Pressure Paint Improves Airplane Design

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UW chemistry professors Martin Gouterman and James Callis have developed a pressure-sensitive paint that is now being used in wind tunnels to optimize the design of airplane wings. The invention seems to be taking off. The UW holds two patents on the technology; and engineers at sites such as NASA Ames and Boeing are using the paint to map the flow of air over a wing.

The paint is based on molecular luminescence, a phenomenon commonly seen in signs that display the daily menu at restaurants.

Molecular luminescence occurs when a molecule absorbs a photon--a tiny packet of light energy--and the molecule's electrons are kicked up into higher energy states. After a brief wait, anywhere from nanoseconds (one billionth of a second) to seconds, the molecule re-emits light of a color that is characteristic of the particular type of molecule. That is, the light emitted represents a sort of molecular "signature."

The emitted light is almost always at a longer wavelength than the absorbed light. So, typically, a molecule might absorb ultraviolet light--light of very short wavelength--and then emit red.

Conveniently in this case, oxygen from the air rains on the parade of the luminescence process. In what is called molecular quenching, oxygen spoils the molecule's excited state, so that the emitted light intensity is reduced (the characteristic color doesn't change, just the amount of light given off). The greater the concentration of oxygen, the greater the quenching. Now, as the atmospheric pressure changes as air flows over an airplane wing, so accordingly does the pressure of oxygen. The effect of the airfoil, or wing-shape, is to reduce pressure on the top of the wing, creating lift that allows airplanes to fly. A luminescing molecule incorporated into a clear polymer film that is painted on the surface of an airfoil mounted in a wind tunnel could sense that changing oxygen pressure. By shining ultraviolet light onto the airfoil surface and taking a video image of the red emitted light, a map of pressure over the surface is created. Voila: pressure paint.

Two pathbreakers/graphics of a Boeing 777 wing are shown in the figure in "false color." That is, the variations in color represent different pressure levels, not colors of light. The pathbreakers/graphics were recorded in a wind tunnel at wind velocities of 0.7 and 0.9 times the speed of sound. The pathbreakers/graphics reflect the greater lift (lower pressure) at the front (leading) edge of the airfoil. At the higher speed there is a shock wave that indicates where the pressure changes sharply. It is indicated by a sharp border between the colors.

Pressure paint holds promise for speeding the design and development of new aircraft and for reducing costs during the early design phase. It is currently in selective use at wind tunnels in the U.S., Russia, and Western Europe. Gouterman expects the paint to be in routine use at wind tunnel facilities by the end of the decade.

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