Collaborative effort at AEDC results in 'standard model' test

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Arnold Engineering Development Center (AEDC) has collaborated with NASA to develop a standard wind tunnel check model for wind tunnel comparison.

The test was part of a complex project that centered around the aerodynamic testing on an existing, but modified, aircraft model to measure flow conditions in Arnold's 16-foot transonic (16T) wind tunnel in October.

Wayne Hawkins, with AEDC's Capability and Integration, Plans and Programs Department, said the test provided an excellent opportunity to demonstrate several AEDC evolving test techniques under the AEDC Technical Excellence Initiative.

"The new test techniques and AEDC technical leads included a pressure sensitive paint phase headed up by Marvin Sellers, planar doppler velocimetery with Dr. Joe Wehrmeyer and background-oriented Schlieren and laser vapor screen work with Fred Heltsley and Ron Porter," he said. "This also included flow seeding headed by Fred Heltsley and Kenneth Scott. Dave Cahill was the lead on the design of experiments. This was the first application of design of experiments methodology in an AEDC wind tunnel environment."

He added that with the test technology techniques demonstrated, it is anticipated the data will be used for computational fluid dynamics comparisons by each center and the AFRL.

NASA will be able to use the test data and the model to compare flow conditions in three of the agency's similar wind tunnel facilities. The joint effort also brought engineers from the different facilities together to share a variety of data and wind tunnel processes and methodologies.

Hawkins said Aerospace Testing Alliance project engineers Dave Cahill and Melissa Minter led the overall AEDC test campaign and coordination on this "challenging" project.

Tom Best, Arnold's technical director of the plans and programs directorate, said Hawkins led a study last year for the Office of the Secretary of Defense comparing Department of Defense and NASA transonic wind tunnels.

"Out of that study the recommendation came to run a standard test at all these facilities to really get a comparison of processes, data quality and flow quality,"
Best said. "In addition, the folks in 16T have long needed a model that they could put in the tunnel every year or so to check and see if the flow has changed - other tunnels have been doing this routinely for many years. This model will serve that purpose for 16T."

Hawkins said, "Last year we did a transonic assessment on behalf of DoD of what we call our national mid-to-large scale wind tunnels. Besides AEDC's transonic 16T, we evaluated similar NASA facilities at Glenn Research Center in Ohio, Langley's National Transonic cryogenic Facility (NTF) in Virginia and the 11-foot Transonic Wind Tunnel at Ames Research Center in California."

He said force and moment test data taken from the model, balance and sting in 16T, will be compared with the data derived from tests conducted by NASA using the same model and instrumentation in each facility.

A wind tunnel simulates the conditions of an aircraft in flight by causing a high-speed stream of air to flow past a model of the aircraft. A check standard aircraft model is used to validate the condition of the tunnel to ensure consistent flow quality and conditions and that established force and moment data on the model has not changed.

"A check model standard is a model that you've established a database on," Hawkins said. "We now have an initial database on 16T - and we can run tests across this Mach number range in selective conditions and if anything in that tunnel has changed this model will pick it up. It does not tell us what exactly has changed - it will give us some good clues, but if nothing's changed, that's it. It's simply a validation of what we had last time. The results are now traceable to a standard."

He said the usual way to calibrate a wind tunnel is by installing a centerline pipe and conducting an expensive test process. A check standard model can be used to establish whether such calibration is necessary.

"Testing a model is far less expensive than doing a centerline check, which can cost between $3 and $4 million today," he said. "Also, sharing this model with other centers and the data produced just makes sense. It should result in a significant cost savings over time and will provide a once in a life-time opportunity for those involved."

The collaboration, which includes working with the Air Force Research Laboratory (AFRL), has already resulted in sharing of data acquisition methods and a range of other wind tunnel and data-related processes between engineers at NASA, AFRL and AEDC.

Hawkins said the opportunity to share such detailed data processes and wind tunnel operations to this level may be unprecedented.

"It includes everything from the initial log data at the start of the test, how we process that data, the filtering techniques that we use, the tunnel flow corrections we use when we need to apply those - we were looking at the methodologies involved - all the way through to the end product," he said.

"Force testing is a core expertise, that we have," he continued. "Part of this overall effort was to get that expertise back because it's in our nation's best interest. The Air Force Research Laboratory has joined the team too because they had lost this expertise. AFRL mothballed their wind tunnels and over the last five years they've been busy bringing them back up and trying to train an operating staff. Jacobs engineering actually has a contract to operate those wind tunnels."

Bob Guyton, technical advisor for AFRL's Experimental Fluid Dynamics Team, said the testing and other aspects of the project provided a great opportunity for engineers at the lab.

"I see important benefits for the wind tunnel community," he said. "Our intent is to change our in-house wind tunnel workforce toward a unit that focuses all resources on conducting wind tunnel experiments. The change requires that we become dependent on central support from elsewhere for highly specialized technical developments. We used to have enough highly experienced people to do both, but how times have changed. Our unit is trimmed for high efficiency and we are "one-deep" in most critical technical areas - enough to operate, but not enough to independently develop capabilities efficiently."

Guyton said the lack of seasoned engineers is not unique to AFRL.

"Most every other wind tunnel site will exist with similar limitations for the long term," he said. "A great responsibility is bestowed on the idea of national 'grey-beard' support centers and national standardization of methods. Many other satellite wind tunnel installations need this to survive as personnel with deep experience leave the work force around the country."

He said the highlights of the collaboration included a "remarkable" level of open exchange of information.

"Our team came away with an appreciation of the process, checklists and regimented procedures," he said. "We also gained an appreciation for the 'test cart' concept - that sparked some ideas."

Guyton said he also saw some room for improvement in future endeavors, although he acknowledged limitations on the part of all parties due to reasons hard to overcome without a concerted effort.

The test article in 16T began as a scaled model of an F-111 Aardvark, a medium-range interdiction and tactical strike aircraft that also served as a strategic bomber, electronic warfare and reconnaissance platform.

AEDC engineers will continue to analyze the data produced and participate in testing at the three other NASA centers in the coming year. A joint NASA/AEDC report will document the final results.