Flaps and slats, which extend out on the wing when an aircraft needs a boost in lift at slow speeds, and landing gear create drag and help aircraft slow down for descent and landing. They are also major sources of noise. The engine roar is only half of the noise equation when a plane is near the ground, according to National Research Council of Canada (NRC) aeroacoustics researcher Jerry Syms.
There has been a desire for a number of years to make aircraft quieter. For example, ICAO and NASA’s Environmentally Responsible Aviation program has set regulations to reduce emissions and noise. In the next 10 years, regulations will be the motivator to reduce noise generated by commercial aircraft by 32 dB relative to the current standard.
Until recently, the industry has focused on measuring and reducing the noise generated by engines alone. As such, turbofan-powered airliners and business jets have become progressively quieter through higher bypass ratios, acoustic materials, and nacelle design techniques to meet the increasingly stringent noise reduction targets.
Because of that focus, some lost sight of the fact that the airframe also generates significant noise because of airflow turbulence.
“Turbulent flow around landing gear generates a significant proportion of the total noise output of an aircraft in close proximity to the ground,” said Stuart McIlwain, Group Leader of fixed-wing aerodynamics at NRC.
Doing its part to attain noise emissions goals, NRC has modified one of its eight wind tunnels in Ottawa with an acoustic liner and precision noise measurement technology so it could detect tiny nuances of noise from landing gear.
“We adapted one of our wind tunnels to measure the noise from undercarriages because it’s the total footprint—not just engine noise—that is measured for ‘Stage 4’ compliance,” said McIlwain.
Researchers took sections of acoustic foam and mounted them inside metal frames. The frames were then covered in a fine mesh to create a smooth surface and cover up the lumps. The foam-filled frames were then mounted on the floor, ceiling, and walls of a wind tunnel that is 2 x 3 m.
A total of 64 microphones were placed inside the wind tunnel, recessed in cavities in the foam so wind would not blow over the mikes. The result is a wind tunnel in which the sounds of air blowing over objects inside it can be accurately measured, morphing the aerodynamic tunnel into an acoustic facility.
“We now have the capability to accurately measure the noise generated by air flowing around aircraft components," said Syms. "We can remove the whole assembly if we need to convert the tunnel back to its original form."
Syms recently completed a project that involved studying the sounds emitted by wind flowing over the full-scale landing gear from a business jet. The gear was mounted in the middle of the converted wind tunnel and then exposed to winds of 145 knot.
“That is a typical approach speed for a plane heading in for landing,” he said.
People on the ground would hear a loud rumbling from a plane of that size traveling at that speed. He added that it’s possible to generate almost twice that speed of wind if needed for other projects.
“We looked at the gear struts and axles individually but also the interaction between such components,” he said. “The drag strut/main strut combination, gear doors, brake lines, wheel wells, and other components generate enough noise on their own to merit attention.”
While the gear was being exposed to the wind, video and audio devices recorded what happened. Researchers then produced sound maps from the test data that showed how much noise is coming from each part of the gear assembly. OEMs can use that information to redesign or alter parts so they create less noise when exposed to fast moving air, said Syms.
“Our job is to search for where the noise comes from and suggest ways to reduce it,” said McIlwain. “The OEMs then can apply and certify their individual solutions to ICAO standards.’’