Early Configuration
Development |
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In 1954, the Navy
selected McDonnell Aircraft to design and develop
an all-weather supersonic fighter. The fighter,
designated the F4H, was to be a fleet defense fighter
that could take off from an aircraft carrier, have
a cruise distance of 250 mi, intercept intruders,
and then return to the carrier 3 hr after takeoff.
The aircraft was to be armed with missiles and
would not carry guns. It would operate as a high-speed
(Mach number of 2), standoff missile launcher that
would not engage in close-in combat.
In mid-1955 the full-scale engineering mock-up
of the twin-engine aircraft featured a swept wing
with no dihedral, and the horizontal tails drooped
down at an angle of 15 deg. At the request of the
Navy, tests began in the Langley Unitary Plan Wind
Tunnel under the direction of Melvin M. Carmel
to determine the supersonic performance and stability
and control characteristics of the original configuration.
Three separate entries in the Unitary Tunnel were
made to evaluate the early F4H design.
In 1968, the Department of Defense (DOD) requested
that NASA respond to the F-15 request for proposals
(RFP) in a manner similar to the industry contractors.
The key person behind the NASA participation was
Dr. John Foster, Director of the Defense Department
Research and Engineering organization. He requested
the participation for two reasons. First, Foster
felt that NASA’s aircraft designs for the
F-15 mission would embody advanced technology and
serve as the upper limit of technology for industry
proposals. Second, NASA and its problem-solving
expertise could minimize risks and problems later
in the development program.
Fighter concepts developed by
NASA for the F-15 mission requirements.
In response to Foster’s request, NASA organized
a design team of about 70 professionals from the
Langley, Ames, Glenn, and Dryden Research Centers.
The team was colocated at Langley under the leadership
of Langley researcher William J. Alford, Jr. Roy
V. Harris, Jr. and A. Warner Robins of Langley
were members of a subteam responsible for configuration
design and many other Langley researchers served
on other subteams. The team spent about 4 months
at Langley developing several fighter concepts
directed at the F-15 mission requirements. The
breadth of studies included analytical and wind-tunnel
tests for the most promising configurations. Four
fighter concepts were studied in great detail:
- LFAX-4—a variable-sweep
configuration
- LFAX-8— a fixed-sweep
version of LFAX-4
- LFAX-9—wing-mounted
twin-engine configuration
- LFAX-10—similar in
external shape to Soviet MiG-25 Foxbat
Industry design teams visited Langley during
the efforts and were continually updated on the
advantages, disadvantages, and technical maturity
of the configurations. The NASA team also briefed
high ranking DOD officials. The LFAX-4 and LFAX-8
embodied features that would subsequently be evident
in the McDonnell Douglas F-15 and Northrop Grumman
F-14 aircraft.
McDonnell Douglas was especially interested in
the NASA fighter study as an extremely valuable
adjunct to the company’s design effort on
the F-15. The LFAX-8 design made an indelible impression
on the McDonnell Douglas design team, which embraced
the fundamental layout of the NASA configuration.
The cranked-wing design of the LFAX-8 had to be
modified by McDonnell Douglas as the requirements
for transonic maneuvering became more important.
Another modification to the LFAX-8 involved the
installation of a larger radar dish in the nose
than the NASA team had allowed for in their design.
The installation required a larger diameter nose
cone, and although the NASA researchers deplored
the increased supersonic drag caused by the larger
nose, the final design incorporated the larger
dish. |
F-15 Source Selection |
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Following the NASA
fighter study, 41 of the 70 NASA researchers became
participants in the detailed evaluation of the
industry proposals by McDonnell Douglas, Fairchild,
and North American Rockwell. The studies and evaluation
efforts, as well as developmental efforts at Langley,
required over 6,000 hr in Langley facilities including
the Unitary Plan Wind Tunnel, the 4- by 4-Foot
Supersonic Pressure Tunnel, the 7- by 10-Foot High-Speed
Tunnel, the 16-Foot Transonic Tunnel, the 16-Foot
Transonic Dynamics Tunnel, the 20-Foot Vertical
Spin Tunnel, and the 30- by 60-Foot (Full-Scale)
Tunnel. During the development of the F-15, Langley
personnel acted as consultants to the Air Force,
and Langley provided a permanent member to the
F-15 Program Office in Dayton, Ohio for improved
liaison and communications.
Following the award of the F-15 contract by the
Air Force to McDonnell Douglas in December 1969,
Langley supported the development of the aircraft
in several key areas.
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Aerodynamic and
Propulsion Integration Studies |
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Previous experiences
with the F-111 and other advanced fighter concepts
indicated that an extremely large portion of the
subsonic cruise drag of modern twin-engine fighters
is contributed by the aft end of the configuration
(approaches 50 percent for some configurations).
Langley researchers learned that careful tailoring
of the engine interfairings and tail surfaces could
prevent excessive aft-end drag. In the course of
F-111 development, Langley researchers in the 16-Foot
Transonic Tunnel under Blake W. Corson, Jr. had
developed test techniques and analysis methods
to minimize this problem.
Aft-end configurations of F-15
models before (top) and after (bottom) Langley
modifications
such as removal of ventral fins, taller vertical
tails, and nozzle interfairing.
A Langley team led by Bobby L. Berrier conducted
extensive tunnel tests on a 4.7-scale model of
the F-15 for a critical assessment of aft-end drag
on the baseline configuration, which at the time
had large ventral fins below the aft fuselage.
Mutual adverse aerodynamic interference effects
were evaluated for the aft-end components, including
the tails, tail booms, nozzles, and fuselage. After
several tunnel entries, the Langley and McDonnell
Douglas research team recommended configuration
changes that significantly reduced the subsonic
cruise drag of the aircraft. Specifically, the
ventral fins were removed, the height of the vertical
tails was increased to compensate for the resulting
loss of directional stability, and nozzle interfairings
(between the nozzles and also between the nozzles
and tail booms) were added. Following briefings
by Berrier to Air Force and DOD managers (including
Dr. John Foster, Director of Defense Research and
Engineering, and Robert Seamens, Secretary of the
Air Force) of the test results and recommendations,
the modifications (except for variable interfairings)
were accepted for all production aircraft. The
variable interfairings were actually built for
flight-test evaluations, but were never flown after
it was learned that the modified production aircraft
met all critical mission requirements without the
interfairing modification.
Upon completion of the drag clean-up studies,
Berrier’s team also provided the subsonic,
transonic, and supersonic aerodynamic data package
for the production aircraft. These data were obtained
during multiple entries in the 16-Foot Transonic
Tunnel and the 4- by 4-Foot Supersonic Pressure
Tunnel. The final aerodynamic data package delivered
by Langley included wind tunnel sting and distortion
corrections and jet exhaust correction increments.
Performance predictions of the final production
aircraft were based on this data package.
In addition to studies of the production F-15,
the Langley team conducted exploratory tests of
the performance benefits of two-dimensional (2-D)
nozzles and other advanced propulsion integration
concepts. These research studies provided the largest
U.S. database for advanced nozzle integration and
served as a valuable foundation for development
of the 2-D thrust-vectoring nozzles used by the
F-15 short takeoff and landing and maneuver technology
demonstrator (STOL/MTD) aircraft and the F-22 fighter.
Langley personnel conducted two other significant
propulsion integration studies on the F-15. The
first study was a wind tunnel to flight correlation
of a highly instrumented powered model in the 16-Foot
Transonic Tunnel and flight tests conducted at
Dryden Flight Research Center. The second effort
was a study of acoustically induced loads on the
F-15 nozzles in the 16-Foot Transonic Tunnel and
the Langley Acoustic and Dynamic Laboratory. The
stimulus for this study, led by Langley researcher
John M. Seiner, was the in-flight loss of external
nozzle leaves (“turkey feathers”) from
operational F-15 aircraft as a result of structural
fatigue. The cause and fix for this phenomena was
identified during the Langley studies, but an alternate
approach, at the expense of cruise drag, of simply
removing the nozzle external leaves before they
could fatigue and fall off was adopted by the Air
Force.
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High-Angle-Of-Attack
and Spin Research |
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The Air Force and
the Navy had expressed notable dismay over the
relatively poor and unforgiving characteristics
of the F-4 aircraft at high angles of attack. The
F-4 exhibited a sudden directional divergence (nose
slice) and other control-induced characteristics
at high angles of attack that made the aircraft
susceptible to loss of control and inadvertent
spins. The two services lost a combined total of
over 100 F-4’s to accidents involving these
characteristics during the operational life of
the aircraft. Understandably, McDonnell Douglas
approached the design of the F-15 with the intention
of providing a high level of stability and spin
resistance for the new air superiority fighter.
Langley technician Ronald White
with one of two F-15 drop models used for research
on spin-entry characteristics.
William P. Gilbert and James S. Bowman, Jr. led
Langley research on the high-angle-of-attack and
spin characteristics of the F-15. Facilities used
for the effort included the Full-Scale Tunnel,
the Spin Tunnel, and a helicopter-launched, radio-controlled
drop model. McDonnell Douglas had done considerable
homework as a result of the F-4 experience, and
entered the F-15 development program with an appreciation
of the stability and control features required.
In addition to careful layout of the airframe
for satisfactory high-angle-of-attack stability
of the F-15, McDonnell Douglas adopted a unique
control augmentation system (CAS) in which movement
of the control stick for lateral control resulted
in deflections of the rudders, rather than the
ailerons, at high angles of attack. With this approach,
McDonnell Douglas restricted spin inducing aileron
inputs while retaining adequate roll response.
Initial analyses of this control concept by Langley
researchers indicated that it would greatly enhance
the spin resistance of the F-15. A demonstration
of the spin resistance of the aircraft through
the use of dynamically scaled free-flying models
was still required. In addition, Langley was requested
to identify the potential spin and recovery characteristics
of the F-15 and the size of the parachute required
for emergency spin recovery for a flight-test aircraft.
Results of tests of a 1/30-scale model in the
Spin Tunnel indicated that the F-15 would exhibit
developed spins if it exceeded the capability of
the CAS to prevent spin entry. However, the ability
of the aircraft control surfaces to recover the
F-15 from spins was predicted to be very good.
Meanwhile, tests in the Full-Scale Tunnel in 1971
on a 0.10-scale free-flying model and with a 0.13-scale
model dropped from a helicopter indicated that
the F-15 would be very stable at high-angle-of-attack
conditions and that spin entry would be very difficult
with the CAS active. Using the drop model, Langley
researchers led by Charles E. Libbey defined a
very limited range of control inputs and aircraft
attitudes that enabled entry into a spin. In summary,
the superior high-angle-of-attack behavior that
had been anticipated by the McDonnell Douglas designers
of the F-15 was validated and vividly demonstrated
by the Langley dynamic-model tests.
In 1972, the Dryden Research Center assessed
the impact of model size and Reynolds number on
the results of the Langley tests. Dryden built
and conducted drop tests with a 0.38-scale unpowered
model, which was launched from a B-52. Results
of the test program correlated well with the results
obtained from the smaller model at Langley, and
further substantiated the spin resistance of the
F-15 configuration.
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The F-15 Short
Takeoff and Landing and Maneuver Technology Demonstrator |
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In 1984, the Flight
Dynamics Laboratory of the Air Force Aeronautical
Systems Division awarded a contract to McDonnell
Douglas for an advanced development STOL/MTD experimental
aircraft. The idea behind the program was to develop
an aircraft that could land and takeoff from sections
of wet, bomb-damaged runway under bad weather conditions
and severe crosswinds without active ground-based
navigational assistance.
The F-15 STOL/MTD aircraft was a direct adaptation
of a configuration developed in Langley sponsored
programs conducted in the 1970’s. Government
and industry studies of nonaxisymmetric two-dimensional
(2-D) nozzles in the early 1970’s had identified
significant payoffs for thrust-vectoring 2-D nozzle
concepts. To better integrate the various 2-D nozzle
technology programs and also to exchange data on
this concept, a joint NASA and DOD workshop was
initiated and sponsored by Langley in September
1975. To strengthen program integration and develop
recommendations for further development of 2-D
nozzles, an ad hoc interagency nonaxisymmetric
nozzle working group was formed at the workshop.
Langley researcher Bobby Berrier was appointed
as the NASA representative to this group, which
also included a member from the Air Force, the
Navy, and the Defense Advanced Research Projects
Agency (DARPA) organizations. The group advocated
for a flight research program on the thrust-vectoring
2-D nozzle concept.
The working group provided reviews and progress
reports to senior management officials throughout
the mid-1970’s to early 1980’s. The
working group advocated for studies on three potential
flight research vehicles. To avoid duplication,
the Air Force studied the F-111 vehicle, the Navy
studied the YF-17/F-18 vehicle, and Langley studied
the F-15 vehicle. In 1977, Langley initiated a
system integration study of thrust-vectoring, thrust-reversing,
2-D nozzles on the F-15 with McDonnell Douglas.
A companion study of 2-D nozzle integration with
the engine was initiated with Pratt and Whitney
by the Glenn Research Center. The eventual F-15
STOL/MTD aircraft with thrust-vectoring, thrust-reversing,
2-D nozzles and a canard for trim of thrust-vectoring
forces is a direct descendent of the configuration
developed in these studies funded by NASA. In addition,
multiple tests for both the F-15 generic and the
F-15 STOL/MTD were conducted at the Langley 16-Foot
Transonic Tunnel to study propulsion integration
and power effect of thrust-vectoring, in-flight
thrust-reversing (including methods for trim),
2-D nozzles on the F-15. As a result of its key
role in developing the technology, Langley provided
nearly all the features of the F-15 STOL/MTD aircraft.
The 16-Foot Transonic Tunnel team, led by Bobby
Berrier, Francis J. Capone, Richard J. Re, Odis
C. Pendergraft, Jr., Mary L. Mason, Laurence D.
Leavitt, and others developed during the 1980’s
the most extensive design database in the world
for low observable, thrust-vectoring, thrust-reversing
(yaw and pitch), 2-D nozzles.
In recognition of Langley’s leadership
role in thrust-vectoring technology, the Air Force
and McDonnell Douglas consulted frequently with
the Langley staff, particularly with the propulsion
integration experts under William P. Henderson
at the 16-Foot Transonic Tunnel and the flight
dynamics experts at the Full-Scale Tunnel. Free-flight
tests conducted by Joseph L. Johnson, Jr.’s
group in the Full-Scale Tunnel demonstrated that
the aircraft would have excellent handling characteristics
at high angles of attack. The most impressive research
results in flight dynamics, however, occurred when
the Langley researchers equipped the free-flight
model with special 2-D nozzles that provided thrust
vectoring in yaw as well as pitch. The superior
control provided by the multiaxis vectoring was
demonstrated when the model was easily flown at
angles of attack up to about 85 deg without vertical-tail
surfaces.
The first flight with the thrust-vectoring nozzles
took place on May 16, 1989. The aircraft was transferred
to Edwards Air Force Base for joint flight tests
by the Air Force and McDonnell Douglas. The 2-D
nozzles were first tested in flight on March 23,
1990. Test flights demonstrated a 25-percent reduction
in takeoff roll, and the thrust-reversing feature
made it possible for the F-15 to land on just 1,650
ft of runway (7,500 ft is required for the standard
F-15). In addition, thrust reversal was used during
up-and-away flight to produce rapid decelerations—a
useful feature for close-in air-to-air combat.
During the flight program, the F-15 STOL/MTD made
vectored takeoffs with rotation demonstrated at
speeds as low as 42 mph. The program ended on August
15, 1991, after accomplishing all of the flight
objectives.
The F-15 STOL/MTD flying with
canards and thrust-vectoring nozzles. |
