Metallic Foam Reduces Airplane Noise

Over the last century, airplanes have revolutionized society. It is possible to traverse the country, coast to coast, to visit far-away family in a matter of hours. We can cross the ocean, leaving at breakfast, and arrive in time for dinner in Paris thanks to airplanes. People and goods from across the globe can travel farther and faster than our great grandparents could have imagined.

For all the benefits of air travel, there are still a few areas of aviation that NASA and the aeronautics industry are continuously striving to improve. One of these important areas is noise reduction.

For people who live around airports, noise created by planes can cause a disturbance. At NASA centers across the country, new and innovative technologies are being researched and tested that can help reduce noise from planes. NASA's Glenn Research Center in Cleveland, Ohio is one of the centers helping new ideas take flight.

Glenn researchers have been working with a metallic foam that is installed around an engine to reduce noise. The firm foam, crafted from stainless steel, looks like a tightly-compacted honeycomb made of silver metal, and feels uniform on the surface—gently abrasive, like a fine grained pumice stone. In the hand it feels lighter than expected for its size.

"This is an open cell foam which is mostly air. The foam is formed by ligaments—like a sponge that you use in your kitchen, except the ligaments are metal," says Cheryl Bowman, an engineer at Glenn.

NASA Glenn's noise reduction experiments with this foam got their start in an entirely different line of inquiry. Several years ago, Dr. Mohan Hebsur (Ohio Aerospace Institute) was working for the Materials Division in the Advanced Metallic Group at Glenn. As he was researching specific material systems for their ballistic impact resistance, he proposed using metallic foams as engine case liners. Through the course of his testing, he sent his foam to NASA's Langley Research Center in Hampton, Virginia, where they noticed something interesting about the metallic foam.

"Mohan started out focusing on one alloy for its good impact resistance but discovered an interesting acoustic benefit that was independent of metal type," Bowman says.

This is where Dan Sutliff got involved. Sutliff, a Glenn engineer and the NASA team lead for the project, has been partnering with Langley for a decade on researching aircraft noise reduction. They knew that open cell materials, like traditional foams, were effective in noise absorption, but they were too combustible for use on an engine. Sutliff and Mike Jones (Co-team lead) from Langley developed an experiment and fine-tuned the properties of the metallic foam.

Partnering with Porvair Advanced Materials (a subsidiary of Selee Corporation) of Hendersonville, North Carolina, they created the metallic foam with exacting properties, including the pore sizes and density that were optimal for engine-generated noise frequencies. They then installed the Foam Metal Liner over the rotor of Glenn’'s Advanced Noise Control Fan, an 8-foot long, 4-foot in diameter test fan that is about the size of an aircraft engine.

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