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Background
The related subjects of stalling
and spinning have received the continuous attention
of aircraft designers throughout the history of
manned flight. High-performance military aircraft
must be capable of extended flight at high angles
of attack near or beyond stall during strenuous
maneuvers without unintentional loss of control
or unrecoverable spins. Commercial civil transports,
business jets, and general aviation aircraft must
exhibit a high degree of stability and controllability
for the low-speed, near-stall conditions associated
with landing and takeoff. In addition, both military
and civil aircraft must display satisfactory recovery
characteristics from inadvertent stalled conditions,
with no tendency to enter unrecoverable poststall
conditions such as an unrecoverable deep stall.
General aviation aircraft in the light, personal-owner
category must display benign, easily controlled
motions during stall maneuvers, with no tendency
to enter dangerous inadvertent spins, particularly
at low altitudes with insufficient height for recovery.
Stalls and spins involve complicated
balances between the aerodynamic and inertial forces
and moments acting on the vehicle. Unfortunately,
the complexities of separated aerodynamic flows
and the high dependence of poststall aerodynamics
on details of aircraft configurations result in
a formidable design problem. The prediction of
spin and spin-recovery characteristics has been
especially difficult during aircraft development
programs, and designers have been challenged since
the first recorded spin and spin recovery occurred
in 1912 when Lieutenant Porte of the British Royal
Navy intentionally spun and recovered his Avro
biplane.
ketch of position of aircraft
while descending
in relatively steep, developed spin to left.
The principal motions of stalling
and spinning involve four distinct phases of flight:
the approach to stall, the stall and incipient
spin, the developed spin, and spin recovery. During
the approach to stall, as airspeed is reduced and
angle of attack is increased, some aircraft may
exhibit large-amplitude rolling or pitching motions,
wing-dropping tendencies, unconventional or ineffective
responses to control inputs, or longitudinal or
directional instabilities. If the motions occur
in a rapid and disorienting matter, the pilot may
inadvertently lose control of the aircraft and
enter the incipient-spin phase. The incipient spin
may also be entered intentionally by the pilot
through applications of rudder-elevator-aileron
controls. In the incipient-spin phase, the flight
path of the aircraft changes from horizontal to
nearly vertical, the angle of attack increases
beyond the value at stall, and the rate of rotation
increases from zero to the rate exhibited in the
fully developed spin. If the loss of altitude is
to be kept to a minimum, the pilot must quickly
recognize the out-of-control situation and apply
corrective controls. If the pilot delays the inputs
or applies the wrong control combinations, the
aircraft may enter the developed-spin phase.
In the developed-spin phase,
the attitude, angles, and motions of the aircraft
tend to be repeatable from turn to turn, and the
flight path is approximately vertical. The spinning
motion is sustained by a balance of the aerodynamic
and inertial moments acting on the aircraft. The
spin consists of a spinning motion about the airplane
center of gravity plus translatory motion of the
center of gravity; however, the motion is primarily
rotary. The developed spin can be very smooth and
steady, or it may be quite oscillatory, violent,
and disorienting to the pilot. In addition, the
spin may be relatively steep, characterized by
a nose-down attitude, an angle of attack ranging
from slightly above the stall angle of attack to
about 30∞, and a relatively large spin radius
(distance of the center of gravity of the aircraft
from the spin axis). The developed spin may also
be fast and “flat,” with a relatively
horizontal fuselage attitude, an angle of attack
approaching 90∞, and the spin axis passing
almost vertically through the center of gravity
of the aircraft with a spin radius of near zero.
Spin recovery is accomplished
by upsetting the balance of aerodynamic and inertial
moments acting on the aircraft by movement of the
aerodynamic control surfaces. The specific control
inputs required for satisfactory spin recovery
for a particular aircraft depend on certain critical
aircraft mass and aerodynamic properties, and the
most effective control sequence varies for different
types of airplanes (for example, fighters and personal-owner
aircraft). Recoveries from steep spins tend to
be less difficult because the aircraft aerodynamic
controls retain a limited degree of effectiveness
slightly beyond the stall. However, recovery from
a flat spin is normally extremely difficult because
the aircraft controls are ineffective at such high
angles of attack.
Because unrecoverable spins
may be encountered during initial aircraft stall/spin
flight tests, spin test aircraft are commonly equipped
with emergency spin-recovery parachute systems,
which can be deployed to terminate the spinning
motion and reduce the aircraft angle of attack
to below stall conditions. The parachute is then
jettisoned by the pilot and conventional flight
resumed.
Unfortunately, when inadvertent
loss of control and spin entry occur at low altitudes,
the pilot may not have enough altitude to terminate
the poststall motions and complete the near-vertical
recovery maneuver before impacting the ground.
Sadly, this scenario occurs frequently in fatal
civil aircraft accidents for personal-owner aircraft
that are piloted by relatively inexperienced individuals,
with minimum exposure to out-of-control maneuvers
and spins.
From a technical perspective,
stalls and spins are very complicated because of
a single factor— aerodynamics. The aerodynamic
characteristics of most aircraft configurations
become extremely nonlinear and ill behaved at angles
of attack beyond stall. Thus, the prediction and
analysis of stall/spin behavior have not been amenable
to theoretical methods. After decades of experience
and national leadership in the area of stall/spin
technology, researchers at the Langley Research
Center have evolved a series of dynamically scaled
model test techniques to provide the information
required for the prediction of airplane spin and
spin-recovery characteristics. Historically, the
quality of correlation of results from dynamic
model tests with full-scale aircraft results has
been extremely good for military aircraft that
employ wings with relatively sharp leading edges
(which are relatively insensitive to Reynolds number
effects at high angles of attack), swept wings,
and/or fuselage heavy loadings. For these configurations,
isolated problems have arisen when fuselage cross-sectional
shapes have shown Reynolds number effects. Langley
has developed artificial model modifications to
correct for many of these effects for military
aircraft models.
Unfortunately, the quality
of correlation between dynamically scaled free-flight
models and full-scale personal-owner general aviation
aircraft has frequently been poor. For these configurations,
the relatively low Reynolds numbers involved in
model tests can sometimes produce erroneous predictions.
The fundamental aerodynamic problem is that the
relatively large radius of the wing leading edges
and the high-lift airfoils used for general aviation
aircraft tend to be extremely sensitive to Reynolds
number. In particular, the wing of a subscale model
at low Reynolds number will generally not produce
as much lift at high angles of attack, and it will
also stall at a lower angle of attack. Many times,
this effect results in the model predicting a more
docile, steeper spin than that exhibited by the
full-scale aircraft. Without corrections for this
Reynolds number phenomenon, the designer may be
surprised by a full-scale aircraft with unsatisfactory
characteristics that were not adequately predicted
by model tests. In summary, spin and spin-recovery
predictions for personal-owner aircraft based on
model tests should be approached with caution.
Langley Research and Development
Activities
NACA researchers at Langley
conducted some of the earliest studies of aircraft
stalling behavior with a Curtiss JN4H Jenny in
1919. In the 1930s, the pioneering efforts of Fred
E. Weick of Langley were directed toward providing
a safe, easy-to-fly design that would be free of
the dangers associated with stalling and spinning.
Weick and his associates developed the highly successful
W-1A experimental airplane, which exhibited radically
improved stalling characteristics compared with
other aircraft of the day. Weick later designed
the famous Ercoupe aircraft, which utilized limited
elevator travel, two-control operation, a nose-down
inclination to the thrust axis, and a carefully
tailored wing stall progression in such a manner
that the entire wing could not be stalled. The
configuration is still in existence today as a
popular antique, and an Ercoupe has never had a
spin accident.
World War II, and the national
focus on military aircraft, virtually eliminated
all NACA research on stall/spin characteristics
of personal-owner type aircraft. Essentially all
NACA resources were devoted to providing support
to high-priority military aircraft development
programs, and Langley’s facilities, such
as the Langley Vertical Spin Tunnel, were busily
engaged in military studies. Hundreds of military
aircraft configurations were tested in the specialized
facilities at Langley; although the focus was not
on civil research, the potential for future applications
to civil aircraft of the data generated in the
military studies was recognized. After World War
II, the large influx of surplus aircraft into the
personal-owner marketplace resulted in an alarming
number of stall/spin accidents, largely because
of the inexperience of new pilots, the high wing
loadings of the aircraft, and aggravated roll instability
at the stall due to power effects on high-lift
systems. In this period, stall/spin accidents accounted
for over 48 percent of all fatal accidents. Interest
in improving the abysmal safety record inspired
new research efforts within NACA, with a particular
emphasis on stalling behavior and leading-edge
devices. In addition, researchers involved in spin
recovery research attempted to summarize and collate
the vast amount of data generated during the war
years in an attempt to provide design guidelines
for light aircraft. During the late 1950s, however,
the pace of research related to general aviation
in NACA slowed to the extent that no new breakthroughs
were forthcoming in the area of stall/spin technology.
In fact, research was virtually nonexistent, and
communications with industry and its needs were
extremely limited. The situation was further aggravated
by the fact that in the postwar years, military
aircraft configurations became markedly different
from general aviation configurations (for example,
wing airfoils, wing sweep, and mass loadings);
thereby, any meaningful transfer of military stall/spin
research results to the civil sector was prevented.
Piper PA-30 Twin Comanche
aircraft mounted in Langley 30- by 60-Foot
(Full-Scale) Tunnel for investigation of power-on
stall characteristics.
Research activities picked
up again in the early 1960s with a series of landmark
flight studies of the handling qualities of representative
single- and twin-engine general aviation aircraft
by the Dryden Flight Research Center. The flight
investigations identified poor stalling behavior
for several of the aircraft tested; this stimulated
follow-on wind-tunnel studies of a series of twin-engine
full-scale aircraft in the Langley 30- by 60-Foot
(Full-Scale) Tunnel. Lead Langley researchers for
these studies included Marvin P. Fink, Delma C.
Freeman, Jr., and James P. Shivers. Some of the
twin-engine aircraft had been found to exhibit
abrupt wing-drop tendencies at stall in flight.
The tunnel tests showed that this unsatisfactory
behavior was caused by asymmetric local upwash
caused by the conventional mode of propeller rotation
(clockwise viewed from the rear). When the propellers
were used in opposite modes (both rotating down
at the wingtip), the asymmetric stall was alleviated.
In addition to providing designers with key information,
these studies helped to provide a foothold for
general aviation research in the NASA aeronautics
program.
James Bowman, Jr., James
Patton, Jr., and Sanger Burk with low-wing spin
research aircraft along with radio-controlled model,
and spin-tunnel model of configuration.
The most progressive era of
NASA stall/spin research for general aviation configurations
was conceived and initiated by James S. Bowman,
Jr., James M. Patton, Jr., and Sanger M. Burk at
Langley in 1972. Together they conceived, planned,
and implemented a research program that was subsequently
augmented with numerous associates at the Langley
and Ames Research Centers. Initially, the program
focused on the validation and interpretation of
design guidelines for spin recovery, but a rapidly
growing research program quickly expanded the scope
of activities. Also, lines of communication with
the general aviation industry were established,
and a mutual trust was cultivated that permitted
practical research to be conducted despite a liability-sensitive
environment. In 1976, a workshop was held at Langley
to discuss and focus the NASA efforts as recommended
by industry, academia, and other government agencies.
Several NASA advisory committees and the General
Aviation Manufacturers Association (GAMA) advocated
for more NASA funding for the area. In response,
the resources were augmented, and for the next
6 years a concentrated effort was conducted at
Langley with support from Ames and academia. Over
100 technical papers were subsequently published
by NASA and its contractors and grantees to document
the results of the program, and a major workshop
was held at Langley in 1980 to disseminate the
results to the industry. The data transmitted included
advancements in the area of aerodynamics at high
angles of attack, factors affecting the spin and
spin recovery, stall/spin prevention concepts,
model/flight test procedures, emergency spin-recovery
systems, and analytical techniques.
In the early 1980s, the emphasis
on other NASA priorities and competition for limited
resources necessarily curtailed the research program.
NASA funding for general aviation stall/spin research
was sharply reduced, and the research activities
were reduced accordingly. Nonetheless, cooperative
research with industry has continued to the current
day and has resulted in progress in several areas,
especially for advanced unconventional designs
and the design of inherently spin resistant wings.
Langley 20-Foot Vertical Spin
Tunnel
Following the initial operations
of a 15-foot-diameter spin tunnel that became operational
in 1935, Langley designed and developed the Langley
20-Foot Vertical Spin Tunnel and initiated operations
in 1941. The Langley Spin Tunnel is a closed-throat,
annular return wind tunnel that operates at atmospheric
conditions. A fixed-pitch fan and drive motor located
above the test section are controlled by a system
that permits rapid changes in fan speed, which
results in rapid flow accelerations in the test
section. Models used in free-spinning tests are
designed and fabricated according to specific scaling
relationships (relative density and Froude number)
similar to some discussed in an earlier section
on aeroelasticity. To study spin characteristics,
the dynamically scaled free-flying models are hand
launched with prerotation into the vertically rising
airstream. The tunnel operator varies the tunnel
speed so that the spinning model remains in equilibrium
in the field of view of multiple video cameras
installed around the test section. Images of the
test are stored on a laser disk and later analyzed
by using a computer code that is capable of calculating
the six-degree-of-freedom position and attitude
of a model at a sample rate of 60 Hz. The resulting
motion time histories are then used for quantifying
spin modes as well as calibrating spin simulations.
Direct observation of the model is possible during
tunnel operations via panoramic control room windows.
The spin-recovery characteristics of the model
are studied by using remote actuation of the model
aerodynamic control surfaces. The size of emergency
spin-recovery parachutes for flight test aircraft
is also determined by using specialized tests of
scaled parachutes attached to the model.
Langley 20-Foot Vertical
Spin Tunnel and adjacent office building.
Cross-sectional view of
Langley Vertical Spin Tunnel.
In addition to free-spinning
tests, the facility permits the measurement of
aerodynamic forces and moments during spin conditions
with a unique rotary-balance apparatus. In this
testing technique, the model is attached to a strain-gauge
balance similar to those used for conventional
wind-tunnel tests; however, in this technique the
model is forced into continuous spinning motions
and the balance signals are transmitted via slip
rings to data reduction equipment. Data obtained
with this technique are used for theoretical studies
of spins, and the apparatus has also been used
to provide electronically scanned pressures on
models during spinning motions.
Typical model mounted on
rotary-balance apparatus for aerodynamic measurements
during simulated spin. Airflow is vertical from
bottom of picture to top.
The rotary-balance test technique
had been used in the Langley spin tunnels during
the 1930s and early 1940s; however, the pressures
of free-spinning tests in the Langley tunnel during
World War II precluded the use of this testing
capability and the test apparatus fell into disrepair.
With new resources to invigorate its program in
the 1970s, Langley contracted Bihrle Applied Research,
Inc., to refurbish and upgrade the testing capability.
The upgrade included state-of-the-art digital data
acquisition equipment and pressure measurement
instrumentation. Data obtained on the rotary balance
cannot be obtained in conventional static wind-tunnel
testing because, during a spin, each location on
the model is subjected to a different local flow
angle due to the rotational kinematics of the spin.
In addition, the trends of forces and moments with
variations in rotation rate are generally very
nonlinear for spin conditions. A new rig with significantly
enhanced capabilities became operational in the
tunnel in 1992.
Radio-Controlled Models
One of the thrusts of the
Langley General Aviation Spin Program was to develop
and evaluate a low-cost testing technique using
radio-controlled models for the prediction of stall/spin
characteristics. Such a technique would provide
industry, and interested individuals, with a method
that could be utilized in lieu of spin-tunnel tests
in the design stages of aircraft development activities.
As a nationally recognized expert in radio-controlled
model technology, Langley technician David B. Robelen
designed, fabricated, and flew several scale radio-controlled
models and correlated model flight results with
those obtained from spin tests of the corresponding
full-scale aircraft. Robelen led the development
of innovative, low-cost hobby-type instrumentation
that used sensors to permit measurement of control
positions, angle of attack, airspeed, angular rates,
and other variables. His efforts even included
the design and use (deployment and release) of
emergency spin-recovery parachute systems for models.
Through the use of special camera tracking equipment
and recorders, relatively sophisticated data were
generated in the model tests.
Emergency Spin-Recovery Parachutes
Parachutes have been commonly
installed on spin test aircraft for backup in the
event that an unrecoverable spin is encountered
during flight tests. The challenges involved in
the design and operation of parachute systems include
a determination of the minimum parachute size required
for satisfactory recovery, the parachute riser
line length, and the design of the mechanical parachute
deployment and release mechanisms. Prior to the
NASA program, a generally accepted approach to
system design for general aviation was not available,
and numerous fatal accidents had occurred during
spin tests of general aviation aircraft because
of improper parachute geometry, deployment, or
jettison characteristics. The parachute canopy
distance for emergency spin recovery is particularly
critical. If the distance from the aircraft to
the parachute (riser plus suspension line length)
is too short, the parachute will probably collapse
in the low-energy stalled wake of the aircraft
and have little effect on the spin. On the other
hand, if the length is too long, the parachute
will trail over to the spin axis, rotate with the
aircraft, and be ineffective in terminating the
spin. The range of information provided by the
NASA program also included design approaches for
the engineering of mechanical deployment/jettison
systems under the leadership of Charles F. Bradshaw.
Extensive testing by Langley researchers led by
Bowman, Burk, and Robelen using spin-tunnel model
tests, radio-controlled models, and full-scale
aircraft tests provided critical information on
the design of parachute systems. Langley test pilots
used the Langley-developed parachute system to
terminate unrecoverable spins 28 times on 4 different
aircraft during flight tests without failure or
incidents. This valuable, life-saving information
on emergency spin-recovery systems was quickly
transferred to the general aviation industry and
further disseminated to the public at seminars
and national meetings such as the annual Oshkosh
Convention.
Impact of parachute canopy
riser line length on spin recovery.
Pyrotechnically deployed
spin parachute developed in Langley program.
Flight Tests
The final, definitive answers
in any analysis of stall and spin behavior are
provided only by flight tests of the full-scale
aircraft. Obtaining these answers, however, involves
potentially hazardous conditions, requiring careful
planning, the provision of emergency spin-recovery
and pilot egress systems, and careful procedural
policies for success. Stall/spin flight tests have
traditionally been a high-risk area for general
aviation manufacturers, particularly in the area
of emergency systems. The NASA program, therefore,
included a concentrated effort to develop and refine
flight test technology.
Four spin research airplanes
shared the Langley flight test activities in the
program. These aircraft resembled production aircraft,
but they were extensively modified for research
with special instrumentation and mass loadings.
The modified research aircraft included a Grumman
American AA-1 Yankee, a Beechcraft C-23 Sundowner,
a Cessna 172 Skyhawk, and a Piper prototype T-Tail.
Over 2,500 spins were accomplished with the aircraft,
including about 8,000 spin turns. Each airplane
was equipped with wing booms that supported sensors
for measurements of angle of attack, angle of sideslip,
and airspeed. Motion-picture cameras were mounted
internally within the wing structure, coupled with
innovative mirrors used to photograph wool tufts
in visualizing the airflow over the wing surface.
In addition to conventional telemetry data, the
aircraft carried onboard instrumentation. Numerous
geometric modifications were made to each aircraft
to assess the effects of various components on
spin-recovery characteristics. Each aircraft was
equipped with a pyrotechnically deployed emergency
recovery parachute system. One of the airplanes
was outfitted with an innovative hydrogen-peroxide
thruster system, which utilized small thrusters
at the wingtips of the airplane to provide a selected
level of moments about the aircraft axes. The test
site for the flight studies was the NASA Wallops
Flight Center, located on Virginia’s Eastern
Shore. Close communication was maintained with
general aviation industry flight test teams regarding
the operational procedures and hardware used in
the Langley effort. Industry teams visited the
test site at Wallops and evaluated the overall
approach to testing. Many of the program elements,
such as parachute system design, have been implemented
and adopted for industry use.
Langley’s chief test
pilot, James Patton during flight test program.
Note symbols denoting successful operations of
spin-recovery parachute system.
Modified AA-1 Yankee research
aircraft in flight.
Tail Damping Power Factor
Following World War II, the
immediate focus in stall/spin technology for general
aviation aircraft was the developed spin and recovery
from the spin. The NACA researchers who labored
in the Spin Tunnel during the war years began to
collate and summarize data from about 100 model
tests that might be applicable to general aviation
aircraft. General results obtained in spin-tunnel
tests indicated that certain critical parameters
dominated the spin and recovery characteristics
for configurations similar to general aviation
designs. These parameters included the relative
density (aircraft density relative to air density),
mass distribution (with extremes of fuselage heavy
or wing heavy), and tail design. These data were
analyzed and used to develop conservative engineering
design guidelines for satisfactory spin characteristics.
The Langley data emphasized
that tail design was very important and that designers
should consider the tail geometry as a first-order
parameter from spin and recovery perspectives.
During spins, a dead-air region exists over much
of the vertical tail because of the stalled wake
of the horizontal tail. Therefore, to have good
rudder effectiveness for spin recovery, a substantial
amount of rudder area must be outside this dead-air
region, either above or below it. Also, a substantial
amount of fixed area should be beneath the horizontal
tail to retard, or dampen, the spinning motion.
These geometric properties were combined in a parameter
known as the tail damping power factor (TDPF),
which quantified the attributes of a specific tail
design through a mathematical equation that estimated
the measure of spin damping provided by the fixed
area beneath the horizontal tail and the control
power provided by the unshielded part of the rudder.
With the passage of time, these guidelines were
extrapolated and eventually evolved into a criterion
for satisfactory spin recovery based solely on
tail geometry; this approach provided the only
existing spin design information for many years.
Sketches of good tail design
(left) and
poor tail design (right) for spin recovery.
Some tail configurations
tested on radio-controlled model of modified AA-1
aircraft.
Several reports of general
aviation aircraft with unacceptable spinning behavior
were brought to the attention of Langley researchers.
These unsatisfactory aircraft had been designed
with the TDPF criterion, yet they displayed dangerous
characteristics. At the other extreme, several
cases were reported wherein aircraft did not meet
the TDPF design criteria yet they had extremely
good spin characteristics. In a national atmosphere
of liability issues and civil lawsuits, it was
imperative that NASA analyze the situation and
assess the validity of its tail-based TDPF criterion.
Accordingly, the initial effort of the Langley
program in the early 1970s was to determine the
validity of the criterion and the effects of other
configuration variables on spin and recovery.
A systematic series of studies
was conducted by Langley in which the tail surfaces
of a modified AA-1 Yankee aircraft were changed
to produce configurations predicted to have either
satisfactory or unsatisfactory behavior according
to the existing TDPF criterion. Four different
tail configurations were tested on spin-tunnel
models, radio-controlled models, and the actual
research airplane. The results of the study showed
that the criterion provided good engineering practice
for the design of tail features that were highly
desirable for spinning. However, the extrapolation
of this criterion to predict the spin-recovery
characteristics for a complete, specific configuration
using only a consideration of tail design could
result in erroneous predictions; this was in agreement
with the earlier reports of deficient predictions
by the general aviation community.
From a technical perspective,
this Langley investigation was a major contribution
because the so-called criterion was properly examined
and evaluated and designers were made aware of
the many configuration features that could overpower
the effects of tail configuration on spin characteristics.
As discussed in the next section of this document,
the effect of the wing was particularly noted to
be important. The effects of power, mass loading,
center-of-gravity position, and fuselage cross-sectional
shape were also determined. These effects were
so powerful that configurations with apparently
ideal tail configurations (such as T-tail designs)
could have unacceptable spin recovery. In addition
to the foregoing studies of aircraft configuration
effects, Langley test pilots examined the effect
of control inputs and recovery procedures for each
of the four full-scale research aircraft. Researchers
and the test pilots participated in numerous workshops,
training symposia, and formal meetings with the
civil pilot community to disseminate their findings
with regard to recovery procedures and the human
factors involved in spinning. This exchange with
private pilots has been extremely important and
beneficial, especially in view of the current lack
of formal requirements for spin training for pilots
of general aviation aircraft.
Applications
The results of Langley’s
research on spin and spin-recovery characteristics
of general aviation aircraft have formed a fundamental
core of knowledge that has been used throughout
the industry in the general awareness and approach
to designing for satisfactory behavior. By providing
data, operational experiences, and critical assessments
of various test techniques to general aviation
manufacturers, home-built aviation enthusiasts,
and private pilots, Langley’s efforts have
made substantial contributions to the safety and
well-being of this sector of the civil aviation
community. The contributions continue to this day,
including applications by industry for the latest
emerging generation of advanced personal-owner
aircraft.
In 1974, Langley and Piper
Aircraft Corporation conducted a joint assessment
of the use of radio-controlled models for stall/spin
evaluation of general aviation aircraft, and in
1975 Langley and the Beech Aircraft Corporation
teamed for a joint program to assess the accuracy
of radio-controlled model tests for the YT-34C
aircraft, which was then being used as a spin trainer
by the U.S. Navy. Spin tests of the configuration
had previously been made in the spin tunnel with
a 1/15-scale model, and over 1,200 spins had been
made with two prototype aircraft. These tests provided
a large database for comparison with the radio-controlled
model results. Langley’s contributions to
the YT-34C program had been made earlier when spin-tunnel
tests indicated that the original aircraft configuration
would have a very flat, unrecoverable spin mode.
This result led to YT-34C modifications prior to
the first aircraft spin test. After flight and
spin-tunnel tests, the final YT-34C configuration
was modified to include dual ventral fins and long
triangular strakes on the aft fuselage, forward
of the horizontal tail. With this configuration,
the YT-34C provides exceptional spin characteristics.
Unfortunately, the results of the YT-34C radio-controlled
model tests were viewed with concern. The model
spin-recovery characteristics correlated very well
with the aircraft results when the spin modes of
the model and aircraft were similar. However, the
radio-controlled model did not develop the same
spin mode as the aircraft. Possible explanations
of the differences included the potential impact
of Reynolds number and other scaling issues.
Radio-controlled model of
YT-34C aircraft used in Beech study of spin characteristics.
Beechcraft Model 77 Skipper,
one of first industry applications of
radio-controlled model test technique for spin
studies.
Despite the unsettling results
of the YT-34C radio-controlled model tests, Beech
included spin-tunnel and radio-controlled model
tests in its development program for the Beechcraft
Model 77 “Skipper” in the mid-1970s.
The Skipper was a T-tail, two-place trainer used
primarily for initial exposure for pilots and the
acquisition of private pilot’s certificates.
The aircraft also incorporated the Langley-developed
GA(W)-1 airfoil for improved performance. The 1/5-scale
radio-controlled model of the Skipper was the first
use of an instrumented radio-controlled model in
an aircraft development program by any general
aviation company. Beech used instrumentation and
a general approach to testing gained through its
experience with the Langley-sponsored T-34 program.
Two radio-controlled models used in the test program
performed a total of 638 spins in the Skipper program.
Beech also committed to conducting spin-tunnel
tests of a 1/11-scale model in the spin tunnel.
This spin-tunnel test was the first fee-paid test
ever run in Langley’s spin tunnel, and the
first such test conducted with general aviation
personnel as participants in the tunnel test operations.
Researcher Charles Fremaux
(left) and technicians Ronald Hermansderfer
and James Hassell conduct tests of Columbia 300
model. Note auxiliary
leading-edge flaps on wings for correction of Reynolds
number effects.
The results of the radio-controlled
model and spin-tunnel model testing for the Skipper
closely matched each other, but they did not completely
match the full-scale aircraft results. Specifically,
the aircraft would only spin if the ailerons were
held against the spin. In contrast, the spin-tunnel
model would not spin with ailerons against the
spin, but would always spin with ailerons with
the spin. The angle of attack of the developed
spin for both the radio-controlled and spin-tunnel
model was about 15∞ less than that of the
aircraft. Based on these results, Beech concluded
that the value of dynamically scaled model tests
for this class of aircraft is limited to an indication
of trends and not quantitative results.
As a result of concern over
potential aerodynamic Reynolds number effects,
Langley made these results known to industry and
provided consultation for the interpretation of
model tests. As would be expected, the potential
existence of Reynolds number effects for general
aviation type configurations could impact the results
obtained from spin-tunnel and rotary-balance tests,
which are also conducted at very low values of
Reynolds number. Because the source of difficulty
is associated with wing aerodynamics, Langley’s
staff has attempted to artificially modify the
wing shape to accommodate scale effects. The success
obtained by using this approach has been marginal
to good.
An example of the impact of
Reynolds number scaling and attempts to circumvent
the problem involved a cooperative study between
Langley and the Lancair Company for the Lancair
Columbia 300 aircraft. In 1997, representatives
from Lancair approached Langley for consultation
regarding spin and spin-recovery testing of the
prototype Columbia 300 aircraft. Lancair requested
assistance in the determination of the emergency
spin-recovery parachute required, the approach
and value of dynamic model testing, and wing design
for spin resistance (to be discussed in the next
section). As a result of mutual interests, a cooperative
experimental program was initiated to conduct spin-tunnel
tests of a model of the Columbia 300 for correlation
with full-scale results and to provide information
on the parachute size required for satisfactory
emergency recovery for the spin test aircraft.
Under the direction of Charles M. Fremaux and Raymond
D. Whipple, free-spin tests were initiated in the
Langley 20-Foot Vertical Spin Tunnel.
Lancair had obtained spin
test results for the full-scale aircraft prior
to the tunnel entry, and the Langley-Lancair team
could quickly correlate the results with the model
data. For the baseline configuration, the developed
spin for the model occurred at substantially lower
angles of attack than for the full-scale aircraft;
this was in general agreement with the other general
aviation results previously discussed. No doubt,
the cause of the poor correlation was the limitation
of the aerodynamic characteristics of the model
wings due to the low Reynolds number of the tunnel
tests.
In an effort to simulate the
higher lift capability of the full-scale aircraft
wings, the Langley researchers modified the wing
of the model to incorporate leading-edge flaps
on the outer wing panels. With this modification,
the spin mode exhibited by the model more closely
approximated that of the full-scale aircraft and
permitted the sizing of the emergency spin-recovery
parachute to proceed with confidence.
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