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Langley Contributions
to the F-22
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The Advanced Tactical
Fighter (ATF) Program |
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By the late 1980’s,
concern arose among military planners about the
aging design (first flight in 1972) of the F-15
and the possible loss of future air superiority
of the fighter. Soviet fighters such as the MiG
29 and the Su-27 had demonstrated remarkable maneuverability
and performance, and were provided to Third World
countries. Also, fighter technology had taken enormous
strides with the introduction of stealth, or low
observable, technology. There was growing concern
over the effectiveness of the Soviet air defense
system, because new surface-to-air missiles posed
a highly lethal environment for the F-15. Therefore,
the Air Force initiated an Advanced Tactical Fighter
(ATF) Program to develop a replacement for the
F-15. Throughout this planning and competitive
period, members of the Langley staff were involved
in briefings and discussions with Department of
Defense (DOD), Air Force, and industry teams about
technical issues and concepts to be exploited by
the new aircraft. In October 1985, the Air Force
issued the original ATF request for proposals (RFP)
to seven competing companies. The next month, the
Air Force revised the RFP to include more stringent
low observable requirements. In June 1986, Lockheed,
Boeing, and General Dynamics signed a teaming memorandum
of agreement, but continued to compete individually
for the ATF contract. Prime contracts were subsequently
awarded by the Air Force to Lockheed and Northrop
for the development of flying prototypes. Lockheed’s
design was designated the YF-22, and Northrop’s
design was the YF-23.
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Value of the Langley
Database |
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The YF-22 and F-22
aircraft benefited from basic and applied aeronautics
research conducted at Langley over the 20-year
period prior to the evolution of the programs.
In particular, Langley research in nonaxisymmetric
thrust-vectoring nozzles, supersonic cruise without
afterburner, and “carefree” maneuverability
with unlimited angle of attack had produced concepts,
data, and assessments of advanced technologies
that ultimately provided the YF-22 and F-22 with
superior characteristics.
The staff of the Langley 16-Foot Transonic Tunnel
had long been recognized as the world leaders in
conceptual research on nonaxisymmetric thrust-vectoring
nozzles and aft-end aerodynamic integration for
advanced fighters. Fundamental studies of isolated
nozzle performance and vectoring efficiency were
coupled with specific configuration evaluations
during the 1960’s, 1970’s, and 1980’s
to the point that risk was considerably reduced,
along with dramatic predictions for improvements
in fighter maneuverability, cruise performance,
and takeoff and landing performance.
The contributions of Langley researchers in the
prediction and reduction of supersonic drag were
also well recognized, including fundamental theories
and design methods to reduce supersonic wave drag.
Langley researchers also participated in in-house
and cooperative studies with industry on supersonic
civil and military aircraft. These researchers,
who used the supersonic capabilities of the Langley
4- by 4-Foot Supersonic Pressure Tunnel and the
Langley Unitary Plan Wind Tunnel, conducted years
of research that came to fruition in the F-22 Program.
The Langley research program on high-angle-of-attack
characteristics of fighter aircraft, which was
initiated in 1969 in response to an alarming number
of fighter aircraft accidents due to deficient
characteristics, provided proven test techniques,
design concepts, and advanced technology for use
by the YF-22 and F-22 design teams. The staffs
of the Langley 30- by 60-Foot (Full-Scale) Tunnel
and the Langley 20-Foot Vertical Spin Tunnel worked
closely with industry in generic and specific applications
of their technologies.
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The YF-22 Program |
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During the ATF competition,
the Air Force established that each contracting
team could draw upon NASA’s unique expertise
and facilities on an equal basis. The Lockheed
YF-22 team intended to place considerable emphasis
on achieving unlimited angle-of-attack capability
and supersonic cruise for their design. Lockheed
requested tests in the Langley Unitary Plan Wind
Tunnel, Spin Tunnel, and Full-Scale Tunnel in support
of this goal.
In mid-1989, static and dynamic tests of the
YF-22 began in the Full-Scale Tunnel under the
direction of Sue B. Grafton to determine stability
and control at high angles of attack. Data was
also obtained to develop aircraft control laws.
Free-flight tests of a YF-22 model to determine
low-speed longitudinal and lateral-directional
response characteristics and departure resistance
were also conducted. The Langley Spin Tunnel was
used to determine the spin and recovery characteristics
of the YF-22 in 1988 and to validate the geometry
of the emergency spin recovery parachute. Tests
were also conducted in the Spin Tunnel to determine
rotational aerodynamics during simulated spinning
conditions for use in the development of control
laws.
The first flight of the YF-22 prototype occurred
on September 29, 1990, and the flight-test phase
of the competition ended on December 28, 1990.
Free-flight tests of the YF-22
model in the Langley Full-Scale Tunnel in 1990.
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Letter of Appreciation |
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The success and
quality of Langley’s contributions to the
YF-22 Program were emphasized in a March 12, 1991,
letter from James A. Blackwell (Vice President
and General Manager of the Lockheed ATF Office)
to Richard H. Petersen, Director of the Langley
Research Center. In the letter, Blackwell stated
On behalf of the Lockheed YF-22
Team, I would like to express our appreciation
of the contribution that the people of NASA Langley
made to our successful YF-22 flight test program,
and provide some feedback on how well the flight
test measurements agreed with the predictions from
your wind-tunnel measurements.
Briefly, the flight test program
was an enormous success, and we achieved all of
our objectives. Between first flight on September
29, 1990, and the last flight on December 28, 1990,
we flew the two prototypes from 80 knots and 60¾
angle of attack to Mach 2+.
We relied on the NASA Langley Unitary
Tunnel to provide the final supersonic drag and
stability and control data that were used for performance
predictions and the flight control law design.
We demonstrated supercruise on both the GE and
P&W-powered prototypes as predicted. The supersonic
drag determined from flight test agreed within
one percent of the predictions that were made using
the Langley Unitary Tunnel data. The supersonic
flying qualities were rated excellent; we flew
to 15¾ angle of attack supersonically; and we did
360¾ full-stick rolls demonstrating excellent roll
rates and roll response.
The highlight of the flight test
program was the high-angle-of-attack flying qualities.
We relied on: aerodynamic data obtained in the
30- by 60-Foot Full-Scale Wind Tunnel to define
the low-speed, high-angle-of-attack static and
dynamic aerodynamic derivatives; rotary derivatives
from your Spin Tunnel; and free-flight demonstrations
in the Full-Scale Wind Tunnel. We expanded the
flight envelope from 20¾ to 60¾ angle-of-attack,
demonstrating pitch attitude changes and full-stick
rolls around the velocity vector in 7 calendar
days, December 11 to December 17. The reason for
this rapid envelope expansion was the quality of
the aerodynamic data used in the control law design
and pre-flight simulations.
Without the help of the NASA Langley
people and your unique test capabilities, it would
have been impossible to complete our YF-22 flight
test program in the time available. We look forward
to working with you and your people when we win
the Full-Scale Development contract to build the
F-22.
The Lockheed, Boeing, and General Dynamics team
was announced as the winner of the ATF competition
on April 23, 1991.
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The F-22 Program |
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Although not immediately
apparent to the uninitiated observer, the external
geometry of the F-22 changed significantly from
the YF-22 prototype. Specifically, the wingspan
was increased, the wing leading-edge sweep was
decreased, the vertical tails were reduced in area
and moved aft, and the horizontal-tail surfaces
were reconfigured. These changes were considered
significant enough to warrant specific tests of
the F-22 in the same unique Langley facilities
as the YF-22.
High-angle-of-attack tests of the F-22 were conducted
in the Full-Scale Tunnel in 1992, and spin and
rotary-balance tests were conducted in 1993 in
the Spin Tunnel. In the F-22 Program, a free-flight
model was not built, and tests in the Full-Scale
Tunnel were limited to conventional static force
and dynamic (forced-oscillation) tests.
Sue Grafton with the F-22 force
test model for the Langley Full-Scale Tunnel.
F-22 model during tests in the
Langley Spin Tunnel to determin
the size of emergency parachute required for spin
recovery.
F-22 model mounted for supersonic
test in the Langley Unitary Tunnel.
A contribution from the Langley Spin Tunnel to
the F-22 Program involved a relocation and assessment
of the attachment point for the emergency spin
recovery parachute. Evaluations of the original
F-22 parachute truss structure and attachment point
were accomplished in 1994; however, the attachment
point had to be moved forward a distance of 3 ft
on the full-scale aircraft to clear the exhaust
plume of the vectored engine in afterburner. These
tests were urgently requested by the Air Force
and Lockheed to complete the installation on the
flight-test aircraft before spring of 1999. The
staff of the Langley Spin Tunnel provided timely
and critical contributions, including the fabrication
of a new parachute truss for the existing F-22
spin model, conducting free-spinning tests in the
Spin Tunnel, and evaluating a broad range of store
loadings and mass asymmetries.
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Vertical-Tail Buffet |
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In collaboration
with the F-22 System Program Office and the Air
Force Research Lab at Wright-Patterson Air Force
Base, Langley researcher Robert W. Moses conducted
tail buffet tests in the Langley 16-Foot Transonic
Dynamics Tunnel. These tests assessed the vertical-tail
buffet environment and explored the potential application
of advanced buffet alleviation systems. In these
tests, the 13.5-scale model of the F-22 that had
been used for tests in the Full-Scale Tunnel was
refurbished with a new starboard flexible vertical-tail
surface that included a rudder driven by a hydraulic
actuator. A rigid, portside vertical tail was instrumented
with pressure transducers for measurements of unsteady
pressures.
F-22 model with instrumented
vertical tails undergoes buffet tests in the 16-Foot
Transonic Dynamics Tunnel.
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