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Background
The breakthroughs in supercritical
airfoil applications further stimulated Langley’s
efforts to develop improved airfoils for broad
classes of general aviation, commercial, and military
aircraft. By 1973, Langley’s efforts in airfoil
research had expanded and become formalized as
a focused research program. Efforts were under
way in experimental and theoretical investigations
for a wide scope of airfoil applications: low-speed
general aviation, low-speed natural laminar flow;
medium speed; supercritical transport type, large
cargo supercritical, laminar-flow control, rotorcraft,
fighter, remotely piloted vehicles (RPVs), and
propellers. The passing of the era of the NACA
airfoil catalogue, ushered in during the 1940s
by Ira H. Abbott and Albert E. von Doenhoff, was
at hand. This new era offered lower cost and more
rapid means to design the desired airfoil properties
for lift, drag, and pitching moment. The results
were envisioned to reduce aircraft fuel consumption,
increase speed and range, reduce landing speeds,
and improve safety at slow speeds.
In 1975, the resurgence of
interest in airfoils resulted in a NASA and Industry
Airfoil Workshop held at NASA Headquarters. Recommendations
and guidelines from industry were solicited, and
NASA’s plans and activities were critiqued
by enthusiastic industry representatives. NASA-wide
leadership of the evolving airfoil program was
provided by Alfred Gessow of NASA Headquarters,
and the leaders from Langley included Robert E.
Bower (Director for Aeronautics) and Percy J. “Bud”
Bobbitt (Chief of the Subsonic-Transonic Aerodynamics
Division). Under the administrative direction of
P. Kenneth Pierpont, a major NASA conference on
Advanced Technology Airfoil Research was held at
Langley in March 1978. The high level of national
interest in research activities was reflected at
this conference by an attendance of over 450 NASA,
industry, university, and DOD participants.
The rapid pace of developments
in advanced computational methods and the refinement
of airfoil analyses changed the general focus of
research in the 1980s and 1990s. Even Richard T.
Whitcomb—widely recognized for his brilliant
experimental approach to research—became
a chief advocate for the new computational methods
and utilized them extensively in his work. Research
efforts became focused on multielement airfoils,
optimization, the prediction of maximum lift, and
the effects of dynamic phenomena. In the 1990s,
research efforts at Langley were directed to three-dimensional
wing design and the fundamental understanding and
modeling of flow physics and Reynolds number effects
exhibited by high-lift systems, including boundary-layer
transition and turbulence.
Langley Research and Development
Activities
Results of the initial research
efforts on supercritical airfoils were quickly
disseminated to the U.S. industry in the early
1970s. Wind-tunnel and flight results were analyzed
independently by designers within the general aviation
industry for potential applications to a wide variety
of aircraft, especially high-speed business jets.
The benefits of the supercritical wing technology
for high-speed cruise were appreciated by the general
aviation industry; however, other beneficial effects
were also appealing to designers of low- and medium-speed
aircraft. In particular, the exceptional high-lift
characteristics of the supercritical airfoils were
viewed with great interest by the general aviation
community.
Following a Langley briefing
to industry on the new supercritical airfoil technology,
Whitcomb was approached by several general aviation
representatives who displayed great enthusiasm
over the potential low-speed, high-lift benefits
of supercritical-type airfoils. The benefits included
higher lift as well as a more docile stall. The
improvement in high-lift characteristics was a
direct result of the increased bluntness of the
leading edge of the supercritical airfoils. At
that time, many of the industry designs used the
NACA 63- and 64-series airfoils, which had tendencies
to stall abruptly at the leading edge. After the
intense interest of the general aviation industry
in improved high-lift characteristics was known
by Whitcomb, he responded with an experimental
and analytical research program to provide specialty
airfoils for this segment of the U.S. aviation
sector. This new airfoil program used a variety
of unique Langley facilities and computer codes.
The Langley Low-Turbulence Pressure Tunnel was
the key experimental facility for wind-tunnel investigations,
and a two-dimensional airfoil computer code developed
by J. A. Braden, S. H. Goradia, and W. A. Stevens
of Lockheed-Georgia, under Langley sponsorship,
was a key tool for the development of the new airfoil
family.
The goal of Langley’s
efforts was not the design of a new series of airfoils
for various applications; rather, Langley adopted
the philosophy of developing and validating theoretical
methods that could be used as useful tools for
designing airfoils for specific applications. Such
an approach was necessitated by the increasing
diversity of airfoil applications, which made the
NACA “airfoil catalog” approach impractical.
Langley’s Low- and Medium-Speed
Airfoil Program was led by Whitcomb; key members
of his research team included P. Kenneth Pierpont,
Robert J. McGhee, and William D. Beasley. Pierpont’s
responsibilities involved management of the day-to-day
challenging wind-tunnel operations, while McGhee
and Beasley led the technical investigations.
The Low-Speed Airfoil Program
was initiated in 1972 with the development of the
GA(W)-1 airfoil, which was analytically developed
by Whitcomb with the previously mentioned computer
code developed at Lockheed-Georgia under Langley
contract. This 17-percent-thick low-speed airfoil
exhibited low cruise drag; high climb lift-drag
ratios; high maximum lift; and predictable, docile,
stall behavior. National interest in this new airfoil
rapidly accelerated as the data were disseminated
in a NASA report. In fact, a rare second printing
of the technical report was required because of
the unanticipated demand. An entire series of airfoils
with varying thickness ratios was subsequently
developed for low-speed applications by Whitcomb’s
team, including a new GA(W)-2 airfoil. The GA(W)-2
section employed a 13-percent-thick profile and
generated considerable interest within the general
aviation community. This low-speed family of airfoils
also included 9-percent- and 21-percent-thick airfoils
that were designed for fully turbulent boundary
layers (negligible laminar flow) and Mach numbers
below about 0.50. Langley conducted both wind-tunnel
and flight research to support the development
and application of these airfoils. Wind-tunnel
tests were conducted to develop trailing-edge flaps
and control surfaces, and flight tests were conducted
to evaluate performance, stall characteristics,
and handling characteristics.
In the mid-1970s, NASA’s
first flight tests for verification of the potential
performance benefits of the GA(W)-1 airfoil were
conducted with a research aircraft known as the
Advanced Technology Light Twin-Engine Airplane
(ATLIT). The ATLIT was a modified Piper Seneca
airplane having a GA(W)-1 wing section, a wing
with increased aspect ratio, full-span Fowler flaps,
and roll-control spoilers; it also incorporated
Langley-developed winglets. The flight tests were
conducted at Langley from 1974 to 1976, under the
management of Joseph W. Stickle, with a test team
that included Bruce J. Holmes (then a graduate
student from the University of Kansas). Holmes
was later employed by Langley, and he subsequently
became an internationally recognized NASA leader
in advanced general aviation transportation systems
and the revitalization of the industry in the 1990s.
Other ATLIT team members included Harold L. Crane,
Joseph H. Judd, Robert T. Taylor, and research
pilots Robert A. Champine and Philip W. Brown.
Following the flight test program in 1976, the
airplane was mounted in the Langley 30- by 60-Foot
(Full-Scale) Tunnel for performance, stability,
and control evaluations, including pressures and
boundary-layer flow instrumentation. Lead researchers
for the study were James L. Hassell, Jr., and Long
P. Yip. The results of the extensive flight and
wind-tunnel testing validated the desirable cruise,
high-lift, and roll-control performance of a wing
designed with the new airfoil, flaps, and spoiler
control system.
Advanced Technology Light
Twin (ATLIT) mounted for tests in Langley 30- by
60-Foot (Full-Scale) Tunnel in 1977.
Aerodynamic research on general
aviation aircraft was especially well suited for
university involvement, and extensive Langley sponsorship
of efforts at Ohio State University (OSU), Wichita
State University (WSU), and the University of Kansas
(KU), among several others, resulted in a tremendous
stimulation and focus on aerodynamic research by
academia. Under the direction of Gerald M. Gregorek
of OSU, the university managed the Langley-sponsored
General Aviation Airfoil Design and Analysis Center
created in 1976 to meet the increasing demands
of industry for assistance in airfoil design, analysis,
and testing services. William H. Wentz of WSU led
extensive wind-tunnel tests to develop design databases
for trailing-edge flaps and control surfaces for
the low-speed series of airfoils. Flight testing
to validate the new airfoils, high-lift systems,
and spoiler-roll-control systems was also conducted
by David L. Kolhman of KU at the university. These
flight tests made use of a highly modified Cessna
177 Cardinal airplane, dubbed “Redhawk.”
First flight tests to evaluate
the characteristics of the GA(W)-2 airfoil were
conducted by the OSU, under Langley sponsorship
in 1976, with a modified Beech Sundowner aircraft.
An OSU team under the direction of Gregorek conducted
detailed in-flight measurements of the aerodynamic
characteristics of the wing (including pressure
distributions at several spanwise stations). The
results validated the predicted performance of
the airfoil in a highly successful flight test
program.
In 1976, a requirement emerged
from the business jet community for airfoils with
higher cruise Mach numbers than the foregoing low-speed
airfoils, while retaining good high-lift, low-speed
characteristics. Thus, two medium-speed airfoils
(13- and 17-percent thick) were developed to fill
the gap between the low-speed airfoils and the
high-speed supercritical airfoils. These new airfoils
were specifically designed for applications to
light executive-type aircraft having cruise Mach
numbers on the order of 0.70.
With the expansion of the
airfoil family from the low-speed to the medium-speed
airfoils, a new airfoil designation system was
put into effect by Langley in 1977. The airfoil
designations were changed to the form LS(1)-xxxx
for the low-speed series. LS(1) indicated the first
series of low-speed airfoils, the next two digits
designated the airfoil design lift coefficient
in tenths, and the final two digits gave the airfoil
thickness in percent chord. Thus, the GA(W)-1 airfoil
became LS(1)-0417 and the GA(W)-2 airfoil became
LS(1)-0413. A similar designation system was developed
for the medium-speed airfoils of the form MS(1)-xxxx.
Langley’s progress in
technology relative to low- and medium-speed airfoils
was summarized at the Advanced Technology Airfoil
Research Conference held at Langley in March 1978.
Together with the directed distribution of technical
reports and close communications with the general
aviation industry, the dissemination of information
led to widespread applications of advanced airfoil
technology and computer design tools.
Applications
Early applications of the
Langley low-speed airfoils included a wide variety
of personal-owner and sport aircraft. Within the
mainstream general aviation industry, the GAW(1)
airfoil saw applications in 1977 by Beechcraft
to the 77 Skipper trainer and by Piper Aircraft
to their new trainer, the PA-38 Tomahawk. The airfoil
was also applied by several independent designers
within their own start-up configurations including
the Bede 5, the American Hustler, and the Rutan
Vari-Eze (main wing and winglets). More recently,
applications have included the Stoddard-Hamilton
Glassair III (LS(1)-0413) and the Saab 340 regional
transport (MS(1)-0316).
Beechcraft Model 77 Skipper
was one of the first applications of GAW(1) airfoil
in 1977.
Natural Laminar Flow Airfoils
The initial emphasis in the
Langley Advanced Airfoil Research Program for low-speed
and medium-speed airfoils was to develop a series
of airfoils that could achieve higher maximum lift
coefficients than those produced by airfoils used
on general aviation airplanes at that time. The
assumption was that the flow over the entire airfoil
would be turbulent because of the riveted sheet
metal construction techniques used by the industry.
Although the new low-speed airfoils did achieve
higher maximum lift, the cruise drag was no lower
than the earlier NACA airfoils used by the industry.
The emphasis in the Langley program therefore shifted
toward natural laminar flow (NLF) airfoils in an
attempt to obtain lower cruise drag while retaining
the maximum lift of the low-speed airfoils.
Research on natural laminar
flow airfoils at Langley dates back to the 1930s
when a team under Eastman N. Jacobs conducted its
famous research for the NACA, which culminated
in the development of the 6-series airfoils that
were applied to many of the famous U.S. military
aircraft of World War II, including the P-51 Mustang.
The 6-series airfoils were not as operationally
successful as low-drag airfoils because the riveted
construction techniques employed at the time introduced
physical disturbances that disrupted laminar flow.
In the mid-1970s, the emergence of smooth, composite
structures led to a resurgence in interest in NLF
research. For decades, the NLF interest resided
in the sailplane community, but the advent of relatively
lightweight composite structures for powered general
aviation aircraft such as the Bellanca Skyrocket
II, Elbert “Burt” Rutan’s family
of aircraft, and the Windecker Eagle stimulated
aerodynamicists to reexamine the feasibility of
NLF airfoils. The Langley research efforts were
pursued with airfoil research as well as substantiating
flight test evaluation and validation.
The Skyrocket II had demonstrated
exceptional performance in flight tests by Bellanca
and had achieved an exceptionally low level of
cruise drag that suggested some amount of laminar
flow was being achieved by the wing. The aircraft
had been designed to use an NACA 6-series airfoil
similar to those developed by Jacobs in the NACA
program. Under a cooperative program stimulated
by Langley’s Joseph Stickle, special flight
tests of the all-composite Skyrocket were conducted
at Langley, under the direction of Bruce Holmes,
to determine the extent of laminar flow on the
aircraft.
The disappointment that had
been experienced in the application of natural
laminar flow airfoils in World War II carried over
into the 1970s, and many critics in the engineering
community doubted that the Skyrocket would exhibit
any significant laminar flow—even with the
smooth composite wing structure. Holmes and his
assistant, Clifford J. Obara, utilized a spray-on
sublimating chemical technique to visually identify
the presence of laminar flow on the Skyrocket at
cruise conditions. In flight, a gray-white area
(aft of the front spar) covered by the sublimating
chemical would indicate laminar flow; the presence
of high surface shear turbulent flow would cause
the gray-white sublimating coating to disappear.
The results of the Skyrocket flight test vividly
demonstrated the presence of laminar flow on the
wing to the point of maximum wing thickness. This
research activity represented a significant milestone
because aerodynamic wing design for future low-
and medium-speed general aviation composite aircraft
could now consider laminar flow as an achievable
goal.
Sublimating chemicals on
right wing of Bellanca Skyrocket indicatin
large extent of natural laminar flow during Langley
flight tests.
Holmes and Obara subsequently
conducted similar laminar flow visualization investigations
of other composite aircraft, including the Vari-Eze,
Long-EZ, and Laser Biplane Racer designed by Burt
Rutan. These researchers also investigated the
limits of the stability of NLF at higher speeds
on a Learjet and Cessna Citation Jet aircraft.
In virtually every case, extensive laminar flow
was detected for the aircraft involved.
Concurrent with the flight
testing, Langley initiated a research program to
develop NLF airfoils that would combine the high
maximum lift capability of the NASA low-speed airfoils
with the low-drag characteristics of the NACA 6-series
airfoils. Langley’s lead researcher in this
effort was Dan M. Somers. Using a computational
design code developed by Richard Eppler of the
University of Stuttgart, Somers designed and tested
a new NLF airfoil in the Langley Low-Turbulence
Pressure Tunnel. Results obtained for the new NLF(1)-0416
airfoil were compared with maximum lift and drag
data for the low-speed GA(W)-1 airfoil as well
as 6-series airfoils. An overriding goal in the
research for the NLF airfoil was that the maximum
lift would not be significantly affected with transition
fixed near the leading edge. This condition represented
the worst-case scenario in which laminar flow was
lost and turbulent flow existed over the airfoil
surface.
The results showed that the
new NLF airfoil, even with transition fixed near
the leading edge, achieved the same maximum lift
as the NASA low-speed airfoils. At the same time,
the NLF airfoil, with transition fixed, exhibited
no higher cruise drag than comparable turbulent-flow
airfoils. Thus, if the new NLF airfoil was used
on an aircraft where laminar flow was not achieved
(due to bug residue and other factors), nothing
was lost relative to the performance of the NASA
low-speed airfoils. If laminar flow was achieved,
however, a very substantial drag reduction would
result.
Because of the very successful
collaboration that occurred between Eppler and
Somers, the Eppler Airfoil Design and Analysis
Code has become one of the most widely used airfoil
codes in the world.
In the mid-1980s, Cessna and
Langley conducted exploratory flight and wind-tunnel
tests of a modified high-wing Cessna T-210 Centurion
to determine the aerodynamic and flight characteristics
of a new NLF wing for the Centurion. The new wing
incorporated a smooth composite upper surface and
had an NLF(1)-0414 airfoil. At Langley, the aircraft
was subjected to the sublimating chemical laminar
flow visualization technique during flight tests;
extensive laminar flow was detected over most of
the upper wing surface. Following the flight test
program, the aircraft was mounted in the Langley
30- by 60-Foot Tunnel where aerodynamic forces
and moments, performance, and controllability were
measured by a Langley and Cessna team led by Daniel
G. Murri and Frank L. Jordan, Jr. Sublimating chemicals
were again used for correlation with flight results.
The detection of large amounts of natural laminar
flow and the correlation of design tools with flight
results resulted in considerable excitement within
the research community at Langley and Cessna.
Chemical sublimation patterns
on wing of modified Cessna Centurion showing large
areas of laminar flow
at 125 knots in NASA-Cessna flight tests. Note
wedges in pattern caused by intentional trips.
The double-pronged approach
of complementary ground-based airfoil research
and flight demonstrations by Langley was extremely
valuable. Almost certainly, neither approach would
have been as successful alone. Together, they had
convinced the industry that the design process
could be used to develop NLF airfoils that could
sustain laminar flow in flight.
One of the first applications
of the new natural laminar flow airfoils to business
jets was by Cessna for the Citation II-S2. Cessna
later incorporated an NLF(1)-0414 (modified) airfoil
in its Citation Jet series, which was announced
at the National Business Aircraft Association (NBAA)
convention in 1989. These aircraft types both exhibited
about a 10- to 20-percent improved range as a result
of NLF. Other applications of the natural laminar
flow technology included the Swearingen SX 300
(NLF(1)-0416); the Lancair 320/360 (NLF(1)-0215F);
the Prescott Pusher (NLF(1)-0215F); and the Mooney
301 (NLF(1)-0315).
Postflight inspection of
Centurion still displays extent of laminar flow
exhibited in-flight at test condition.
Following highly successful
flight test program, Centurion was tested in Langley
30- by 60-Foot Tunnel in 1985.
Cessna Citation Jet with
natural laminar flow airfoil.
Although NLF airfoils have
become widely used in the homebuilt and certified
personal aircraft segments, the primary roadblock
in their adaptation to the higher speed end of
the general aviation spectrum was their incompatibility
with current deice systems, which permit ice to
build on the leading edge before breaking it free
with inflatable boots (typically used by the propeller-driven
aircraft family), or anti-ice systems, which prevent
the formation of ice with heated leading edges
(typically used by jet aircraft). Achieving laminar
flow across the junction aft of a leading-edge
deice device currently requires an enormous amount
of postproduction work. Because this junction is
typically at 5- to 15-percent chord, achieving
laminar flow beyond this point was extremely difficult
in production. Thus, extensive use of laminar flow
technology for higher speed aircraft is hindered
by airframe ice protection technology. This NASA
research on NLF airfoils provided tolerance data
for surface imperfections for the mating of a deicing
leading edge to a production-painted metal or composite
wing surface. The challenge of laminar-compatible
ice protection systems for future aircraft was
largely solved by the AGATE design database developed
in wind-tunnel testing at WSU and Glenn Research
Center in the late 1990s. These data provide laminar
flow tolerance information for installing smooth
pneumatic boots as well as their alternatives (electroexpulsive
or heated leading edges).
Wing sweep represents an additional
physics challenge for NLF. On straight (unswept)
wings, the stability of laminar flow is relatively
greater than on wings with more than modest amounts
of sweep. The Langley NLF research produced a clearer
understanding of these limitations.
Large nose-down pitching moments
and the associated high-magnitude control-surface
hinge moments, in addition to trim drag, are another
concern for NLF-type airfoils. Similar to supercritical
airfoils, as previously mentioned, a large amount
of aft camber is required to achieve high maximum
lift on airfoils with extensive laminar flow. Some
industry teams believe that new NLF airfoils should
have little or no trailing-edge reflex. In their
opinions, future NLF airfoils will have to exhibit
much lower pitching moments, and the required maximum
lift will have to come from advanced, yet simple,
high-lift systems. The advent of the latest knowledge
and computer design tools provides the means for
designers to tailor the desired pitching-moment
characteristics to meet their unique requirements.
The Fall and Resurgence of General
Aviation
In 1978, piston aircraft manufacturers
in the United States had a record year, with shipments
of 14,398 airplanes. It had been the best year
for general aviation in the history of the business,
and few had anticipated the dramatic downturn in
the light aircraft industry during the 1980s. Troubled
by product liability lawsuits, poor management
decisions (especially regarding technology readiness),
and a stale economy, the general aviation industry
suddenly experienced a free-fall drop in sales.
By 1986, Cessna stopped making single-engine personal-owner
planes altogether, and Piper filed for Chapter
11 bankruptcy. In 1990, the industry only shipped
608 airplanes and it appeared that the general
aviation industry would disappear from the domestic
scene. The demise of the general aviation industry
curtailed any interest in advanced technology,
and applications of the emerging NASA low- and
medium-speed airfoil technology in the 1980s and
early 1990s were few and far between.
In the early 1990s, an upswing
in the business jet sector brought some relief
to the legacy industry members such as Beechcraft
(now Raytheon Aircraft) and Cessna. At the same
time, however, two factors began to reshape the
national interest in technology for light personal-owner
aircraft. The first important factor was the emergence
of a new breed of small, enthusiastic companies
that offered the interested private pilot a range
of relatively low-cost, advanced aircraft in the
form of kits or fabricated designs. Advanced technologies,
such as composite construction and advanced aerodynamics,
became strong selling points and led to numerous
evaluations of new wing and airfoil designs. The
second major factor that stimulated technology
insertion in the 1990s was the implementation of
the NASA Advanced General Aviation Transport Experiment
(AGATE) Program, which recognized the potential
of the general aviation aircraft industry to relieve
the impending saturation of the major air carrier
transportation system. This revolutionary program
rapidly accelerated the transfer and implementation
of NASA-developed and NASA-sponsored research and
technology.
Cirrus SR20.
NASA’s Columbia 300
research aircraft showing shape of natural laminar
flow airfoil used by the wing.
Because of these two stimuli,
advanced airfoil technology has again become of
widespread interest, and the emerging stable of
new aircraft incorporates many of the airfoils
and design tools produced by the Langley program
of the 1970s. Typical of these advanced aircraft
are the Stoddard-Hamilton Glassair III kit, which
uses the LS(1)-0413 airfoil; the Lancair Columbia
300, which uses a natural laminar flow airfoil;
and the Cirrus SR22, which also utilizes a natural
laminar flow section. Each of these applications
has demonstrated performance gains because of laminar
flow. In January 2001, the Langley Research Center
acquired production versions of the Lancair Columbia
and the Cirrus SR22 for research studies in its
follow-on program to AGATE, known as the Small
Aircraft Transportation System (SATS) Program.
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