OEMs: airframe mods key to noise reduction

 - May 2, 2008, 6:02 AM

Helicopter manufacturers are exploring a number of technologies to reduce noise, both that perceived from the ground and inside the cabin. European manufacturers are working on several demonstration programs to reduce noise by changing the helicopter’s airframe dynamics.

According to Rogelio Ferrer, an expert at Eurocopter’s research department in Marignane, France, “the main rotor, the tail rotor and the engine, in that order, are the three major noise sources” for those on the ground. Bob Meissbach, Mesa, Ariz.-based MD Helicopters’ director of engineering and integration, said this also depends on the flight phase. “At flyover, a traditional tail rotor will be the first source, although a quiet-technology tail rotor or a Notar [MD Helicopters’ antitorque technology] leaves the first place to the main rotor. At takeoff and climb, this depends on how quickly the maneuver is performed; the more aggressive the maneuver, the more antitorque the helicopter needs, the more tail rotor noise,” he said.

Blanche Demaret, director of helicopter programs at French aerospace lab Onera, noted that the engine at takeoff, producing full power close to the ground, is especially noisy.

Perceived noise also depends on weather conditions, Meissbach noted. “There is a big difference between the noise heard in hot and dry versus cool and humid conditions,” he said. Whatever the weather, noise reduction has to target the rotors, the turboshaft and flying procedures.

On the main rotor, researchers and engineers have modified the blades’ planform and profile to reduce perceived noise. “To break the blade tip vortex, the tip can be turned upward, thus creating a dihedral effect,” Ferrer said. The EC 225 Super Puma and the NH90 military transport have such dihedral blade tips.

Most modifications on a blade are so small as to be nearly invisible. They are tiny refinements in the blade’s twist and profile and on the trailing edge. The EC 225 is the only model in Eurocopter’s line to feature the latest in blade profile.

An EC 225 rotor blade uses the OA4 profile, the fourth generation of profiles developed jointly by the OEM and Onera. On a rotor blade, the profile is not uniform along the blade’s span. It changes according to the expected aerodynamic load. The result is profiles, breaking down the blade into several segments.

Researchers are now working on an OA5 profile. Separately, Eurocopter is planning flight tests late this month as the conclusion of another “promising” research program with Onera. “We plan to use the results on our entire range of products,” Ferrer told AIN.

Another possibility for reducing rotor noise is blade shape control, the active method of using piezoelectric actuators to change the blade’s shape in flight. Under a joint program with AgustaWestland, Eurocopter has conducted laboratory tests and is now preparing for wind-tunnel trials.

Blade-shape active control is different from using piezoelectric-actuated trailing-edge flaps to reduce vibration strains on the rotor mast and noise. A project led by Eurocopter Deutschland, where this technology was flight tested on a BK 117 in 2005, is now on hold.

Main-rotor noise can also be reduced through operational changes. Variable-speed rotors would enable pilots to choose between more power–when close to terrain, for example–and less noise, when flying over populated areas. “The EC 145 already has an automated variable-speed rotor, where rpms depend on the power requested,” Dominique Orbec, Eurocopter’s marketing vice president, told attendees at a seminar on aerospace and the environment. In principle, rotor speed control can be automated or given to the pilot via a trim.

The second noise source, the tail rotor, has been a special concern for Eurocopter because the company’s fenestron (shrouded tail rotor) is noisier than the average anti-torque device. Eurocopter has solved part of the problem by dephasing–or unevenly spacing– the fenestron’s blades. This reduces the typical high-frequency noise of such a tail rotor, but more research is needed to make the fenestron quieter. The manufacturer was thus planning to recalibrate fenestron airflow modeling software programs and conduct new optimization studies.

MD Helicopters claims its Notar technology is quieter, especially at flyover. Instead of a tail rotor, an MD helicopter tailboom consists of air-blowing ducts, nozzles and slots. “The Notar fan is buried, which thwarts noise,” Meissbach said.

Engine Noise Reduction
Hushing the engine depends a lot on how the airframer installs it, said Gérard Paty, Turbomeca’s head of research and technology programs, adding that burying the engine inside a full cowling, integrating the engine to the fuselage, is therefore a better way to tackle noise. The airframer, however, must balance noise concerns with weight penalties.

Adding layers of noise-absorption materials–padding or drill-holed sheet-metal panels, for example–is also effective. “We demonstrated minus 3 dB with an Arrius on a ground testbed,” Paty said, referring to a recent European research program called Silencer. The main challenge was to find heat-resistant materials to soundproof the exhaust nozzle.

In an ongoing research program with Eurocopter and AgustaWestland, called Friendcopter, Turbomeca is working on the air intake and the exhaust nozzle. “Both receive the same kind of treatment but for different ranges of frequencies,” Paty said. At the air intake, the main noise source is the compressor, with frequencies between 8 and 10 kHz. At the exhaust, noise comes from the combustor (200 Hz) and the turbine (5 kHz).

“We want to work closer to the noise source,” Paty said. This should allow better control of noise propagation. A 2008-2011 research project, dubbed Teeni, will focus on combustion noises.

By 2015 Turbomeca wants to reduce engine noise by 5 dB. To do that, it plans to take noise into account as early as during compressor design. “Being quiet and aerodynamically efficient can be contradictory, so this will require a lot of work,” Paty conceded.

Again in the European Commission-funded Friendcopter research program, “Onera is analyzing the directionality of noise radiation around the engine,” Demaret told AIN. In other words, researchers want to understand where engine noise goes. The first results are expected next year.

An additional avenue of research in Friendcopter is optimal trajectories. Researchers have designed a software tool to work out the quietest flight path. “Input the helicopter type and the details of the trajectory from A to B, including the location of populated areas, and the program will lay out the best flight path, including headings and slope angles,” Ferrer explained.

Separately, depending on rotor rotation direction and whether the pilot turns left or right during the final approach, power has to be increased or decreased, which affects the level of noise reported by those on the ground, Orbec said.

Inside the cabin, the number-one source of noise is the main gearbox. “It is terribly noisy because the gears mesh quickly,” Ferrer pointed out. The frequency heard is the rotation speed multiplied by the number of teeth on a gear wheel. With a rotation speed of about 100 Hz and 30 pinions, for example, the resulting noise is heard at 3 kHz. Most gearbox noises are in the 500 Hz-5 kHz bracket– the human ear’s favorite band, said Ferrer. They are sharp spectrum lines and sound like saws.

A Quieter Cabin
In terms of cabin soundproofing, there is little a designer can do to blunt gearbox noise because the gearbox is above the cabin’s ceiling. Soundproofing materials, such as foams, can be used only in thin layers, and the total thickness must be less than one inch, otherwise the encroachment on the cabin’s size is too severe. However, Onera is working on active panels, fitted with actuators to counter noise.

Eurocopter specialists are working on the shape of the gear’s teeth, and after some research they have designed a quieter gearbox. Its first application will be the EC 175 medium twin, which the company is developing jointly with China’s Harbin Aircraft.

Another noise source inside the cabin is air conditioning. Usually, engine bleed air is mixed with cool air and sent at the appropriate temperature into ducts that channel air to nozzles in the cabin. But that air is blown at a relatively high speed, which makes it noisy. In the future, a heat exchanger might use hot bleed air without mixing it with cooler air. The secondary air will move more slowly. A demonstrator is to fly next month.

“To cut noise in the cabin, the first thing to do is to insulate drive trains to avoid noise radiation and transmission,” said MD Helicopters’ Meissbach. For example, a pump should be insulated from its mount–or the mount from its environment. Meissbach noted that the Notar fan, which blows the air to be used for antitorque control, is located inside the cabin, necessitating the installation of noise-absorption panels.

Paradoxically, the main rotor is heard in the cabin but the way its vibrations are felt is not actual noise. Rather, low-frequency pressure variations are felt on all the body’s organs, notably the eyes. This translates into fatigue and sometimes giddiness. Eurocopter has plans to test counter vibrations as a solution.