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NASA Has Helped Hush Aircraft Engine Noise for Decades

NASA Has Helped Hush Aircraft Engine Noise for Decades

Modern jet engines are loud, but they used to be much louder. NASA’s Glenn Research Center has been at the forefront of the nation’s efforts to reduce aircraft engine noise for over 70 years. During this time, the center has built an array of test facilities to carry out this work, culminating in the Aero-Acoustic Propulsion Laboratory (AAPL), a world-class noise-reduction research facility.

The AAPL, referred to as “the dome,” contains multiple test rigs enclosed in a large, echo-free chamber. The unique 130-foot diameter and 65-foot-high hemispherical structure stands out on Glenn’s campus. Its triangular sections make it appear like a golf ball rising from the ground. The interior is covered in spiky, fiberglass sound-dampening wedges and an overhead array of microphones that capture engine noise data.

A white, half-dome shaped building with triangular sections on its exterior is illuminated at nighttime on NASA Glenns campus. Through the facilitys open door, test rigs can be seen inside.
The Aero-Acoustic Propulsion Laboratory in October 1991, shortly after the dome was completed. The Nozzle Acoustic Test Rig is to the left and the Powered Lift Facility is to the right.
Credits: NASA/Quentin Schwinn

Jet propulsion has been a cornerstone of NASA Glenn’s research since the center’s establishment as a National Advisory Committee for Aeronautics (NACA) laboratory in the early 1940s. The lab’s improvements to the nation’s first generations of turbojet engines contributed to making the first jet airliners possible in the mid-1950s.

Jet aircraft are much louder than their piston-engine counterparts, though, and communities near airports complained about the increased noise. The center began studying its causes in the early 1950s. Since it is difficult to evaluate noise in the wind tunnels and test chambers normally used to test engines, Glenn researchers mounted engines on simple steel stands near the lab’s hangar and placed microphones around them to measure noise levels.

A black-and-white image of a metal frame holding up a jet engine outside at NASA Glenn. A researcher inspects the frame, sitting below the engine.
A General Electric J-47 installed on a thrust rig outside NASA Glenn’s hangar in 1954. It was being used to test the effectiveness of a screen behind the nozzle to reduce on-the-ground noise at airports.
Credits: NASA/Bill Bowles

Researchers found that the primary source of noise was not the mechanical elements of engines, but the mixing of the cold atmosphere with their narrow, high-speed stream of hot exhaust gases. In response, researchers tested unique nozzle and ejector designs and worked to balance sound-reducing hardware’s effects on engine performance and weight.

The introduction of turbofan engines, which offered increased efficiency and lowered noise levels, proved to be a breakthrough in the 1960s. But with more airliners, new Federal Aviation Agency standards, and an increased interest in vertical and short-takeoff and landing (V/STOL) aircraft, continued noise reduction research was needed.

In the late 1960s and early ’70s, NASA Glenn constructed a new, more robust array of outdoor test stands at sites across campus to study fan noise, including three rigs at the site of the current APPL. The Powered Lift Facility (PLF) was added to the site in 1986 to provide data on V/STOL concepts. Complaints from the local community about the PLF’s noise levels, however, led to the suspension of testing in 1989.

Metal hardware, pipes, and machinery for the Powered Lift Facility, pictured outside on NASA Glenns campus.
The Powered Lift Facility as it appeared in November 1989 after operations were suspended due to complaints about the noise from nearby residents. Credits:
Credits: NASA/Angela Coyne

In 1990, NASA initiated the High-Speed Research (HSR) program to develop technologies for supersonic transportation, so Glenn started designing the Nozzle Acoustic Test Rig (NATR), a 53-inch diameter wind tunnel that could test acoustic and aerodynamic performance of nozzles. This testing was inherently loud, so the center decided to construct a structure to cover both the PLF and NATR.

An overhead shot of construction equipment and partially built triangular frame segments for the dome for NASA Glenns Aero-Acoustic Propulsion facility.
Construction of the geodesic dome over the Powered Lift Facility in January 1991.
Credits: NASA/Marvin Smith

The new facility was completed in 1991 and named the Aero-Acoustic Propulsion Laboratory (AAPL). Its geodesic dome not only prevents noise from exiting the facility, but also protects against outside sounds and weather.

A Learjet engine nozzle peeks out from NASA Glenns Nozzle Acoustic Test Rig, a cylinder surrounded by tan wedges. The walls of the Aero-Acoustic Propulsion Facility, also covered in wedges, glow blue in the background.
Testing of a Learjet engine nozzle in the Nozzle Acoustic Test Rig in 2018.
Credits: NASA/Rami Daud

The NATR has become the center’s workhorse for acoustic testing. In addition to the HSR program, the NATR also performed testing for the Advanced Subsonic Technology program and conducted the first successful test of a chevron nozzle in 1997.

The Advanced Noise Control Fan (ANCF) rig and the Small Hot Jet Acoustic Rig (SHJAR) were added in the mid-1990s and 2000, respectively. Researchers used the ANCF to study acoustic liners and fan noise, and the SHJAR was more economical to operate for smaller tests than the NATR.

In the early 2000s, the AAPL was outfitted with a new acoustic arena that allows simultaneous testing on multiple rigs and the measurement of flyby and sideline noise. Use of the PLF declined over the years, and it was eventually decommissioned in 2007. The ANCF was removed in 2021 to create space for the DGEN AeroPropulsion Research Turbofan (DART) rig, a small-scale engine testbed useful for a variety of research.

The DGEN AeroPropulsion Research Turbofan test rig is illuminated by lights inside of NASA Glenn’s Aero-Acoustic Propulsion Laboratory. The walls of the facility, covered in wedges, glow blue in the background.
The DGEN AeroPropulsion Research Turbofan (DART) test rig includes a DGEN 308 turbofan engine that is used to conduct an array of research prior to testing with full-scale turbofan engines.
Credits: NASA/Bridget Caswell

The AAPL continues to play a key role in NASA’s testing efforts. Researchers are comparing noise data collected from scale-model nozzles inside the facility to data from Learjet 25 flight tests. This work may improve researchers’ ability to predict takeoff noise for future supersonic commercial aircraft.

Robert S. Arrighi
NASA’s Glenn Research Center