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Power of pink provides NASA with pressure pictures

05 August 2016

Pink paint is helping NASA’s aeronautical innovators test new aircraft designs that could cut fuel use in half, pollution by 75 percent and noise to nearly one-eighth.

An aircraft design that could reduce fuel use, emissions and noise is set up for a test in a wind tunnel at NASA's Ames Research Center in California in which pink-colored pressure-sensitive paint is applied to the vehicle. (Credit: NASA)

“PSP is great because as long as you can apply paint to the area you want to test, illuminate it with a lamp, and view it with a camera, you can gather data you might not otherwise be able to get,” said Nettie Roozeboom, an aerospace engineer with NASA’s Ames Research Centre in California.

Engineers need to know how pressure is distributed across an airplane’s surfaces as it moves through the air so they can ensure, among many other critical variables, that they understand the loads the vehicle is experiencing at given wind tunnel conditions.

When testing scale models of aircraft designs in wind tunnels, pressure readings are traditionally taken using pressure taps, little plastic tubes that are strung through the model’s interior up to the surface through a small hole in key places. 

“The challenge is you can only install so many of these taps, otherwise your whole model becomes one big pressure tap,” Roozeboom said. “And in some models, there just isn’t enough room inside the model to accommodate the taps.”

Case in point: Recent tests of a transonic truss-braced wing aircraft design, part of Boeing’s Subsonic Ultra Green Aircraft Research project, involved a model in which some of the parts were too tiny for documenting air pressure any other way than PSP.

“The wing struts in particular were very thin, and it was difficult to machine taps where the wing met the fuselage, so PSP really was the ideal solution,” Roozeboom said.

Power of pink

Here’s how PSP works:

• A thin coat of the special paint, about one-and-a-half thousandths of an inch thick, is sprayed on to the model that will be tested in the wind tunnel and allowed to dry.

• The model is then installed in the wind tunnel, which also is equipped with a series of blue LED lights and a complement of specially equipped black and white cameras to record the test.

• With the wind tunnel active, air flows over the model resulting in varying surface pressures. The blue lights excite molecules known as luminophores within the paint causing them to fluoresce, or shine.

Pressure-sensitive paint covers the blade tips of a helicopter being tested in a wind tunnel at NASA's Langley Research Center in Virginia. (Credit: NASA)

• At the same time, due to the nature of the paint’s chemistry, oxygen molecules quench the luminophores. High-pressure areas have more oxygen, so the pink shines dimmer. Lower pressure areas have less oxygen, so the pink shines brighter.

• The difference in how much the paint fluoresces or shines is recorded by the cameras, and the resulting black and white images are analysed. The intensity of the different shades of grey are converted to a colour scale indicating the varying pressure levels.

Researchers use PSP as one of many tools to better understand how a new aircraft design might perform. It also can be used to help make other tools, such as the computer modeling tool known as CFD, more accurate.

“When you start to marry all these technologies to arrive at the best answer, I think you can walk away knowing more,” Roozeboom said.

Pink plans

Today, NASA uses PSP as a research tool in the wind tunnels at Ames, Glenn Research Centre in Ohio, and Langley Research Centre in Virginia, and engineers continue to look for ways to make PSP even more effective for testing both aeronautical and space vehicles.

Roozeboom said the next big thing for PSP is to perfect a new version of the paint in which the luminophores molecules get excited faster and are less sensitive to temperature – work sponsored by NASA’s Aeronautics Research Mission Directorate.

Known as Unsteady PSP, the paint would reveal changes in pressure at the microsecond level, providing a more detailed look at what is happening on the surface of an aircraft or launch vehicle being subjected to high stress during a high-speed flight or launch.

But one thing that is not expected to change is the bright pink colour of the specialty paint.

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