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Langley Contributions
to the F-16
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Early Assessments |
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In early 1972, the
Langley Research Center was requested by the Department
of Defense (DOD) to participate in assessments
and tests of competing YF-16 and YF-17 designs
for the Air Force Lightweight Fighter (LWF) Program.
Langley researchers, as members of DOD source evaluation
teams, assessed technical claims of each of the
competing contractors. The YF-16 configuration
underwent extensive wind-tunnel tests at Langley,
especially in the 30- by 60-Foot (Full-Scale) Tunnel,
the 20-Foot Vertical Spin Tunnel, the 7- by 10-Foot
High-Speed Tunnel, the 16-Foot Transonic Dynamics
Tunnel, and the Unitary Plan Wind Tunnel. Extensive
studies of enhanced control systems for high-angle-of-attack
conditions were also conducted in the Langley Differential
Maneuvering Simulator (DMS) with pilots from Langley,
Lockheed Martin Tactical Aircraft Systems (LMTAS)
Division, and the Air Force. The results of these
studies provided information on the capabilities
of the YF-16. Following the selection of the YF-16
for production as the F-16 in 1974, the precursor
tests served as an excellent knowledge base for
development of the new F-16 fighter.
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Transonic Performance |
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The air war experience
in Vietnam, where the lack of maneuverability of
U.S. fighters at transonic speeds provided advantages
to nimble enemy fighters, was the stimulus for
the YF-16 program. The Air Force and designers
of the YF-16 therefore placed great emphasis on
achieving unprecedented transonic maneuver capability
with excellent handling qualities. At that time,
Langley researchers under the leadership of Edward
C. Polhamus were conducting in-depth studies in
the 7- by 10-Foot High-Speed Tunnel of approaches
to obtain near optimum aerodynamic maneuver performance
for wings, including the use of fixed and variable-camber
concepts. Some of the earliest systematic wind-tunnel
tests determined the most effective geometries
for leading- and trailing-edge wing flaps. In addition,
studies were conducted to develop methodologies
for the prediction and minimization of undesirable
buffet characteristics. The program was coordinated
with flight tests of actual high-performance fighters
at the NASA Dryden Flight Research Center.
Polhamus and his group (including Edward J. Ray,
Linwood W. McKinney, Blair B. Gloss, and William
P. Henderson) provided valuable guidance to the
LMTAS design team. The insight and understanding
provided by the broad database from Langley tests
permitted development of the extremely effective
leading- and trailing-edge flaps used by the F-16.
The F-16 (and most other high-performance fighters)
uses specific schedules of flap deflection with
Mach number and angle of attack for superior maneuverability
at transonic conditions.
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Vortex Lift |
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In the early 1960’s
worldwide interest in the phenomenon known as “vortex
lift” increased as a result of aerodynamic
studies of highly swept configurations such as
the Concorde supersonic transport. Two events contributed
to the initiation of a world-class Langley vortex-lift
research program led by Edward Polhamus.
The first event was a detailed experimental and
theoretical study of canard configurations at high
subsonic speeds led by researcher Linwood McKinney.
McKinney became interested in the favorable effects
of vortex on lift that were demonstrated during
development of the Swedish Viggen canard configured
aircraft. McKinney’s study indicated that
the favorable effects of the canard trailing vortex
on the lifting capability of a close-coupled wing
might also be extended to higher angles of attack
by the strong leading-edge vortex flow of a slender
lifting surface.
F-16 in a hard turn with wing
leading-edge flaps deflected and vortices produced
by the sharp wing-body strakes illustrated by condensation.
The second event that led to the vortex-lift
work at Langley was a cooperative Langley and Northrop
study of hybrid wings that centered on the use
of relatively large, highly swept wing extensions
at the wing-fuselage intersection to promote strong
beneficial vortex-flow effects.
Stimulated by the promise of this revolutionary
aerodynamic concept, Polhamus and his associates
put together a vortex-lift research program that
became internationally recognized for its experimental
database, analytical procedures, and aircraft applications.
In addition to Polhamus and McKinney, key members
of this team included Edward Ray, William Henderson,
John E. Lamar, and James M. Luckring. Their research
into the fundamentals and applications of vortex
flows allowed Langley to aid U.S. industry in the
design of highly maneuverable advanced fighters.
The evolution of the YF-16 design at LMTAS included
studies of configuration variables such as wing
design, maneuvering devices, number and location
of engines, control surfaces, number and location
of tail surfaces, and structural concepts. As the
configuration options matured, two candidate configurations
competed for priority. The first configuration
was a simple wing, body, and empennage design,
while the second design was a twin-tailed, blended-wing
body with vertical and horizontal tails on booms.
The LMTAS team selected the best features of both
configurations for the final YF-16 design. After
considerations of performance, stability, and control
were addressed, the YF-16 configuration incorporated
a rather wide, blended forebody that produced strong
vortices at moderate angles of attack. LMTAS had
attempted to weaken the strength of the vortices
by promoting attached flow, but these attempts
were not successful.
A team from LMTAS visited the Polhamus group
and requested guidance for control of the vortical
flows emanating from the forebody of the YF-16.
In a historically significant meeting, the Langley
team suggested that a completely different approach
be used to control the vortical flow. Specifically,
Langley suggested that the leading edge of the
blended forebody be sharpened to increase (rather
than decrease) the strength of the vortices, which
could be exploited for additional lift. This modification
allowed the forebody vortices to dominate and stabilize
the flow field over the aircraft at high angles
of attack, improve longitudinal and directional
stability for the single-tail configuration, and
stabilize the flow over the outer wing panels.
The LMTAS team accepted the recommendation, and
subsequent wind-tunnel tests verified the lift-enhancing
effect of the sharpened wing-body strake. In addition,
the sharpened strake significantly reduced buffet
intensity at transonic maneuvering conditions.
The wing-body strake of the F-16 is regarded as
a key contribution to its success as a maneuvering
fighter.
When the YF-16 team analyzed the effects of deflected
leading- and trailing-edge flaps and the sharp-edged
wing-body strake on directional stability at high
angles of attack, they found that the stability
contributions of a single vertical tail were significantly
enhanced. However, the contributions of twin vertical
tails were markedly degraded. As a result of this
analysis, the YF-16 was configured with a single
vertical tail. Thus, the Langley recommendation
for a sharpened wing-body strake favorably impacted
other configuration features of the aircraft.
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High-Angle-Of-Attack
Stability and Control |
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Increased maneuverability
for the YF-16 necessitated extended flight at high
angles of attack where aerodynamic deficiencies
caused by separated airflow can result in sudden
decreases in stability and controllability. Therefore,
special emphasis was placed on tests in Langley
facilities to insure that the YF-16 could provide
the pilot with “carefree” maneuverability.
Langley facilities used included the Full-Scale
Tunnel, the Spin Tunnel, and the DMS. Helicopter
drop-model tests for the YF-16 and the F-16 were
not conducted at Langley because of concurrent
resource demands of the B-1 and F-15 programs and
the fast pace of the LWF Program.
Researcher William A. Newsom,
Jr. with the free-flight model
of the YF-16 used in tests in the Full-Scale Tunnel.
To provide superior handling characteristics
at high angles of attack, any undesirable handling
characteristics were pushed out of the operating
envelope of the aircraft and the flight envelope
was limited with an advanced fly-by-wire flight
control system by LMTAS. This concept has proven
to be highly successful and has been used in all
variants of the F-16.
Researchers in the Full-Scale Tunnel conducted
exhaustive tests of the YF-16 and the F-16 configurations
at high-angle-of-attack conditions. Data gathered
in these studies identified the aerodynamic contributions
of components of the airframe, deficiencies in
stability and control, and potential solutions
to these deficiencies. These data also formed the
basis for piloted simulator studies at Langley
and LMTAS that helped LMTAS design the flight control
system for critical high-angle-of-attack conditions.
Langley and LMTAS engineers recognized that reliance
on the flight control system to insure satisfactory
behavior at high angles of attack required research
on the ability of fly-by-wire control systems to
limit certain flight parameters during strenuous
air combat maneuvers. The YF-16 and F-16 employ
the concept of “relaxed static stability”
in which the aircraft is intentionally designed
to be aerodynamically unstable while the flight
control system provides integrated stability by
sensing critical flight variables and making the
control inputs required to stabilize the aircraft.
Cooperative piloted simulator studies were conducted
in the Langley DMS to identify critical control
system components, schedules, and feedback gains
to stabilize the aircraft and pilot system for
the most demanding maneuvers for high-angle-of-attack
conditions. Of particular concern was the ability
of the horizontal tails and longitudinal control
system to limit the aircraft’s angle of attack
during maneuvers with high roll rates at low airspeeds.
Such maneuvers are critical because rapid rolling
maneuvers produce large nose-up trim changes due
to inertial effects, whereas the aerodynamic effectiveness
of the horizontal tails becomes significantly reduced
at low airspeeds and high angles of attack.
Under the leadership of Langley researchers Luat
T. Nguyen and William P. Gilbert, extensive studies
were conducted in the DMS with pilots from Langley,
LMTAS, and the Air Force. These studies verified
the effectiveness of the flight control system
and identified critical maneuvers that would be
tested during the flight development program. Gains
in the flight control system were modified and
incorporated into the aircraft system. New control
elements, such as a yaw rate limiter, a rudder
command fade-out, and a roll rate limiter that
maximized the maneuvering envelope with minimal
adverse effects were developed. The DMS was flown
by the LMTAS YF-16 and F-16 test pilots who later
flew the prototypes. These pilots cited the accuracy
of the preflight predictions and the valuable training
and exposure to potentially hazardous flight-test
conditions provided by the DMS.
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Curing Deep Stall |
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Early in Langley
research, tests of a YF-16 model in the Full-Scale
Tunnel indicated that if angle of attack was not
limited by the flight control system, the aircraft
could pitch up and attain an undesirable trimmed
condition at very high angles of attack with insufficient
nose-down aerodynamic control to recover normal
flight. The Langley researchers viewed this “deep”
stall as a serious problem that would require significant
research for resolution. The ability of the YF-16
to enter this deep stall was demonstrated in piloted
simulation with the Langley DMS and the results
were formally reported to the LMTAS team and the
Air Force. Unfortunately, other NASA and industry
wind-tunnel tests of YF-16 models contradicted
the Full-Scale Tunnel results—indicating
that the problem would not exist. The engineering
community generally agreed that the deep stall
was not an issue for the F-16. Flight tests of
the YF-16 aircraft were not extensive enough to
determine susceptibility to deep-stall phenomenon.
However, the follow-on flight-test program for
the F-16 proved that the favorable projections
were wrong and that the deep-stall condition actually
existed for the aircraft.
High-angle-of-attack test results obtained on
the early production version of the F-16 configuration
in the Full-Scale Tunnel showed the same deep-stall
trimmed condition that was noted in the YF-16 results.
Again, contradictory wind-tunnel results obtained
elsewhere convinced the engineering community that
the aircraft would not exhibit the problem. However,
in subsequent high-angle-of-attack flight evaluations
at Edwards Air Force Base, an F-16 that had been
subjected to rapid rolls at diminishing airspeeds
in vertical zoom climbs suddenly entered a stabilized
deep-stall condition and the pilot was unable to
recover the aircraft with normal aerodynamic controls.
The conditions for the deep stall agreed very well
with predictions based on the earlier test results
from the Langley Full-Scale Tunnel. Fortunately,
the test aircraft was equipped with an emergency
spin recovery parachute that was deployed to recover
the aircraft to normal flight conditions. This
event brought all high-angle-of-attack flight tests
of the F-16 to a standstill while a solution to
the deep stall could be found.
The excellent correlation of the Langley wind-tunnel
data with the events that occurred in flight and
the ongoing evaluation of the F-16 in the DMS at
Langley provided the tools to work the problem.
A joint NASA, LMTAS, and Air Force team arrived
at Langley and aggressively sought interim and
permanent fixes under Nguyen’s lead. Working
with the evaluation pilots, the team devised a
“pitch rocker” technique in which the
pilot pumped the control stick fore and aft thereby
setting up an oscillatory pitching motion that
forced the aircraft out of the deep stall and allowed
recovery to normal flight. The concept was adopted
and validated in the F-16 flight-test evaluation
at Edwards and was incorporated in the flight control
system as a pilot selectable emergency mode. A
longer term fix was developed in cooperative wind-tunnel
tests in the Full-Scale Tunnel and in LMTAS wind
tunnels. The ultimate fix for the problem (which
also improved takeoff performance) was increasing
the size of the horizontal tail about 25 percent.
This solution has been incorporated in all F-16
production aircraft.
Langley also identified and developed an automatic
spin prevention control system concept that could
prevent inadvertent spins for the F-16. Nguyen
and his group demonstrated the effectiveness of
the system with the DMS and analytical studies.
The fundamental elements of the system were further
refined by LMTAS and incorporated in the F-16 fleet.
As a result of Langley supporting analysis and
tests, the advanced fly-by-wire flight control
systems of the YF-16 and F-16 were designed with
confidence that inadvertent loss of control incidents
that had plagued many earlier fighters would be
eliminated.
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Spin Recovery |
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The YF-16 and F-16
configurations underwent extensive tests in the
Langley Spin Tunnel to determine spin and spin
recovery characteristics, especially for the configurations
with external store loadings. In addition, these
tests determined the size of emergency spin recovery
parachute that is required for the flight trials.
The results of these tests indicated trends that
were fairly typical of high-performance fighters.
The models predicted two types of spins—one
spin was a relatively well-behaved moderate spin
from which recovery was easily effected, and the
second spin was a potentially serious flat spin
at very high angles of attack with poor recovery.
The success of the sizing of the spin recovery
parachute was proven in its successful deployment
in the unexpected deep-stall encounter. The parachute
can be credited with saving the invaluable test
aircraft as well as the pilot. The development
and implementation of the production automatic
spin prevention system by the Langley and LMTAS
team has minimized operational encounters with
spins.
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Fly By Wire and
Sidestick Controller |
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In 1954, flight
tests of the first fly-by-wire aircraft, a modified
F9F Panther jet, were initiated at Langley. The
primary objective of the tests was to evaluate
various automatic control systems, including those
based on rate- and normal-acceleration feedback.
However (as is the case in many research investigations),
the most valuable result of the flight test was
related to secondary objectives—in this case
the introduction and evaluation of fly by wire
and a sidestick controller for pilot inputs.
In a serendipitous approach, Langley researchers
decided to avoid the relatively large expense and
time required to modify the existing hydraulic
flight control system for the F9F. Instead they
chose to implement an auxiliary system based on
a fly-by-wire analog concept and a small (4 in.)
sidestick controller mounted at the end of the
armrest at the side of the pilot. The sidestick
controller was used as the maneuvering flight controller
throughout the investigation. Rapid and precision
maneuvers such as air-to-air tracking, ground strafing
runs, and precision landings were evaluated.
Langley test pilot William L.
Alford checks out the side-stick controller in
1954 studies.
The objectives of the flight program were completed
with great success, and the information on various
types of automatic control feedback was used for
numerous aircraft development programs. However,
the very successful use of the rudimentary fly-by-wire
and sidestick controller concepts generated considerable
excitement within the research community, especially
those visionaries that anticipated the weight saving
advantages for future aircraft. Additional research
was conducted at Langley on these systems, including
the use of a sidestick controller for the Apollo
mission. The concept of the fly-by-wire control
system was later refined and greatly improved in
cooperative efforts between Langley and Dryden
that used a modified Vought F-8 Crusader and surplus
Apollo digital computers.
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Flutter Clearance |
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Flutter clearance
tests of both the YF-16 and F-16 were conducted
in the Langley 16-Foot Transonic Dynamics Tunnel
(12 tunnel entries for the F-16) under the leadership
of Moses G. Farmer, Raymond G. Kvaternik, Jerome
T. Foughner, and Frank W. Cazier. Numerous external
store configurations were investigated and several
potential flutter problems were identified and
solved. LMTAS requested a large number of flutter
tests to identify critical conditions that would
be demonstrated in flight, thereby minimizing very
expensive flight-test time. The YF-16 and F-16
did not demonstrate flutter within their flight
envelopes in the tunnel tests, but valuable improvements
in flutter margins were identified. For example,
a fuel-usage sequence was established for the three-bay
external tanks that significantly raised the flutter
speed. Mass balancing of the wingtip missile launcher
corrected a potential flutter problem with the
wingtip missile and underwing missile combination.
Several TDT studies were directed at active flutter
suppression for the F-16 with stores. Beginning
in 1979, a joint NASA, General Dynamics, and U.S.
Air Force Wright Aeronautical Laboratories team
initiated a series of tests that continued over
an 8-year period. These highly successful studies
progressed through an evaluation of a digital adaptive
(no prior knowledge of the aircraft configuration)
system. A Langley concept for flutter suppression
known as the “decoupler pylon” was
designed and evaluated by Wilmer H. Reed, Jr.,
Cazier, and Farmer, including flight tests on an
F-16 aircraft; however, the concept was not implemented
in the F-16 fleet.
Jerome T. Foughner checks out
the F-16 model with external stores
mounted in the 16-Foot Transonic Dynamics Tunnel
for flutter tests.
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The F-16XL |
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In the mid-1970’s
the U.S. Air Force became interested in a fighter
aircraft capable of “supercruise”—the
ability to cruise supersonically without an afterburner
while retaining respectable maneuver, takeoff,
and landing characteristics. The supercruise requirement
drove aircraft configurations to highly swept wing
platforms. LMTAS appreciated the fact that the
modular construction of the YF-16 allowed for relatively
simple replacement of the outer wing panels and
that a supercruiser demonstrator aircraft with
a highly swept wing would undoubtedly attract considerable
interest within the Air Force.
Langley’s staff had developed a research
program known as the Supersonic Cruise Integrated
Fighter (SCIF) Program under the leadership of
Roy V. Harris, Jr. As participants in previous
national and NASA civil supersonic transport programs
(SST), the Langley staff were leaders in the development
of databases and design methods for efficient SST
configurations. Several in-house supercruiser fighters
were designed and tested across the speed ranges
at Langley. Subsequent to the SCIF program, Langley
joined several industry partners in cooperative,
nonproprietary studies of supercruiser configurations.
In 1977 Langley and LMTAS agreed to a cooperative
study to design a new cranked-arrow wing for the
F-16 to permit supersonic cruise capability. Personnel
from LMTAS worked alongside the NASA researchers
under the direction of Charles M. Jackson at Langley
during the studies. The project leader for supersonic
design was David S. Miller. The results of the
wind-tunnel and analytical studies indicated that
a viable wing could be designed to satisfy the
supersonic and transonic requirements. With these
results, LMTAS initiated a company funded development
of an F-16 derivative with supersonic cruise capability.
Following the spirit of the previous wing design
cooperative venture with NASA, a cooperative agreement
was signed for mutual efforts on the new demonstrator,
which was called the Supersonic Cruise and Maneuver
Prototype (SCAMP).
Project engineer Sue B. Grafton
with the free-flight model of the F-16XL.
F-16XL prototype in flight.
Extensive tests for SCAMP took place in Langley
facilities, including the Unitary Plan Wind Tunnel,
the 7- by 10-Foot High-Speed Tunnel, the 16-Foot
Transonic Dynamics Tunnel, the Full-Scale Tunnel,
the DMS, the Spin Tunnel, and a helicopter drop
model. During these tests, a team led by researcher
Joseph L. Johnson, Jr. identified low-speed stability
and control issues that required modifying the
wing apex with a rounded planform. Research on
the SCAMP configuration by Langley researchers
identified numerous advanced concepts for improved
performance, including the application of vortex
flaps on the highly swept leading edge for improved
low-speed and transonic performance, automatic
spin prevention concepts, and optimized wings for
supersonic cruise. The final configuration became
known as the F-16XL (later designated the F-16E),
which displayed an excellent combination of reduced
supersonic wave drag, utilization of vortex lift
for transonic and low-speed maneuvers, low structural
weight, and good transonic performance. The F-16XL
flutter envelope was cleared in the 16-Foot Transonic
Dynamics Tunnel by Charles L. Ruhlin without significant
problems.
Two (a one-seat and a two-seat) F-16XL demonstrator
aircraft were subsequently built and entered flight
tests in mid-1982. In recognition of Langley’s
many contributions to the F-16XL, LMTAS management
sent letters of recognition to Langley and senior
NASA management. Marilyn E. Ogburn of Johnson’s
group was an invited participant at flight-test
evaluations of the F-16XL at Edwards Air Force
Base. The results of flight tests validated the
accuracy of Langley wing design procedures, wind-tunnel
predictions, and control system designs based on
DMS tests. Unfortunately, the interest in supersonic
cruise was replaced by an urgency to develop a
dual role fighter with ground strike capability.
Although the relatively large wing of the F-16XL
carried a significant amount of weapons, the Air
Force ultimately selected the F-15E in 1983 for
developmental funding and terminated interest in
the F-16XL.
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