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Langley Contributions
to the F/A-18
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Vortex Lift and
Maneuvering Flaps |
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In the early 1960’s,
the Northrop Company noticed an improvement in
the maximum lift of the F-5 aircraft because of
a small flap actuator fairing that extended the
wing root leading edge. This phenomenon spurred
interest in the effects of inboard vortex flows
and led to a cooperative NASA and Northrop study,
which was conducted in the Langley 7- by 10-Foot
High-Speed Tunnel with a group led by Edward C.
Polhamus. The cooperative study of hybrid wings
centered on the use of relatively large, highly
swept wing extensions at the wing-fuselage intersection,
which promoted strong beneficial vortex-flow effects.
The scope of the study included parametric studies
to maximize the lift- and stability-enhancing effects
of the wing extension concept, which became known
at Northrop as the leading-edge extension (LEX).
Studies were also directed at cambering the leading
edge of the LEX to suppress the vortex at low angles
of attack, and thereby minimize drag at cruise
conditions. Northrop applied a large highly swept
LEX to the YF-17 prototype aircraft to enhance
lift and stabilize the flow over the YF-17 main
wing at high angles of attack.
From these initial cooperative studies with Northrop,
Polhamus and his associates put together a world-class
vortex-lift research program that became internationally
recognized for its experimental database, analytical
procedures, and aircraft applications. In addition
to Polhamus, key members of this team included
Linwood W. McKinney, Edward J. Ray, William P.
Henderson, John E. Lamar, and James M. Luckring.
Their extraordinary research into the fundamentals
and applications of vortex flows placed the Langley
Research Center in an excellent position to aid
the U.S. industry in the design of highly maneuverable
advanced fighters. This experienced pool of experts
would subsequently provide invaluable guidance
and analysis to industry design teams in the development
of the F-16 for the Air Force and the F/A-18 for
the Navy.
F/A-18 with wing leading-edge
flaps deflected and LEX vortices made visible by
condensation.
In Vietnam, the lack of maneuverability of U.S.
fighters at transonic speeds provided key advantages
to nimble enemy fighters. Industry, the Department
of Defense (DOD), and NASA were all stimulated
to sponsor research to achieve unprecedented transonic
maneuverability while maintaining excellent handling
qualities. Langley researchers, under the leadership
of Polhamus, conducted studies in the 7- by 10-Foot
High-Speed Tunnel 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 were
conducted by the group to determine the most effective
geometries for wing leading- and trailing-edge
flaps. In addition to tests of aerodynamic performance
and stability and control, buffet studies were
conducted to understand and develop methodologies
for the prediction and minimization of undesirable
buffet characteristics. The program was closely
coordinated with flight tests of actual high-performance
fighters at the NASA Dryden Flight Research Center.
Flight evaluations of the effects of maneuver flaps
on a YF-17 were also later conducted in the Dryden
program.
Numerous discussions with Polhamus and his staff
provided valuable guidance to the Northrop design
team and the McDonnell Douglas team for the subsequent
F/A-18 design. 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 YF-17 and the
F/A-18. The F/A-18 and similar high-performance
fighters use specific, computer-controlled schedules
of flap deflection with Mach number and angle of
attack for superior maneuverability throughout
the flight envelope.
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Development of
the YF-17 |
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On January 6, 1972,
the Air Force issued a request for proposals (RFP)
for a Lightweight Fighter (LWF) Program. In March
1972, the Langley Research Center was requested
by DOD to participate in assessments and supporting
tests of the competing YF-16 and YF-17 designs
for the LWF. Langley researchers became members
of DOD source evaluation teams to assess and check
technical claims by each of the contractors. The
sponsoring LWF Program Office requested that certain
services of Langley be made available on an equal
basis to the two competing teams. This remarkable
arrangement provided each team with analysis and
support if they desired.
Northrop placed a high priority on superior high-angle-of-attack
characteristics and a high degree of inherent spin
resistance for the YF-17. The company had also
placed priorities in these areas during the development
of the F-5 and T-38 aircraft, which had become
known for outstanding resistance to inadvertent
spins. Langley support was therefore requested
for tests in the 30- by 60-Foot (Full-Scale) Tunnel
and the 20-Foot Vertical Spin Tunnel.
To provide superior handling qualities at high
angles of attack for fighter aircraft, Northrop
provided the airframe with the required levels
of aerodynamic stability and control characteristics
without artificially limiting the flight envelope
with the flight control system. This approach proved
to be highly successful for the YF-17 and has been
adopted by McDonnell Douglas (now Boeing) and used
in all variants of the F/A-18 aircraft.
Researcher Sue B. Grafton conducted exhaustive
tests in the Full-Scale Tunnel of the YF-17 configuration
at high angles of attack in 1973. The results of
the Langley tests revealed that Northrop had done
an outstanding job in configuring the YF-17 design.
The integration of the large LEX surfaces and the
placement of the twin vertical tails provided exceptional
tail effectiveness at high angles of attack. The
small strakes added to the forward fuselage nose
by Northrop resulted in extremely high directional
stability at high angles of attack. Free-flight
tests of the YF-17 model in the Full-Scale Tunnel
confirmed the excellent flying characteristics
predicted by the wind-tunnel data and provided
Northrop with highly positive predictions for upcoming
flight tests of the two YF-17 prototypes at Edwards
Air Force Base. The wind-tunnel data also formed
the basis for piloted simulator studies at Langley
and Northrop that helped Northrop design the flight
control system for critical high-angle-of-attack
conditions.
Project engineer Sue Grafton
with the free-flight model of the YF-17 in 1973.
Spin and recovery tests in the Spin Tunnel also
provided positive results for the YF-17. The scope
of the tests in the Spin Tunnel was relatively
broad for the fast-paced LWF Program and included
the determination of the size of the emergency
spin recovery parachute that would be required
for flight tests. The results of the Spin Tunnel
tests showed that the YF-17 would have remarkably
good spin and recovery characteristics. In fact,
the YF-17’s characteristics were the best
noted for any fighter configuration to that time.
Although not specifically requested by the Air
Force, Langley had already conducted Air Force
approved studies of the high-angle-of-attack characteristics
of the YF-16 in the Langley Differential Maneuvering
Simulator (DMS) and approval was given to conduct
similar studies of the YF-17. Under the leadership
of Langley researchers Luat T. Nguyen and William
P. Gilbert, extensive studies were conducted in
the DMS to verify the impressive behavior predicted
by the wind-tunnel and free-flight model tests
for more realistic air combat conditions. In addition,
the simulation was used to refine certain elements
of the flight control system for high-angle-of-attack
conditions. The results of the simulator investigation
showed the YF-17 to be highly maneuverable and
departure resistant throughout the operational
angle-of-attack range and beyond maximum lift.
A YF-17 prototype in flight with
the open slots in the LEX adjacent to the fuselage.
YF-17 model in the Langley 8-Foot
Transonic Pressure Tunnel for drag assessment studies.
The Langley predictions for the YF-17 behavior
were subsequently confirmed in 1974 when two YF-17
prototypes began flight evaluations at Edwards
Air Force Base. Handling characteristics at high
angles of attack were excellent. The YF-17 could
achieve angles of attack of up to 34 deg in level
flight and 63 deg could be reached in a zoom climb.
The aircraft remained controllable at indicated
airspeeds down to 20 knots. Northrop consequently
claimed that their lightweight fighter contender
had no angle-of-attack limitations, no control
limitations, and no departure tendencies within
the flight envelope used for the evaluation.
Langley researchers conducted a cruise-drag test
of the YF-17 in the Langley 8-Foot Transonic Pressure
Tunnel in 1973. The test was initiated when Northrop
questioned the high transonic drag levels predicted
by the Air Force, which were based on results from
other wind tunnels. The Air Force agreed that an
independent NASA analysis would be appropriate
and requested the test.
Langley researchers found that their results
agreed with the Air Force predictions. Researchers
then identified the major contributors to drag
and provided recommendations to reduce it. Dr.
Richard T. Whitcomb reviewed the YF-17 drag results
and concluded that the wing design was the major
factor in the unexpected drag levels. Whitcomb’s
suggestion for a wing redesign to solve the problem
was unacceptable to Northrop.
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F/A-18A to F/A-18D
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Development of
the F/A-18 |
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In April 1974, the
LWF Program changed from a technology demonstration
program to a competition for an Air Force Air Combat
Fighter (ACF), and the flight-test programs for
the YF-16 and YF-17 were rushed through in a few
months instead of the planned 2 years. On January
13, 1975, the Air Force announced that the General
Dynamics YF-16 would be the new ACF. Congress decreed
that the Navy adopt a derivative of one of the
LWF designs as the new Naval Air Combat Fighter
to complement the F-14. On May 2, 1975, the Navy
announced that the YF-17 design would form the
basis of their new F/A-18 fighter-attack aircraft.
Northrop, inexperienced in the design of naval
fighters, teamed with McDonnell Douglas, which
had extensive experience with a highly successful
line of naval aircraft, including the F-4 Phantom.
McDonnell Douglas became the prime contractor for
the F/A-18 with Northrop the prime subcontractor.
The formidable task of converting the land-based
YF-17 lightweight day fighter into an all-weather
fighter-attack aircraft capable of carrier operations
with heavy ordnance loads required significant
changes from the earlier configuration. Structural
strengthening and a new landing gear design were
required for catapult launches and arrested landings.
The aircraft gross weight rapidly grew from 23,000
lb for the YF-17 to a projected weight over 33,000
lb.
The required approach speeds for carrier landings
resulted in modifications to the wing and LEX surfaces
of the YF-17 configuration to provide more lift.
McDonnell Douglas consulted the Langley research
staff, and several individuals participated in
the analysis of wind-tunnel tests that had been
conducted at NASA Ames Research Center and McDonnell
Douglas facilities. As a result of the analysis,
changes were made to the aircraft configuration.
The geometric shape of the YF-17 LEX was extended
farther forward on the fuselage and the plan view
of the LEX was modified to produce additional lift
while retaining the good high-angle-of-attack characteristics
exhibited by the YF-17.
The deflections of the wing leading- and trailing-edge
flaps were increased and the ailerons were programmed
to droop in low-speed flight to augment lift. Finally,
a “snag” or discontinuity was added
to the leading edges of both the wing and horizontal
tails to provide more lift.
Formal Navy requests for specific NASA studies
in support of the evolving F/A-18 configuration
were received and accepted by Langley. The excellent
correlation of Langley predictions for high-angle-of-attack
and spin characteristics of the YF-17 prompted
the Navy to request the full suite of tests at
Langley for these characteristics. A request for
tests in the Spin Tunnel and the Full-Scale Tunnel
and helicopter drop models was received in late
1976.
The first preproduction F/A-18 made its first
flight on November 18, 1978, and entered the initial
phases of flight tests at the Patuxent River Naval
Air Station in Maryland. The preproduction flight-test
program lasted from January 1979 to October 1982,
and the Langley staff was called on to help solve
several critical developmental problems.
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Cruise Drag |
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Initial results
from flight evaluations at Patuxent River in 1979
indicated that the cruise performance of the F/A-18
was significantly below expectations, with a shortfall
of about 12 percent in cruise range. The performance
deficiency became a weapon for those who sought
the termination of the F/A-18 Program. A number
of reasons for the poor performance were identified.
Modifications to the engines, computer-controlled
schedules for the deflection of leading- and trailing-edge
flaps, and other changes reduced the cruise range
deficit to about 8 percent, but aerodynamic drag
remained a problem.
In response to this critical threat to the program,
a Navy and NASA F/A-18 working group was formed
in late 1979. The NASA members were all Langley
personnel led by researchers Richard Whitcomb,
Edward Polhamus, and William J. Alford, Jr. With
the addition of members from the Navy, McDonnell
Douglas, and Northrop, the group totaled about
20 participants. After consideration of several
approaches to reduce the drag of the aircraft,
the group recommended wind-tunnel and flight studies
of modifications to several configuration features.
Modifications included increasing the wing leading-edge
radius, variations in the LEX camber, and filling
in the slots in the LEX-fuselage juncture. The
Langley members identified the slots as a particularly
undesirable feature with potentially high drag
characteristics. Tests with favorable results were
conducted in the Langley 8-Foot Transonic Pressure
Tunnel in early 1980. These changes were implemented
on the F/A-18 test aircraft at Patuxent River where
they were found to favorably increase the cruise
range of the aircraft. The impact of filling in
the LEX slot on high-angle-of-attack characteristics
was found to be acceptable in additional tests
at Langley and F/A-18 flight tests.
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High-Angle-Of-Attack,
Spin, and Spin Recovery Characteristics |
|
Langley responded
to the Navy’s request for stall-spin tests
in support of the F/A-18 with spin tunnel tests
(1978), free-flight model tests (1978), and drop-model
tests (1979). The results of the Langley spin tunnel
and drop-model tests were very favorable. The F/A-18
configuration was found to be extremely resistant
to spins. (The pilot was required to maintain prospin
controls for over 20 sec to promote a spin.) When
spins were entered, recovery could be effected
very quickly. In the spin tunnel tests, the F/A-18
model demonstrated the best spin recovery characteristics
of any modern U. S. fighter (as had the YF-17 configuration).
During the limited model tests for spins, the phenomenon
known as “falling leaf” was not encountered,
but it became a problem in operational usage as
will be discussed in a later section.
The F/A-18 free-flight model tests that were
conducted in the Full-Scale Tunnel were very controversial;
however, the model tests subsequently proved to
be a major contributor to the success of the F/A-18
development program.
As previously discussed, early high-angle-of-attack
tests had been completed in other NASA and McDonnell
Douglas wind tunnels to tailor the geometry of
the F/A-18 for good flight characteristics. The
removal of the stabilizing nose strakes that were
on the YF-17, which would have interfered with
the radar performance on the F/A-18, and the revision
of LEX shape had been carefully analyzed and designed
from these results. When the Langley free-flight
model underwent tests, the LEX and leading-edge
flap schedule had been defined for flight tests.
When the Langley model was tested, however, the
results obtained with this particular model indicated
that the F/A-18 would exhibit a moderate yaw departure
near maximum lift. Although not a flight safety
concern, this result would infringe on the precision
maneuverability of the aircraft. These undesirable
results had been obtained at low wind-tunnel test
speeds with a relatively large model and were dismissed
with skepticism by the engineering community as
having been caused by erroneous scale effects.
In 1979, an F/A-18 test aircraft at Patuxent
River suddenly and unexpectedly departed controlled
flight during a wind-up turn maneuver at high subsonic
speeds. None of the baseline wind-tunnel data predicted
this characteristic, and the F/A-18 Program was
shocked by the event. The fact that the free-flight
model had also exhibited such a trend did not go
unnoticed, and a joint NASA, Navy, and McDonnell
Douglas team was formed to seek solutions with
the free-flight model at Langley. Following exhaustive
wind-tunnel tests in the Full-Scale Tunnel, the
team recommended that the wing leading-edge flap
deflection be increased from 25 deg to 34 deg at
high angles of attack. Following the implementation
of this recommendation on the test aircraft (via
the flight control computers), no more departures
were experienced, and the flap deflection schedule
was adopted for production F/A-18’s.
F/A-18 drop model prepared for
flight in 1978.
The F/A-18A free-flight model
during tests in the Langley 30- by 60-Foot Wind
Tunnel in 1978.
This was not the first time that results from
large free-flight models in the Full-Scale Tunnel
had proven to accurately predict the flight behavior
of an aircraft at high angles of attack, despite
the low speeds of the wind-tunnel tests. (See Langley
Contributions to the F-16, for example.)
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The NASA High-Angle-Of-Attack
Technology Program |
|
The emergence of
a generation of highly maneuverable fighter aircraft
such as the F-14, F-15, F-16, and F-18 in the late
1970’s resulted in a new perspective on operating
high-performance aircraft at high angles of attack.
Previous U.S. military experience with aircraft
such as the F-4 and A-7 during the Vietnam era
had been tormented with unacceptably high accident
losses of these aircraft from inadvertent departures
and spins from maneuvers at high angles of attack.
Operational procedures required very careful and
precise pilot inputs at high angles of attack to
avoid loss of control, and handbook restrictions
were placed on the operational use of angle of
attack. With the advent of the new generation of
fighters, flight at high angles of attack became
a common occurrence and was no longer feared or
avoided by pilots. Exploitation of high angles
of attack provided the potential for new maneuvers
and options for air combat tactics. These interests
resulted in several major programs within DOD,
industry, and NASA to develop the analysis and
design methodologies for superior performance over
an unlimited range of angle of attack.
In the early 1980’s, NASA initiated a High-Angle-of-Attack
Technology Program (HATP) among the aeronautics
research centers (Langley, Ames, Dryden, and Glenn).
The program included all the critical elements
of high-angle-of-attack technology: aerodynamics,
flight controls, handling qualities, stability
and control, and the rapidly evolving area of thrust
vectoring. In recognition of its extensive accomplishments
in this field, Langley was designated the technology
lead center for the program, and Dryden was designated
the lead for flight research and operations. The
Langley leader in the initial phases of the program
was William Gilbert, who was followed by Luat Nguyen
in later years.
A critical element in the HATP plan was the correlation
and validation of experimental and analytical results
with results from flight tests of a high-performance
aircraft. The NASA team considered several aircraft
configurations for this important task, including
the F-15, F-16, F/A-18, and the X-29 forward-swept
wing technology demonstrator aircraft. The advantages
and disadvantages of each aircraft for the NASA
program were thoroughly considered, and the F/A-18
was unanimously chosen as the desired test vehicle
for several reasons. In flight, the F/A-18 had
shown aerodynamic and aeroelastic phenomena of
interest (vortex flows and tail buffet), stall-free
engine operation at high angles of attack, excellent
spin recovery characteristics, an advanced digital
flight control system, and a large angle-of-attack
capability (up to 60-deg trim capability at low
speeds). The Navy’s response to a request
from NASA for a preproduction F/A-18 vehicle for
the program was very positive. The Navy initially
offered NASA an early production F/A-18A aircraft;
however, NASA targeted the specially equipped preproduction
F/A-18 (ship no. 6) that had been used at Patuxent
River for the spin evaluation phase of the development
program. The aircraft had completed its spin tests
and had been stored in a hangar and stripped of
its major components as a spare-parts aircraft.
However, this particular aircraft was equipped
with a special emergency spin recovery parachute
system worth several million dollars and a programmable
digital flight control computer adaptable to the
variations desired in the HATP. The Navy approved
the transfer of the stripped aircraft to NASA,
and it was trucked to Dryden and reassembled by
experts at Dryden with the Navy’s help into
a world-class research aircraft to be known as
the F/A-18 High Alpha Research Vehicle (HARV).
F-18 HARV during flow visualization
studies using smoke ejected at apex of LEX and
wool tufts on upper surfaces.
The HATP studies were conducted in three phases.
In the first phase, emphasis was placed on static
and dynamic aerodynamic phenomena. This phase included
extensive correlations of wind-tunnel and computational
results with data from flight instrumentation on
the HARV. These data correlations were conducted
with common data sensor locations on the wind-tunnel
models and the HARV. As a result of these aerodynamic
experiments, the capabilities of emerging computational
fluid dynamics tools and the interpretation of
wind-tunnel test techniques were aggressively accelerated.
Detailed studies of vortical flow structures emanating
from the F/A-18 LEX and interactions with the vertical
tail surfaces resulted in rapid progress in the
understanding, prediction, and minimization of
vertical-tail buffet phenomena. Finally, in-depth
analysis of the flow fields shed by the nose and
forebody of the F/A-18 resulted in wind-tunnel
test procedures and validation of computational
codes for future fighters.
An excellent example of the impact of this fundamental
research is the LEX upper-surface fences mounted
on F/A-18C/D aircraft. These devices greatly alleviate
the vertical tail buffeting associated with sudden
bursting of the core of the strong vortical flow
shed by the LEX at high angles of attack. McDonnell
Douglas developed the fence during parametric wind-tunnel
experiments in its own wind tunnel; however, they
did not have the resources or time to identify
the flow mechanisms created by the fence. Within
the scope of the HATP, Langley developed a laser
vapor screen flow visualization technique that
permitted rapid, global assessments of interactive
flow fields. Using this technique, Langley researcher
Gary E. Erickson conducted diagnostic tests on
an F/A-18C model in the Navy David Taylor Research
Center (DTRC) 7- by 10-Foot Transonic Tunnel, and
he precisely identified the vortex-flow mechanisms
associated with the benefits of the fences. The
success of the vapor screen system so impressed
the Navy and industry that they requested the Langley
vapor screen system on other tests to understand
nonlinear flow behavior at subsonic and transonic
speeds.
In the second phase of the HATP, the HARV incorporated
a relatively simple and cheap thrust-vectoring
system for studies of aerodynamics at extreme angles
of attack, and engineering development and evaluations
of the advantages of thrust vectoring for high-angle-of-attack
maneuvers. Previous efforts with the Navy involving
experiments with thrust-vectoring vanes on an F-14
inspired the NASA team to adopt this simple concept
instead of a program to develop internal engine
vectoring at a cost of many more millions of dollars.
(See Langley Contributions to the F-14.)
Dryden managed and directed a contract with McDonnell
Douglas to outfit the HARV with deflectable external
vanes mounted behind the engines. The engine exhaust
nozzle divergent flaps were removed and replaced
with a set of three vanes for each engine, which
resulted in both pitch and yaw vectoring capability.
The specific vane configuration was analyzed and
defined by tests of a powered model in the Langley
16-Foot Transonic Tunnel and the Langley Jet Exit
Test Facility. Meanwhile, the aircraft flight computer
was modified to permit research evaluations within
a broad spectrum of thrust-vectoring parameters.
The modification of the HARV proved to be extremely
challenging, and several members of the staffs
from Langley and Dryden assisted in the final implementation
of the hardware and software of the thrust-vectoring
system.
Extensive piloted simulator evaluations of the
HARV with thrust vectoring were conducted in the
DMS, with results indicating that the implementation
of both pitch and yaw vectoring provided powerful,
unprecedented controllability and precision for
maneuvers at high angles of attack.
The results of the F/A-18 HARV thrust-vectoring
flights were remarkable. The HARV became the first
high-performance aircraft to conduct multiaxis
thrust-vectoring flights, and the powerful effectiveness
of thrust vectoring at high angles of attack was
vividly demonstrated. The control precision of
the aircraft at extreme angles of attack was significantly
enhanced, and the vehicle could be stabilized at
the conditions for high quality aerodynamic studies.
Thrust-vectoring vane installation
on the F-18 HARV free-flight model with
rectangular box above the vanes representing the
container for the spin parachute.
In the final phase of the HATP, earlier studies
to understand and predict the strong vortex flows
emanating from the long pointed fuselage forebody
of the F/A-18 were extended. The studies now included
the ability to control these flows for aircraft
maneuvers at extreme angles of attack, where yaw
control provided by conventional aerodynamic rudders
becomes relatively ineffective. Based on in-depth
studies conducted by Langley in several wind tunnels,
the HARV was equipped with deflectable foldout
strake surfaces on the forward forebody, and the
control system was modified to permit the pilot
to use the strakes for roll control. The strake
hardware was engineered and fabricated in Langley
machine shops, and the flight computer interface
was provided by Dryden. Results of the flight evaluation
of the nose strakes were extremely impressive.
The time required to roll the aircraft through
a specific roll attitude change was significantly
reduced due to the crisp, powerful control provided
by the strakes. At the higher subsonic speeds,
the effectiveness of the strakes approached levels
provided by thrust vectoring in yaw.
The HATP efforts represent a decade of invaluable
research and development on key technologies that
will be required for future highly maneuverable
high-performance fighter aircraft, as well as the
F/A-18 configuration. The data gathered by NASA
has been widely disseminated among design teams
at NASA sponsored national conferences and during
site visits. The program has been cited for its
high quality, pioneering value on numerous occasions
by industry and DOD. The results of the program
are being used in the development of the next generation
F/A-18E/F and other military aircraft.
|
The Falling-Leaf
Maneuver |
|
The falling-leaf
maneuver originated during World War I as a flight
training exercise. In this exercise, pilots intentionally
stalled the aircraft and forced a series of incipient
spins to the right and left. The aircraft descends
as it rocks back and forth, much as a leaf does
falling to the ground. In the early 1980’s,
an unintentional falling-leaf mode surfaced as
a severe out-of-control problem during developmental
flight tests of the F/A-18A. The out-of-control
falling-leaf mode is a highly dynamic mode where
the aircraft oscillates so that it is very difficult
to reduce angle of attack and recover. The term
“alpha hang-up” was used to describe
this problem with the F/A-18 and it was a key driver
in establishing the aft center of gravity and the
maneuvering limits for the aircraft. During early
operational use of the F/A-18, the falling-leaf
mode was rarely encountered; however, by the early
1990’s increasingly aggressive maneuvering
had exposed a susceptibility to the falling-leaf
mode with numerous incidents and losses of aircraft.
Langley began studying the falling-leaf mode
in 1994 at the request of the Naval Air Systems
Command, following an F/A-18 falling-leaf incident
during a routine training flight that nearly resulted
in loss of the aircraft. Under the technical leadership
of Langley researcher John V. Foster, the cause
for the falling-leaf mode was identified and linked
to a common aerodynamic stability design method
that had been used for many years. Using the DMS,
the critical entry maneuver that excites the mode
was identified and correlated with fleet incidents,
and various recovery methods were studied. In addition,
the necessary requirements for high-fidelity prediction
of the falling-leaf motion in piloted real-time
simulations were defined.
In light of the growing falling-leaf problem
on early models of the F/A-18, there was concern
that the emerging F/A-18E/F, which was then preparing
for developmental flight tests, would have the
same problem. In response, McDonnell Douglas proposed
a control law design approach, supported by Foster’s
research, that specifically targeted falling-leaf
suppression, which was later implemented on the
aircraft.
Because no methods existed specifically for falling-leaf
flight tests, NASA again used the DMS facility
to develop reliable flight-test techniques that
were subsequently validated on the NASA F/A-18
HARV at Dryden. Using these test techniques, falling-leaf
susceptibility was extensively evaluated on the
F/A-18E/F during the high-angle-of-attack flight-test
program. While the unaugmented aircraft was shown
to exhibit the falling-leaf mode, the new control
system design was shown to be very effective in
suppressing the mode and the falling-leaf problem
was considered solved for that aircraft. Largely
due to the success of the F/A-18E/F program, the
Navy is considering retrofitting earlier models
of the F/A-18 with the updated control law for
the purpose of eliminating the falling-leaf problem.
While the F/A-18 was the most visible recent
manifestation of the falling-leaf problem, numerous
other aircraft configurations have exhibited the
mode. For example, the AV-8B exhibited a severe
falling-leaf mode that the Foster criteria showed
to be a result of a minor configuration change.
As new highly maneuverable configurations are developed
and the angle-of-attack envelope expands, Langley’s
F/A-18 research has brought a necessary and timely
focus to a critical flight problem.
|
Flutter Clearance
and Tail Buffet |
|
In a departure from
most fighter programs, flutter clearance tests
of both the YF-17 and the F/A-18A were not conducted
in the Langley 16-Foot Transonic Dynamics Tunnel.
Research studies were, however, conducted in the
areas of active flutter suppression and buffet
alleviation. A cooperative NASA, Northrop, and
Air Force research study was later conducted to
assess the application of active controls for flutter
suppression for the YF-17 wing-store-pylon combination
in the tunnel in late 1977. The results indicated
that active control of wing-store-pylon flutter
was possible and that wing leading-edge surfaces
could be effective in such a control system. In
1979 interest in the concept resulted in an international
study of several active flutter suppression systems
designed by British Aerospace and the Royal Aeronautical
Establishment from the United Kingdom, the Office
of National d’Etudes et de Recherches Aerospatiales
from France, and Messerschmitt-Bolkow-Blohm from
the Federal Republic of (West) Germany. The TDT
tests showed that all the control systems were
highly effective in supressing flutter. The YF-17
testing continued through 1982.
The vertical-tail buffet experienced by the F/A-18A
and other high-performance aircraft resulted in
cooperative research by industry, DOD, and NASA
on buffet alleviation from smart materials and
actively controlled rudders. Because international
allies of the U.S. use the F/A-18, this research
effort included Australia and Canada. The Actively
Controlled Response of Buffet Affected Tails (ACROBAT)
Program led by Robert W. Moses investigated the
use of an actively controlled rudder, actively
controlled piezoelectric devices, and other novel
concepts to alleviate vertical-tail buffeting.
For the ACROBAT Program, a 1/6-scale rigid full-span
model of the F/A-18 A/B aircraft with flexible
tails was tested in the 16-Foot Transonic Dynamics
Tunnel. Numerous control laws were tested, and
many demonstrated buffeting alleviation over a
large angle-of-attack range. These tests have been
followed up with international ground-based tests
of a buffeting alleviation system on a full-scale
F/A-18.
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Thrust-Vectoring
Research |
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In the early 1970’s,
the Navy conducted a vertical and short takeoff
and landing (V/STOL) nozzle study for potential
applications to high-performance aircraft. Following
this study, the augmented deflector exhaust nozzle
(ADEN) was selected as the best nozzle concept
for Navy V/STOL requirements. The ADEN nozzle had
a hood, which deployed from a long upper ramp,
that was capable of thrust vector angles from 0
deg to over 90 deg. Nozzles of this type (long
flap on one side of the nozzle and no deploying
hood without VTOL capability) were later called
single expansion ramp nozzles (SERN).
F/A-18 model with 2-D convergent-divergent
(CD) nozzles tested in Langley 16-Foot Transonic
Tunnel.
In 1975, a joint DOD and NASA two-dimensional
(2-D) nozzle workshop held at Langley formed an
ad hoc interagency nonaxisymmetric nozzle working
group that eventually recommended a flight test
of 2-D, thrust-vectoring, in-flight thrust-reversing
nozzles. To avoid duplication, studies on potential
flight-test vehicles were split up among NASA (F-15),
the Air Force (F-111), and the Navy (YF-17 and
F/A-18).
The first full-scale tests, which were sponsored
by the Navy, of a 2-D nozzle (ADEN) were conducted
in a Glenn Research Center altitude test cell in
1976. In support of the Navy studies of advanced
nozzles, Langley conducted powered-model tests
of an early F/A-18A configuration with 2-D nozzles
in the Langley 16-Foot Transonic Tunnel.
Since the F/A-18A represented the state of the
art in fighter design, Langley expanded the test
objectives to include propulsion-airframe integration
studies of a number of nozzle types (wedge, 2-D
convergent-divergent (CD), SERN), thrust-vectoring
concepts, and in-flight thrust-reversing concepts.
The studies also examined F/A-18A vertical-tail
loads, particularly during in-flight thrust-reversing
operation. Unfortunately, Navy and NASA attempts
to secure funding for the F/A-18A flight tests
were unsuccessful, and the F/A-18A studies of thrust
vectoring were terminated.
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F/A-18E/F
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The Super Hornet |
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The F/A-18E/F Super
Hornet was funded in June 1992 as the replacement
for the cancelled Navy A-12 aircraft and as the
replacement for the early F/A-18A aircraft and
other Navy and Marine Corps aircraft. The F/A-18E/F
is a larger version of the F/A-18C/D Hornet with
extended mission capabilities. The E/F is roughly
25 percent larger than the C/D, with a 25 percent
increase in operating radius and a 22 percent increase
in weapons load capability. First flight of the
F/A-18E/F occurred on November 29, 1995, and preproduction
flight tests began in February 1996, at Patuxent
River.
F/A-18E flight test aircraft
prepares to land at Patuxent River Naval Air Station.
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Cruise Performance |
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NASA’s involvement
with the F/A-18E/F began in the early stages of
the proposed aircraft development when the Office
of the Secretary of Defense (OSD) became concerned
with the accuracy of McDonnell Douglas’ range
estimates for the vehicle. At the request of OSD,
a three-member NASA, DOD, and industry team conducted
an independent review of McDonnell Douglas’
F/A-18E/F fighter-escort mission range estimates
in April 1992. Langley researcher Gary Erickson
represented NASA and called on supporting analyses
from several Langley specialists. Results of the
independent review substantiated the McDonnell
Douglas estimates and were briefed to the Assistant
Secretary of Defense. The favorable results from
this review were critical to the aircraft program
proceeding to the Defense Acquisition Board for
funding advocacy.
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Redesign of the
Leading-Edge Extension |
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Many operational
requirements were revisited as the F/A-18C/D grew
to the larger, heavier F/A-18E/F. Critical performance
issues included lift requirements for carrier approach
and wave offs, high-angle-of-attack stability,
recovery from extreme attitudes, and minimization
of adverse aerodynamic phenomena such as tail buffet.
McDonnell Douglas explored a number of configuration
changes to satisfy these requirements, including
adding a snag to the wing leading edge, reshaping
the LEX, and using moveable spoilers on the upper
surfaces of the LEX to ensure recovery from extreme
angles of attack.
Low-speed wind-tunnel tests by McDonnell Douglas
had indicated that a deflected spoiler on the upper
surface of each LEX would alleviate buffeting of
the vertical tails, promote good lateral-directional
stability, and provide trim changes for nose-down
control at extreme angles of attack. In May 1992,
a series of tests were conducted in the Langley
8-Foot Transonic Pressure Tunnel to determine the
effects of the spoiler concept at higher speeds
(Mach numbers from 0.6 to 1.2). Unfortunately,
results of the Langley tests indicated that the
deployed spoilers would cause an unacceptable decrease
in maximum lift of over 15 percent. With these
negative results, a redesign of the LEX surfaces
began. The LEX spoilers have been retained on the
F/A-18E/F as speed brakes and nose-down control
devices.
F/A-18E/F model in 8-Foot Transonic
Pressure Tunnel for evaluations of the LEX spoiler
concept.
F/A-18E/F (foreground) with redesigned
LEX in flight with F/A-18C chase aircraft.
Redesigning the LEX was the job of a 15-member
industry, DOD, and NASA team, which included Langley
researchers Daniel G. Murri, Robert M. Hall, and
Gary Erickson. This team, which was active for
the first 6 months of 1993, initially explored
small modifications to the size and shape of the
original F/A-18C LEX to help provide the required
lift and improve lateral-directional stability.
However, subsequent wind-tunnel tests showed that
this incremental approach would not be successful
and much larger changes to the LEX configuration
would be required. Based on his prior research
with other configurations, Murri proposed more
radical LEX candidates that would potentially satisfy
these requirements. One of the LEX configurations
with favorable wind-tunnel results that was recommended
by Langley was accepted for further refinements
and met all design goals. This configuration was
the basis for the final design adopted as the wing
LEX configuration for the production F/A-18E/F.
Extensive tests of the F/A-18E/F with the redesigned
LEX were conducted in several NASA and industry
wind tunnels to completely define the aerodynamic
characteristics of the configuration. Data from
these tests were used to generate the aerodynamic
databases for flight simulation and to develop
the flight control software for the aircraft. NASA
engineers worked closely with engineers from McDonnell
Douglas and the Navy to assure that design requirements
and national goals were met.
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High-Angle-Of-Attack
Characteristics |
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Stability and control
characteristics of the F/A-18E/F at high-angle-of-attack
flight conditions were evaluated in numerous wind-tunnel
tests at Langley. In the Full-Scale Tunnel, a combination
of static, dynamic, and powered tests was conducted
to define and develop a database for the high-angle-of-attack
aerodynamic, stability, and control characteristics
of the aircraft. In addition to tests of the basic
configuration, tests were conducted to study the
impact of fuselage-mounted and wing-mounted stores
on aerodynamic and stability characteristics, to
assess aerodynamic damping characteristics, and
to assess the magnitude of thrust-induced aerodynamic
effects on the configuration. Free-flight tests
were also conducted to provide confirmation of
the stability and flight dynamic characteristics.
This database has been used by McDonnell Douglas
and the follow-on Boeing organization, NASA, and
the Navy to conduct flight-simulation studies and
to aid in the development of the aircraft’s
flight control system. An extensive unpiloted and
piloted simulation of the F/A-18E/F has been implemented
in the Langley DMS and is being used to analyze
flight control modifications and the impact of
aircraft parameters on high-angle-of-attack behavior.
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Spin Tunnel and
Drop-Model Tests |
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Hundreds of tests
in the Langley Spin Tunnel quantified the spin
modes and spin recovery characteristics of the
F/A-18E/F, determined the acceptable emergency
spin recovery parachute size, and identified the
optimal spin recovery procedures prior to flight
tests. Time histories of model motions from these
tests were used by McDonnell Douglas to validate
their spin simulation. Data from rotary-balance
tests conducted in this facility provided inputs
for an analytical assessment of spin modes, spin
recovery characteristics, and a database for incorporating
rotational aerodynamic characteristics into the
flight simulation.
F/A-18E/F drop model immediately
after release from the helicopter.
A highly sophisticated F/A-18E/F drop model was
tested by Langley at the NASA Wallops Flight Facility
to provide risk reduction for the high-angle-of-attack
part of the flight-test program. These tests, which
used a 0.22-scale remotely piloted model, supplemented
the aircraft flight-test program by providing flight
dynamics data for the aircraft at conditions outside
the planned operating envelope.
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Flutter Tests |
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F/A-18E/F flutter
clearance tests were conducted at Langley by Moses
G. Farmer in the 16-Foot Transonic Dynamics Tunnel
during four tunnel entries from 1993 to 1995. Phase
I tests insured that each pair of dynamically scaled
surfaces (wings, horizontal tails, and vertical
tails) was clear of flutter throughout the scaled
flight envelope. Phase II of the tests studied
the configuration both with and without stores
(bombs and fuel tanks) mounted on the wings. These
tests used the unique two-cable-mount system of
the tunnel, which allows the model to actually
fly in the center of the tunnel with assistance
from a pilot in the control room. The tests verified
that the aircraft was free from aeroelastic instabilities,
including flutter, within its flight envelope.
As a result of the Langley tests, the number of
expensive flight tests dedicated to flutter clearance
could be minimized.
In view of Langley’s corporate knowledge
of the F/A-18E/F and its predecessors, the Navy
requested NASA involvement when the F/A-18E/F began
flight tests. NASA continued to work closely with
the Navy and McDonnell Douglas during the engineering
and manufacturing development (EMD) phase. Langley
support included flight-test planning and data
evaluation, especially in the high-angle-of-attack
regime. In 1997, Langley assigned an engineer to
Patuxent River as a participant in the flight tests
and as a liaison and technical consultant for the
test team.
F/A-18E/F model mounted for flutter
tests in the Langley 16-Foot Transonic Dynamics
Tunnel.
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Wing Drop |
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The most recent
Langley contribution to the F/A-18E/F Program explored
the phenomenon known as wing drop. In March 1996,
during flight tests at Patuxent River, an F/A-18E/F
experienced wing drop—an unacceptable, uncommanded
abrupt lateral roll-off that randomly occurred
and involved rapid bank angle changes of up to
60 deg. The problem was viewed as extremely serious
and posed a threat to operational tests and the
overall development schedule. During the first
year of flight tests, the wing-drop phenomenon
was only seen at high altitudes because load restrictions
prevented the aircraft from reaching the relevant
range of angle of attack at low altitudes. As the
loads test program opened the flight envelope to
7.5g at all altitudes, the full extent of
the wing-drop problem became evident. Objectionable
wing-drop events occurred throughout the flight
envelope at Mach numbers between 0.5 and 0.9, and
this deficiency became a significant threat to
the technical and political health of the F/A-18E/F
Program.
A joint Navy and McDonnell Douglas team concluded
that the wing drop was caused by a sudden, abrupt
loss of lift on one of the outer wing panels during
maneuvering. Though the basic cause of the wing
drop was determined, how to moderate the airflow
separation differences between the left and right
wings was not. A variety of solutions was explored.
For example, 25 potential wing modifications were
tested in wind tunnels, computational fluid dynamics
studies were undertaken, and two of the potential
fixes were flown with mixed results. In one approach,
the use of stall strips in the vicinity of the
wing-fold fairing eliminated wing drop, and the
aircraft exhibited excellent flight qualities throughout
the envelope. However, the impact of the stall
strips on range and turn performance were severe,
and they were discarded as a viable option.
Langley engineer Robert Hall also served on a
DOD blue ribbon panel to review the approach taken
by McDonnell Douglas to resolve the wing drop.
The panel also participated on various McDonnell
Douglas and Navy “tiger teams” that
were created to resolve issues related to the wing-drop
problem. Roy V. Harris, a distinguished Langley
retiree, was selected by DOD to head the blue ribbon
panel, which subsequently cited the lack of technical
understanding and design tools available in the
technical community to address the high subsonic
and transonic wing-drop problem. The panel strongly
recommended that a research program be undertaken
to develop the design methods required to avoid
this problem in future fighter aircraft. The recommendation
was accepted, and a joint NASA and Navy Abrupt
Wing Stall (AWS) Program was initiated. Langley
researchers Jeffrey A. Yetter and Robert Hall served
as AWS Program Comanager and Technical Comanager,
respectively, with their Navy partners.
As a low impact “80-percent solution”
to the F/A-18E/F wing-drop problem, a revised deflection
schedule for the leading-edge flaps was evaluated
in flight tests in early 1997, with very favorable
results. Although the leading-edge flap schedule
modification significantly reduced the magnitude
of the problem, the aircraft still exhibited smaller
wing drops at many test conditions. The original
wing-drop problem had been viewed as a potential
safety hazard and a roadblock to productive load
tests. After the modification, the problem was
reduced to a flying qualities issue that allowed
other tests to continue. Wind-tunnel tests were
conducted at sites other than NASA to evaluate
wing changes that might eliminate the residual
wing-drop tendencies. Langley researchers Gary
Erickson and Robert Hall participated in the tests,
which continued through the summer of 1997.
During the winter of 1997 to 1998, the Navy asked
NASA for additional assistance in resolving wing
drop. In November 1997, a diagnostic flight test
of an F/A-18E/F with the wing-fold fairing removed
was conducted and the results showed that wing
drop had been eliminated from most of the flight
envelope. The fairing-off configuration was not
a viable approach for production aircraft, and
Langley engineers led by Steven X. S. Bauer suggested
that the flight program apply porosity, a passive
technology developed at NASA, to the wing in the
fold area. Langley researchers had been conducting
experiments with passive porosity to control shock
locations and other characteristics for several
years. During the initial concept evaluation on
the F/A-18E/F, the porous fairing was simply a
standard wing-fold fairing with areas cut out and
a screen mesh substituted. Langley provided design
guidelines for the porosity and thickness of the
mesh. This solution, refined by the NASA, Navy,
and Boeing team, resolved the wing-drop problem
and permitted continued production of the aircraft.
Additional activities are being conducted to
support the joint NASA and Navy AWS Program, including
tests of an F/A-18E/F model in the Langley 16-Foot
Transonic Tunnel in September 1999 under the leadership
of Robert Hall and S. Naomi McMillin.
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