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Langley Contributions
to the C-5
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C-5 Evaluations |
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In 1964, the Air
Force awarded study contracts to Boeing, Douglas,
and Lockheed for the design of a heavy-lift transport
for the Cargo Experimental–Heavy Logistics
System (CX–HLS). In December 1964, the three
manufacturers were invited to submit proposals
for the new transport, which was intended to carry
bulky and heavy military equipment that could not
be accommodated in the C-141. All three industry
designs incorporated high-wing configurations with
four large turbofan engines in underwing nacelles
and front and rear doors with ramps for flow-through
loading and unloading. The Boeing and Douglas designs
had conventional tail configurations, whereas the
Lockheed design incorporated a T-tail configuration.
At the request of the Department of Defense (DOD),
Langley conducted aerodynamic assessments of all
three designs in the Langley 8-Foot Transonic Pressure
Tunnel in 1965 under the leadership of Langley
researcher Dr. Richard T. Whitcomb. The results
of these tests were provided to a committee to
select the winning C-5 design. In an extremely
controversial decision based on proposal cost estimates,
industry workloads, and geopolitical considerations,
the Air Force announced in October 1965 that Lockheed
had been selected to proceed with development of
their C-5 design.
The C-5 design submitted by Boeing was found
to have superior aerodynamic cruise performance
in the transonic wind-tunnel tests performed at
Langley. Boeing’s experience with the C-5
competition coupled with Boeing management’s
vision of the marketability of jumbo civil transports
(and interest from Pan American Airlines) led to
the development of the Boeing 747, which enabled
Boeing to dominate the world market with a new
product line. Although the 747 was a completely
new aircraft design (low wing, passenger-carrying
civil aircraft), the general configuration influence
of the earlier C-5 candidate is in evidence.
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Research on Sting
Interference Effects |
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Analysis of the
data from the Langley 8-Foot Transonic Pressure
Tunnel tests of the three C-5 configurations indicated
that the high degree of aft-fuselage bottom upsweep
of all the configurations increased cruise drag
(especially the Douglas design, which had 19 deg
of upsweep). Because of the critical nature of
the aft-end drag, concern arose over potential
interference effects caused by the model support
sting on drag measurements.
Donald L. Loving and Arvo A. Luoma conducted
tests in 1965 of all three configurations with
conventional stings, dummy stings, and dorsal strut-support
systems over a limited range of test variables
from just below to just above the design cruise
condition of each configuration. The results of
their investigation indicated that the sting interference
effects were of very small magnitude.
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Research on Effects
of Test Section Size |
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Also in 1965, a
study was conducted by Langley researchers Arvo
Luoma, Richard J. Re, and Donald Loving to address
concerns over the effect of model size during transonic
tests of large models in the 8-Foot Transonic Pressure
Tunnel. In wind-tunnel tests, the largest model
possible is generally desirable so that higher
model Reynolds numbers are obtained, but if the
model is too large potential tunnel wall effects
and data corrections become concerns, especially
at high subsonic and transonic speeds. Comparative
aerodynamic data were obtained for the same 5-ft-span
model of the C-5 in the Langley 8-Foot Transonic
Pressure Tunnel and the Langley 16-Foot Transonic
Tunnel for Mach numbers from 0.75 to 0.83. The
5-ft-span model was two to three times larger than
usual models tested in the Langley 8-Foot Transonic
Pressure Tunnel.
Competing C-5 configurations
during tests in the Langley 8-Foot Transonic
Pressure Tunnel. Top to bottom: Douglas, Lockheed,
and Boeing designs.
C-5 model mounted for tunnel
test section study in the Langley 16-Foot Transonic
Tunnel.
Langley 8-Foot Transonic Pressure Tunnel and
the Langley 16-Foot Transonic Tunnel for Mach numbers
from 0.75 to 0.83. The 5-ft-span model was two
to three times larger than usual models tested
in the Langley 8-Foot Transonic Pressure Tunnel.
The results of the study indicated that the data
obtained in the two tunnels were in good agreement
and that large models could be tested in a slotted
tunnel such as the 8-Foot Transonic Pressure Tunnel
at subsonic speeds with acceptable results.
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Propulsion Integration
Research |
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In 1966, Langley
researchers James C. Patterson, Jr. and Stuart
G. Flechner conducted parametric studies in the
Langley 8-Foot Transonic Pressure Tunnel to determine
the effects of large high-bypass engines on the
interference drag of wing-nacelle configurations
at cruise conditions. In the study, a large powered
semispan model that incorporated tip-driven, nitrogen-powered
engine simulators was used. The baseline model
configuration represented the C-5; however, the
horizontal and vertical positions of the engines
relative to the wing were varied so the effects
of the engine exhaust wakes could be studied in
detail. The results of the study indicated that
the interference drag effects could actually be
beneficial for certain combinations of wing-nacelle
geometric parameters.
Powered C-5 semispan model in
the Langley 8-Foot Transonic Pressure Tunnel.
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Low-Speed and Wake
Characteristics |
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In late 1965, the
Air Force requested that tests be conducted in
the Langley 30- by 60-Foot (Full-Scale) Tunnel
to investigate the application of the externally
blown flap to the C-5 aircraft. Close examination
of the C-5 configuration indicated that the engine
exhaust would blow strongly on the trailing-edge
flaps and that the aircraft would probably have
considerable jet-flap lift effect in its basic
configuration. The jet-flap lift effect might be
sufficient to promote marked improvements in takeoff
and landing performance that had not been anticipated.
However, the jet-flap effect could induce pitch
and roll trim problems that were also not anticipated.
The Air Force request included tests of a semispan
powered model, as well as a full-span powered model.
The examination by Langley of the C-5 configuration
indicated that several features of the wing and
flap did not lend themselves well to adaptation
of a high efficiency jet-flap arrangement. For
example, the flap configuration was not optimum
for blown flap applications. As a result of concern
expressed by Langley, a two phase program was conducted.
The first phase of the program measured the effects
of power on lift and drag characteristics and the
effect of power (and engine-out conditions) on
the longitudinal and lateral trim requirements,
control, and stability of the basic C-5 configuration.
The second phase of the research program consisted
of modifying the wing and flap system to provide
a more optimal configuration for an efficient jet
flap.
In 1966, a powered semispan model of the C-5
underwent two test entries in the Full-Scale Tunnel
to determine a limited amount of power effects,
including the effects of thrust reversers. Unfortunately,
Lockheed changed the trailing-edge flap system
of the C-5 from a double-slotted configuration
to a Fowler flap configuration, and the tests had
to be conducted with an outdated flap system.
In 1967, a full-span 0.057-scale model of the
C-5 was tested in the Full-Scale Tunnel to determine
the effect of the externally blown flap concept
on the aircraft. The initial work was done with
the original, nonoptimized double-slotted flap
design. Subsequent work was done with a new flap
system that was designed specifically for the blown
flap concept by the Langley staff under the leadership
of Joseph L. Johnson, Jr.
Powered C-5 model mounted in
the Langley Full-Scale Tunnel for externally
blown flap assessments with the double-slotted
flap configuration.
Langley engineer Charles C. Smith,
Jr. conducts flow visualization studies on the
modified C-5 model.
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Tail Flutter |
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The staff of the
Langley 16-Foot Transonic Dynamics Tunnel (TDT)
led research on flutter characteristics of the
T-tail configuration as a design feature of large
transport aircraft with transonic cruise capabilities
in the late 1950’s. Although several aerodynamic
theories had been developed for predicting subsonic
and supersonic flutter of T-tails, the transonic
regime posed special challenges because transonic
flutter speeds tend to be lowest and are accompanied
by complex shock patterns that make flutter analyses
difficult. TDT staff members Charles L. Ruhlin
and Maynard C. Sandford had been actively involved
in the flutter issues that faced the C-141 and
had contributed to the databases for the design
of flutter-free T-tails for future aircraft. Their
pioneering efforts in the C-141 program resulted
in new test procedures, validation of computational
methods, and fundamental research of the complex
aerodynamic and structural coupling for the new
tail designs. Two types of T-tail flutter had been
identified by research: symmetric tail flutter,
which was dominated by pitching-type relative motions
of the surfaces, and antisymmetric flutter, which
was characterized by yawing and rolling relative
motions of the tail surfaces. Antisymmetric flutter
is especially complex, and more data were needed
to advance the understanding and design tools.
Clipped wing model of the C-5 in the Langley 16-Foot
Transonic Tunnel for flutter tests.Full-span model
of the C-5 in the Langley 16-Foot Transonic Dynamics
Tunnel.
Clipped wing model of the C-5
in the Langley 16-Foot Transonic Tunnel for flutter
tests.
Full-span model of the C-5 in
the Langley 16-Foot Transonic Dynamics Tunnel.
Ruhlin and Sandford developed an innovative approach
to testing large, isolated-tail and aft-fuselage
models, which permitted more accurate simulation
of structural and aerodynamic properties of full-scale
aircraft. In late summer of 1966, they began studies
of an isolated 1/13-scale model of the C-5 empennage,
fuselage, and inner wings in the TDT. The T-tail,
fuselage, and inboard wings were geometrically,
dynamically, and elastically scaled. Two models
of the T-tail were tested—one was built with
the C-5 design stiffness, while the second had
only half the design stiffness.
The tail flutter speeds for the designed C-5
empennage were beyond the required flutter demonstration
speeds; however, a pronounced decrease in flutter
speed was observed slightly above the maximum Mach
number (between 0.92 and 0.98). As a result of
these tests, the fin spar stiffness was increased,
and flutter clearance for the full-span C-5 model
in the TDT was subsequently obtained.
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Load Alleviation |
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In the initial design
of the C-5, Lockheed implemented an aggressive
weight reduction program to meet performance requirements.
The wing weight was reduced by using higher design
stress levels and reducing primary component thickness.
The higher stress levels proved to be a problem,
and wing cracks were found early in full-scale
ground fatigue tests in July 1969. After the aircraft
had been in service several years, a wing tear-down
inspection on one aircraft with a high number of
flight hours revealed significant cracks. Lockheed
proposed several approaches to restore C-5 wing
fatigue life to a specified level of 30,000 flying
hours. These approaches included (1) an active
aileron system to alleviate gust loads on the wing,
(2) local wing modifications to improve fatigue,
and (3) redistribution of fuel within the wing
to reduce bending moments.
Langley researchers Ruhlin and Sandford conducted
tests of the C-5 active lift distribution control
system (ALDCS) in the 16-Foot Transonic Dynamics
Tunnel in 1973. The results of this study validated
the use of active control technology for the minimization
of aircraft aeroelastic response and showed that
scaled aeroelastic wind tunnel models can be used
in developing active control systems.
Active ailerons were retrofitted to 77 C-5’s
in 1975 through1977. That approach, however, was
superseded by a redesign of the wing that included
a new center wing, two inner wing boxes, and two
outer wing box sections, which were manufactured
from advanced aluminum alloys that were unavailable
when the original wings were produced in the late
1960’s and early 1970’s. As a result,
all C-5 aircraft were modified with the new wings.
Active load alleviation test
of the C-5 in the Langley 16-Foot Transonic Dynamics
Tunnel.
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Ditching Tests |
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Langley conducted
ditching investigations for military and civil
aircraft (including the Space Shuttle) for many
years in a water tank facility known as the Langley
Impacting Structures Facility. Following World
War II, aircraft shapes and sizes did not vary
significantly from the existing database that was
generated by the Langley research. However, the
introduction of the C-5 and other large wide-body
civil transports required the prediction of ditching
characteristics for heavier and larger configurations
that were not included in the database. The design
configuration and structural features of large
cargo and transport aircraft also required that
a dynamic model be investigated to determine overall
motions, accelerations, and the approximate location
and amount of damage that might be expected during
a ditching at sea. In addition, the large number
of main landing gear wheels (24) for the C-5 and
the ability of the landing gear to be extended
to various positions offered the possibility of
an optimal ditching configuration, since previous
wheels-down dynamic-model investigations had shown
a wide variation of ditching performance with landing
gear extended.
C-5 model in Langley Impacting
Structures Facility for ditching tests.
C-5 ditching model with simulated
structural skin on bottom of model.
At the request of the Air Force, Langley researcher
William C. Thompson conducted ditching studies
of a 1/30-scale model of the C-5 in the Impacting
Structures Facility in 1969. The model was highly
instrumented to measure accelerations, and a special
scaled-structure lower fuselage was included to
determine the structural damage incurred in ditching.
The tests included various impact attitudes, flaps
up and down, and landing gear retracted and down
in simulated calm and rough (simulated sea state
4) water. Results of the study indicated that the
most favorable condition for ditching was a 7-deg
nose-high attitude with the flaps down 40 deg,
the nose gear retracted, and the main gear fully
extended. For these conditions, damage to the fuselage
bottom would occur, and most of the main landing
gear would probably be torn away.
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Wind-Tunnel Test
to Flight-Test Correlation |
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Analysis of flight
data obtained with the C-5 indicated that significant
differences existed between pressure measurements
made in the wind tunnel and flight. Similar issues
had previously arisen during the C-141 program.
These issues involved difficulties with scaling
effects in wind-tunnel tests. Extensive fundamental
research was initiated in the Langley 8-Foot Transonic
Pressure Tunnel under the direction of Richard
Whitcomb. The research was initiated to develop
an approach of simulating high Reynolds number
flows by using specific placement of artificial
boundary-layer trip strips on the wing upper and
lower surfaces of the wind-tunnel model. The test
program, which was conducted by Donald Loving,
Arvo Luoma, and James A. Blackwell, Jr., successfully
identified a methodology that more properly simulated
the appropriate boundary-layer thickness in transonic
wind-tunnel tests. This technique was used extensively
in the development of the supercritical airfoil.
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