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Background
The acceleration
of airflow over the upper surfaces of wings of
conventional subsonic jet transports at high subsonic
speeds results in a region of supersonic (supercritical)
flow above the upper surface of the lifting airfoil
that terminates in a shock wave. The formation
of the shock wave causes an increase in drag known
as wave drag, which dramatically increases the
thrust required to cruise at higher speed conditions.
In addition, flow through the shock wave eventually
encounters unfavorable conditions that may lead
to shock-induced separation of the boundary layer,
which typically causes an additional increase in
drag, potential aircraft stability and control
problems, and buffet. These locally supersonic
flow characteristics are experienced by the wings
of subsonic jet transports even though the aircraft
is flying at subsonic conditions (typically at
about Mach 0.80).
The supercritical airfoil,
developed at the Langley Research Center, uses
a unique geometric shape to control the characteristics
of the supersonic flow in a manner to minimize
drag and enhance the cruise efficiency of the transport.
The curvature of the middle region of the upper
surface of the supercritical airfoil is significantly
reduced and carefully tailored to result in a more
rearward location and substantial decrease in the
strength of the shock wave, and drag for a given
lift coefficient is reduced. The onset of boundary-layer
separation is also substantially delayed to a higher
Mach number. The relatively small amount of lift
lost by reducing the curvature of the upper surface
of the airfoil is regained and substantially augmented
by the larger extent of supersonic flow on the
upper surface and by incorporating a substantial
camber into the rear portion of the airfoil. The
leading-edge shape of the supercritical airfoil
is also considerably blunter than those of conventional
airfoils; this provides improved high-lift performance
of the wing at cruise and landing conditions. The
supercritical airfoil enables the performance-enhancing
options of cruising at higher values of Mach number
or cruising at the same Mach number with a substantial
increase in wing thickness, which permits wing
planform (aspect ratio) and structural design trade-offs
to enhance cruise efficiency.
Airflow fields over conventional
airfoil and
supercritical airfoil at high subsonic speeds.
Langley Research and Development
Activities
After the first U.S. swept-wing
jet transports became operational in the late 1950s,
most of the research community in the United States
regarded the design of subsonic jet transports
as a mature science, and many began to join the
national effort that was emerging to develop a
Mach 3 supersonic civil transport. In contrast,
the European community continued to develop advanced
airfoils for increased high-speed subsonic cruise
efficiency. In Britain, H. H. Pearcey of the National
Physical Laboratory showed, in 1962, that airfoils
whose curvature decreased abruptly downstream from
the leading edge could exhibit shock-free recompression
and a significantly weaker upper-surface shock
at high-speed conditions. With this modification,
the critical Mach number (Mach number where drag
abruptly increases) could be delayed by about 0.03.
Following his brilliant scientific
development of the area rule (to be discussed in
a later section), Richard T. Whitcomb of Langley
became deeply involved in the NASA research activities
of the U.S. Supersonic Transport (SST) Program.
Whitcomb had developed a particularly efficient
SST design in his research efforts. However, the
relatively high fuel and operating costs of the
supersonic transport configurations that were evolving
in the U.S. program represented formidable barriers
to the economic feasibility of these configurations,
regardless of aerodynamic efficiency. Recognizing
the tremendous penalties of the supersonic transport
relative to advanced subsonic transports, Whitcomb
terminated his efforts on supersonic transports
and searched for more productive research in support
of advanced subsonic aircraft.
Initially, Whitcomb explored
the potential application of the area rule to the
fuselages of conventional subsonic transports.
However, the industry saw no cost-effective applications
for the concept and did not provide the advocacy
required for additional research. (Whitcomb, however,
did return to fuselages with area rule in the maturation
of supercritical wing technology, as mentioned
in a later section.) While Whitcomb searched for
new concepts to improve subsonic transports, a
milestone event occurred during day-to-day business
that resulted in yet another major breakthrough
by Whitcomb. As frequently happened, Whitcomb was
requested by the Langley Director for Aeronautics,
Laurence K. Loftin, Jr., to review wind-tunnel
data that had been obtained by the Ling-Temco-Vought
(LTV) Company for a radical new vertical takeoff
and landing (VTOL) concept known as Air Deflection
and Modulation (ADAM). The ADAM concept used a
unique configuration that incorporated a slot in
the upper wing surface from which engine air was
blown for lift augmentation in VTOL flight. Whitcomb
noted that at high-speed conditions, air flowing
from the slot resulted in a substantial increase
in the drag-divergence Mach number. He hypothesized
that the increase in Mach number was caused by
delayed shock-induced separation, and he immediately
envisioned an application to the swept-wing subsonic
transport.
Based on this inspiration,
Whitcomb and his staff initiated research in 1964
in the Langley 8-Foot Transonic Pressure Tunnel
that would ultimately lead to the first NASA supercritical
airfoils. Whitcomb’s chief assistant and
lead researcher for the airfoil studies was Charles
D. Harris. Initially, tests were conducted on a
two-dimensional airfoil with a self-actuated slot
from which high-pressure air was ejected. Research
objectives focused on delaying and reducing the
shock-induced separation losses. After several
iterations of airfoil shapes, and led by Whitcomb’s
intuitive wisdom, the work focused on a slotted
airfoil with a flattened upper surface ahead of
the slot, which naturally resulted in large negative
camber. A large portion of the lift was carried
by a short, positively cambered portion aft of
the slot. This innovative airfoil shape showed
a substantial increase in drag-rise Mach number.
However, concern arose over the feasibility of
scaling, fabricating, and maintaining the geometric
tolerances of the slot, as well as operational
problems and costs. Thus, Whitcomb and his staff
conducted additional research that enabled the
slot to be eliminated with only a small penalty
in the onset of drag rise and with considerable
simplification in the potential fabrication and
application to three-dimensional wings. This so-called
integral airfoil was first tested in 1966. Later,
in recognition of the structural problems that
might arise with the thin trailing edge of the
integral airfoil, a thickened trailing edge was
developed and underwent subsequent modifications
through 1974.
At the time of Whitcomb’s
unique, experimental development of supercritical
sections, no computer-based theories were available
to guide his work, and the development work at
Langley was entirely experimental. In 1969, Loftin
recognized that the development and application
of this technology would be severely impeded if
industry applications demanded unique experimental
facilities and the methodical experimentally based
development process employed by Whitcomb and his
staff. Loftin pushed for the development of an
analytical design method, and with the recommendations
of Clinton Brown of NASA Headquarters, Paul R.
Garabedian of the Applied Mathematical Department
of New York University was chosen to develop a
practical theoretical program that could be used
by designers as a routine airfoil design tool.
Jerry C. South, Jr. served as Langley’s manager
for the grant, with I. Edward Garrick also monitoring
this breakthrough activity. Garabedian, along with
other pioneering individuals (including Anthony
Jameson and Earll Murman) developed and validated
the theory. Garabedian led this breakthrough in
computational research for the supercritical airfoil
in 1971, and Jameson led the way in computational
wing design in 1977. Both Garabedian and Jameson
later received major NASA awards in recognition
of their brilliant contributions. Although this
pioneering theory has now been superseded by more
sophisticated analysis methods, it served as a
critical historical building block in the development
of transonic aerodynamic analyses now used by industry.
As the supercritical wing
technology continued to mature, Whitcomb and his
team provided critical design information on the
integration of supercritical airfoils into three-dimensional
wings and the effects of advanced propulsion systems
on supercritical wing performance. Key Langley
team members for studies of supercritical applications
included Thomas C. Kelly, Dennis W. Bartlett, Charles
D. Harris, and James A. Blackwell. In addition,
Stuart G. Flechner, James C. Patterson, Jr., and
Paul G. Fournier conducted extensive tests on advanced,
energy-efficient subsonic transport models in the
Langley 8-Foot Transonic Pressure Tunnel to add
to the rapidly growing national database on the
effects of engine nacelle and pylon cant angle
and engine longitudinal and vertical position on
cruise performance, including the effects of powered
nacelles. The substantial database and design guidelines
supplied by the staff of the 8-Foot Transonic Pressure
Tunnel provided extremely valuable information
to both the airframe and engine industries.
Another key member of Whitcomb’s
staff, Theodore G. Ayers, led studies on the potential
applications of supercritical airfoil technologies
to military aircraft, including fighter-bombers
(F-111), transports (YC-15), fighters (F-14), and
bombers (B-1). Ayers later transferred to NASA
Dryden Flight Research Center where he played a
key leadership role in the highly successful flight
tests of a modified F-8 aircraft in a joint Langley-Dryden
program, as well as other flight programs on supercritical
wings and advanced airfoils.
Evolution of Langley supercritical
airfoil concept.
Flight Evaluations
The first flight demonstration
project of supercritical wing technology was initiated
in 1969, when the Navy and NASA cosponsored a joint
project to demonstrate the ability of the supercritical
wing to accommodate increased wing thickness with
no reduction in cruise Mach number. The benefits
realized from an increase in thickness included
improved structural efficiency (and reduced wing
weight) and increased internal wing volume. For
example, if a supercritical wing of 17-percent
thickness could be designed to have the cruise
efficiency of a 12-percent-thick conventional wing,
the internal fuel volume could be increased by
about 40 percent and the amount of wing volume
devoted to fuel could be increased by over 50 percent.
Initial wind-tunnel tests by North American Rockwell,
Columbus Division, of wings with this range of
thickness values indicated that the drag-divergence
Mach number of a 17-percent-thick supercritical
airfoil shape was equal to that for a conventional
12-percent-thick airfoil, that the buffet onset
was considerably higher, and that the low-speed
maximum lift was increased considerably. With these
promising results in hand, the interest turned
to flight verification of the wind-tunnel results.
Basic T-2 (left) and modified
T-2 with 17-percent-thick supercritical wing (right).
The test aircraft for the
project was a T-2C Buckeye trainer aircraft on
loan from the Navy. With Whitcomb and his staff
serving as technical consultants, Rockwell International
designed, developed, and manufactured a modified
17-percent-thick supercritical wing. A standard
production T-2C wing was modified by the addition
of balsa wood and fiberglass to obtain the desired
thickness and configuration. A 0.09-scale model
of the modified T-2C was tested in the Langley
8-Foot Transonic Pressure Tunnel for correlation
with flight. First flight of the modified T-2C
occurred in 1970, and the results indicated that
for the Mach number of interest (about 0.60), the
17-percent-thick supercritical wing provided the
same performance as the conventional 12-percent-thick
wing.
In fulfilling his responsibilities
as Director for Aeronautics, Loftin recognized
that the new supercritical technology would have
to be tested, matured, and demonstrated during
actual flight tests of full-scale aircraft before
the technology would be considered mature enough
for applications to future aircraft. In 1967, he
strongly advocated for a flight demonstration program
to be conducted at the NASA Dryden Flight Research
Center with an F-8 Crusader aircraft obtained from
the Navy. The F-8 was an ideal test bed for the
supercritical wing demonstration. It could achieve
the speed of sound at high altitudes, and its unique
variable incidence wing could be replaced with
a modified wing by simply lifting the original
wing off the aircraft. A joint Langley-Dryden program
was approved in early 1968. The wing investigated
in this test program was designed and developed
in the 8-Foot Transonic Pressure Tunnel by Whitcomb
and his staff for a cruise Mach number of about
0.98. This particular wing was not intended for
retrofitting to the F-8 but was representative
of wings that might potentially be used on a near-sonic
transport with an area-ruled fuselage. The leading-edge
sweep of the baseline F-8 was increased to about
42∞ and the wing aspect ratio was increased
to 6.8. The gradual increase in cross-sectional
area required by the area rule for near-sonic conditions
required a modified fuselage as well as a graceful
extension of the inboard leading edge of the wing.
Richard T. Whitcomb with
model of F-8 supercritical wing
configuration in Langley 8-Foot Transonic Pressure
Tunnel.
F-8 supercritical wing test-bed
aircraft in flight with an area-ruled
fuselage and underwing leading-edge vortex generators.
Preliminary wind-tunnel tests
on the modified F-8 configuration indicated that
the new high-aspect-ratio wing with more sweepback
exhibited severe longitudinal instability (pitch
up) at transonic, high-lift conditions. The problem
was caused by spanwise flow toward the wingtips
on the wing upper surface. Charles D. Harris and
Dennis W. Bartlett conducted wind-tunnel tests
that ultimately developed small vertical vortex
generator surfaces on the lower outboard part of
each wing panel that created vortices on the upper
surface which acted as aerodynamic “fences”
to block the spanwise flow and eliminated the problem.
In 1969, Rockwell International,
North American Aircraft Division, received a contract
to fabricate the supercritical wing for the F-8,
which was delivered to NASA in December 1969. Thomas
C. Kelly was the Langley project engineer for the
program, whereas John McTigue served as Dryden’s
Supercritical Wing (SCW) program manager.
Project pilot Tom McMurtry
flew the first F-8 SCW flight on March 9, 1971;
the last flight of the aircraft was piloted by
Ron Gerdes on May 23, 1973, ending an 86-flight
program. Results of the flight tests vividly demonstrated
the significant improvements afforded by the supercritical
wing. The concept had improved the transonic efficiency
of the F-8 by as much as 15 percent, and the projected
benefits to transport aircraft were very significant.
F-8 on static display at
Dryden.
Today, the F-8 SCW is on permanent
display at Dryden. A plaque mounted at the aircraft
site reads as follows:
This research aircraft was the
first airplane to demonstrate the transonic performance
capabilities of a supercritical wing. This airplane
demonstrated a drag-rise Mach number of 0.96 at
cruise lifting conditions. The resulting technology
base permitted an increase in cruise Mach number
for transport aircraft from approximately 0.82
to above 0.9.
On February 29, 1972, NASA
reported to industry and the Department of Defense
(DOD) on the progress of the F-8 and T-2C flight
programs at a classified conference held at the
NASA Dryden Flight Research Center.
Intense interest over the
results coming from the F-8 Supercritical Wing
Program spurred NASA and the Air Force to modify
an F-111A to explore the application of supercritical
wing technology to maneuverable military aircraft.
The joint Transonic Aircraft Technology (TACT)
Program was approved in 1969. By 1971, NASA and
General Dynamics Corporation had over 1,600 hours
of wind-tunnel time on perfecting a suitable wing.
Whitcomb and his staff determined its shape, twist,
and airfoil coordinates. General Dynamics built
the wing, and the Air Force Flight Dynamics Laboratory
provided the funding for the aircraft modification
and flight tests. The first flight of the F-111
TACT aircraft occurred in 1973. The supercritical
wing substantially improved the performance of
the aircraft. The wing delayed the drag rise at
transonic speeds, delayed the onset of buffet,
and produced substantially more lift than the conventional
wing.
Applications
The aerodynamic performance
improvements provided by the supercritical airfoil
technology are extremely significant. The drag-rise
Mach number can be significantly delayed, the onset
of buffet is also delayed, and high-lift performance
is improved. However, most applications of the
supercritical airfoil have utilized the concept
and its performance-enhancing characteristics to
permit the use of thicker wings and lower sweep
(enabling higher wing aspect ratio), rather than
to increase the drag-rise Mach number. Thicker
wings can be structurally lighter; thereby, larger
payload fractions and improved operational economics
are provided. The use of higher aspect-ratio wings
contribute directly to increased performance and
economic benefits.
Ironically, the application
of this innovative concept has involved uses (more
efficient flight at existing speeds) that were
not envisioned during the initial development process
(flight at near-sonic speeds). Thus, the experience
of the supercritical wing once again underscores
the fact that it is often impossible to identify
in advance all the real-world applications and
justification for a research activity.
In recognition of his outstanding
accomplishment in supercritical wing technology,
Whitcomb was awarded the maximum $25,000 prize
by NASA for the invention in 1974, and that same
year he also won the Wright Brothers Memorial Trophy
of the National Aeronautic Association for his
enduring contributions to aeronautics.
The revolutionary gains provided
by the supercritical wing were initially directed
at increasing the cruise speed of subsonic transport
configurations. In mid-1970, NASA initiated a new
focused research program known as the Advanced
Technology Transport (ATT) Program to provide technology
for a superior subsonic long-haul aircraft that
could cruise just below the speed of sound. Langley’s
initial manager for the program was William J.
Alford, Jr. In the ATT Program, NASA initiated
several studies of the near-sonic transports that
utilized the benefits of the supercritical wing
to drive the cruise speed up to the point where
area ruling of the fuselage became necessary. Industry
studies of candidate near-sonic transports by The
Boeing Company, McDonnell Douglas Corporation,
and General Dynamics were undertaken by using aerodynamic
data obtained for a representative transport configuration
in the 8-Foot Transonic Pressure Tunnel. Results
of the industry studies indicated that a new aircraft
of this design would have operating costs similar
to those of aircraft at that time, but the new
aircraft would have an increase in speed of about
20 percent and be capable of cruising at a Mach
number of about 0.98. As a matter of interest,
during this time frame Boeing was actually involved
in preliminary design of the Boeing 767 transport,
and one of the initial 767 configurations employed
the trademark area-rule shaping to its fuselage
and wings—far different than its eventual
production shape.
Near-sonic transport wind-tunnel
model with area ruling and advanced supercritical
wing.
Unfortunately, about 2 months
after the industry studies were completed, the
Organization of Petroleum Exporting Countries (OPEC)
tripled the price of crude oil and the airlines
were no longer interested in flying faster. Rather,
the industry wanted technology that would reduce
fuel consumption. Whitcomb and his team justifiably
terminated the work under way on near-sonic transports
and redirected their efforts to using the trades
provided by supercritical technology to obtain
more aerodynamic efficiency. At that time, the
program was also renamed the Advanced Transport
Technology Program.
The industry-wide design approach
to using supercritical wing technology was to increase
the thickness ratio of the wing airfoil, reduce
the wing sweep to reduce the structural weight,
and increase the aspect ratio of the wing to reduce
the drag due to lift. This design approach resulted
in significant increases in fuel efficiency and
has basically been the major application of supercritical
technology used by designers of today’s new
subsonic transports.
As mentioned in an earlier
section, the NASA Aircraft Energy Efficiency (ACEE)
Program initiated a number of technology thrusts
to increase the fuel efficiency of subsonic transports.
Within this program, an element known as the Energy
Efficient Transport (EET) Program addressed the
improvements promised by advances in aerodynamic
technologies that included supercritical wings,
winglets, advanced high-lift systems, and active
load alleviation. The EET Program was the stimulus
for extensive Langley activities in advanced transport
aerodynamics—especially the supercritical
wing.
As previously mentioned, the
supercritical airfoils developed by Whitcomb embodied
three primary geometrical factors: a relatively
uncambered upper surface, an increased leading-edge
radius, and a cambered trailing edge. Industry
applications of the technology have included specific
configurations that include all three features.
Other applications include only the upper-surface
and leading-edge principles because of concerns
over pitching moments resulting in increased trim
drag and trailing-edge flap actuator implementation
caused by the reflexed trailing-edge shape.
The design and analysis of
advanced airfoils and wing performance within industry
is conducted with extensive proprietary computer
codes and sophisticated methods. Many computational
methods developed by Langley researchers have been
embedded in industry design tools. The flexibility
provided by the codes significantly reduces the
scope of design variables and reduces the number
of potential wing designs to be studied in new
aircraft programs.
The evolution of supercritical
airfoil technology embodied in current jet transport
aircraft is depicted in the accompanying sketch
of typical airfoil shapes and camber distributions
used by industry for conventional, intermediate,
and aft-loaded airfoils. As discussed earlier,
virtually all subsonic transports exhibit a region
of supersonic flow on the upper surface of the
wing in cruise flight; therefore, the use of the
word “supercritical” has somewhat confused
the layman’s understanding of the aerodynamic
design of aircraft of the 1990s. As depicted in
the sketch, the conventional airfoil (A), which
is typical of aircraft such as the older DC-10,
is characterized by forward and mid-camber distribution.
The intermediate airfoil (B), which is typical
of aircraft such as the Boeing 767, utilizes a
relatively mild degree of aft camber to provide
a limited amount of the performance benefits provided
by the more aggressive supercritical airfoils.
A typical aft-loaded supercritical airfoil (C)
is characterized by its blunt nose, virtually no
leading-edge camber, and a relatively high level
of trailing-edge camber (about 2 percent). This
type of airfoil is employed by the more modern
Boeing 777.
Airfoils showing typical
geometric shapes (upper)
and distribution of camber along airfoils (lower).
Following the dissemination
of flight results for the F-8 and T-2C to industry
in 1972, initial applications of the supercritical
wing technology to U.S. civil aircraft occurred
within the business jet aircraft community. The
first application of supercritical technology in
the U.S. to a production civil transport aircraft
was by Cessna Aircraft Company for the Citation
III in 1981. The supercritical wing design process
for the Citation III was a joint effort between
Cessna, McDonnell Douglas, and NASA. Working with
Boeing, Cessna subsequently developed the supercritical
wing design of the Citation X. This advanced business
jet incorporates a highly swept, second-generation
supercritical wing and is capable of Mach 0.92
and a maximum altitude of 51,000 feet.
Cessna Citation III was
the first application of supercritical technology
to production aircraft in the U.S.
Cessna Citation X.
Other U.S. business jet applications
include the Sabreliner 65 and the Gulfstream V,
which uses certain principles of supercritical
technology (leading-edge radius and upper-surface
contouring) to obtain exceptional speed and efficiency.
The winglets of the Gulfstream V also utilize supercritical
technology. During 1997, the Gulfstream V aircraft
demonstrated its capabilities by setting 46 world
and national records consisting of 21 speed records
and 25 performance records. The Gulfstream V was
named the winner of the 1997 Collier Trophy for
“the most significant aeronautical achievement
in the United States.” Other U.S. business
jets using supercritical technology include the
Hawker Horizon, which is designed, built, and marketed
by the Raytheon Aircraft Company.
Applications of supercritical
wing technology to larger transport aircraft in
the United States began with prototype military
transports in 1976—Boeing’s application
to the YC-14 and McDonnell Douglas’ application
to the YC-15. The first application of aggressive
supercritical wing technology to a large production
transport in the United States was by McDonnell
Douglas (now Boeing) for the military C-17 Globemaster
III. The McDonnell Douglas technology and design
expertise for this application was built on Langley’s
pioneering efforts in developing supercritical
airfoils, McDonnell Douglas design activities sponsored
by the ACEE Program, and the YC-15 design experience.
McDonnell Douglas was awarded the Collier Trophy
for 1994 in recognition of the superior design
of the C-17.
The application of supercritical
wing technology to large U.S. commercial transport
aircraft has been more conservative. Several factors
are responsible for this situation, including extremely
important cost/benefit considerations that trade
off increased aerodynamic performance with production
costs and other variables. The wings of the Boeing
757, Boeing 767, and McDonnell Douglas MD-11 employ
certain principles of supercritical technology
such as an increased aft loading, although these
applications are not the more aggressive supercritical
technology design features identified by earlier
NASA research. The most recent U.S. large transport
to incorporate a more aggressive supercritical
wing is the Boeing 777, which entered commercial
service in May 1995. Supercritical technology principles
and proprietary supercritical airfoils used by
Boeing in the design of the 777 are much more advanced
than those used by earlier Boeing transports and
provide exceptional structural and aerodynamic
efficiency. In recognition of its superior efforts
in the design of the 777, Boeing was awarded the
Collier Trophy for 1995.
Boeing 777 with supercritical
wing technology.
In-flight photograph of
Boeing 777 shows lower wing trailing-edge curvature
associated with supercritical airfoils.
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