Early NASA Fighter
Studies |
|
During the development
of the variable-sweep wing at the Langley Research
Center, researchers recognized the advantages of
applying the concept to multimission aircraft.
One ideal application was for naval fleet defense
fighters, which must be able to quickly intercept
threats and yet slowly approach aircraft carriers
to land. Variable-sweep wings in the fully swept
(high-speed) configuration permit efficient supersonic
dash and the carrier-approach requirements could
be met with the wing in the unswept (low-speed)
position. Inspired by the potential of this application
of variable-sweep technology, Langley conducted
several in-house studies of fighter configurations
for naval applications. In 1967, Langley published
the results of in-house studies of a variable-sweep
fighter configuration known as LFAX-4 that incorporated
several features that are evident in the F-14 aircraft.
Some of these features are
- Short propulsion package
to minimize weight
- Engines placed forward for
balance
- Horizontal ramp engine inlets
for good performance at high angles of attack
- Tailored twin-engine aft-end
spacing and interfairing for efficient subsonic
cruise conditions
Results of studies of the performance, stability,
and control characteristics of the LFAX-4 obtained
across the speed range of a representative advanced
naval fighter were outstanding, and Langley researchers
conducted extensive briefings of their findings
at industry and Department of Defense (DOD) sites.
One result of the LFAX-4 study that the Langley
team emphasized was the critical location of the
pivot for the movable wing panels. To minimize
drag during transonic maneuvers typical of air-to-air
combat, the pivots must be located in a relatively
outboard position. Langley’s experience with
the test and analysis of the F-111 revealed that
large penalties in trim drag occurred if this key
design factor was not adequately appreciated. Although
the F-111 incorporated the variable-sweep concept,
the full advantages of the concept were not realized
because the pivot locations were relatively inboard.
As a result, the F-111 suffered excessive trim
drag at transonic and supersonic conditions. The
designers of the F-14 were made aware of the significance
of pivot locations by NASA briefings. Comparison
of the NASA results for the LFAX-4 to those of
the F-111 helped convince Grumman to locate the
F-14 pivots in a more favorable outboard position.
The Langley LFAX-4 fighter configuration.
The LFAX-4 studies also pointed out the importance
of the placement of the horizontal tail relative
to the wing for stability and control. These recommendations
were also included in the F-14 design. |
NASA Independent
Assessment |
|
On January 14, 1969,
the Navy announced the award of the contract for
the VFX fighter, now designated F-14, to Grumman.
The Assistant Secretary of the Navy requested that
NASA make a timely independent assessment of the
technical development of the F-14. A NASA F-14
Study Team of over 40 Langley personnel led by
Langley researcher William J. Alford, Jr. was organized.
A briefing of the study results was given in August
1969 at the Naval Air Systems Command by a team
led by Laurence K. Loftin, Jr., Mark R. Nichols,
and William Alford. This briefing (which covered
results in cruise and maneuvering performance,
aeroelasticity and flutter, propulsion integration,
stability, and control) identified several areas
where further research would enhance the F-14’s
capabilities. Following the briefing, Dr. John
Foster, Director of Defense Research and Engineering,
requested the support of Langley in the development
of the F-14.
|
Propulsion Integration |
|
Initial flight-test
evaluations of the performance of the F-14 by the
Navy revealed higher drag levels at high subsonic
and transonic speeds than had been predicted by
Grumman wind-tunnel tests. The Navy requested Langley
support in analyzing and providing solutions to
the problem. Langley experience with the F-111
and other advanced fighter concepts indicated that
an extremely large portion of the high subsonic
and transonic cruise drag of modern twin-engine
fighters is contributed by the aft end of the configuration.
For example, a relatively poor aft-end design could
produce almost 50 percent of the cruise drag for
some configurations. In the course of the F-111
development program, Langley researchers in the
16-Foot Transonic Tunnel had developed test techniques
and analysis methods to minimize this problem,
and they went to work on the aft-end aerodynamic
characteristics of the F-14 configuration.
Based on their extensive experience, the 16-Foot
Transonic Tunnel team conducted tests in 1972 to
determine the characteristics of the critical engine-fuselage
fairing (pancake) at the rear of the aircraft.
Several geometric variations were evaluated to
determine more effective pancake shapes, with an
appreciation of the trades that are necessary to
minimize component interference drag while adhering
to the area rule developed by Langley researcher
Dr. Richard Whitcomb. In addition, certain regions
had to be preserved for the F-14 fuel jettison
system and the landing arrestor hook. The researchers
cut away areas of the pancake, reshaped the geometry
and added a “handle” in the shape of
a bulbous pod at the rear of the pancake. The design
recommended by the Langley tests proved extremely
effective in reducing cruise drag and was incorporated
into the F-14 configuration.
In addition to the pancake modification, Langley
researchers recommended that generous “speed
bump” (area added to shape the aircraft to
comply with Whitcomb’s area rule) fairings
be added to the forward bottom area of the vertical
tails. The suggestion was accepted by Grumman and
incorporated into the production F-14 fleet.
Pancake configurations evaluated
in cruise-drag studies in the
Langley 16-Foot Transonic Tunnel. Original shape
is at left.
Aft-end pancake handle and vertical
tail root speed bumps on F-14 aircraft.
|
High-Angle-Of-Attack
and Spin Research |
|
In early 1970, initial
tests conducted in the Langley 20-Foot Vertical
Spin Tunnel at the request of the Navy indicated
that the F-14 would exhibit two types of spins.
The first spin involved relatively steep, nose-down
spins from which recovery would be relatively easy
for the pilot. However, the results also showed
that the F-14 might exhibit a relatively flat unrecoverable
spin in which the aircraft would rotate rapidly
(2 sec per turn) about a vertical axis through
its center of gravity, while descending vertically
with the fuselage in a relatively horizontal attitude.
Because of the high rate of rotation of the flat
spin, the g-forces at the cockpit location
would be very high (approximately 6.5 longitudinal
g’s outward) and would probably incapacitate
the pilot if sustained for even a moderate period
of time. Under the direction of Langley researcher
James S. Bowman, Jr. exhaustive spin tunnel tests
were conducted to define a spin recovery procedure,
but the aerodynamic control surfaces of the F-14
were ineffective at these flat attitudes. In fact,
even a scaled version of a very large 35-ft diameter
spin recovery parachute (the largest that could
be carried by the F-14 was 21 ft) could not recover
the model from the spinning motions. The only concept
that provided marginal recovery was the simultaneous
application of normal recovery controls, deployment
of the emergency parachute, and extension of auxiliary
canards on the nose of the model.
Unrecoverable flat spins have been exhibited
by many fighter configurations in the Langley Spin
Tunnel, and such characteristics are viewed with
concern. However, additional types of model tests
are required to judge the seriousness of the problem.
In particular, drop-model tests are conducted to
determine if the aircraft can enter the spin from
initial flight conditions. Spin tunnel tests are
conducted with the model launched in a flat attitude
into a vertically rising airstream—conditions
very favorable for the spin to stabilize. However,
in actual flight conditions, many aircraft lack
the control power required to reach these conditions.
For example, although the F-5 aircraft exhibited
a flat spin in Langley Spin Tunnel tests, it was
virtually impossible for pilots to intentionally
enter the flat spin.
F-14 model in spin recovery tests
in the Langley Spin Tunnel.
F-14 drop model being calibrated
in flight electronics lab prior to flight tests.
Two F-14 drop models were under fabrication when
the spin tunnel results became known. The Langley,
Grumman, and Navy team had planned to equip the
models with extensive instrumentation to measure
flight variables for correlation with analytical
studies of the spin, but the installation process
would have taken several months. Because the Navy
required an immediate answer about the susceptibility
of the F-14 to enter flat spins, Marion O. McKinney
directed the team to install limited instrumentation
in one of the models, and to obtain answers as
quickly as possible. With the approval of this
approach by the Navy, Charles E. Libbey and his
team installed a miniature movie camera in the
engine inlet of the model and pointed it forward,
where it could monitor the relative angle of a
simple wooden angle-of-attack vane mounted on a
nose boom. This innovative approach gave the Navy
answers in a few weeks, rather than months. The
team conducted as many as 4 drop tests a day in
a very successful test series. More refined instrumented
model tests were completed later for a total of
55 drop tests in the program.
Libbey and his team concentrated their initial
studies on the susceptibility of the F-14 model
to enter the spin by manipulating the controls
after the model had been dropped from a helicopter.
Results of the tests showed that the model could
be pitched up in an aggressive manner with no tendency
to enter either the steep or flat spins. However,
if the roll control (differential deflection of
the horizontal tails) was used in normal fashion
to pick up a down-going wing at high angles of
attack, the model would depart controlled flight
in a direction opposite of the intended input because
of adverse yaw caused by large yawing moments produced
by the horizontal tails. If the pilot held in the
roll control, the model would enter a flat spin.
Recovery from flat spins requires the use of an
emergency parachute, special nose canards, and
full differential tail deflections.
Although the drop-model results predicted that
entry into the flat spin was possible, it was recognized
in this test technique that the ground-based pilot
controls the model from a remote position, thereby
losing the natural physical cues available to the
pilot of the actual aircraft. In addition, the
scaled model moves much faster than the full-scale
aircraft, so the time available to the pilot of
the model for recovering from an impending spin
is very limited. The analysis of the F-14 therefore
turned to the use of the Langley Differential Maneuvering
Simulator (DMS) where the actual human environment
could be more accurately simulated.
In 1972, a team headed by Langley researcher
Luat T. Nguyen programmed the DMS with aerodynamic
data from Langley wind-tunnel test results and
F-14 aircraft mass characteristics. Arrangements
were made for Langley research pilots and the Grumman
chief test pilot Charles Sewell to participate
in the evaluation. Simulated maneuvers flown in
the DMS confirmed the drop-model results—that
is, the aircraft could be aggressively pitched
to extreme attitudes without loss of control, but
if roll control was maintained at high angles of
attack, the aircraft would depart from controlled
flight. In addition, with simulation of full-scale
conditions, it appeared that the pilot had time
to make corrective controls before hazardous spin-entry
conditions occurred.
Nguyen and his team had participated in the development
of the flight control system for the F-15 for high-angle-of-attack
conditions. (See Langley Contributions to the
F-15.) They recognized that the concept used
by the F-15 to reduce the adverse effects of the
horizontal tails as roll control devices at high
angles of attack would be an ideal solution to
the F-14 problem. With this knowledge, an automatic
rudder interconnect (ARI) for the F-14 was implemented
and evaluated in the DMS. The ARI system automatically
phased out movement of the tails for roll control
and phased in deflections of the rudders at high
angles of attack. The concept was refined and matured
in the simulator studies. The pilots who flew the
F-14 with the ARI system were enthusiastic, and
the system allowed the pilot to maneuver the aircraft
without regard to angle of attack or switching
from differential tails to rudders. The Grumman
team regarded the ARI concept developed by Langley
as a highly desirable addition to the F-14 aircraft.
The production contract for the early F-14 aircraft
called for the implementation of an ARI system.
At this point, Langley closed out its F-14 research,
while Grumman pursued the development of the final
ARI system.
Unfortunately, the early F-14 aircraft also included
another late developing preproduction concept—deployable
wing leading-edge maneuver slats for improved maneuverability.
Early Grumman flight tests revealed that the F-14
modified with both the ARI system and the maneuver
slats displayed unsatisfactory air combat maneuvering
characteristics because the ARI rudder inputs aggravated
lightly damped rolling oscillations (wing rock)
induced by the slats during maneuvers. Because
of this incompatibility, the Navy deactivated the
ARI systems on all fleet F-14 aircraft.
The F-14 proved to be a relatively forgiving
aircraft to fly, and pilots adapted to manually
switching from using differential tails for roll
control at low angles of attack to using rudders
at high angles of attack. However, the F-14 fleet
began to experience spin losses at the rate of
about one aircraft per year. In 1978, a joint NASA,
Navy, and Grumman program was initiated to develop
a new ARI system to increase the spin resistance
of the F-14. Nguyen and his team once again examined
the F-14 and used the DMS to develop a new ARI
that provided adequate damping of the wing rock,
while retaining the spin resistance of the original
ARI system developed by Langley. A flight-test
F-14 was modified with a spin parachute, battery
driven hydraulic pumps for emergency power, and
the special foldout canards on the fuselage forebody
that were recommended by the earlier spin tunnel
tests. Fitted with the new ARI, flight tests were
conducted over a 2-year period at NASA Dryden Flight
Research Center with Langley personnel on site
for the flight tests. Over 100 flights by 9 pilots
were made up to low supersonic speeds.
The results of the flight-test program were extremely
impressive. Wing rock was suppressed, inadvertent
spins were eliminated, and the handling qualities
throughout the air combat envelope were improved.
Several years passed before funding constraints
permitted the Navy to develop the ARI within plans
to equip the F-14 fleet with a new advanced digital
flight control system (DFCS). Following further
refinements during Navy flight evaluations at Patuxent
River Naval Air Station in Maryland, the Navy implemented
the DFCS with the ARI. The first F-14 deployments
with the ARI occurred during the Kosovo operations,
and glowing reports from the F-14 squadrons indicated
that the new system was a success.
The F-14 responds to an abrupt
stick pull by Dryden test pilot Fitz Fulton
during evaluations of the Langley designed Automatic
Rudder
Interconnect (ARI) control system at Dryden. |
Flutter Tests |
|
Flutter clearance
tests of the F-14 in the Langley 16-Foot Transonic
Dynamics Tunnel required five entries from 1970
to 1973 under the leadership of Moses G. Farmer.
The utilization of variable-sweep wings by the
F-14 introduced a unique flutter problem that had
been unanticipated prior to the tests. The challenge
of providing an upper-fuselage covering for the
variable-sweep wing panels had been addressed by
Grumman with relatively flexible inner wing covers.
Early in the flutter tests, large deflections and
buffeting of the over wing panels were observed
and viewed as a potentially serious flutter problem.
With knowledge of the Langley results, Grumman
engineers designed a set of external stiffening
strakes for the wing covering that eliminated the
flutter problem. An additional favorable impact
of the strakes was local straightening of the airflow
over the upper fuselage, which resulted in performance
benefits. With the strake modification, the F-14
passed flutter clearance tests in the Langley tunnel.
Initially, it was proposed that this modification
would only be applied to the preproduction flight-test
F-14 aircraft while a redesign of the wing cover
could be accomplished. However the modification
proved to be extremely robust and similar strakes
were incorporated in all production F-14 aircraft.
During the flutter tests, the Langley staff observed
considerable buffeting of the vertical tails, particularly
at moderate angles of attack. The staff of the
16-Foot Transonic Dynamics Tunnel modified the
cable-mount system to permit tests at high-angle-of-attack
conditions where the buffeting became more intense.
Langley expressed concern that damage to the structural
integrity of tails or tail-mounted avionics and
antennae might be encountered, but Grumman did
not accept this concern as an issue. Subsequently,
the F-14 fleet experienced structural damage and
the replacement of tail-mounted radar warning units.
As a result, the vertical tails of F-14 aircraft
were stiffened.
The experience of the engineering community with
vertical-tail buffeting in the F-14 led to the
development of design analysis tools and special
wind-tunnel test techniques for follow-on aircraft
including the F/A-18 and F-22.
F-14 model mounted in preparation
for flutter tests in the 16-Foot Transonic Dynamics
Tunnel.
Close-up view of the external
over wing cover strakes added to prevent panel
flutter.
|
F-14 Yaw Vanes |
|
In the early 1980’s,
researchers in the Navy community became interested
in the potential benefits of using thrust vectoring
for control augmentation of the F-14. Their interest
had been spurred by a cooperative Langley and Navy
piloted simulator study that was conducted in the
Langley DMS. The study defined the benefits to
a representative fighter aircraft of maintaining
the control power required for satisfactory V/STOL
flight in conventional flight. In this study, an
existing Langley simulator model of the F-14 was
modified to incorporate the control modifications
under the leadership of Luat Nguyen. Langley and
DOD pilots flew the simulated flights.
Results of the simulator study showed that the
most important benefit occurred when the yaw control
was augmented at high angles of attack (normally,
yaw control provided by conventional rudders is
markedly reduced at high angles of attack). With
the increased yaw control capability, pilots could
consistently win against a variety of adversaries
in simulated air-to-air combat. Analysis of the
desired control levels in the simulator results
indicated that deflecting the engine thrust on
aircraft similar to the F-14 would provide the
necessary control.
To pursue the development and demonstration of
the effectiveness of yaw vectoring, the Navy conducted
a series of tests to evaluate the turning effectiveness
and structural integrity of external vanes mounted
behind the nozzles of the F-14 engines. These activities
were augmented by tests led by Bobby L. Berrier
and David E. Reubush in the Langley 16-Foot Transonic
Tunnel. Data obtained in these tests were used
to define the geometry and thrust-vectoring effectiveness
for the Navy evaluations.
Full-scale vanes were fabricated and initially
ground tested behind an F-14 aircraft at the Patuxent
River facility. Flight tests of a modified F-14
were subsequently conducted to demonstrate the
structural integrity and thrust-vectoring performance
of the vane concept over a limited flight envelope.
F-14 model equipped with deflectable
yaw vanes in the Langley 16-Foot Transonic Tunnel.
F-14 with experimental yaw vanes
in flight over Patuxent River Naval Air Station.
Further applications of the yaw vane concept
to the F-14 did not develop beyond this exploratory
program, but the concept of external vanes as relatively
inexpensive devices for thrust vectoring at high
angles of attack was successfully used in the NASA
F-18 High-Angle-of-Attack Technology Program and
the DARPA X-31 Enhanced Fighter Maneuverability
Program.
|
