AEDC team provides specialized techniques to test novel aircraft

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A team at AEDC recently conducted aerodynamic testing, using a combination of more conventional methods and the application of pressure sensitive paint (PSP) on a scale-model of a tailless aircraft for the Air Force Research Laboratory in the center's four-foot transonic wind tunnel.

The test article represented a supersonic research aircraft that employed a jet effects opening or slot on the top of one wing and a solid spoiler on the other wing to compare the effectiveness of each in providing yaw and roll control for stability in flight, said Brant Maines, the Lockheed Martin Corp. program manager on the test.

"The main objective of the test program was to develop an aerodynamic database that we could use to validate our computational predictions of virtual fluidic surfaces - it's kind of what we call them - essentially fluidic spoilers, jet spoilers and other similar applications," he explained.

"We ran steady air at different locations and with different jet exit geometries on the model. We were able to measure forces and moments and take a lot of pressure sensitive paint data in order to get pressure distributions that we could then compare directly to the computational predictions."

Maines said the use of PSP was of particular value in acquiring high quality data they needed from the test. PSP is a technique that 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.

"Pressure sensitive paint gives us a near real-time visualization of the entire pressure distribution over the upper surface of the wing," he said. "So, instead of having the individual pressure taps sporadically placed on the surface, we get a complete picture that then is very easily compared to the computational fluid dynamics (CFD) estimates and simulations that we performed on the model."

A spoiler, whether it's a metal plate or pressurized air from an opening in the upper surface of a wing, reduces lift and increases drag. Creation of these aerodynamic forces can then be used to control the vehicle in flight.

Judy Bergmann, the center's Aerospace Testing Alliance (ATA) project manager on the entry, said the test was a technology demonstration of jet effects on a one-eighteenth scale model of an Air Force conceptual aircraft. She said the aircraft itself was not the focus of the test, but rather the data gleaned from using the unconventional routing of air over the aircraft to control it.

"We supplemented force and moment data with the information we could get by applying PSP," explained Marvin Sellers, the ATA engineer who has the most experience with PSP at AEDC. "We were really only interested in the top surface of the model and seeing what happens to the pressure distribution from the jet, and compare that to their computational fluid dynamics solutions. So, the PSP was a tie-in to the CFD so they could see the distribution of the pressure on the model.

The balance was the primary measurement of the effectiveness of the jet or pitch, yaw, roll control."

Sellers and Bergmann said the customer's representatives were analyzing the data while they were at AEDC and saw some unexpected things as well as some anticipated results.

"When we started putting pressurized gas out of the jet they expected the rolling moment to go one direction, and it actually went the opposite direction," Bergmann explained. "They were not sure why that was happening, but they were able to look at the PSP and see pressure distributions on the wing that would back up what the balance was measuring. They could see what was going on and what processes were happening around the jet to cause that. That helped them understand that issue. That's the reason you test."

Sellers pointed out that the test provided a first look at the aerodynamic interactions between the airflow from the jet and around and over the aircraft model.

"It was all preliminary; an initial study of the jet," he said. "It gives them a little bit of an understanding on what's happening so they can decide what to do in the future."

The customers were not the only ones who were venturing into some new territory.
"Probably the biggest challenge we had, from my viewpoint, was the air system setup our team did," Bergmann said. "Reggie Riddle, Dave Beard and Marvin Sellers did a tremendous amount of work on getting that system in place because there isn't a standard system like that in 4T.

They used things from the 16-foot transonic wind tunnel system that we brought in, including one trailer full of high pressure air and another one full of high pressure helium to provide the gases."

She said two different gases were used to simulate jet exhaust.

"Air is easy to use for simulation of jet exhaust, but it doesn't match the true characteristics, the properties of the high temperature gases that might actually be used to do this," she explained. "High pressure helium simulates that better than air. They call it the maximum flux - this is what they were trying to match - the max flux which deals with temperature density and pressure and gas properties.

You can either raise the temperature of the gas to make it match the mass of the true gas or you can knock the density down which should also simulate jet exhaust gases more closely. So, that's what the helium is for. And the helium actually provided better results, closer to what they expected."