Pressure sensitive paint could save millions of dollars

  • Published
  • By Janae Daniels
  • Arnold Engineering Development Center Public Affairs
ARNOLD AIR FORCE BASE, Tenn. (AFPN) --  The Arnold Engineering Development Center has reached a milestone with its pressure sensitive paint system. A recent demonstration test was conducted on the F-35 Lighting II joint strike fighter model installed in the center's 16-foot transonic wind tunnel.

PSP applied to a joint strike fighter force-and-moment model provided data for comparison with data acquired earlier this year from a pressure-instrumented model. The excellent agreement between the two models validated that pressure data can be acquired without building an expensive pressure-instrumented model. The model had 13 pressure taps added to the surface for direct correlation with the PSP data.

"The goal of using PSP is to have the capability to acquire surface pressure data on a wind-tunnel model without pressure taps," said Marvin Sellers, system engineer and developer of AEDC's PSP system. Mr. Sellers said it is conceivable that several million dollars and months could be saved.

"Typically, customers fabricate a special wind-tunnel model with several hundred pressure taps to measure the pressure distribution," Mr. Sellers said. "These models can require months to design and fabricate, and cost several million dollars.

"In addition, a special entry into the wind tunnel is required with the model to measure the pressures, and this will cost several million dollars as well."

Mr. Sellers thinks the selling point for PSP to AEDC customers is the acquisition of pressure data early in the vehicle design stage.

"PSP presents AEDC customers with the opportunity to obtain critical wind-tunnel data earlier in the design cycle, reducing the risk to the program," Mr. Sellers said. "Also, as aircraft configurations can change during the test and evaluation phase, PSP can provide pressure data on the latest configuration without the expensive and time-consuming process of constructing a new pressure instrumented model."

AEDC first became involved with PSP in 1993. The technique uses a special paint and illumination source combined with an extremely sensitive camera to obtain surface pressure data. The paint is applied to the model in two layers, a white undercoat and the PSP layer. The white undercoat provides a uniform reflective surface for the PSP layer. 

The illumination source excites the PSP layer, which fluoresces with intensity inversely proportional to the surface pressure on the model. The brighter areas indicate lower pressure and dimmer areas indicate higher pressure. This is called the "intensity" technique because the intensity variations are captured with digital black and white cameras. Two images are acquired; one with the wind off and one with wind on, and the ratio represents the pressure distribution. 

Special image processing is performed to convert the intensity ratio to pressure and combine the images from eight cameras onto a three-dimensional grid that represents the model. The pressure values are correlated to different colors for easy visualization of the pressure variation on the model surface.

The intensity technique has been used on several test vehicles including the Evolved Expendable Launch Vehicle, the Dornier Alpha Jet, the Global Hawk and the F-16 Fighting Falcon.

However, several limitations with the intensity technique prevented the acquisition of pressure data at all conditions. Fortunately, new camera and lighting technology permitted the use of a new technique based on measuring the decay rate of the fluorescence. Called the "lifetime" technique, two images are acquired with the wind on and are separated by time relative to a light pulse. The decay rate increases with increasing pressure, which means that high-pressure areas will be dim and low pressure areas bright.

"The PSP system was converted to use the lifetime technique between 2001 and 2004 under the Advanced Instrumentation Data and Control System program," said Mr. Sellers. "The first test to use the new system was the Space Shuttle program in October 2004. The detailed pressure information obtained by the PSP was exceptional and NASA was extremely pleased."

The last demonstration of the new technique was on the JSF model with a few added pressure taps and the results will demonstrate to customers the advanced state of this technology.