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NASA is creating a tool to predict the noise of supersonic jets at launch

NASA researchers recently conducted a series of flights to record the sound of jet engines with the goal of using the data to predict how future engines designed for use in supersonic aircraft might sound at takeoff. An acoustic flight test of the Learjet took place at Niagara Falls International Airport in New York, where the two engines of a Learjet 25 aircraft owned by Calspan Corporation of Buffalo, New York, provided sound for the aircraft in flight in various conditions.

Brenda Henderson, NASA flight test principal investigator said “We chose the Learjet 25 because its engines are similar to what a future commercial supersonic aircraft might sound like”.

Creating a modeling tool capable of predicting the noise levels of future supersonic aircraft during takeoff and landing could help US and international regulators make informed decisions about setting noise level standards acceptable to the public. The Learjet flew a total of four combat sorties during which it completed 73 overflights at an altitude of approximately 500 feet. Six different engine conditions were studied.

At the same time, a small remote-controlled drone hovered safely nearby, collecting weather data to understand the atmospheric conditions around the Learjet. The drone and its operators are based at NASA’s Langley Research Center in Virginia.

Henderson said “The Langley group let us borrow their equipment, provided expertise in its use, and made a flight to collect our weather data, their contributions have been key to our success”.

Supersonic jets and subsonic speeds

During takeoffs and landings, supersonic aircraft fly subsonic—or slower than the speed of sound. But even at these speeds, future supersonic aircraft may sound different from those designed for subsonic flight due to design differences, including different types of engines.

Unlike modern commercial airliners, supersonic aircraft engines are expected to have a low bypass ratio. This means that the jet engine’s fuel-burning core produces most of the aircraft’s thrust, rather than driving a large fan to produce thrust. Because engines of this design are not commonly used in commercial aviation today, there is limited research data available on what they would sound like.

However, with commercial supersonic travel expected to make a comeback with industry showing interest and NASA’s Quest mission trying to enable silent supersonic flight over land collecting this data could help shape that future. That’s where the Learjet 25 comes in. Although it’s a subsonic aircraft, its CJ610 jet engines create a noise similar to what future commercial supersonic jets might produce.

As of 2019, NASA researchers have been testing scale models of CJ610 Learjet engines at the Aero-Acoustics Propulsion Laboratory test facilities at NASA’s Glenn Research Center in Cleveland. In the process, they collected a lot of data about the sound the engines should produce.

The Learjet Acoustic Flight Test aims to validate the accuracy of the wind tunnel tests, allowing researchers to make improvements and adjustments to their datasets and prediction models to match the real world. However, to achieve the goal of creating a predictive modeling tool, the Niagara test was performed differently from other tests performed in the past.

While most jet noise tests are used to verify that the aircraft is quiet enough, this one captured the relevant mechanics of the sound source itself under different conditions. While state-of-the-art microphones were installed on the inactive runway at Niagara Falls International Airport, the Learjet was equipped with numerous sensors and positioning equipment to record conditions inside the engines as well as the aircraft’s exact position relative to the microphones.

Henderson said “We used a state-of-the-art GPS system in the aircraft for precise positioning and sensors that recorded engine temperatures, pressures and exhaust conditions, everything was time-stamped, so every moment the plane was flying, we knew exactly where it was and how the engines were working”.

Years later, scale models of the Learjet engines were again tested on the test facilities at NASA Glenn – this time under the same engine and flight conditions as the flight test. Researchers are currently validating data from ground tests.

With this new and improved information in hand, the researchers will create a predictive modeling tool that in concert with further research can help government, industry, and academia work to usher in a new era of publicly acceptable commercial supersonic travel.

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