Air Force Academy engine used for collaborative AEDC test project

  • Published
  • By Philip Lorenz III
  • AEDC/PA
ARNOLD AIR FORCE BASE, Tenn. --  An Arnold Engineering Development Center (AEDC) team of engineers collaborated with officials at the Air Force Academy in Colorado Springs, Colo., over the summer to conduct a series of ground tests on one of the school's F109 turbofan engines.

The F109 was a small turbofan engine developed for the Fairchild T-46 light jet trainer aircraft by the Garrett Air Research Corp. The Air Force cancelled the T-46 program in 1986 and the few engines that had been produced were donated to universities, including the Air Force Academy.

It sounds like a routine ground test of an aircraft engine, a significant part of what engineers at AEDC do on a regular basis.

However, this test was anything, but routine.

Maurice May, AEDC's Test Systems Division program manager for instrumentation and diagnostics, said the test represented a milestone, accomplishing multiple objectives to advance cutting-edge technologies and methodologies to resolve aircraft engine operability and structural challenges on the most advanced military aircraft.

David Beale, ATA's Technology and Analysis Branch project manager for the F109 work, said, "Prior to this test project, we did not have swirl data characterizing the way the engines with these inlets function under simulated flight conditions. This data is critical in two ways. It both supports the development of methodologies for addressing swirl in the inlet-engine integration process and the development of methodologies for evaluating engine durability."

AEDC Test Technology Branch's project also had support from Dr. Milt Davis, formerly an ATA engineer specializing in turbine engine modeling and simulation and now the aeropropulsion lead for AEDC's Air Force Analysis Branch.

Dr. Davis said the modeling and simulation part of the project was productive, but more work remains to be done to improve the model and integrate it into subsequent tests.

The modeling and simulation part of the project had two purposes. One was to validate the model being developed for evaluating the engine stability in the presence of swirl and then secondly, to use modeling and simulation online as part of the test process, to influence the test.

Regarding engine operability work the team accomplished on the F109, May said the test had already yielded valuable data, and paves the way for future work with additional configurations - on different aircraft engines and fans.

"Because of serpentine duct designs on modern airplanes, we need to further characterize swirl effects on engine stability," he said. "The next generation of fighters or aircraft, not necessarily just fighters, would also be subjected to this as well. This database needs to be put together so that we can start characterizing turbulence, swirl, total pressure, temperature and distortion with the goal of suggesting and making design modifications to mitigate those effects early on. We also need to understand how those effects interact and impact engine performance.

"This is where ground testing comes in, to evaluate those effects and implement design changes when and where they're needed. Then you continue the ground testing process to evaluate those modifications. If this hasn't been satisfactorily accomplished prior to getting out to flight test, then it's real hard to make changes, from a momentary as well as a design point-of-view."

The test on the F109 engine represented an important step in an ongoing and iterative process. It was clear to the team that it is imperative to address the effects resulting from multiple variables and complex interactions of air flow through the inlet and into the engine.

"For example, one of the things that haven't been determined yet, is how does swirl combine with total pressure," May continued. "We see total pressure distortion now and we haven't put the two together or mixed temperatures into a test. There are all these other interactions that go on, too."

The engineers working on the structures-related part of the test collected and compared data from non-intrusive stress measurement system (NSMS) probes and strain gauges on the engine's fan rotor blades. They were able to make correlations between the strain gauges and the NSMS probes and looked at a comparison of multiple NSMS probe technologies and their uncertainties analyses.

"It is important to gain a perspective on the importance of this project," said Lt. Col. August J. Rolling, the Air Force Academy's propulsion discipline director and the director of the aeronautics laboratory. "Fifth generation fighters are being asked to do more with continuous engine operation than ever before.

"Such things as high angle-of-attack maneuvers, hot-gas ingestion from missile launch and serpentine inlets do bad things to flow entering the engine. Although static distortion testing has been around for decades, the testing techniques and analysis processes for such dynamic environments need to be improved and capabilities such as swirl distortion added."

May said gaining this type of information is invaluable because the turbine engine community and the acquisition community do not have the needed database. The testing the team accomplished has provided a baseline of data for future developments.

The test also represented a long-standing relationship that has been established between AEDC and the academy.

"It was a real pleasure working with all the guys from AEDC," said Jerry Stermer, Air Force Academy test facility chief technician. "The professional approach over the past several years resulted in compiling a huge amount of valuable data on flow quality. The use of our F109 engine was a perfect fit as we were able to modify the fan shroud and exhaust duct to support exactly what was needed.

"We were able to make modifications to the F109 that included NSMS probes at several locations around the fan shroud [and] a motorized cone that could be moved into the exhaust duct, causing the fan to go into a stall condition at our command. We were able to produce a real-time display of the stall line so we knew exactly where that stall would occur before it actually happened."

Colonel Rolling said the effort over the summer was complimentary both academically and in providing a much-needed test asset and setting.

"This effort is critically important to our success from both perspectives," he said. "Looking at the curriculum, we are able to leverage this new knowledge directly into the classroom.

"From our perspective, we are customer focused, and supporting such an important customer as AEDC proves that the U.S. Air Force Academy is a great institution to bring tough research projects to, expecting world-class results."

Dr. Davis shared the colonel's view and said the ongoing collaboration with the academy may impact the students in ways no one has anticipated.

"It provides an interaction between the academy's educational program and the need for test assets that are available and a lot less expensive," he said. "When you go into a university setting, you really don't have the expenses that you have at AEDC. The other thing is by connecting with the academy, those second lieutenants who have graduated from the Academy, might be the [future] commanders of AEDC 15 to 20 years from now."