Test before flight
This T-38 Northrup Talon supersonic jet trainer model was tested in the U.S. Air Force Arnold Engineering Development Center's 16-foot transonic wind tunnel in 1958. The T-38 flew in 1959. (AEDC file photo)
AEDC partners with Edwards AFB to test legacy jet as part of propulsion modernization program



by Philip Lorenz III
AEDC/PA


6/16/2008 - ARNOLD AIR FORCE BASE, Tenn. -- A long-standing collaboration between the Air Force Flight Test Center (AFFTC) at Edwards AFB and the Arnold Engineering Development Center (AEDC) through its Technology and Analysis Program is helping propulsion integration engineers quantify performance improvements of an updated version of a legacy airplane.

Capt. Chuck McNiel, aeropropulsion technology program lead in the 704th Test Systems Group at AEDC, said the partnership between the test centers is providing near and long-term benefits for the T-38 and all of its users.

"The T-38 is a critical Air Force asset for training future fighter and bomber pilots," he said. "The analysis and evaluation provided by AFFTC and AEDC in conjunction with T-38C flight testing has greatly enhanced the decision making process for the users in choosing the best aircraft configuration for the Air Education and Training Command."

The T-38, a twin-engine, high-altitude jet, was the world's first and most prolifically produced supersonic trainer. Originally designed in the 1950s, the aircraft remains in service with air forces around the world, including the T-38's biggest user, the U.S. Air Force.

David Kidman, the AFFTC's technical expert in the propulsion integration flight test branch, said that T-38 analysis efforts benefited from an on-going AFFTC/AEDC technical excellence initiative in which both centers are collaborating to develop and apply state-of-the-art analysis capabilities to strengthen the government's role in test and evaluation and acquisition processes.

"An over-arching goal of the technical excellence initiative is for the two centers to integrate ground test, flight test, and modeling and simulation to reduce the time and cost of developmental testing and maximize knowledge derived from tests," said Dr. Don Malloy, a member of Aerospace Testing Alliance's (ATA) Technology and Analysis Branch, technical lead and project manager for collaborative projects between the two centers. 

ATA is the support contractor for Arnold.

AEDC Air Force and ATA engineers remotely supported recent T-38C/Propulsion Modernization Program tests at Edwards AFB. Paul Burns, a University of Huntsville aerospace engineering senior, who has been working for ATA as a co-op student, provided on-site support at Edwards.

"With Dr. Malloy's guidance, Burns provided valuable on-site support to recent flight testing of the T-38C aircraft," said Propulsion Integration Engineer Benjamin Tomlinson with the 773rd Test Squadron attached to the 412th Test Engineering Group at Edwards AFB. "We introduced Paul to flight test execution, by including him in flight test missions. He brought a beta version of the operability analysis software which AEDC used to analyze the flight test data."

Burns has been to Edwards AFB three times this year to help with the engine testing. He said development of the model-based compressor analysis software he brought with him began in November 2007.

"We have and will continue to refine the analysis tool," Burns explained. "The analysis tool is used to analyze pressure, temperature and speed data from flight tests to help determine where the compressor is operating and estimate the loss in stability pressure ratio due to pressure and temperature distortion and Reynolds number effects. We want to predict what flight conditions could possibly cause the compressor blades to stall."

Tomlinson said it was important to understand how the center's recent support role in testing the J85-5 engine in flight evolved to fully appreciate the significance of the collaboration.

"Don's team discovered the compressor speed measurement on the flight test aircraft did not respond quickly enough to assess the loss in compressor stability margin during transient engine operation," he said. "Once we were alerted to the lag in the speed measurement we were able to bypass the legacy speed measurement system on the production aircraft and use Edward's signal processing equipment to produce the high-quality time dependent data required for the Air Force's assessment of engine operability."

Tomlinson added, "Even more important, the AFFTC/AEDC team was able to determine that the measurement uncertainty wasn't good enough to meet the test objectives prior to initiation of engine operability testing."

Dr. Malloy explained that recent upgrades to the T-38's engine and airframe challenged propulsion integration engineers.

"The Air Force modernized the engine to get more life out of the engine and reduce maintenance costs" he said. "They also upgraded the inlet and ejector nozzle on the engines to provide better hot-day takeoff performance and lower specific fuel consumption at cruise conditions."

Tomlinson said that a goal of the propulsion modernization program was to obtain the highest possible performance out of the engine and aircraft without adversely affecting engine operability.

"The J85-5 engines in the T-38 aircraft do not have modern digital engine controllers found in the current generation of Air Force fighter engines," he said. "Without a full authority digital electronic controller, compressor stalls during aircraft maneuvers are possible. Engine and aircraft manufacturers were challenged to improve the performance of the integrated engine and aircraft system while ensuring that the occurrence of compressor stalls remained below the stall rate with the legacy engines."

A compressor stall is characterized by an abnormal airflow inside a jet engine that can result from a stall of the blades within the compressor's rotor, similar to what happens when the airflow over an aircraft's wing fails to produce lift. Compressor stall can lead to a complete reversal in the direction of airflow and loss of compression throughout the engine, commonly known as a compressor surge.

A compressor stall causes a loss of engine power, which can result in a temporary loss of power or a complete shutdown of the engine.

Dr. Malloy said two key challenges that the AEDC team helped their counterparts at Edwards tackle were base-lining compressor operation and estimating the effects of temperature distortion in the absence of measurements in the engine-inlet to measure pressure recovery and temperature distortion.

"Our role was to help characterize compressor operation during propulsion flight tests and one of the challenges is they don't have the same instrumentation we have available during ground tests," Malloy said. "Safety of test and flight are critical and any instrumentation added to the engine has to be flight certified. As a result, few internal engine measurements are typically available in flight and model-based analysis approaches are required to characterize compressor operation with a limited number of flight test measurements."

It was more than just an analysis effort - the AFFTC/AEDC team had to come up with an approach to maximize the knowledge from a limited number of flight test measurements using historical ground test data and predictions from engine and aircraft manufacturer models."

Tomlinson described the collaborative effort with AEDC as being "an excellent experience."

He said, "The tools being developed are exactly what a legacy aircraft like the T-38 needs. We will continue to work with AEDC to upgrade our analysis tools to support T-38 engine operability flight tests at AFFTC later this summer."

Brian Binkley, a T-38 project team member and engineer in ATA's technology and analysis branch, said a key to successful collaborative development of analysis tools is a common development platform.

"Collaborative efforts between AFFTC and AEDC utilize MATLAB as the development environment," he said. "The data analysis capabilities inherent in MATLAB facilitate rapid development of engineering tools. This allows engineers to focus on data analysis and problem solving rather than coding of analysis algorithms."

MATLAB, created by Mathworks, Inc., is a numerical computing environment and programming language.

"It is clear that AEDC/AFFTC analysis is on par or better than airframe and engine manufacturer analysis in support of the T-38 program," Kidman said. "It is also clear the Air Force's technical excellence initiatives to develop and apply state-of-the-art analysis capabilities to strengthen the government's role in test and evaluation and acquisition are working."