Ice Accretion Formations on a NACA 0012 Swept Wing Tip in Natural Icing Conditions

28 Pages·1.6 MB·English

by  Vargas, Mario

NASA/TMm2002-211357 AIAA-2002-0244
Ice Accretion Formations on a NACA 0012
Swept Wing Tip in Natural Icing Conditions
Mario Vargas, Julius A. Giriunas, and Thomas P. Ratvasky
Glenn Research Center, Cleveland, Ohio
January 2002
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NASA/TM--2002-211357 AIAA-2002-0244
Ice Accretion Formations on a NACA 0012
Swept Wing Tip in Natural Icing Conditions
Mario Vargas, Julius A. Giriunas, and Thomas P. Ratvasky
Glenn Research Center, Cleveland, Ohio
Prepared for the
40th Aerospace Sciences Meeting and Exhibit
sponsored by the American Institute of Aeronautics and Astronautics
Reno, Nevada, January 14-17, 2002
National Aeronautics and
Space Administration
Glenn Research Center
January 2002
Acknowledgments
The authors would like to thank all of the personnel at Aircraft Operations for their help
in all phases of the experiment and Mr. Chris Lynch from the Imaging Technology Center
for his assistance with the photographic work.
Available from
NASA Center for Aerospace Information
National Technical Information Service
7121 Standard Drive
Hanover, MD 21076 5285 Port Royal Road
Springfield, VA 22100
Available electronically at http://gltrs.grc.nasa.gov/GLTRS
AIAA-2002-0244
ICE ACCRETION FORMATIONS ON A NACA 0012 SWEPT WING TIP
IN NATURAL ICING CONDITIONS
Mario Vargas', Julius A. Giriunas", Thomas P. Ratvasky""
NASA G/enn Research Center, C/eve/and, Ohio
Introduction
Abstract
Past experimental investigations on the formation
An experiment was conducted in the DeHavilland
of ice accretions on swept wings at glaze ice
DHC-6 Twin Otter Icing Research Aircraft at NASA
conditions provided understanding of the
Glenn Research Center to study the formation of
mechanisms of formation, the role of local effects
ice accretions on swept wings in natural icing
on roughness elements, the role of feather growth,
conditions. The experiment was designed to obtain
and allowed identification of some of the important
ice accretion data to help determine if the
parameters involved. Since nearly all of those
mechanisms of ice accretion formation observed in
investigations were conducted in icing tunnels, it is
the Icing Research Tunnel are present in natural
critical to determine if the formation of ice
icing conditions. The experiment in the Twin Otter
accretions in natural icing conditions (in-flight)
was conducted using a NACA 0012 swept wing tip.
shows the same mechanisms observed in tunnel
The model enabled data acquisition at 0°, 15°, 25°,
experiments.
30°, and 45 o sweep angles. Casting data, ice
shape tracings, and close-up photographic data
This report presents the results of an experimental
were obtained. The results showed that the
investigation carried out in the DeHavilland DHC-6
mechanisms of ice accretion formation observed
Twin Otter Icing Research Aircraft at NASA Glenn
in-flight agree "well with the ones observed in the
Research Center to study the formation of ice
Icing Research Tunnel. Observations on the end
accretions on swept wings in natural icing
cap of the airfoil showed the same strong effect of
conditions. The experiment in the Twin Otter was
the local sweep angle on the formation of scallops
conducted using a swept NACA 0012 wing tip. The
as observed in the tunnel.
model enabled data acquisition at 0°, 15°, 30°, and
45° sweep angles. When the plane was in icing
Nomenclature
conditions, a hatch port on the upper part of the
fuselage of the plane was opened and the airfoil
A Sweep angle, degrees was moved into the air stream using a specially
Critical distance, millimeters
designed stand. After the ice encounter the airfoil
FSSP Forward Scattering Spectrometry Probe
was lowered back into the aircraft cabin, and
LWC Cloud liquid water content, g/m3
photographic data was taken at a location on the
MVD Water droplet median volume diameter,
leading edge and along the end cap of the airfoil.
pm
After the photographic data was taken, ice tracings
T Temperature, °F
were made and the ice accretion was removed and
Ice accretion time, minutes
stored in a freezer. At the end of the flight, a wax
V Velocity, mph
mold was made of each ice accretion. Later a
casting of the ice accretion was obtained from the
* Aerospace Engineer, Icing Branch, NASA Glenn Research wax mold. Over a period of two and half weeks,
Center, Member AIAA data was obtained from 25 icing encounters.
** Aerospace Engineer, Icing Branch, NASA Glenn Research
Center, Member AIAA
Previous work on ice accretions in a natural icing
*** Aerospace Engineer, Icing Branch, NASA Glenn Research
Center, Member AIAA environment on a swept wing was done by
Reehorst 1. He conducted an experiment in the
Copyright © 2002 by the American Institute of Aeronautics and DeHavilland DHC-6 Twin Otter Icing Research
Astronautics, Inc. No copyright is asserted in the United States
under Title 17, U.S. Code. The U.S. Govemment has a royalty- Aircraft using a swept NACA 0012 wing tip airfoil.
free license to exercise all rights under the copyright claimed He obtained ice tracings and photographic data.
herein for Governmental Purposes. All other rights are The sweep angles employed were 0°, 30° and 45°.
reserved by the copyright owner
NASA/TM--2002-211357 1
American Institute of Aeronautics and Astronautics
He reported that the ice accretions were mostly of definition of the
glaze ice feathers zone. When the
the glaze type and he observed scallop formations critical distance
is zero only the glaze ice feathers
at 30° and 45osweep angles. LEWICE 3D was
zone ispresent.
modified and code predictions were compared to
ice accretion flight data.
The feathers in the glaze ice feathers zone are
inclined into the flow1°. They have a tooth shape,
The presence of scallop formations on swept wings
and therefore a preferred direction of growth (the
has been documented in past studies of ice
direction along which a feather is growing faster
accretions on swept wings 234. Reehorst and
laterally, that is, the direction of the larger axis for
BidwelP did an experiment in the NASA Glenn Icing
the tooth shape of the top of the feather). The
Research Tunnel (IRT) to study the effect of tunnel
preferred direction of growth is oriented perpendicular
parameters on the presence or absence of
to the external streamlines. The angle that the
scallops. Hedde and Guffond6proposed a ballistic
model of scallop growth. preferred direction of growth makes with respect to
the attachment linedirection is 90°forfeathers located
on the attachment line(streamlines are parallel to the
A study of the fundamental physical mechanisms
attachment line direction at this location). The angle
that lead to the formation of scallops on swept
decreases as the distance from the attachment line
wings was conducted by Vargas and Reshotko 78
increases (the streamlines are turning as they move
They also conducted a parametric experimentai
away from the locationof the attachment line).
study on the effect of velocity and sweep angle on
the critical distance do,9and astudy on the effect of
Three types of ice accretions were identified onswept
LWC and temperature _oon the critical distance.
wings at glaze ice conditions: complete scallops,
incomplete scallops, and no-scallops.
The present work continues the investigations into
the formation of ice accretions on swept wings by
When complete scallops form_0(figure 2a), the ice
studying the formation of these ice accretions
in-flight. accretion is covered with glaze ice feathers with a
preferred direction of growth that is perpendicular to
the external streamlines. The feathers have
Background
developed from roughness elements that reached a
certain height. The value of the critical distance is
Previous studies on formation of ice accretions on
zero, and only the glaze ice feathers zone is present.
swept wings at glaze ice conditions7,8found that
The feathers join along the preferred direction of
scallop and incomplete scallop formation isgoverned
growth to form ridges, with incipient scallop tips
by local effects on roughness elements. Roughness
formed by the feathers at the end of each ridge. As
elements formed at the beginning of the ice accretion
the ridges grow they form scallop tips. The scallop
process develop into glaze ice feathers when they
tips have different inclinations with respect to the
reach a given height, and are located beyond agiven
surface of the airfoil. Because of their different
distance from the attachment line. This distance is
inclinations, as the scallop tips grow in height and
called the critical distance dcr (see figure 1), and it
along the preferred direction, they merge with
depends on sweep angle and tunnel conditions_o
adjacent scallop tips by joining at the top of the
When the critical distance is not zero it defines two
feathers that form each scallop tip.This mechanism is
zones on the ice accretion. A zone called the
responsible for the growth of the scallop tips, their
attachment linezone, where the roughness elements
increase in size, and the enhancing of the spacing
do not become feathers, and another zone called the
between scallops. Along the attachment lineareathe
glaze ice feathers zone where the roughness
feathers tend to join other feathers around them by
elements develop into feathers (figures la, lb). The
touching, bridging and filling, in this way they form
attachment line zone begins at the attachment line
areas of solid ice that are also covered with
proper (the line inthe spanwise direction, alongwhich
roughness elements. As the scallop tips grow by
the flow splits over and under awing) and extends on
joining at their tops, the top surface tends to fill with
each side to the location where roughness elements
water and areas of solid ice can be observed with
develop into feathers (beginning of the glaze ice
roughness elements present.
feathers zone). The glaze icefeathers zone begins at
the endofthe attachment linezone and extends some
When the critical distance is not zero and only
distance chordwise. Only feathers that are an active
roughness elements located beyond a given distance
part of the main ice accretion are included in the
from the attachment line grow to become feathers,
NASA/TM--2002-211357
American Institute of Aeronautics and Astronautics
and the ice accretion looks almost like complete
incomplete scallops or no-scallops may develop. The
scallops. In a no-scallop, in some cases the feathers
formation of incomplete scallops or no-scallops
may grow faster than the attachment line ice in the
depends on the angle that the preferred direction of
beginning of the ice accretion, but the ice of the
growth of the feathers makes with respect to the attachment line eventually will cover them and the
attachment line direction.
feathers will cease to play a role in the formation of the
ice shape. Because of the fast growth of the feathers
When incomplete scallops form1° (figure 2b), the
inthe initial stage of the formation of the ice accretion,
feathers in the glaze ice feathers zone will form
the ice shape will show horns, and resemble some of
scallop tips. The mechanisms of formation of these
the ice shapes that are observed in 2D ice accretions.
scallop tips were found to be identical to the
mechanisms observed for the formation of the scallop
In all cases of formation of glaze ice accretions on a
tips in the complete scallop case. The formation of
swept wing, the distance from the attachment line
the scallop tips depends on the angle that the
beyond which the roughness elements become glaze
preferred direction of growth of the feathers makes
ice feathers (the critical distance dcr)is a critical factor
with respect to the attachment line, which in turn
inthe kind of ice accretion that will form.
depends on the distance from the attachment line at
which those feathers are located. The angle must be
A Parametric experimental study on the effect of
large enough to allow the mechanisms of formation of
velocity and sweep angle on the critical distance 9
scallop tips to operate and form scallop tips, and also
conducted on a swept NACA 0012 wing tip showed
must be large enough to prevent the ice of the
that at a given velocity and tunnel condition, as the
attachment line zone from covering all the glaze ice
sweep angle is increased from 0° to 25°, the critical
feathers zone and burying the incipient scallop tips.
distance slowly decreases. As the sweep angle is
increased past 25°, the critical distance starts
When no-scallops form '° (figure 2c), scallop tips are
decreasing more rapidly. For 75 and 100 mph it
not present in the ice accretion. Three situations may
reaches a value of 0 millimeters at 35 °. For 150
lead to the formation of no-scallops. In one case the
and 200 mph it reaches a value of 0 millimeters at
critical distance may be so large that only the
40°. On the ice accretion, as the sweep angle is
attachment line zone is present. In the second case
increased from 0° to 25°, the extent of the
the attachment line zone and the glaze ice feathers
attachment line zone slowly decreases but overall,
zone are both present, but the angle of the feathers
the ice accretions remain similar to the 0° sweep
with respect to the attachment line direction is not
angle case. As the sweep angle is increased above
large enough for the mechanisms of formation of
25°, the extent of the attachment line zone
scallop tips to operate. In the last case, the
decreases rapidly and complete scallops form at
attachment line zone and the glaze ice feathers zone
350 sweep angle for 75 and 100 mph, and at 40° for
are both present, and the angle of the feathers with
150 and 200 mph.
respect to the attachment line direction is large
enough for the mechanisms of scallop tip formation to
A study on the effect of LWC and temperature on
operate, but not large enough to prevent the ice of the
the critical distance 1° conducted on a swept NACA
attachment line from totally covering the scallop tips.
0012 wing tip airfoil found that decreasing the LWC
When the feathers are totally covered they cease to
to 0.5 g/m3 decreases the value of the critical
play a role inthe formation of the ice accretion.
distance at a given sweep angle compared to the
baseline case, and starts the formation of complete
When the attachment line zone and the glaze ice
scallops at 30°sweep angle. Increasing the LWC to
feathers zone are both present, in general one will
1.0 g/m 3increases the value of the critical distance
tend to prevail over the other in the first stages of the
compared to the baseline case, the critical distance
formation of the ice accretion, and this will influence
remains always above 0 millimeters and complete
the final ice shape. This interaction between the ice of
scallops are not formed. Decreasing the total
the attachment line zone and the feathers in the glaze
temperature to 20 °F decreases the critical
ice feathers zone depends on how fast each kind of
distance with respect to the baseline case and
ice grows. In an incomplete scallop when the critical
formation of complete scallops begins at 25° sweep
distance is small and the feathers in the glaze ice
angle. When the total temperature was increased
feathers zone grow faster than the ice of the
to 30 °F, bumps covered with roughness elements
attachment line, they will form large scallop tips. As
appear on the ice accretion at 25° and 30° sweep
ice accretion time increases the large scallop tips
angles, large ice structures appear at 35o and 40°
wash out the presence of the attachment line zone
NASA/TM--2002-211357 3
American Institute of Aeronautics and Astronautics
sweep angles, and complete scallops are formed at
hatch in the cabin of the aircraft and into the
45 °sweep angle.
airstream.
Experimental Procedure
A heater was installed on the area where the
Icing Research Aircraft measurements and collection of ice shapes were
going to be performed. The heater was powered
by a 28 DC, 1 amp, power supply. A grid was
The icing research aircraft is a modified
painted on the surface of the heater to allow
DeHavilland DHC-6 Twin Otter Icing Research
identification of the flow direction on the
Aircraft (figure 3). It is a twin engine commuter
photographic data and to serve as a distance scale
type aircraft powered by two 550 shaft horsepower in some pictures.
turboprop engines. It has been modified for flight
in icing environments and to permit the acquisition
of icing data. A small laboratory type freezer (figure 5) was used
to store the ice samples during flight operations.
The freezer weighs 60 Ibs and has 1.5 cu-ft of
InstrumentaUon and Data Reduction
storage. It draws a peak current of 11 amps at
startup, but rapidly drops to 1 amp during
The Twin Otter Icing Research Aircraft is equipped
continuous operation. The freezer uses a small
with standard flight instrumentation and with
amount of R-134A refrigerant as the working fluid
instrumentation specific to icing research. The
and can maintain the ice samples between -4 °
measured flight data consisted of airspeed, outside
and 20° F. Ice samples were laid on bubble-wrap
air temperature, angle of attack, angle of sideslip,
material to prevent damage to them during storage.
droplet size (MVD), liquid water content (LWC),
The freezer door was modified with a safety latch
and altitude. The LWC was measured using a King
to keep it securely closed while in flight and during
probe. Droplet size (MVD) measured by a Forward
take-offs/landings. Ice packs were also installed in
Scattering Spectrometry Probe (FSSP). The
the freezer to help keep ice samples frozen in the
outside temperature was measured with a
event the power to the freezer was interrupted for
Rosemount probe mounted on the fuselage. any extended period of time.
Airspeed, altitude, angle of attack and sideslip were
measured with a Rosemount 858 probe. Test Procedure
All data was acquired using Science Engineering
Each day of testing started with identification of
and Associates (SEA) M200 data acquisition
icing conditions. The pilots and the test engineer
software and was stored in a binary format. SEA
held a telecon with National Center for Atmospheric
Playback software read the binary formatted data
Research personnel to determine the best altitude
and produced ASCII files which were analyzed
and location to intercept icing conditions. The
using a spreadsheet software. The time series for
aircraft was launched. When the location for icing
each parameter was plotted and time averages and
conditions was reached a determination was made
standard deviations were calculated. Table 1
if the conditions were sufficient or if better icing
shows the summary of the flight data.
conditions were needed. If better icing conditions
Model and Related Hardware were needed, communication was established by
Satcom with NCAR personnel, and a better
location for icing conditions was located (if it
The model chosen for this study was a NACA 0012
existed). If the conditions were found to be
swept wing tip (figure 4). The airfoil is made of
sufficient, a holding pattern was established and
wood, with a 0.381meter (15-inch) chord measured
the experiment was started. The hatch on the
normal to the leading edge, and a 0.609-meter (24-
ceiling of the aircraft was opened, and the airfoil
inch) span. It was fitted with a rounded end cap. It
was elevated through the hatch into the airstream.
was mounted on a base that allowed pivoting of the
airfoil to sweep angles from 0° to 45° in 50
Because of the size of the hatch opening, the airfoil
increments. The base of the airfoil was attached to
was elevated at 0° sweep angle through the hatch
a specially designed stand (figure 5). The stand
and once in the airstream it was set at the selected
consisted of a system of pulleys connected to a
sweep angle. The starting time for the icing
crank that allowed the airfoil to be raised through a
encounter was recorded. During the icing
NASA/TM--2002-211357 4
American Institute of Aeronautics and Astronautics
Table 1 presents the complete summary of the
conditions the meteorological data was recorded by
flight data. Tables 2 through 5 present the flight
the data acquisition system and constantly
data in each one of the groups A through D.
monitored by the test engineer. About midpoint
into the ice encounter, the researcher requested
Group A: glaze ice conditions
from the test engineer a reading of the
meteorological conditions and wrote them in a data
The data in this group (Table 2) have average icing
sheet, these conditions were used as a rough
conditions close to V = 150 mph, T_ota_= 25 °F, LWC
indication of the icing conditions.
= 0.4 g/m 3,MVD = 13 _m.
Once the ice accretion was formed, the airfoil was
For the in-flight test the sweep angles tested were
moved to 0° sweep angle and lowered into the
0°, 15°, 25°, 30°, and 45°.
cabin. The ending time of the ice encounter was
recorded. Photographic data of the ice accretion
For a sweep angle of 45°two ice accretions belong
was taken. Three cuts were made in the ice
to group A (figures 6,7). The average conditions
accretion using an aluminum template. The cuts
for both cases are about the same with slight
were made perpendicular to the leading edge. A
difference in the LWCs (0.48 g/m3 versus 0.37
cardboard tracing was made in the middle cut.
g/m3).
Once the tracing was done, the heater was
activated and two samples of the ice accretion
For the higher LWC (figure 6) the scallop tips are
were removed from the airfoil and placed in a
well defined and starting at the attachment line.
freezer. The freezer was kept at a temperature
The ice shape is a complete scallop. The scallop
near 10 °F. The airfoil was cleaned and prepared
tips are very close and the spaces between are
for the next data point. At the end of the flight, the
filled with ice. This ice shape is different from the
ice accretions were placed in a Styrofoam
ice shape in figure 7. The ice shape is almost
container and moved to the location in the hangar
identical to ice shapes obtained at a lower total
where a wax bath was maintained. A wax mold of
temperature (figure 23) and also to an ice shape
each ice accretion was made and labeled. Later a
obtained at lower LWC (figure 20).
casting of the ice accretion was obtained from the
wax mold.
The ice shape in figure 7 is a complete scallop. A
side view of the ice accretion (figure 8) shows the
Castings feather formation of the scallop tips. It compares
well with an ice accretion obtained in the IRT
For each of the wax molds a urethane casting was
(figure 9) at A = 45°, V = 150 mph, Ttota_= 25 °F,
made of the ice shape. The castings allowed
LWC = 0.5 g/m3, MVD = 20 _m, 1; = 5 minutes.
measurement of the critical distance and
Close up photographic data of the in-flight
observation of the ice accretion.
accretion (figure 10) and the IRT accretion (figure
The castings were made by dipping the ice shape 11) show that they share the same icing
characteristics. The scallop tips are similar in
several times in a bath of bees-wax at 150 °F to
formation and not well defined. They are made of
form a wax mold, draining the water from inside the
wax mold, and filling the mold with liquid urethane. feathers joining along their preferred direction of
The urethane was allowed to solidify, and then a growth, in both ice accretions large feathers with a
solvent was used to remove the wax. preferred direction growth can be seen along the
attachment line. These large ice feathers along the
Results and Discussion attachment line have been observed in past studies
of ice accretions on swept wings 1°. They typically
start appearing at large sweep angles when the
For the purpose of data analysis, the flight data
LWC is lowered from 0.75 g/m _ to 0.5 g/m 3, or
was divided into four groups labeled A through D.
when the total temperature is lowered from 25 °F to
The data in a given group have average icing
conditions within a close range of values. This 20 °F starting at a baseline condition of 150 mph,
division of the data allowed observation of the 25 °F, 0.75 g/m 3,20 _m MVD.
effect of the sweep angle and in some cases
comparison to Icing Research Tunnel data.
NASA/TM--2002-211357 5
American Institute of Aeronautics and Astronautics
At a sweep angle of 30°the ice shape (figure 12) is
increase in the local sweep angles on the end cap
a complete scallop. Comparison with an accretion
affect the iceshape.
obtained in the tunnel (figure 13) at A = 30°, V =
150 mph, Ttota=p 25 °F, LWC =0.5 g/m3,MVD = 20
At the bottom of Table 2 there are three data points
pm, "_ = 10 minutes, shows that the two ice below the heading "Data points with lower LWCs".
accretions have the same overall characteristics. These data points have similar icing conditions to the
For both ice accretions large feathers appear along other ones but the LWCs are closer to0.3 g/m3than
the attachment finearea. Only thetop of the feathers
to 0.4 g/m3. The three pointsare for sweep angles of
can beobserved and the space between them is filled 45°,30°,and 25°.
by ice with roughness elements. The scallop tips are
present in both cases but they are small and not well
Inthe case for asweep angleof 45° (figure 20)the ice
defined.
accretion issimilar tothe iceaccretion infigure 6. Itis
acomplete scallop. Scallop tips are well defined. Ice
At a sweep angle of 15°the ice shape is ano-scallop isfillingthe space between the scallop tips.
(figure 14). Comparison with an ice accretion (figure
15) obtained in the tunnel at A = 15d,V = 150 mph, Inthe case for asweep angle of30°(figure 21) the ice
T_ola=p25 OF,LWC = 0.5 g/m3,and MVD =20 _m, "c accretion is a complete scallop. The overall ice
= 10 minutes, shows that the two ice accretions accretion is similar to the case with the higher LWC
share similar characteristics. In both cases the discussed above (figure 12), but the ice forming the
attachment line and the glaze ice feathers zone are ice shape islesstransparent, more towards rime ice.
present. In both cases the attachment linezones are
covered withroughness elements. Thefeathers inthe In the case for a sweep angle of 25° (figure 22), the
glaze ice feathers zones have apreferred direction of ice accretion is an incomplete scallop. The scallop
growth oriented inthe direction ofthe attachment line. tips are present although not well defined. The
In both cases the critical distance (dcr)is present. For attachment line zone and the glaze ice feathers zone
the tunnel ice accretion dcr= 7 mm, for the in-flight ice can be observed. The attachment line zone is
accretion d_r=8mm. covered with large roughness elements. The tops of
large icefeathers seen at largersweep angles are not
At asweep angleof 00the ice accretion isanoscallop present.
(figure 16). Comparison with an ice accretion (figure
17)obtained inthe tunnel at at A = 0°, V= 150 mph, Group B
Ttota_=25 °F, LWC =0.5 g/m3,and MVD =20 _tm,"¢
= 5 minutes, shows that the same icing The data inthis group (Table 3) have average icing
characteristics are present. Both ice shapes have an conditions close to V = 150 mph, Ttota_= 20°F, LWC
attachment line zone covered with roughness = 0.5 g/m3, MVD = 12 _tm. The main difference
elements and a reduced glaze ice feathers zone. For with the data from group A is the lower total
the in-flight ice shape de,= 9 mm, and for the tunnel temperature of 20 °F versus 25 °F. The effect of
ice shape dc,=11.5 mm. lower temperature can be observed when
comparing the ice accretions in both groups.
Observations on the end cap of the airfoil allow
studying the effect of changing the local sweep angle Icing test points were obtained for sweep angles of
45d,30°, 15°and 00.
for the same icing conditions. This is especially
important in the in-flight data because the conditions
are unsteady. Figures 18 and 19 present the For 45° sweep angle (figure 23) the ice accretion is
photographic data onthe end cap when the airfoil was a complete scallop. The ice accretion is similar to
set a sweep angle of 15°. On the span of the airfoil the ice shape in figure 6. Well-defined scallop tips
the ice accretion is a no-scallop (figure 14). As the can be observed. The space between the scallop
local sweep angle changes along the end cap of the tips and the area around the attachment line are
covered with ice.
airfoil, the critical distance collapses and scallops
made of feathers with a preferred direction of growth
can be observed (figure 18). A side view of the For 30° sweep angle (figure 24) the ice accretion
scallops onthe endcap (figure 19)shows their feather looks similar to the ice accretion from group A for
formation. These observations are consistent with the same sweep angle but in this case the area
observations made in IRT experiments of how the around the attachment line is covered with ice
(figure 25) and there is no indication of large
NASA/TM--2002-211357 6
American Institute of Aeronautics and Astronautics