AFFTC T-38C #64-13302 lands at Edwards AFB, CA
Photo: USAF / Chad Bellay
6/24/2008 - ARNOLD AIR FORCE BASE, TN -- Arnold Engineering
Development Center recently collaborated with the Air Force
Flight Test Center at Edwards Air Force Base, Calif., to help
quantify performance improvements of a T-38 Talon.
The twin-engine, high-altitude T-38 was the first world's first
supersonic trainer, originally designed in the 1950s.
"The T-38 is a critical Air Force asset for training future
fighter and bomber pilots," said Capt. Chuck McNiel, 704th
Test Systems Group technology program lead at AEDC. "The
analysis and evaluation provided by Edwards and AEDC, in conjunction
with T-38 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 partnership between the test centers is providing near
and long-term benefits for the T-38 and all of its users, Captain
The analysis efforts benefited from an on-going technical excellence
initiative, said David Kidman, AFFTC propulsion integration
flight test branch technical expert. Both Arnold Engineering
Development Center and the AFFTC are collaborating to develop
and apply state-of-the-art analysis capabilities, strengthening
the government's role in test and evaluation and acquisition
"An over-arching goal of the technical excellence initiative
is for the two centers to integrate ground test, flight test
and modeling and simulation," said Dr. Don Malloy, technical
lead and project manager for collaborative projects between
the two centers. "This is to reduce the time and cost of
developmental testing and maximize knowledge derived from testing."
Recent upgrades to the T-38's engine and airframe challenged
propulsion integration engineers, Dr. Malloy said.
"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."
One of the goals 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," said Benjamin Tomlinson, propulsion
integration engineer with the 773rd Test Squadron at Edwards.
"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. They needed to
ensure 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, Mr. Tomlinson said. This is similar
to what happens when the airflow of 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.
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, Dr. Malloy
"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," Dr. 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. model-based
analysis approaches are required to characterize compressor
operation with a limited number of flight test measurements."
The collaborative effort with AEDC was an excellent experience,
Mr. Tomlinson said.
"The tools being developed are exactly what a legacy aircraft
like the T-38 needs," he said. "We will continue to
work with AEDC to upgrade our analysis tools to support T-38
engine operability flight tests at the Air Force Flight Test
Center later this summer."
The Arnold Engineering Development Center and AFFTC analysis
is on par or better than airframe and engine manufacturer analysis
in support of the T-38 program, Mr. Kidman said.
"It is also clear the Air Force's technical excellence
initiatives to develop and apply state-of-the-art analysis capabilities
are working," he said.
Source: USAF Edwards AFB Press Release by by Philip Lorenz