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Langley Contributions
to the C-17
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The CX Competition |
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In the late 1970’s,
the U.S. military recognized a growing demand for
rapid deployment of military forces and equipment
that would exceed the capabilities of the existing
C-141, C-5, and C-130 fleets. Early in 1980, the
Department of Defense (DOD) issued a request for
proposals (RFP) for a new Cargo Experimental (CX)
Program. Boeing, Lockheed, and McDonnell Douglas
submitted variants of civil transports, derivatives
of the prototype YC-14 and YC-15 aircraft, and
completely new aircraft in response to the RFP.
In August 1981, the Air Force announced that it
had selected the Douglas Aircraft Company Division
of McDonnell Douglas to develop the CX, now known
as the C-17.
Langley’s contributions to the development
of the C-17 included years of consultation and
cooperative research with the Douglas team, providing
unique test facilities, and several innovative
technological concepts.
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The Externally
Blown Flap Concept |
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The specifications
for the C-17 transport required advanced concepts
for superior short takeoff and landing (STOL) performance.
As a result of Langley research on powered high-lift
systems for over 35 years, the innovative externally
blown flap (EBF) concept had matured to the point
that it could be incorporated in the C-17 transport.
The EBF enables the C-17 to make slow, steep approaches
with heavy cargo loads to touch down precisely
on the spot desired on limited runway surfaces.
Because of this technology that was developed by
Langley, the C-17 can carry the same loads as a
C-5 and use the same airfields as a C-130.
John P. Campbell of Langley conceived the EBF
concept in the mid-1950’s as a relatively
simple approach to augment wing lift for low-speed
operations. In this concept, the exhaust from pod-mounted
engines impinges directly on conventional slotted
flaps and is deflected downward to augment the
wing lift. The magnitude of lift augmentation is
extremely large, and the resulting lift can be
as much as twice the value for a conventional aircraft.
However, no serious consideration was given to
the EBF concept initially because of the severe
high-temperature impingement on the wing and flap
surfaces from the turbojet (no bypass or fan flow)
engines used at that time. Also, the relatively
small mass flow from such engines was a limiting
factor for lift augmentation. In addition, considerable
concern was expressed over potential control problems
in the event that an engine became inoperative
during flight at low speeds with high-power settings.
With the advent of turbofan engines, however, the
efflux from the engines was relatively cool, and
large quantities of air became available for increased
airflow through the flaps. The turbofan engine,
therefore, provided the breakthrough mechanism
that permitted Langley researchers to evolve and
mature the applications of the EBF concept.
At Langley, extensive wind-tunnel tests in the
Langley 30- by 60-Foot (Full-Scale) Tunnel and
the Langley 300-MPH 7- by 10-Foot Tunnel explored
the fundamental lift-augmentation capability of
the EBF for various wing-engine nacelle-flap combinations.
These tests defined the optimum wing and flap geometries
for application to aircraft configurations. Led
by Joseph L. Johnson, Jr., a team of researchers
in the Full-Scale Tunnel studied the aerodynamic
performance of two- and four-engine EBF transport
configurations. The scope of their research included
detailed studies of projected aircraft performance,
potential control problems and solutions, and design
guidelines for the general geometric layout of
the aircraft. The advantage of using a T-tail empennage
configuration was identified as a desirable approach
to avoid the large local downwash angles experienced
at conventional tail locations due to the increased
wing circulation produced by the EBF concept. The
engine-out control issue was addressed and solved
by defining the vertical tail and rudder size required
to maintain directional control.
Langley researcher John P. Campbell
(l) and Gerald G. Kayten (r) of NASA Headquarters
inspect a free-flight model of an EBF configuration
in the Langley Full-Scale Tunnel.
Dynamically scaled models of transport configurations
vividly demonstrated the high-lift potential of
the EBF concept, while exhibiting satisfactory
stability and control characteristics, in free-flight
wind-tunnel tests. Aerodynamic data generated by
these studies were then used for piloted-simulator
studies of EBF transports at Langley, during which
more realistic evaluations of the handling qualities
could be conducted and flight control systems could
be configured. Throughout the years that the EBF
concept developed and matured, Langley researchers
coordinated with U.S. industry and DOD. Langley
provided briefings and invaluable technical reports
that summarized the findings of the research studies
and the revolutionary capability offered by this
new concept in aeronautics.
In the late 1960’s, the progress on the
EBF concept had advanced considerably, and most
of the major issues had been adequately addressed
and resolved. The concept was now ready for flight
evaluation on an actual aircraft. Langley advocated
for a special flight demonstrator program and conducted
several studies of the feasibility of modifying
existing aircraft as appropriate test beds. The
Douglas A3 Skywarrior twin-engine, high-wing aircraft
operated by the U.S. Navy appeared to be a desirable
option; however, a new DOD program known as the
Advanced Tactical Transport Program superseded
the Langley plans.
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The YC-15 |
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In 1972, the Air
Force issued a request for proposals (RFP) for
an Advanced Medium STOL Transport (AMST) that would
ultimately replace the C-130 tactical transport.
The program emphasized innovative technologies
and the capability of conducting STOL operations
from 2,000-ft runways. Boeing and McDonnell Douglas
were each awarded preliminary design contracts
for the construction and testing of two transport
prototypes, respectively designated YC-14 and YC-15.
Boeing based their YC-14 design on another powered-lift
concept known as upper surface blowing (USB), which
had also been developed at Langley in efforts led
by Joseph Johnson and Oran W. Nicks, Deputy Director
of Langley. In the USB concept, the engines were
mounted so that the exhaust spread over the upper
surface of the wing for enhanced circulation and
lift augmentation in STOL operations.
McDonnell Douglas used the EBF concept with a
four-engine configuration and large double-slotted
flaps that extended over 75 percent of the total
span for the YC-15 prototypes. The first of two
YC-15 prototypes made its first flight on August
26, 1975, and was joined by the second prototype
in December of that year. The YC-15 demonstrated
exceptional STOL performance in its flight-test
program with an approach speed of only 98 mph and
a field length of 2,000 ft at a landing weight
of 150,000 lb.
During the flight-test program for the YC-15,
Langley researchers participated in evaluations
and analysis of STOL capabilities, including an
assessment of lift augmentation in ground effect.
In addition to participating in the flight tests
of the YC-15, Langley conducted a cooperative wind-tunnel
test in the Langley 8-Foot Transonic Pressure Tunnel
to evaluate the effectiveness of the emerging winglet
concept, which was developed by Langley, on the
YC-15 configuration. Dr. Richard T. Whitcomb’s
winglet designs are small, wing-like vertical surfaces
located at each wingtip that enable an aircraft
to fly with greater efficiency. Winglets are strategically
located at the wingtip to produce a forward force
on the aircraft, similar in many respects to the
sail on a sailboat.
Although the YC-15 was never placed into production,
the experiences gained by McDonnell Douglas in
advanced wing design (supercritical airfoil and
winglets), the EBF concept, and advanced controls
for STOL operations gave the company confidence
in future applications of these technologies to
the C-17.
YC-15 model in the Langley 8-Foot
Transonic Pressure Tunnel for an evaluation of
winglets.
One of the McDonnell Douglas
YC-15 prototypes in flight.
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The C-17 Supercritical
Wing, Winglets, and Aerodynamic Studies |
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The YC-15 was the
first military transport to use supercritical wings,
a major innovative technology conceived and developed
through wind-tunnel research by Richard Whitcomb
at Langley. Whitcomb’s supercritical wings
incorporate advanced airfoils that enhance the
range, cruising speed, and fuel efficiency of aircraft
by producing weaker upper-surface shock waves,
thereby creating less drag and permitting higher
efficiency. McDonnell Douglas subsequently incorporated
supercritical wing technology in the C-17 design.
Whitcomb’s brilliant development of the
winglet concept was another product of research
at Langley that was ideally suited for the C-17.
The C-17 is a large aircraft with a relatively
small wing. The wingspan of the C-17 was dictated
by an Air Force requirement for three aircraft
to maneuver on a ramp measuring 90 m by 122 m that
is connected by a 15-m-wide taxiway. The aerodynamic
contribution of the winglets permits the C-17 to
employ a shorter wing span while retaining the
efficiency of a larger wing span. The C-17 winglets
also employ supercritical airfoil sections.
The successful application of winglets to the
C-17 also required consideration and analysis of
the flutter characteristics of the wing-winglet
combination. This area was of particular concern
because flow separation in the wing-winglet juncture
could provide an aggravating mechanism that might
lower the flutter speed to within the flight envelope
of the C-17. Led by Charles L. Ruhlin, a Langley,
McDonnell Douglas, and Air Force team conducted
flutter tests on the C-17 wing-winglet configuration
in the Langley 16-Foot Transonic Dynamics Tunnel
and successfully cleared the aircraft for flight
tests.
Several cooperative wind-tunnel test studies
were conducted by McDonnell Douglas and Langley
in the National Transonic Facility (NTF) at the
Langley Research Center to assess and optimize
the cruise aerodynamic performance of the C-17.
The unique capability of the NTF to more properly
simulate full-scale aerodynamic flight conditions
has been an extremely valuable contribution to
the program.
Semispan model for the C-17 wing-winglet
configuration in the
Langley 16-Foot Transonic Dynamics Tunnel for flutter
tests.
C-17 model mounted in the National
Transonic Facility at Langley for aerodynamic studies.
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Composite Materials |
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The initiation of
the Aircraft Energy Efficiency (ACEE) Program by
NASA in 1976 in response to the energy crisis accelerated
the development of concepts to improve the efficiency
of advanced aircraft. New methods of producing
unique, lightweight materials were one of the major
thrusts of the ACEE Program, with emphasis on durable
composite materials for aircraft structures. The
Structures Division at Langley played a key role
in developing the technology, which was ultimately
incorporated on several components of the C-17.
One of the most valuable Langley contributions
in the ACEE Program was the development of graphite-epoxy
upper aft rudders for the DC-10. The rudders have
accumulated over 500,000 flight hours since they
were introduced into regular airline service in
1976, thereby providing extensive experience for
applications to other aircraft, including the C-17.
Several major components of the C-17 are made
of advanced composites (ailerons, rudders, elevators,
vertical and horizontal stabilizers, flap hinge
fairings, main landing gear pod panels, and winglets).
Over 16,000 lb of composites are incorporated in
the design, with composites accounting for about
8 percent of the structure.
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Fly-by-Wire Control
System |
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The extensive database
and literature produced by pioneering research
at Langley on fly-by-wire flight control systems
and handling qualities of STOL transports provided
McDonnell Douglas with a rich source of information
in the development of the C-17 flight control system.
Although not directly involved in this facet of
the C-17 development program, Langley contributed
fundamental research information that helped build
the confidence and risk reduction required for
the application of the sophisticated C-17 systems.
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Avoiding the Deep
Stall |
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The T-tail empennage
configuration offers significant aerodynamic advantages
over conventional designs. The relatively high
location of the horizontal tail places the tail
in a relatively undisturbed airflow at normal cruise
conditions, thereby maximizing the contribution
of the tail to stability and control. Many military
and civil design teams have adopted this tail configuration
very successfully. However, the application of
the T-tail requires consideration of critical aerodynamic
factors—especially an analysis in wind-tunnel
tests to ensure satisfactory handling characteristics
at extreme pitch attitudes. At high angles of attack
associated with wing stall, the low-energy wake
of the stalled wing can impinge on the horizontal
tail and result in a loss of longitudinal stability
(pitch up) and markedly reduced longitudinal control
effectiveness. As a result of these flow phenomena,
the angle of attack can increase to a deep-stall
condition, in which the aircraft enters a stable
but uncontrollable trim point with a very high
rate of descent.
In the early 1960’s, a British BAC 111
T-tail transport experienced a fatal accident in
which the aircraft entered a deep stall and descended
in an uncontrollable condition at a very high rate
of descent with an almost horizontal fuselage attitude
until impact. Worldwide interest in the causes
of this accident resulted in a research program
at the Langley Research Center on the behavior
of T-tail configurations at high angles of attack.
Under the leadership of Robert T. Taylor and Martin
T. Moul, extensive wind-tunnel and piloted-simulator
studies were conducted to determine the aerodynamic
characteristics associated with deep-stall trim
conditions and to develop design methods and pilot
techniques to recover from such conditions. A large
number of aircraft configurations were studied
in the Langley 7- by 10-Foot High-Speed Tunnel,
with an emphasis on designs with aft-fuselage-mounted
engine nacelles. Studies at Langley on the T-tail
deep-stall phenomenon provided a valuable database
that has been used extensively in the design of
numerous T-tail civil and military transport configurations.
Perhaps the most valuable contribution of this
work is the guidance and approach it suggests to
the designer during early wind-tunnel tests and
configuration layout.
The C-17 design team was very familiar with the
existing NASA database and design procedures for
avoiding the deep-stall problem. The quadruple
redundant digital fly-by-wire flight control system
of the C-17 provides automatic limiting of angle
of attack for high-angle-of-attack conditions,
thereby preventing any tendency of the aircraft
to enter an uncontrollable deep-stall condition.
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Recognition Visit |
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On May 24, 1996,
McDonnell Douglas and the Air Force brought a C-17
of the Air Mobility Command from Charleston Air
Force Base, S.C., to Langley as a gesture of thanks
to Langley and its employees for their contributions
to the design of the new military jet transport.
The visit and formal ceremony acknowledged the
contributions of all four NASA Aeronautics Centers
(Langley, Ames, Dryden, and Glenn) to the design
and development of the C-17. The NASA research
contributions cited by the visitors included the
areas of powered-lift systems, short takeoff and
landing control systems, head-up display technology,
supercritical wing and winglet, fly-by-wire systems,
engine technologies, and composite materials.
C-17 in front of the hangar during
a visit at Langley on May 24, 1996.
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