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Background
Despite decades of research
and development on spin and spin-recovery characteristics,
stall/spin accidents continued to plague the military
and civil communities up to the 1970s. In the 1970s,
however, two concepts suddenly dominated research
activities and resulted in dramatic improvements
in the stall/spin behavior of aircraft configurations.
One engineering concept was the technical approach
of using emerging advanced flight control systems
for automatic spin prevention and spin recovery.
For years automatic flight control systems could
recognize the loss of control and incipient-spin
conditions more quickly than the human pilot and
could apply corrective controls before the aircraft
could enter a developed spin. In fact, if the control
loops were tight enough, the control system could
be tuned to prevent the incipient spins; this would
provide carefree maneuvers and flight operations
for the pilot. This concept was particularly appealing
for advanced military aircraft configurations,
which were frequently flown in the hazardous high-angle-of-attack
environment. Unfortunately, the flight control
systems used prior to the 1970s did not utilize
the flight parameters necessary for automatic spin
prevention. If a unique auxiliary spin-prevention
system had been implemented during that time period,
it would have operated very infrequently, and the
probability of failure or maintenance problems
were major issues that blocked the implementation
of the concept. However, in the 1970s, flight control
systems of advanced military aircraft began using
feedback from virtually all flight parameters;
this permitted the design and integration of automatic
spin-prevention systems into the normal flight
control system. Such systems have had a profound
beneficial impact on current military aircraft
and significantly improved the flying qualities
of high-performance aircraft at high angles of
attack and spin resistance, as well as avoiding
the loss of pilot lives and the cost of aircraft
destroyed in accidents.
The second engineering concept
that emerged in the 1970s involved a change of
emphasis in stall/spin research for personal-owner
civil aircraft. Because most stall/spin accidents
for this class of aircraft occurred at low altitudes,
where the altitude was insufficient to even obtain
a developed spin before ground impact, it became
obvious that the major research thrust should be
changed from an emphasis on the developed spin
and spin recovery to an emphasis on spin avoidance
and increased spin resistance. In other words,
the historical approach of concentrating on the
developed spin was finally recognized as working
the wrong end of the stall/spin problem. Thus,
Langley researchers involved in the General Aviation
Stall/Spin Program began to turn their efforts
toward concepts that might be utilized to achieve
these goals.
Langley Research and Development
Activities
Several approaches might be
used to increase the spin resistance of personal-owner
light aircraft. For example, commercial civil transports
have successfully used pilot stall-warning systems,
such as stick shakers, for many years to provide
an awareness of stall proximity. Some T-tail transports
have used automatic stick pushers to actively prevent
inadvertent stalls to avoid entry into potentially
dangerous deep-stall conditions. High-performance
military fighters successfully use complex control
system feedbacks and schedules which permit strenuous
maneuvers at high angles of attack. Another approach
to providing spin resistance was used by Weick
to design the spin-proof Ercoupe aircraft mentioned
in the previous section. His approach involved
restricted control surface deflections and limited
center-of-gravity travel. Finally, research prior
to the 1970s had indicated that the selection of
wing airfoils and wing stalling characteristics
had significant potential for improved spin resistance;
and several aircraft programs within the civil
sector indicated that canard-type configurations
could be designed to be inherently stall proof.
Each approach to improve the spin resistance of
an aircraft involves consideration and trade-offs
of various levels of complexity, cost, and compromise
in the performance and utility of the aircraft.
For a comprehensive discussion
of the details of Langley’s efforts in spin
resistance for civil aircraft (including extensive
references), the reader is referred to the excellent
paper by H. Paul Stough III and Daniel J. DiCarlo
listed in the bibliography section of this document.
Control System Concepts
Control system concepts for
increased spin resistance are very attractive for
personal-owner aircraft because pilots of this
class of vehicle are usually not as experienced
as professional commercial or business pilots.
Therefore, their ability to recognize and correct
for inadvertent stalls and spin entry (particularly
during disorientation) would be significantly enhanced
by automatic control systems. Unfortunately, relatively
inexpensive personal-owner aircraft cannot reasonably
be implemented with expensive, maintenance-intensive
control systems, especially concepts similar to
those used by military aircraft.
Researcher Dale Satran inspecting
full-scale powered model of AA-1 research aircraft
during
tests of automatic stall-prevention concepts in
Langley 30- by 60-Foot (Full-Scale) Tunnel.
Long Yip with full-scale
model of Rutan VariEze aircraft in Langley 30-
by 60-Foot Tunnel.
Note outer wing extended leading-edge-droop modifications
on aft main wing.
In the mid-1970s, Langley
researchers led by Eric C. Stewart and Dale R.
Satran participated in joint studies with academia
to develop and assess active control concepts that
might be suitable for personal-owner aircraft within
the cost and maintenance constraints associated
with this class of aircraft. Analytical studies,
piloted simulator investigations using a General
Aviation Cockpit Simulator at Langley, wind-tunnel
tests in the Langley 30- by 60-Foot (Full-Scale)
Tunnel, and flight investigations were conducted
in individual programs with Mississippi State University
and Texas A&M University to assess stall-deterrent
systems that used angle-of-attack sensors and automatic
longitudinal control concepts. Although the results
of the research studies indicated that such automatic
control concepts were extremely effective in the
prevention of stalls, the relative cost, maintenance,
and certification issues limited interest in this
approach to spin resistance.
Canard Configurations
It has long been recognized
that aircraft with canard surfaces might be designed
for inherent (passive) stall and spin resistance.
For a typical canard configuration, the canard
tail surfaces are mounted forward on the fuselage
and are designed to stall before the aft-mounted
main wing. The mechanism of canard stall (and the
associated loss of canard lift and the effectiveness
of canard-mounted elevators) results in an inherent
limiting of angle of attack to values lower than
that required to stall the main wing. Langley’s
interest in pursuing the potential benefits of
canard configurations for spin resistance led to
a cooperative study with noted aircraft designer
Burt Rutan to obtain detailed aerodynamic, performance,
and stability and control characteristics of his
homebuilt VariEze canard configuration in the early
1980s. As a firm believer in the advantages of
canard-type aircraft, Rutan has embodied the concept
in most of his designs. The scope of this cooperative
study included wind-tunnel force and free-flight
studies of a subscale VariEze model and wind-tunnel
force, moment, and pressure studies of a full-scale
VariEze model. The program was initiated and managed
by Joseph R. Chambers and Joseph L. Johnson, Jr.,
and the Langley 30- by 60-Foot (Full-Scale) Tunnel
was the site of the investigations. Key Langley
researchers in the studies included Long P. Yip,
Dale R. Satran, and Paul F. Coy.
A full-scale VariEze aircraft
was fabricated from a commercial homebuilt kit
by the Langley fabrication shops and prepared for
testing in the 30- by 60-Foot Tunnel. Extensive
aerodynamic measurements, pressure instrumentation,
and flow visualization studies provided data to
help quantify the stallproof character of the VariEze
configuration. The thick high-lift airfoil of the
unswept canard surface stalled well before the
swept aft wing. Augmented by free-flying model
tests in the 30- by 60-Foot Tunnel, the information
gathered in the joint program has provided a broad
database for the understanding, engineering analysis,
and design of advanced canard configurations. One
of many highlights of this research program was
an assessment of the effects of a discontinuous
wing leading-edge droop on the outer main wing
of the VariEze. The outer wing droop eliminated
tip stalling of the main wing at extremely high
angles of attack; thereby large-amplitude wing-rocking
motions of the configuration were eliminated for
centers of gravity beyond the aft limit. The discontinuous-droop
concept was a key factor in other Langley research
projects on wing design for increased spin resistance
as discussed in the next section.
Free-flight model of VariEze
aircraft undergoing flight tests to evaluate stall
resistance.
The database provided by other
Langley studies of canard civil configurations
included a wind-tunnel study of the potentially
degrading effects of power for a tractor propeller
canard configuration and the attributes of “Three-Surface”
configurations that use a forward-mounted canard
as well as a conventional aft-mounted tail.
Spin-Resistant Wing Design
The fact that the aerodynamic
characteristics and stalling behavior of the typically
unswept wings of personal-owner aircraft often
dominate the spin resistance of these configurations
has been well-known for many years. Certain stalling
characteristics (especially abrupt leading-edge
flow separation) produce sudden, asymmetric wing
drop and highly autorotative rolling moments, which
can result in rapid rolling and yawing motions
that precipitate spin entry. Wing leading-edge
devices such as slots, slats, and flaps can significantly
improve the autorotative resistance of unswept
wings at stall, and early research at Langley by
the NACA demonstrated the effectiveness of these
devices. However, many of these devices proved
to be impractical because of complexity, maintenance
requirements, cost, and degradation of aerodynamic
cruise performance.
In the late 1970s, NASA researchers
at the Ames and Langley Research Centers began
to reassess the effectiveness of various leading-edge
devices on stall control for unswept wings. Initial
cooperative efforts by T. W. Feistel of Ames and
R. A. Kroeger of the University of Michigan were
directed at avoiding the abrupt and precipitous
drop in lift curve associated with relatively small
increases in angle of attack above stall displayed
by wing configurations that were prone to autorotate.
As a goal, their efforts involved the use of separate
leading-edge slat segments to control the shape
of the lift curve, eliminate the sudden drop in
lift curve at stall, and produce a “flat-top”
lift-curve shape to angles of attack far beyond
the stall. These initial efforts proved very promising.
The results indicated that, with auxiliary slats
on the inner and outer wing segments (no slat on
the middle wing section), the shape of the lift
curve for rectangular wings representative of those
used by general aviation aircraft was essentially
flat to an angle of attack of approximately 32∞—far
in excess of values that were believed to be adequate
for spin resistance. The value of maximum lift
obtained was about the same as for the unmodified
wing, but the flat top of the lift curve indicated
that favorable, more benign stalling characteristics
would be expected. In addition, the effectiveness
of conventional ailerons was noted to be significantly
improved with the leading-edge modifications.
Inspired by these fundamental
studies, Langley researchers under the direction
of Joseph R. Chambers undertook studies to more
fully explore the impact of various leading-edge
modifications on aerodynamics and to extend the
studies to explore the impact on autorotative characteristics
and aircraft stall/spin behavior. The scope of
these initial tests in 1977 consisted of static
and dynamic wind-tunnel tests of a subscale wind-tunnel
model of the NASA AA-1 experimental research aircraft
used in the general aviation stall/spin program
as discussed in the previous section “Spin
Technology.” Sanger Burk led the first wind-tunnel
tests to develop wing configurations that attempted
to provide the flat-top lift-curve characteristic
displayed by the Ames and University of Michigan
studies. In collaboration with Chambers and his
assistant, Joseph L. Johnson, Jr., Burk examined
a series of leading-edge modifications, including
a “discontinuous” leading-edge configuration
in which the airfoil of the outer wing panel was
extended and drooped. The Langley team projected
that this obligation would have a minimal impact
on the cruising performance of the wing and might
be a more acceptable modification if it improved
stalling characteristics and increased spin resistance.
During the initial test program
for the discontinuous leading-edge modification,
Burk reported difficulty in achieving a flat-top
lift curve. Instead, he obtained data showing a
lift curve that exhibited a first break in linearity
at stall, followed by an increasing lift-curve
slope with increasing angle of attack to extreme
angles of attack well beyond stall—on the
order of 40∞. After examining these remarkable
data and associated flow visualization results,
the Langley team realized that the unique lift-curve
variation was indicative of a wing stall progression
that started at the trailing edge of the midspan
position and progressed forward as angle of attack
was increased to stall (the first stall break in
Burk’s data). However, the increase in lift-curve
slope beyond stall was caused by the fact that
the outer wing panel continued to produce lift
to extreme angles of attack, as would be expected
from a low-aspect-ratio (about 1) unswept wing.
Using flow visualization tests, Burk was able to
show that the leading-edge discontinuity produced
vortical flow that prevented the low-energy stalled
flow of the inner wing from progressing spanwise
and stalling the outer wing. Thus, the discontinuity
worked as an aerodynamic fence to prevent outer
panel stall. When the discontinuity was eliminated
with a fairing, the lift curve exhibited by the
model reverted to the sudden, undesirable break
displayed by the baseline unmodified configuration.
Armed with these extremely
promising results, Burk and technician David B.
Robelen used an existing 1/5-scale radio-controlled
model of the AA-1 aircraft in early 1978 in the
first flight tests to evaluate the impact of the
discontinuous leading edge on spin resistance.
During these radio-controlled model flight tests,
the basic unmodified configuration easily entered
spins following deliberate prospin control inputs.
With the discontinuous outboard leading-edge modification,
the spin resistance of the model was significantly
improved. The model exhibited only a very slow
steep rotation from which recovery could be achieved
immediately by removing prospin control inputs.
Following additional exploration
with the radio-controlled model, the Langley researchers
were ready for full-scale flight test validation
and assessments by Langley test pilots. When high-priority
approval for the proposed flight test program was
given by then Division Chief Robert O. Shade, the
Langley fabrication shops completed (in a period
of only about a week) a wood and fiberglass leading-edge
modification for the full-scale aircraft, NASA
501, which was concurrently undergoing spin technology
testing at the NASA Wallops Flight Facility. A
project team that was led by engineer Daniel J.
DiCarlo and included H. Paul Stough III, Langley
Chief Test Pilot James M. Patton, Jr., and research
pilot Philip W. Brown directed the tests at Wallops.
Initial research flights of the modified aircraft
by Patton on June 6 and 7, 1978, validated the
results previously obtained with the radio-controlled
model. The marked improvement in the airplane stall/spin
characteristics with the leading-edge modification
correlated extremely well with the model results.
Subsequent flight tests of the aircraft with the
discontinuity faired over indicated that the improved
spin resistance provided by the modification had
disappeared; this showed that the discontinuity
was a key feature of the modification and also
in agreement with the results of the model tests.
Sketch of discontinuous
outer wing
leading-edge droop on AA-1 configuration.
The very positive results
of these initial tests resulted in a complete shift
in emphasis of the Langley General Aviation Stall/Spin
Research Program from the developed spin and spin
recovery to the topic of spin resistance and the
evaluation of wing configurations that significantly
enhanced aircraft characteristics. The scope of
full-scale aircraft configurations that had been
included in the original Langley program proved
to be invaluable for this research on spin resistant
wings. The AA-1 configuration used for the early
tests incorporated a rectangular (untapered) untwisted
wing. Langley’s other research aircraft included
a modified Beech C-23, which incorporated a rectangular
twisted wing; a modified Piper PA-28 T-tail aircraft
with a tapered twisted wing; and a Cessna C-172
high-wing configuration with a tapered twisted
wing. The availability of these flight-test aircraft
provided Langley researchers with a broad range
of configuration variables for the wing studies.
Summary of results for spin
attempts for four NASA research aircraft.
Cessna 172 research aircraft
with outer wing leading-edge-droop modification.
Aircraft models ranging from
subscale to full scale were tested in both static
and dynamic flight conditions in the Langley 20-Foot
Vertical Spin Tunnel, the Langley 30- by 60-Foot
(Full-Scale) Tunnel, the Langley 12-Foot Low-Speed
Tunnel, and the Glenn L. Martin Tunnel at the University
of Maryland. Rotary-balance testing and radio-controlled
model tests rounded out this unique set of facilities
and research tools for the task at hand. Throughout
the 1980s, Langley researchers conducted extensive
research on the geometric variables involved in
the discontinuous leading-edge concept, and a detailed
database was developed to define the most effective
location of the leading-edge discontinuity, the
impact of airfoil variations, and other key geometric
features. Unique flow visualization tests using
fluorescent light techniques in the Glenn L. Martin
Tunnel provided considerable insight into the flow
mechanisms involved in the stalling behavior of
the aircraft, and an overall approach to design
assessments of the lift-curve variations produced
by wing leading-edge modifications was developed.
Throughout this Langley research
effort, consistent results were obtained regarding
the impact of the discontinuous wing leading-edge
modification on spin resistance. Tested on a wide
range of configurations, the concept was truly
effective in increasing spin resistance with a
minimal impact on aircraft cost, performance, or
other key factors. One of the most impressive measurements
of the effectiveness of this wing modification
on spin resistance was obtained by examining the
frequency of spin entry following the intentional
application of prospin control inputs by the pilot
for each of the four NASA research aircraft. The
basic airplanes entered spins in 59 to 98 percent
of the intentional spin-entry attempts, whereas
the modified aircraft entered spins in only 5 percent
of the attempts and required prolonged, aggravated
control inputs or out-of-limit loadings to promote
spin entry. These impressive results are indicative
of the powerful influence that wing aerodynamics
can have on the spin resistance of personal-owner
aircraft, and they offer considerable promise that
simple, inexpensive wing designs can significantly
improve the safety of this class of aircraft.
Applications
The international leadership
of the NASA Langley Research Center in the area
of spin resistance has produced contributions that
have been widely utilized within the military and
civil aircraft sectors. Langley’s contributions
to military aircraft of the 1990s are documented
in NASA SP-2000-4519 Partners in Freedom. Because
spinning is not a major concern for commercial
civil aircraft, the industry approach of providing
adequate stall warning and (sometimes) active angle-of-attack
limiting has proven to be satisfactory and very
successful. Thus, few technical interactions have
occurred between Langley and the commercial transport
industry in this area. However, the continuing
national effort to reduce the number of accidents
and fatalities due to inadvertent spin entries
for personal-owner aircraft has resulted in extensive
Langley and industry cooperative interests and
assessments of Langley-developed technology.
Although Langley’s research
activities and sponsored studies indicated that
it might be possible to limit the angle of attack
of personal-owner aircraft to values below stall,
thereby avoiding inadvertent spin accidents, the
issues of cost, complexity, and maintainability
presented formidable barriers to the implementation
of this technology. As a result, none of the personal-owner
aircraft of the 1990s incorporated the active controls
approach to providing increased spin resistance.
Aerodynamic data provided
by Langley research on canard configurations represent
a significant design resource for industry. The
relative lack of popularity of canard-type aircraft,
because of other considerations, has limited the
applications of this particular approach to spin
resistance at the present time. This experience
emphasizes that the ultimate application of technology
depends on a broad spectrum of user requirements
(i.e., performance, cost) beyond safety issues
such as spin resistance.
Unquestionably, the most important
contribution of the Langley Research Center in
the area of spin resistance for civil aircraft
has been the development and demonstration of the
discontinuous wing leading-edge droop. During the
course of NASA research studies, daily communications
with interested industry observers were commonplace,
and the flight-test studies conducted within the
NASA program were especially effective in demonstrating
firsthand the improved aircraft characteristics
noted with the wing modifications. For example,
following the first significant flight tests of
the modified AA-1 research aircraft in 1978, Grumman
American Aviation Corporation personnel and a test
pilot conducted flight tests of the modified aircraft
at Wallops. In 1982, industry flight evaluations
of the NASA PA-28 with the wing modification were
performed by Piper. Cessna and Beech also conducted
flight evaluations of the same aircraft in 1983.
In addition to the dissemination of results to
the industry via company visits, cooperative projects,
and technical symposia, Langley ensured that this
information was provided to other organizations,
such as the FAA, the homebuilt aircraft community,
and emerging aircraft companies. In 1983, the FAA
Kansas City Office visited Wallops and participated
in an assessment of the modified PA-28 with a view
toward certification requirements.
Industry applications of the
spin-resistance technology developed by Langley
immediately faced a challenge because of the lack
of FAA certification requirements for spin-resistant
aircraft. At the time Langley initiated its research
program, the stall/spin certification standards
for personal-owner aircraft considered two types
of aircraft spin behavior for aircraft in the so-called
Normal Category (nonaerobatic). Specifically, the
stall/spin certification requirements had been
defined for either a spinproof aircraft (characteristically
incapable of spinning) or aircraft capable of recovery
from a one-turn spin. The provision for spinproof
aircraft had been essentially unused by industry
because the absolute nature of the regulation made
compliance a very lengthy and technically difficult
process. On the other hand, compliance with the
one-turn spin and recovery forces the aircraft
configuration to be spinnable. Thus, regulations
had not been included in the certification procedures
to provide manufacturers with an incentive to develop
a spin-resistant aircraft.
In reaction to a continuing
concern over stall/spin accidents, in October 1981
the General Aviation Manufacturers Association
(GAMA) hosted a workshop on General Aviation Stall/Spins
that highlighted the need for certification requirements
that would promote the development of aircraft
with spin-resistant characteristics. After the
workshop, in 1982 the GAMA proposed to the FAA
that a new certification category be developed
for spin-resistant aircraft. However, before such
a regulation could become effective, the FAA required
the formulation of specific criteria. Langley researchers
led by Stough, DiCarlo, Patton, and Brown and others
participated in joint flight tests and analysis
using NASA’s research aircraft, that formulated
spin-resistance criteria in cooperation with industry
and FAA partners. GAMA subsequently used these
data as the basis for its proposed spin-resistance
certification standards that were submitted to
the FAA on May 2, 1985. FAA representatives who
had experienced the remarkable characteristics
displayed by the modified NASA research aircraft
were key participants in the development of these
criteria, and they championed the development and
acceptance of the proposal by the FAA. Subsequently,
the new regulation emerged from an extensive review
process as Amendment 23-42 to the Federal Aviation
Regulations (FAR) Part 23 dated February 4, 1991,
which officially incorporated criteria to allow
for spin-resistance certification.
Initial efforts to apply the
discontinuous wing leading-edge concept were undertaken
by several emerging general aviation companies.
Under the leadership of Joseph L. Johnson, Jr.,
Langley responded to numerous proposals from these
companies for cooperative studies of the application
of the concept. Research efforts at Langley were
led by Long P. Yip, Holly M. Ross, and David B.
Robelen. In addition to the Rutan and Langley VariEze
application discussed earlier, several new aircraft
configurations incorporated the concept. Unfortunately,
for other reasons, many of these aircraft never
progressed to flight certification and production.
One of the first NASA and industry cooperative
programs conducted during the mid-1980s focused
on a radical new high-wing, canard, turboprop pusher
configuration known as the OMAC Laser 300. Long
P. Yip led a NASA and OMAC test team during wind-tunnel
tests in the Langley 12-Foot Low-Speed Tunnel to
assess the overall stability and control characteristics
of the configuration, with emphasis on high-angle-of-attack
characteristics and stall/spin resistance. The
results of the tests indicated that the configuration
would have unacceptable longitudinal stability
at high angles of attack, and an extension to the
wing trailing-edge flap was designed to minimize
this problem. The discontinuous leading-edge droop
installed on the outer portion of the main wing
also benefited longitudinal stability and kept
the flow attached in the region for angles of attack
up to about 35∞. In addition to the spin
resistance provided by the droop concept, the canard
configuration provided a nose-down pitching moment
at stall, enhancing the stall resistance of the
aircraft. Although a prototype of the aircraft
was flown, the Laser 300 was never certified or
produced.
OMAC Laser 300 prototype
in flight.
Model of OMAC 300 in Langley
12-Foot Low-Speed Tunnel.
Note leading-edge droop and extended chord of wing
trailing-edge flap.
The DeVore 100 Sunbird aircraft,
a two-place, high-wing, single-engine pusher configuration,
was designed with the droop concept. Cooperative
DeVore-Langley wind-tunnel and radio-controlled
model tests indicated that an abrupt, uncontrollable
roll departure at stall was eliminated by the droop
and that the modified configuration would exhibit
extreme spin resistance. A prototype of the Sunbird
aircraft was first flown in October 1987. Unfortunately,
the Sunbird aircraft did not enter production.
The Questair Venture, a low-wing,
tractor-propeller, kit-built aircraft incorporated
the discontinuous droop concept as a result of
cooperative wind-tunnel and radio-controlled model
tests with Langley in 1987. Designed as a relatively
short-coupled, high-aspect-ratio aircraft with
emphasis on high cruise speeds, the Venture incorporated
an NACA five-digit airfoil that was expected to
have poor stalling characteristics.
As a result of several cooperative
studies with Langley (involving graduate students
onsite at Langley), the aeronautical engineering
staff of the North Carolina State University (NCSU)
was aware of Langley’s discontinuous outer-wing-droop
concept and brought the concept to the attention
of Questair with a proposal to form a cooperative
Langley, NCSU, and Questair team to develop and
assess the discontinuous droop concept for the
Venture aircraft. Yip and Ross led the activities
at Langley, and John N. Perkins of NCSU and his
graduate students contributed to the cooperative
study.
The researchers faced two
technical challenges in the project. First, the
Venture incorporated a high-aspect-ratio wing (10.4),
which was expected to exhibit different stall progression
characteristics than those exhibited by the lower-aspect-ratio
wings (about 7.0) previously involved in Langley’s
research. This feature would probably require a
different leading-edge-droop configuration to control
stall progression. The second challenge was created
by the fact that the design of the Venture was
focused on high-speed capability. Thus, any modification
to the wing had to result in a minimal impact on
aerodynamic performance. Yip and Ross found that
a single leading-edge-droop segment would not provide
the necessary spin resistance for the high-aspect-ratio
wing configuration. A Langley contractor, D. V.
Rao of ViGYAN, Inc., had conducted research on
a new wing-slot stall-control concept for high-aspect-ratio
wings, and Yip and Ross included the concept in
their study. The team subsequently found that the
combination of leading-edge droop and wing slot
operated synergistically to provide significantly
more spin resistance than could have been obtained
with each individual concept for this particular
wing design; the Venture incorporated a single
outboard-droop segment together with a small slot
for spin resistance. The challenge of minimizing
performance penalties due to wing modifications
for spin resistance had been previously addressed
by Pat King, a Langley graduate student, who used
the Eppler airfoil design code in an optimization
study to design wing-droop shapes with minimal
impact on aircraft drag. D. Bruce Owens, an NCSU
graduate student, applied the technique to the
Questair wing and developed an appropriate droop
shape.
Long Yip and David Robelen
prepare radio-controlled model of DeVore Sunbird
for flight tests.
Note discontinuous leading-edge-droop segments
on outer wing.
The basic Venture wing was
expected to exhibit unpredictable and abrupt stall
characteristics, and the original prototype aircraft
displayed unsatisfactory stall behavior. The pilot
for this aircraft reported unpredictable roll offs
at stall and generally unacceptable characteristics.
When the wing-droop–slot modification was
incorporated, however, the aircraft exhibited a
gentle, very controllable stall with no tendency
for wing drop. In carefully controlled performance
tests, the penalty in cruise performance was found
to be imperceptible—about 1 knot. Lateral
control was shown to be effective throughout the
entire stall maneuver, even with full elevator
deflection. The Questair Venture was subsequently
produced and sold in kit form.
Model of Questair Venture
aircraft in Langley 12-Foot Low-Speed
Tunnel during tests to develop spin-resistant wing
configuration.
The Schweizer SGM 2-37A motorglider,
which first flew in 1986, incorporated two spanwise
segments of wing leading-edge droop to improve
stall characteristics. The development of this
unique wing modification was stimulated by cooperative
studies of Langley and the University of Maryland
to further explore the stall progression and spin
resistance of high-aspect-ratio wings with discontinuous
leading-edge modifications. With an unusually high
aspect ratio of 19, this aircraft required multiple-droop
segments, as predicted, based on oil flow studies
in the Glenn L. Martin Tunnel at University of
Maryland.
As part of a cooperative research
program between the Langley Research Center and
the Smith Aircraft Corporation, wind-tunnel tests
involving the discontinuous wing leading-edge droop
were performed on a 1/6-scale model of a proposed
general aviation trainer configuration in the Langley
12-Foot Low-Speed Tunnel. Although the full-scale
aircraft program never proceeded into certification
or production, this activity is noteworthy because
of the innovative application of the discontinuous-droop
concept. One focus of the aircraft development
program was to develop wing leading-edge modifications
that would tailor the stall/spin characteristics
of the aircraft. The configuration was designed
to be a trainer aircraft with two different training
roles. The first role was to provide an aircraft
in which a student pilot could learn spin-entry
and spin-recovery techniques. The second training
role was to provide a spin-resistant aircraft that
could be safely flown by student pilots without
fear of inadvertent spins. It was thought that
the two very different types of training could
be accomplished with one aircraft design by modifying
the wing leading edges differently to alter high-angle-of-attack
characteristics. The leading-edge modification
for the spinnable version would be used to provide
a more gentle, controllable stall without allowing
the aircraft to attain too high an angle of attack,
which could make entering a spin more difficult
and harder to recover from. For these reasons,
the leading-edge modification would need to be
relatively small and kept on the outboard wing
only. In contrast, the spin-resistant configuration
should have a leading-edge modification that protects
the outboard wing to very high angles of attack
to provide good roll damping past the stall. The
cooperative wind-tunnel test program identified
candidate leading-edge modifications for the trainer
configuration, but the aircraft program was canceled
before production.
Model of Smith Aviation
Corp. trainer in Langley 12-Foot Low-Speed Tunnel
with outer-wing leading-edge droops.
On June 14, 1994, the Advanced
Aerodynamics and Structures Jetcruzer 450, a single-engine,
pusher-propeller, canard, six-seat transport became
the first aircraft to receive FAA certification
as spin resistant. On October 23, 1998, the Lancair
Columbia 300 and the Cirrus SR20 advanced aircraft,
both of which employ discontinuous outboard-wing
leading-edge droop to enhance spin resistance,
received FAA certification using selected spin-resistance
certification requirements. Although neither aircraft
was certified as fully spin resistant, they both
exhibited an exceptional level of safety in the
stall/post-stall regime. Furthermore, the aircraft
were found to provide a definite increased level
of safety in safeguarding against loss of control
and low-altitude stall/spin accidents that have
been so prevalent in general aviation.
Cirrus SR20 with outboard
drop concept.
NASA Lancair 300 research
aircraft with outboard leading-edge droop.
Other applications of the
discontinuous-droop concept included a military
application for the U.S. Marine Corps Exdrone delta-wing
remotely piloted vehicle. As discussed in Partners
in Freedom, the incorporation of droop to the outboard
wing significantly improved the departure resistance
of the vehicle and greatly improved its operational
viability. This unique and unusual transfer of
technology from the general aviation community
to the military is extremely noteworthy.
After 20 years of research
and development, the extremely promising concept
of inherent spin resistance through a specific
approach to wing design has reached fruition and
applications. Hopefully, additional applications
and experiences in the future will validate the
potential benefits on safety and result in an attendant
reduction of fatal accidents in the general aviation
community.
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