROSAS or Route Gorton. “They in the past several years ambient conditions. “We have to be Optimization for Survivability Against particularly have become extremely able to model the atmospheric effects,” Sensors. At the Helicopter Association interested in helicopter noise because says Ms. Gorton. “How do you do that in International (HAI) HeliExpo in there’s been an explosion of helicopter the daily, varying environment when a March, NASA plans to present some noise complaints.” front comes through or an atmospheric lessons learned from recent flight AHS has also played an important inversion occurs?” She adds, “We don’t tests that will highlight the noise role with its Community Noise have good models for things like differences and reductions that can be Initiative, says Ms. Gorton. “The AHS reflections in what some people call an obtained through executing maneuvers International noise meetings at the ‘urban canyon’ with tall buildings.” The with alternative control inputs – this recent Annual Forums have provided same uncertainty applies to reflections information can be applied to the Fly insights into noise problems and have off bodies of water such as Long Island Neighborly noise abatement guidelines. fostered interaction among many Sound where noise complaints around As part of its research plans, NASA is different pieces of the community, the Hamptons became national news, working to develop a noise tool that can including manufacturers, operators, and and resulted in drastic flight restrictions. predict the noise impact along a flight government agencies.” The Langley Aeroacoustics Branch path. A long-term, stretch goal for NASA AHS also lobbied Congress for deploys a Mobile Acoustic Facility (MAF) is to develop real-time acoustic cockpit additional funding for NASA rotorcraft for noise measurements in different displays that help direct pilots how to research – some of which is now being environments. The MAF links up to fly the aircraft quietly, safely and avoid applied to the noise problem – and is 32 wireless microphones to control noise-sensitive areas. HAI president working to educate the manufacturers, and instrumentation trailers and and CEO Matt Zuccaro routinely enters regulators and operators, in concert with includes tethered weather balloon and helicopter noise disputes and says, HAI. ground-based LIDAR (Light Detection “What we’re finding is data has been Ms. Gorton notes, “As NASA moves and Ranging) systems that measure absent in most cases when we’re dealing forward with planning our research wind speed, direction and other

Vol. 61, No. 5 33 AW609 tilt rotor and Sikorsky S-97 Raider coaxial compound helicopter exploit innovative configurations to cut noise. AgustaWestland claims the AW609 in flyover does not have the high-amplitude, low-frequency noise associated with a helicopter. The eight- bladed Sikorsky Raider leverages low rotor tip speed and the ability to de- clutch the tail propeller at lower speeds to reduce noise. “The manufacturers are working that issue,” acknowledges Matt Zuccaro. “However, we can show those [improvements] on statistical charts, but the average person out there on the street has trouble discerning the difference even though an aircraft is quieter . . . . It’s still perceived as a helicopter making noise.” Bell and HAI launched the Fly Neighborly initiative in 1982. General Acoustic measurements were made with a light civil AS350B (shown at Salton Sea) and a medium Fly Neighborly guidance advises pilots military EH-60L helicopter at three different elevations with the same pilots flying identical to plan routes that avoid noise-sensitive profiles. (NASA photo) areas, fly as high as safely possible parameters. An Aircraft Navigation and drive community reaction. Researcher enroute, avoid noisier flight conditions Tracking System (ANTS) documents Vincent Mestre explains, “For 40 years, and descend using the quietest safe the position of flight test vehicles. social survey data has been done approach for the specific aircraft. MAF noise measurements are used to around airports asking people to rate Sikorsky acoustics technical fellow Eric conduct fundamental research into their annoyance with fixed-wing noise. Jacobs notes: “Altitude guidance is based rotor noise mechanisms and gauge the Very little similar work has been done on the physical benefits of increasing detectability or annoyance potential of for helicopters.” The Schultz Curve that source-receiver distance, although in rotorcraft. matches noise dose with annoyance some cases that must be balanced Just why helicopter noise is factor around runways and railroads against directivity effects; i.e., increased especially annoying is another area of may or may not apply to helicopters. altitude in some situations can expose investigation. Helicopters are singled Researchers will measure noise a greater proportion of the community out for complaints in notably noisy around heliports before and during to higher source-noise levels emitted Manhattan. “It doesn’t appear to me to neighborhood surveys and expect to beneath or near beneath the helicopter.” be exactly loudness,” says Ms. Gorton. have results by June 2016. “The end More specific Fly Neighborly “In some communities, loudness would game is to find out if there is a way to procedures are tied to individual be acceptable, some not . . . . The noise recreate the fixed-wing noise survey for aircraft. While pilots of Russian Mil and metrics we have do not take into helicopter noise.” French-designed account the tonal nature of helicopter Ironically, as noise sensitivity and should put noise-sensitive areas on the noise – not enough to predict the complaints increase, helicopters are right, pilots of most other helicopters impact on the community.” NASA growing quieter. According to Albert must keep them on the left. Mr. Jacobs Langley has an Exterior Effects Room Brand, Bell Helicopter director of Flight explains, “Noise abatement guidance to measure observer noise response Technology, Research and Simulation, for the S-76 and S-92 helicopters in a laboratory environment. “It’s got all Bell helicopters in production today were defined through flight testing an amazing sound system in there,” meet Federal Aviation Administration performed by Sikorsky Aircraft. Turn explains Ms. Gorton. “We can take Stage 3 noise limits for takeoff, flyover direction primarily impacts tail rotor human subjects and have them listen and approach. The five-bladed Model anti-torque requirements and, in some to real sounds – recordings from the 525 Relentless first flown in July has a cases, blade vortex interaction effects.” field played back to them – or sounds “significantly quieter rotor system” than Understanding the interactions has generated by analysis and simulation.” previous models. Dr. Brand says, “It is special relevance to Sikorsky efforts The Transportation Research Board anticipated that the Relentless will be to build helicopter autonomy. “Flight of the National Academies of Science, quiet enough to meet the next stage of guidance technology is an enabling Engineering and Medicine is meanwhile FAA noise regulations expected to be technology for automated low-noise halfway through a real-world survey to adopted in the next 10 years.” flight procedures that would minimize assess tools that measure community The Airbus Bluecopter demonstrator noise footprints.” annoyance with helicopter noise. The with Blue Edge main rotor blades and study conducted by airport consultants acoustically refined Fenestron aims to Landrum & Brown will document reduce further conventional helicopter acoustic and non-acoustic factors that noise. Meanwhile, the AgustaWestland Vol. 61, No. 5 34 INNOVATION INTELLIGENCE® 35 VERTIFLITE September/October 2015 Fly, Listen and Predict ASA noise footprint studies have typically characterized Nrotorcraft noise in steady-state flight over a linear microphone array perpendicular to the flight track. The technique measures lower hemisphere noise as a function of airspeed and glideslope. A straight-and-level flyover defines a rectangular footprint. Flight tests with a Bell-supplied Model 430 over the NASA Mobile Acoustic Facility at Eglin Air Force Base in 2011 marked the first time NASA tried to measure noise with a maneuvering helicopter. The collaborative tests collected noise measurements in transient pitch-ups, pushovers, longitudinal and rolling maneuvers, and velocity changes. Other measurements were taken in steady- state 80 and 130 kt (148 and 241 km/h) With proprotors in airplane mode, the AW609 tilt rotor avoids the high-amplitude, low-frequency level flight and descending at 70 kt noise associated with a helicopter. Helicopter-like landings nevertheless put the revolutionary (130 km/h) with a flight path angled rotorcraft back in noise-sensitive environments. (AgustaWestland) 9°. FRAME code predicted some 500 acoustic spheres covering airspeeds AH-64 Apache, the wide-bladed HH-60M from 40 to 130 kt (74 to 241 km/h) in Black Hawk, and tilt rotor CV-22 Osprey Forum 71 Best Papers climb and descent with various load in a wider experimental envelope, he prestigious Alfred Gessow factors. according to Ms. Gorton. “You can put Award for the best technical The investigation showed the refined them through maneuvers you might not paper at the 71st Annual FRAME-QS model accurately estimated T necessarily do with a civil configuration Forum was awarded to Troy Schank the near-horizon noise radiated in – higher G-loading maneuvers.” and Kynn Schulte, Bell Helicopter, transient helicopter maneuvers and Other recent tests flown in California for their paper, “A Smart Position demonstrated an approach that can with AS350B light civil and EH-60L Sensor for Articulated Rotors,” part generate a source noise modeling medium military helicopters were of the Condition Based Maintenance database for different flight conditions. replicated by the same pilots flying (CBM)/Health and Usage Monitoring “With our old predictions, we were identical profiles: at Salton Sea just Systems (HUMS) sessions. missing the noise by twice the level of below sea level, at Sierra Army Depot The summary reads: “A novel loudness,” summarizes Susan Gorton. at 4,000 ft (1.2 km) and at Sweetwater, rotor blade position sensor for Accurate models could be used to part of the US Marine Corps’ Mountain articulated rotors was developed generate low-noise mission profiles. “In at Bell Helicopter. The device has Warfare Training Center along the a perfect world, where we could have been successfully demonstrated Nevada border at 7,000 ft (2.1 km). information coming to the pilot, I would to measure the coupled flap, lag “What we saw a hint of and decided to like to have a flight director cueing and pitch displacements of a explore was the actual acoustic data system that shows it knows where you spherical joint as realized through changes with altitude,” says Ms. Gorton. want to go and here’s the low-noise an elastomeric bearing. The device “It’s non-linear, so you can’t just make an path to get there. In order to do that will be used in flight tests of the 525 extrapolation from sea level to altitude.” and do it safely, you’re going to have to Relentless, where accuracy of the flap The wealth of experimental data have some terrain information . . . some and lag measurements is anticipated is still being digested, and NASA and atmospheric information, and real-time to be within ±0.26° flap, ±0.21° lead- the Army are still formulating future lag, and ±0.95° degrees in feathering.” aircraft trim state.” acoustic test plans. “If we could boil it The winning Gessow paper will be Any quiet flight path would also down to a few parameters, that would presented at the European Rotorcraft have to take into account safe handling. be wonderful,” says Ms. Gorton. “It’s a Forum (ERF), held September 1-4 A steep, 12° glideslope, for example, very complex, interactive system. We’re this year in Munich, Germany. The reduces noise on the ground. “It’s safe, trying to get our arms around the best award is named after Alfred Gessow, but it’s not something the pilots like, and way to proceed in that arena.” a helicopter pioneer, researcher, it may not be something with which the author, professor and founder of the passengers are comfortable.” rotorcraft center at the University of Follow-on acoustic maneuvering Maryland. Find this and the 18 other tests in 2013 and 2014 flew very best papers at: different aircraft at different weights. The www.vtol.org/Gessow-Forum-71 MAF at Eglin collected noise data on the

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