Residents of communities near airports may someday
breathe an audible sigh of relief as a result of NASA tests
using one of the world’s largest wind tunnels to evaluate a
variety of ‘quiet’ aircraft technologies.

Aircraft are at their ‘loudest’ when landing gear, wing flaps
and slats are deployed, creating large amounts of wind
turbulence and generating lots of noise. Engineers at NASA
Ames Research Center, Moffett Field, Calif., with support
from NASA Langley Research Center, Hampton, Va., will use
Ames’ 40 x 80-foot (12.2 x 24.4-meter) subsonic wind tunnel
to test design modifications that reduce aircraft noise on a
26 percent-scale model of a Boeing 777 aircraft.

“This wind tunnel test is the culmination of eight years of
work to make aircraft quieter during the time when the most
noise is produced — take-off and landing. This is the first
time that all of these noise-control devices will be tested
together. Each device works separately, but this test will
determine how well they work together,” said engineering lead
Kevin James of Ames.

“These technologies, developed by NASA and an industry
airframe-noise reduction team, will revolutionize the design
of future generations of commercial aircraft. Communities
will experience less noise, and their citizens’ quality of
life will be improved, with the implementation of the
advanced technologies to be demonstrated in this critical
experiment,” said Bill Willshire, noise reduction program
manager at Langley. The research to develop quieter aircraft
is funded under the Aerospace Vehicle System Technology Noise
Reduction Program managed by Langley.

The model — known as the Subsonic Transport Acoustic
Research (STAR) model — consists of the left half of the
aircraft. It will be mounted with the wing vertical in the
test section. The model is complete with a wing, landing
gear, leading-edge slats and flaps that are fully extended to
duplicate take-off or landing conditions.

“This is one of the most detailed wind tunnel models of a
commercial aircraft ever tested. The model has all the
control surfaces and parts of the real aircraft. It has a
semi-wingspan of 26 feet (7.25 meters) and is built to a
tolerance of 0.030 of an inch (.0762 centimeters),” said
James. “The Boeing 777 was picked for the development of
these quiet modifications because it is already a relatively
quiet aircraft. We wanted the challenge of making it even
quieter,” he added.

To determine noise levels, the test will use one large, fixed
acoustic-sensor array and one mobile acoustic (or
“microphone”) array that can cover the entire length of the
model. Acoustic arrays enable engineers to measure the noise
generated from small portions of the model. With the
microphone array, noise generated from the flap edge can be
separated from noise generated from the slat. The benefit of
the microphone array is very much like that of a microscope,
allowing researchers to look at individual noise-generating
parts. The model is also heavily loaded with sensors that
monitor wind-speed, turbulence and pressure.

“We’ve learned a great deal over the past several years and
have significantly improved our sensor array designs. A
mobile array was needed because some of the noise produced is
very directional. The sensor needs to be in position to ‘see’
the noise in order to accurately detect the noise,” said
James.

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