Background |
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Langley participated
in the X-31 Enhanced Fighter Maneuverability (EFM)
Program during four separate activities. From 1973
to 1984, Langley was active in the planning, testing,
and analysis of the remotely piloted Highly Maneuverable
Aircraft Technology (HiMAT) research vehicle. From
1984 to 1985, Langley cooperated in a program with
Rockwell International to develop a representative
fighter configuration that could demonstrate the
advantages of exploiting high-angle-of-attack maneuvers
during close-in air combat. From 1986 to 1991,
Langley participated in the analysis and configuration
development of the International (United States
and Federal Republic of (West) Germany) X-31 Program.
From 1991 to 1995, Langley supported the flight-test
program, which was conducted at NASA Dryden Flight
Research Center by the International Test Organization.
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The HiMAT Program |
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Following the Vietnam
conflict and renewed emphasis on close-in air-to-air
combat, the U.S. military became interested in
aircraft maneuverability. As a result, the requirement
for high speeds, long considered the key factor
in successful air combat, became a secondary objective.
NASA initiated a joint program with the Air Force
known as the Highly Maneuverable Aircraft Technology
(HiMAT) Program. The staffs of the Langley, Ames,
and Dryden Research Centers all participated in
planning the HiMAT Program, with William P. Henderson
serving as the technical lead and coordinator for
Langley. The focus of the HiMAT Program was flight
research and maneuverability demonstrations of
a representative advanced configuration in the
form of a remotely piloted subscale vehicle at
Dryden. The goals of HiMAT included a 100-percent
increase in aerodynamic efficiency over 1973 technology,
and maneuverability that would allow a sustained
8-g turn at a Mach number of 0.9 and an
altitude of 25,000 ft. The program ultimately achieved
all goals.
The original HiMAT model with
the thrust-vectoring, wedge nozzle in the Langley
16-Foot Tunnel.
HiMAT during flight tests at
Dryden.
In August 1974, Rockwell International was awarded
a contract to construct a reduced scale model of
the HiMAT design. Rockwell submitted a canard configuration
with twin vertical tails on a highly swept wing
and a high aspect ratio, pitch thrust-vectoring,
wedge nozzle. The thrust-vectoring wedge nozzle
was later replaced with a fixed, axisymmetric nozzle
to reduce program costs. The first flight of the
HiMAT occurred on July 27, 1979, and research continued
through January 1983. The success with the HiMAT
configuration inspired Rockwell to examine the
benefits of a derivative fighter that exploits
high angles of attack for new tactical maneuvers
during close-in air combat.
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The SNAKE Program |
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In 1984, Rockwell
proposed a cooperative program to Langley to assess
and develop a Rockwell advanced design known as
the Super Normal Attitude Kinetic Enhancement (SNAKE)
configuration. Joseph R. Chambers and Joseph L.
Johnson, Jr. determined that the proposal was in
concert with many Langley research interests in
high-angle-of-attack technology, and the cooperative
program on the SNAKE configuration was begun. Langley
researcher Mark A. Croom was assigned the role
of lead engineer, and he began a decade of personal
participation in the X-31 evolution and flight-test
programs.
The initial SNAKE configuration bore a superficial
resemblance to the earlier HiMAT design (canard
and wing-mounted twin vertical tails); however,
the new configuration was designed analytically
with computers and a minimum amount of wind-tunnel
tests. Unfortunately, Croom’s aerodynamic
tests of the initial SNAKE configuration in the
Langley 30- by 60-Foot (Full-Scale) Tunnel indicated
unacceptable stability and control characteristics.
The configuration was unstable in pitch, roll,
and yaw for all angles of attack.
Based on their extensive experience with stability
and control characteristics of advanced fighters,
Croom and Johnson provided the Rockwell team with
several recommendations to cure the problems exhibited
by the SNAKE configuration. The configuration modifications
resulted in satisfactory characteristics, and the
aerodynamic deficiencies of the initial SNAKE design
had been eliminated. Rockwell was grateful for
the guidance and innovation contributed by Langley
in the evolution of the SNAKE configuration.
The original Rockwell SNAKE configuration
with downturned wingtips and thrust vectoring paddles.
The modified SNAKE configuration
after the Langley tests with upturned wingtips,
wing
leading-edge flaps, enlarged vertical tails, enlarged
wing-fuselage strake, and nose strakes.
Modified SNAKE model flying at
extreme angle of attack using thrust vectoring
in the Langley Full-Scale Tunnel.
In the early 1980’s, an awareness of the
benefits of thrust vectoring for dramatically improved
control at high angles of attack surfaced. In addition
to studies of advanced engine concepts with vectoring
nozzles, interest arose over the use of simple
thrust-vectoring paddles in the engine exhaust
to deflect the thrust for control augmentation.
As discussed in Langley Contributions to the
F-14, the Navy, with Langley’s assistance,
had taken the lead in this area with flight tests
on an F-14 modified with single-axis yaw-vectoring
paddles. In addition, during a cooperative program
with Rockwell led by Langley researcher Bobby L.
Berrier, Langley provided design data for multiaxis
thrust-vectoring paddle configurations using the
Jet Exit Test Facility in 1985. Based on these
fundamental research studies, Rockwell incorporated
multiaxis thrust-vectoring paddles into the SNAKE
configuration. Free-flight tests of the modified
SNAKE model in the Full-Scale Tunnel by Croom’s
team in 1985 provided an impressive display of
the effectiveness of thrust vectoring at extreme
angles of attack.
In West Germany, Dr. Wolfgang Herbst of Messerschmitt-Bolkow-Blohm
(MBB) aggressively touted the advantages of post-stall
technology (PST) for increased effectiveness during
close-in air combat. Herbst’s conclusions
were based on wind-tunnel tests of a German advanced
canard fighter configuration known as the TKF-90
and piloted simulator studies during which the
application of simulated thrust vectoring resulted
in rapid directional turns at high angles of attack
had increased the turn rate by over 30 percent.
Technical discussions between the Rockwell SNAKE
Program managers and Herbst were initiated in 1983,
and planning for a mutual program on PST ensued.
Discussions with the Defense Advanced Research
Projects Agency (DARPA) were very positive. When
funding for collaborative international activities
became available from the U.S. (the Nunn-Quayle
research and development initiative in 1986) and
West German governments, the technical expertise
of Rockwell and MBB were joined under DARPA sponsorship
in the X-31 Program. In view of Langley’s
extensive experience in high-angle-of-attack technology,
unique test facilities, and contributions to the
Rockwell SNAKE Program, DARPA requested in 1986
that Langley become a participant in the X-31 development
program.
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X-31 Configuration
Evolution |
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The Rockwell and
MBB X-31 design team merged their configuration
candidates into a canard fighter powered by a single
General Electric F404 engine with a single vertical
tail. The initial design included an F-16 canopy
for cost-saving purposes. Extensive tests of the
initial X-31 configuration were carried out at
Langley during 1987. These tests included static
wind-tunnel tests and configuration component evaluations
in the Langley 14- by 22-Foot High-Speed Tunnel,
rotary-balance tests in the Langley 20-Foot Vertical
Spin Tunnel to determine aerodynamic characteristics
during spins, and dynamic force tests in the Langley
Full-Scale Tunnel. Unfortunately, in 1988 the X-31
configuration was revised, and an F-18 canopy was
incorporated. This change was regarded as significant,
and a major portion of the previous wind-tunnel
tests had to be repeated for the revised configuration.
Rotary-balance tests of the revised configuration
were conducted in 1988, and spin tests and static
and dynamic tests were completed in 1989 for the
updated configuration. In 1989, a 0.19-scale model
of the X-31 underwent extensive aerodynamic and
free-flight tests in the Langley Full-Scale Tunnel.
Results from these ground-based studies indicated
that the X-31 might have marginal nose-down control
at high angles of attack and that the configuration
might exhibit severe, unstable lateral oscillations
(wing rock) that would result in a violent, disorienting
roll departure and an unrecoverable inverted stall
condition. Fortunately, the results also indicated
that a simple control law concept could prevent
the aircraft from entering a spin. The awareness
that such phenomena might exist for the full-scale
aircraft enabled the X-31 design team to configure
the flight control system for maximum effectiveness.
An exhaustive test, which included 498 paddle
and nozzle configurations of the multiaxis thrust-vectoring
system, was conducted by Langley researcher Francis
J. Capone in the Jet Exit Test Facility during
1988. These data were used to select the final
paddle and nozzle multiaxis thrust-vectoring configuration.
These data were also critical to the design of
the X-31 flight control system, since vectored
thrust imposes large forces and moments in addition
to the normal aerodynamic parameters.
A 0.27-scale drop model was used by Langley to
evaluate the post-stall and out-of-control recovery
characteristics of the configuration. The model,
which weighed about 540 lb and included extensive
instrumentation, was flown without an engine to
assess the capabilities and characteristics of
the basic airframe. The objective was to demonstrate
that the X-31 would be agile and have satisfactory
characteristics without the additional augmentation
provided by thrust vectoring. The drop-model test
identifies characteristics and large amplitude
flight motions that cannot be assessed in conventional
wind or spin tunnels. In the X-31 Program, the
technique proved to be invaluable as an early indicator
of the highly unconventional behavior of the configuration.
In particular, the violent roll departure indicated
by tests of the free-flight model was encountered
during the drop-model tests. Several control schemes
were evaluated to eliminate this problem. In addition,
the drop-model test technique provided solutions
to barrier problems during the full-scale flight-test
program.
X-31 with F-16 canopy during
tests in the Langley 14- by 22-Foot Low-Speed Tunnel.
Free-flight tests of the X-31
in the Langley Full-Scale Tunnel.
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X-31 Flight Demonstration
Program |
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The first flight
of the first X-31 aircraft occurred at Palmdale,
California, on October 11, 1990, and the second
aircraft made its first flight on January 19, 1991.
During the initial phase of flight-test operations
at the Rockwell facility at Palmdale, the two aircraft
were flown on 108 test missions. On the test missions,
the aircraft achieved thrust vectoring in flight
and expanded the post-stall envelope to 40-deg
angle of attack. Operations were then moved to
Dryden in February 1992, at the request of DARPA.
At Dryden, the International Test Organization
(ITO) expanded the flight envelope of the aircraft,
including military utility evaluations that compared
the X-31 to similarly equipped aircraft for maneuverability
in simulated combat. The ITO, managed by DARPA,
included NASA, the U.S. Navy, the U.S. Air Force,
Rockwell Aerospace, the Federal Republic of Germany,
and Deutsche Aerospace (formerly Messerschmitt-Bolkow-Blohm).
The first NASA flight under the ITO took place
in April 1992. As the X-31 full-scale aircraft
flight tests began at Dryden, the Langley staff
maintained a close support role for consultation
and ground testing capability.
Two problems surfaced during the X-31 flight-test
program, and both were considered significant enough
to curtail flight tests until solutions were found.
The first problem was encountered in the flight-test
program when it became apparent that the pitch
control effectiveness of the aircraft at post-stall
conditions (particularly at aft center of gravity
conditions) was marginal. Pilots reported that
their ability to obtain positive, crisp, nose-down
aircraft response was unsatisfactory and that increased
control effectiveness was required if the X-31
was to be considered tactically responsive at high
angles of attack. As part of the X-31 Team, Langley
was requested to conduct wind-tunnel tests to explore
options to provide the increased control at high
angles of attack. Mark Croom and his team quickly
responded and evaluated 16 configuration modifications
to improve nose-down recovery capability in the
Full-Scale Tunnel. Results of the investigation
recommended that a pair of 6- by 65-in. strakes
be mounted along the fuselage afterbody to promote
nose-down recovery. The Langley recommendations,
which were given within a week of the test request,
provided a timely solution to the problem. The
aft-fuselage strakes were incorporated in the X-31,
and the pilots reported that the nose-down control
was significantly improved.
One of the X-31 aircraft in flight.
Langley researcher Mark Croom
(l) discusses the X-31 drop-model program with
an X-31 program manager.
Mark Croom points to the aft-fuselage
strakes defined by
Langley tests and subsequently incorporated on
the X-31 aircraft.
The second problem that occurred in the X-31
full-scale flight test was caused by large out
of trim asymmetric yawing moments at high angles
of attack. Shortly into the high-angle-of-attack,
elevated-g phase of the envelope expansion,
a departure from controlled flight occurred as
the pilot was performing a maneuver at 60-deg angle
of attack. Data analysis by the X-31 team indicated
that a large asymmetric yawing moment, in excess
of the available control power, had triggered the
departure. In response to an urgent request for
solutions, Croom and the Langley team conducted
tests in the Langley Full-Scale Tunnel to design
nose strakes that would minimize the problem. Once
again, Langley responded rapidly with a strake
configuration that permitted the flights to continue.
The X-31 Program logged an X-plane record of
524 flights in 52 months with 14 pilots from NASA,
the U.S. Navy, the U.S. Marine Corps, the U.S.
Air Force, the German Air Force, Rockwell International,
and Deutsche Aerospace.
Evaluation of the X-31 as an enhanced fighter
maneuverability demonstrator by the ITO concluded
in early 1995.
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Role of the X-31
in High-Angle-of-Attack Technology |
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The accompanying
photograph shows three thrust-vectoring aircraft,
each capable of flying at extreme angles of attack,
cruising over the California desert in March 1994.
The F-18 HARV (top), the X-31
(middle), and the F-16 MATV (bottom) in flight.
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