Wind tunnel tests may yield quiet aircraft

Engineers at NASA are conducting 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 Centre, with support from NASA Langley Research Centre, will use Ames’ 12.2 x 24.4-metre 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,’ said engineer Kevin James of Ames. ‘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.’

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 7.25 metres and is built to a tolerance of 0.0762 centimetres,’ 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 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,’ added James.

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