Planes Under Pressure: Glowing Paint Reveals Flight Physics

R.C. Dohare B.E. (mech), M.E.(E.S.E.), PGDCSc.

TO THE NAKED EYE:

• Pressure-sensitive paint (PSP) looks unassuming to the naked eye. But under short-spectrum light and with special filtering cameras, it can sport a rainbow of information. The paint is sprayed—much like automotive paint—onto models, such as the one here being tested at Arnold Air Force Base near Tullahoma, Tenn. The paint is removed with a solvent after an aircraft or prototype is wind tunnel tested.

IN THE RIGHT LIGHT:

• After planes are painted, dried and put into a wind tunnel, operators switch on short-wavelength light—either blue or ultraviolet—which bathes the tailless airplane model seen here, activating dye molecules in the paint. Although the human eye cannot pick up the subtle glow that the paint emits, the change can be picked up by special cameras and analyzed by computers.

UNDER PRESSURE:

• When the wind tunnel is cranked up to simulate flight conditions, the brightness of the paint's glow on the model's surfaces depends on the varying pressure at different points. The paint responds to oxygen levels in the air—thus, the more oxygen or pressure a point gets the less it will glow. The paint only puts off light in the red spectrum, but thanks to computer analysis, engineers can read the pressure map in a full spectrum. The colorful portrait allows them to better understand the aerodynamics of planes like this Dornier Alpha Jet.

FULL-COLOR F16 • To get these data-rich maps of a

plane's air surfaces, each camera actually takes two snapshots of an area: a control shot without airflow, then one during the wind test. The computer then analyzes the ratio of the paint's glow from the two images to calculate the pressure. Here, an image of an F–16 was gleaned from PSP tests. The results read out on a scale: red is the highest pressure and blue is the lowest.

PSYCHEDELIC SHUTTLE

• The PSP technology also works on a model of the space shuttle. One way to make these readings especially useful to engineers, notes Wim Ruyten, an associate fellow at the American Institute of Aeronautics and Astronautics who has worked with the paint at Arnold Air Force Base, is to generate 3-D grids of the models. To do that, models are outfitted with small reference dots that then help them turn two-dimensional snapshots into three-dimensional digital models.

HOT, HOTTER, HOTTEST

• Using much the same idea as PSP, temperature-sensitive paint has been used to show which parts of an object are absorbing the most heat. Here, a test of a prototype of NASA's Orion Crew Exploration Vehicle shows the section of the craft's heat shield likely to endure the most thermal stress.