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Friday, April 1, 2011

Quiet Down Up There!

Rotorcraft noise unsettles passengers, disrupts cockpit communication and annoys people on the ground. Can helicopters ever be hushed?

By Mark Robins

The Fenestron shrouded tail rotor features a stator and asymmetrically spaced blades to reduce noise emission ensure safety.

Rotorcraft noise is inevitable and almost taken for granted. To the general public, a helicopter’s positive image is one of a provider of emergency medical services, law enforcement and fire fighting. However its disruptive noise can lead to responses of annoyance, fear, privacy invasion, sleep disturbance and military detection.

Often, the acceptability of helicopter noise is a function of the aircraft’s intended operation and the configuration for that operation. A VIP-configured aircraft typically requires the lowest noise levels, but other flight operations such as EMS or offshore helicopters will tolerate higher noise levels to meet operational requirements. All helicopter noise is distinct and much of it is produced from the rotors’ aerodynamic forces. Thickness noise is caused by the fluid displacement of rotor blades pushing on the air. Loading noise is caused by accelerating aerodynamic distribution forces on blades. This includes blade-vortex-interaction (BVI), a noise produced from rotor blades hitting wakes from other blades. High-speed impulsive noise results from nonlinearities in the flow field. There also are broadband noises like turbulence ingestion noise, self noise and trailing-edge noise. All of these noises are influenced by the helicopter’s weight, speed, rotor blade thickness, altitude and rate of descent.

Also, engine noise contributes 0.2 to 1 EPNdB for current helicopters and up to 3 EPNdB for high-technology helicopters. There are transmission noises from produced from the drivetrain, hydraulic systems and gearbox. “Structures vibrating at 500-3,000 Hz generate interior noise,” says Dr. Ed Smith, professor of aerospace engineering at Penn State University. “Teeth meshing inside gearboxes are the major source of this noise. The trouble is, the lighter a structure is, the more noise it radiates. Modern materials technologies to reduce a gearbox and airframe weight can present a challenge to noise control engineers.”

Can all this ‘copter clatter hurt the aircraft itself? “The internal noise in the cabin can lead to structural fatigue and damage to the airframe if it is intense enough,” cautions Kenneth Brentner, professor of aerospace engineering at Penn State. “This is rare however. The internal noise comes largely from vibrations in the aircraft due to the aerodynamic noise and transmission, engine and drivetrain.”

Internal cabin noise has a negative effect on pilots, passengers and ground crews. Duration to rotorcraft noise exposure should be limited. The National Institute of Occupational Safety and Health makes recommendations on its noise findings to regulatory agencies. It recommends against eight-hour workdays at 85 db, stating this contributes to an 8 percent chance of hearing loss. An eight-hour workday at 95 db increases this chance to 25 percent. Cabin noise can make voice communication for passengers without intercom capability difficult and often confusing. “On one occasion I flew a paradrop mission in a UH-1H,” says Samuel Evans, research associate at Penn State’s Aerospace Engineering department and retired U.S. Army colonel/aviator. “When the jumpmaster yelled the preparatory command of ‘Get Ready,’ one confused jumper, obviously not clearly hearing the jumpmaster’s command, coupled with an intense motivation and/or fear of the jumpmaster, left the aircraft as the helicopter was climbing. He was injured due to the low altitude which didn’t allow his parachute to fully deploy.”

      Rotorcraft noise is distinct and produced from various locations.

There have been attempts to improve or maintain hearing protection for pilots. Specialized helmet ear cups initially provided much of the hearing protection for Army pilots and crewmembers; but as aircraft noise increased there was a need to use earplugs in addition to the helmets. Evans says this configuration (helmets supplemented by earplugs) better protected the pilot’s hearing, but made it difficult to clearly hear radio transmissions. Ten years ago the army developed a device called the communication electronic earplug (CEP) that improves the ability of pilots to hear aircraft radios without reducing hearing protection.

Noise cancellation headphones using active noise control can assist in-flight communication by reducing ambient sounds. “The unwanted sound is sensed by a microphone and then a digital signal processor produces an anti-sound [out-of-phase] signal,” says Don Weir, engineering fellow and technical manager of acoustics at Honeywell. “A speaker produces the anti-sound signal in a manner that constructively interferes with the unwanted sound in the headphones, reducing the level heard.”

Complex rotorcraft noise can be measured with a microphone measurement system. The variation in acoustic pressure vs. time is measured. This signal is analyzed and represented by a sound pressure level spectrum, which indicates the amplitude of the various harmonic (and non-harmonic) components in the acoustic signal. Eric Jacobs, principle engineer of aerodynamics and external noise at Sikorsky, uses microphone recordings and other recording devices like sound level meters for external and internal noise measurements. “We do noise certification recordings to exact requirements that are international standards and in FAR Part 36 and in the 14CFR Part 36.”

The internationally agreed measurement procedure of rotorcraft external noise (ICAO, FAR noise certification) is based on three ground microphones arranged perpendicularly to the flight path. “Prescribed flight conditions are take-off, level flight and a six-degree descent flight,” says Andreas Dummel, acoustic expert at Eurocopter in Munich. The measuring metric is the Effective Perceived Noise Level, EPNL. Further units, used by individuals or local authorities are the A-weighted level LA (dBA), the sound exposure level (SEL) and the equivalent sound pressure level. Cabin noise can be described by the speech interference level (SIL) and also by the A-weighted level, according to Dummel.

Rotorcraft noise can be mitigated in several ways. The three most common are: acoustic treatments to reduce cabin noise, improved low-noise rotor design, and low-noise (“fly neighborly”) flight path operations. Can acoustic modifications really reduce cabin noise? Yes, says Sikorsky’s Jacobs, but he insists that there are trade-offs. “In most cases you have weight penalties, because you have to do interior cabin treatments,” he says. “Depending on the configuration, you may need to incorporate materials and designs that minimize the noise into the cabin space. This will incur some weight as you’re doing that and so the more noise you need to take out, the more weight you’re putting into that interior.”

Evans believes that most noise mitigation is reactive. “If there is a mission requirement to reduce noise, then steps will be taken to add soundproofing and sound deadening in an aircraft after the fact,” he says. A better method would be to design the aircraft to a noise standard specified as part of the requirements document.” One way to reduce rotor noise is to reduce the tip speed of the rotors. Blade tip velocity is the most powerful driver of noise. Rotors with more blades also reduce the loading per blade and thus reduce the rotor noise. Tail rotors can use unequal blade spacing to spread the noise harmonics across the spectrum and make the noise more “broadband-like,” which is generally considered more acceptable by listeners. The rotation speed of the rotor can be reduced, but only within limits. Safety considerations must be balanced. “Thinner blades can reduce high-speed noise; more blades reduce the loading noise as do lighter weight vehicles,” says Brentner. “The blade planform can be designed to limit blade-vortex interaction noise.”

Eurocopter has lowered noise by reducing rotor speed, advancing bladeshapes, and Fenestron, a shrouded tail rotor that shields ground observers from the noise source. More recently, Eurocopter’s Blue Pulse technology has integrated “intelligent” piezoelectric actuators into blades’ trailing edges. It uses three flap modules located at the trailing edge of each rotor blade. The blade’s actuators move the rotor flaps 15 to 40 times per second in order to completely neutralize the “slap noise” typically associated with helicopters during descent. While technology can quiet the skies, an equal and potentially more efficient way to reduce noise is via appropriate flight procedures and fly-neighborly operations. “Avoiding flight profiles which lead to high blade vortex interaction noise and avoiding ‘high-g’ maneuvers can reduce noise on the ground,” says Marc Gervais, acoustic expert at Eurocopter.

The helicopter industry as a whole has been developing noise abatement flight procedures which include aggressive implementation efforts. Even NASA has an active rotorcraft noise reduction effort under the Fundamental Aeronautics program.

Manufacturers are taking responsibility and seeing (and hearing) results. “Sikorsky tests for noise certification where the flight conditions measure the worst case or maximum noise source levels,” says Jacobs. “We’ve demonstrated on our S-76 in full autopilot in a decelerating approach that we’ve eliminated BVI noise. Our fly-neighborly program includes a noise abatement short course for pilots. We are a good 10 decibels quieter in our interior than what we were over 20 years ago.” Since the mid-to-late 1980s, FAA has required certification of new civil helicopters in a similar manner to fixed wing commercial aircraft. All new vehicles must meet noise certification rules to be allowed to enter service. The FAA has an integrated noise model for community noise estimates around airports and heliports.

The FAA regulations and international standards for noise certification limits have two key principles behind them: technical feasibility and economical reasonableness. “They are not set to minimize noise but rather ensure that new aircraft design are reflecting ‘the state-of-the-art’ during the design certification process,” says Jacobs. “Over time, those noise limits are adjusted to reflect current noise technology and design capabilities.” There are many current and proposed regulatory requirements for rotorcraft noise. CFR 14 Part 36 establishes aircraft type noise standards and air-worthiness certification. Others regulations pertain to helicopter weight, flight path height speed, approach angle, rate-of-climb takeoff and maximum normal operating rotor RPM. In August 2010 the Aeronautics Committee report to the NASA Advisory Council addressed rotorcraft technical challenges. It proposed to develop and demonstrate technologies to enable variable-speed rotor concepts like an integrated aeromechanics/propulsion system. These technologies should enable very high-speeds, efficient cruise and hover, reduced noise and increased range.

If the FAA and residential noise coalitions aren’t enough to quiet helicopters, economic considerations may provide the incentive, especially in the United States. “If U.S. manufacturers don’t step up effort to reduce noise signatures, they will continue to lose market share in Europe,” cautions Smith. Despite regulation, innovation and economic situations, one sound can be sure: the noise over helicopter noise will be heard for some time.

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