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Langley Contributions
to the B-1B
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Early Strategic
Bomber Studies |
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Activities that
led to the development of the B-1B bomber began
in 1961, when the Air Force initiated studies under
the Subsonic Low Altitude Bomber (SLAB) Program.
The mission requirements and name of the bomber
program continued to change during the early 1960’s
and became the Advanced Manned Strategic Aircraft
(AMSA) Program in 1965. NASA’s contributions
in the definition of the FX Advanced Tactical Fighter
(F-15) Program had not gone unnoticed by Air Force
leaders involved in the AMSA Program. The close
working relationships and mutual respect that existed
between the Air Force and NASA are exemplified
in excerpts from letters of request from the Air
Force to NASA Headquarters for NASA participation
in the AMSA effort.
From Lieutenant General Holzapple (Deputy Chief
of Staff, Research and Development) to Dr. Mac
Adams (NASA Associate Administrator for Advanced
Research and Technology) on March 27, 1968:
It appears appropriate to have
an independent review of the technical status of
the (AMSA) air vehicle/engine configurations that
have evolved. Informal discussions with Mr. Mark
Nichols of NASA Langley indicate that NASA may
be able to make a significant contribution in the
form of evaluation and constructive criticism.
Specifically, we have in mind verification of the
latest preliminary designs and associated performance
calculations and wind-tunnel results to ensure
that our optimism is justified. In addition, such
activity by NASA at this time would provide the
appropriate background for NASA participation in
any contractor source selection that might eventually
come to pass.
From General McConnell (Air Force Chief of Staff)
to Mr. James Webb (NASA Administrator) on June
13, 1968:
The continued technical support
that NASA has provided the Air Force in defining
the FX Advanced Tactical Fighter has been most
useful. I feel that our cooperative efforts have
resulted in a very positive gain for the Air Force…
Because of the value we place in NASA’s expertise
I would also ask you to consider supporting our
AMSA effort. This should serve not only to improve
our product but also to increase the confidence
of others in our work.
In response to these requests, Langley participated
extensively in AMSA reviews, planning discussions,
and exploratory wind-tunnel tests of candidate
configurations. The expertise of the Langley staff
in supersonic aerodynamics, and their active participation
in the U.S. Supersonic Transport Program were especially
valuable to the AMSA Team. The evolving requirements
for the AMSA included challenging mission capabilities
at supersonic speeds and low altitudes. As the
studies began, it became obvious that application
of the variable-sweep wing concept developed at
Langley to the candidate bomber configurations
was extremely beneficial.
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The B-1A Program
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In June 1970, the
Air Force awarded development contracts to North
American Rockwell to build the B-1A bomber airframe
and to the General Electric Corporation to provide
the engines for the advanced bomber. Four B-1A
prototypes were ordered for the development program.
Langley immediately received a request from the
Air Force to assist in the assessment and development
of the B-1A.
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High-Angle-of-Attack
Research |
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The Air Force requested
that Langley conduct the complete suite of tests
for high-performance military aircraft, including
wind-tunnel free-flight model tests, spin tests,
and radio-controlled drop-model tests. Rockwell
engineers were interested in the potential variation
of handling characteristics with wing-sweep position
and were anxious to uncover any problems, along
with the solutions.
Free-flight model tests of the B-1A were conducted
in the Langley 30- by 60-Foot (Full-Scale) Tunnel
in 1972, by Langley researchers William A. Newsom,
Jr. and Sue B. Grafton. Preliminary wind-tunnel
tests of the model indicated that a severe longitudinal
instability (pitch-up) would be encountered for
angles of attack immediately above wing stall.
This undesirable result was attributable to the
relatively high position of the horizontal tail,
which placed it in an adverse flow field at post-stall
angles of attack. Subsequently, Langley radio-controlled
drop-model tests by Charles E. Libbey vividly demonstrated
the undesirable pitch-up characteristic. Piloted
simulator studies were conducted at Langley by
William D. Grantham and Langley pilot Perry L.
Deal to evaluate the stall recovery characteristics
of the B-1A under simulated mission conditions,
such as hook-up and disengagement during air-to-air
refueling. Grantham and his team of Langley and
Air Force research pilots found that recovery from
the pitch-up was very demanding and that a high
potential for inadvertent secondary stalls (and
pitch-ups) existed for the basic aircraft. The
Langley, Rockwell, and Air Force team evaluated
several control system concepts to artificially
limit the angle of attack of the B-1A and thereby
eliminate the post-stall instability from the operational
envelope. Based in part on the Langley studies,
an angle-of-attack limiter was subsequently incorporated
into the B-1A flight control system.
Langley researcher William Newsom
with the B-1A free-flight model.
B-1A model in free-flight tests
in the Langley Full-Scale Tunnel in 1972.
B-1A drop model being prepared
for helicopter drop test in 1974.
Free-spin tests of a B-1A model were conducted
in the Langley 20-Foot Vertical Spin Tunnel by
James S. Bowman, Jr.; however, the angle-of-attack
limiter system precluded serious concern of inadvertent
spins with the B-1A.
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Cruise Drag |
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At the request of
the Air Force, Langley researchers analyzed and
measured the cruise-drag characteristics of the
B-1A during the design process and the development
program.
At the request of the Air Force, Langley researchers
William P. Henderson and Bobby L. Berrier participated
in a special B-1A drag audit in the summer of 1972.
The audit, which was requested by the B-1 Systems
Program Office (SPO), consisted of a detailed assessment
of the external and internal drag and installed
thrust of the B-1A to evaluate the progress of
the program and identify any high risk areas. The
audit team consisted of 12 members from the Air
Force and NASA.
Drag reduction studies for the B-1A were also
conducted in the Langley 8-Foot Transonic Pressure
Tunnel under the leadership of Theodore G. Ayers.
The studies were directed at two objectives: the
reduction of aerodynamic drag caused by the relatively
large over-wing fairing and the potential application
of supercritical wing aerodynamics. A cooperative
Langley and Rockwell investigation of supercritical
aerodynamics promised significant results; however,
the Air Force directed Rockwell to turn their design
information over to General Dynamics for possible
application to the FB-111, and the B-1A application
was not revisited.
B-1A model in the Langley 16-Foot
Transonic Tunnel.
Aircraft such as the B-1B that incorporate advanced
configuration features such as variable-sweep wing
and variable nozzle throat and exit areas for engines
have detailed configuration features that can result
in excessive drag at cruise conditions. For example,
variable-sweep wings can have steps and gaps in
the wing and glove juncture that can produce high
drag. Variable exhaust nozzles when closed down
to cruise settings often have large boat tail angles
that result in high drag. Also, for twin-engine
configurations with closely spaced exhaust nozzles,
the large surface slopes in the gutter between
the nozzles can cause flow separation and high
drag.
To improve the drag characteristics of the B-1A,
an investigation of several modifications on a
0.06-scale model was conducted in 1974 in the Langley
16-Foot Transonic Tunnel. Langley researchers Richard
J. Re and David E. Reubush, with Rockwell and Air
Force participants conducted tests to determine
the effects of nozzle configuration, wing and glove
fairings, fuselage underfairings, and other configuration
modifications.
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Flutter |
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The B-1A configuration
underwent tests in the Langley 16-Foot Transonic
Dynamics Tunnel (TDT) during several entries to
determine flutter characteristics of the tail surfaces,
obtain flutter clearance for the complete configuration,
and investigate a unique outer wing oscillation
phenomenon. Langley researchers Charles L. Ruhlin
and Moses G. Farmer led these tests, which began
in late 1972. Flutter clearance for the B-1A was
obtained during this test program, and flight tests
of the aircraft proceeded. During the flight tests
at high altitude maneuvering conditions, an unusual
aeroelastic phenomenon was encountered in which
the outer wings of the aircraft exhibited a relatively
low frequency, low amplitude undamped oscillation
for certain values of wing sweep and flight conditions.
Several entries in the TDT were conducted from
1973 to 1987 to further analyze the driving mechanism,
which was identified as periodic shock-induced
separation. The oscillations occurred near critical
Mach number conditions for the wing airfoil and
only at high positive angles of attack. The instability
was demonstrated in the wind tunnel, however, at
slightly different conditions than in flight.
On June 30 1977, President Jimmy Carter announced
that the United States would not proceed with the
production of the B-1A bomber. However, flight
tests of the four B-1A prototype aircraft continued
until April 1981.
B-1A flutter model mounted in
the Langley 16-Foot Transonic Dynamics Tunnel.
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The B-1B Program
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As the B-1A Program
was terminated, the DOD initiated a study through
the Air Force Scientific Advisory Board to determine
the need and direction for future strategic bombers.
The results of the study concluded that a derivative
of the B-1A, the B-1B, was the best candidate to
fulfill the nation’s needs within the envisioned
mission requirements and the projected deployment
date. Although the B-1B retained the same general
geometrical shape of the B-1A, the shift in emphasis
on penetration of highly defended targets resulted
in modifications to the more crucial aircraft systems,
especially defensive systems. The B-1B has a maximum
speed of only half that of the B-1A, but it incorporates
many more advanced concepts for enhanced survivability.
Then known as the Long Range Combat Aircraft (LRCA),
the B-1B was selected as the next strategic bomber
and endorsed for production by President Ronald
Reagan in October 1981.
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Dynamic Loads on
Engine Nozzle Flaps |
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The B-1B experienced
excessive dynamic loads on the engine external
nozzle flaps, which led to premature failures of
the flap attachment system. In 1987, a joint Langley,
Rockwell, and Air Force wind-tunnel and full-scale
flight investigation of dynamic loads on engine
nozzle flaps was conducted. A 0.06-percent full-span
model of the B-1B with powered engine nacelles
was tested in the Langley 16-Foot Transonic Tunnel
under the leadership of Langley researchers John
M. Seiner, Francis J. Capone, and Odis C. Pendergraft,
Jr. The Langley researchers had previously participated
in similar dynamic loads tests for the F-15. (See
Langley Contributions to the F-15.) Seiner
had led analysis of the principal mechanisms responsible
for high loads on the external flaps that are caused
by twin-plume supersonic resonance phenomena. The
results of the B-1B study were similar to the previous
F-15 analysis and contributed further understanding
for improved design and analysis methods. Although
the study team identified several solutions to
the dynamic loads problem, the Air Force eventually
decided to fix the problem by permanently removing
the external flaps from the nozzles, which increased
cruise drag, but reduced weight.
B-1B model in an inverted position
for nozzle flap load investigation in the Langley
16-Foot Transonic Tunnel.
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