Mario Vargas and Richard E. Kreeger

Glenn Research Center, Cleveland, Ohio

Measurement of the Critical Distance Parameter 

Against Icing Conditions on a NACA 0012

Swept Wing Tip

NASA/TM—2011-216966

February 2011

AIAA–2009–4123
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Mario Vargas and Richard E. Kreeger

Glenn Research Center, Cleveland, Ohio

Measurement of the Critical Distance Parameter 

Against Icing Conditions on a NACA 0012

Swept Wing Tip

NASA/TM—2011-216966

February 2011

AIAA–2009–4123

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

First Atmospheric and Space Environments Conference

sponsored by the American Institute of Aeronautics and Astronautics

San Antonio, Texas, June 22–25, 2009
Acknowledgments

The present work is funded under the NASA Integrated Resilient Aircraft Controls (IRAC) Project of the Aviation Safety Program.

The authors would like to thank Mr. Gene Addy and Dr. OA Guo for their support. Thanks to Mr. Ed Hermann and Mr. Mike 

Beyman for their help with the experiment in the Goodrich Icing Wind Tunnel (IWT). Thanks to the Icing Research Tunnel (IRT) 

personnel for their help with the experiment. Thanks to Mr. John Oldenburg for his help in facilitating the tunnel time exchange 

between the NASA IRT and the Goodrich IWT. Special thanks to Mr. Chris Lynch and Mr. Quentin Schwinn from the Imaging 

Technology Center for their support of the experiment and for developing and improving the measurement technique used.

Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

5301 Shawnee Road

Alexandria, VA 22312

Available electronically at http://www.sti.nasa.gov

Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2011-216966 1 

Measurement of the Critical Distance Parameter Against Icing 

Conditions on a NACA 0012 Swept Wing Tip 

Mario Vargas and Richard E. Kreeger 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

This work presents the results of three experiments, one conducted in the Icing Research Tunnel 

(IRT) at NASA Glenn Research Center and two in the Goodrich Icing Wind Tunnel (IWT). The 

experiments were designed to measure the critical distance parameter on a NACA 0012 Swept Wing Tip 

at sweep angles of 45, 30, and 15. A time sequence imaging technique (TSIT) was used to obtain real 

time close-up imaging data during the first 2 min of the ice accretion formation. The time sequence 

photographic data was used to measure the critical distance at each icing condition and to study how it 

develops in real time. The effect on the critical distance of liquid water content, drop size, total 

temperature, and velocity was studied. The results were interpreted using a simple energy balance on a 

roughness element. 

Nomenclature 

dcrit Critical distance, mm 

 Sweep angle, degrees 

V Velocity, mph 

Ttotal Total temperature, F 

LWC Cloud liquid water content, g/m3

MVD Water droplet median volume diameter, m 

NACA 0012 Wooden NACA 0012 Swept Wing Tip airfoil 

SLD  Supercooled Large Droplets 

IRT Icing Research Tunnel 

ITC Imaging Technology Center at NASA Glenn Research Center 

IWT Goodrich Icing Wind Tunnel 

TSIT Time-Sequence Imaging Technique 

   Local collection efficiency

ts  Surface temperature, 32 F for glaze ice 

Qin Heat flux gain for a roughness element, energy/unit area unit time 

Qout Heat flux loss for a roughness element, energy/unit area unit time 

I. Introduction 

Scallop tips are complex icing structures that may be part of ice accretions on swept wings. The 

airfoil geometry, the sweep angle and the icing conditions determine if the scallop tips are present and the 

distance from the attachment line where they form. Previous studies (Refs. 1 to 7) showed that the scallop 

tips are made of icing feathers and their interactions. The location where the feathers begin forming with 

respect to the attachment line determines where the scallop tips begin and consequently the type of ice 

accretion that will form. The distance from the attachment line to where the feathers begin forming is 

called the critical distance. Measurements of the critical distance against icing conditions are needed to 

provide fundamental data for its modeling and prediction. Prediction of the critical distance will allow 
NASA/TM—2011-216966 2 

icing codes to anticipate the type of ice accretion that will form and specifically to anticipate if a 

scalloped ice accretion will be present. 

Past measurements of the critical distance were made from images taken at the end of the ice 

accretion process. Because it was not possible to capture the details of the ice accretion when small 

amounts of ice were present, large ice accretion times were used. The large ice accretion times made it 

difficult to take accurate measurements and the technique did not allow determining how the critical 

distance developed in real time. Recent improvements in time sequence imaging techniques have made it 

possible to obtain more accurate measurements and to observe its real time development. 

This report presents the results of three experiments to measure the critical distance. Two experiments 

were conducted at the Goodrich Icing Wind Tunnel (IWT) and one at the Icing Research Tunnel (IRT) at 

NASA Glenn Research Center. The objective of the experiments was to measure the effect of changes in 

liquid water content, drop size, total temperature and velocity on the critical distance, and to observe its 

real-time development on a NACA 0012 swept wing tip at 45, 30, and 15 sweep angles. A time 

sequence imaging technique (TSIT) developed by personnel of the Imaging Technology Center (ITC) at 

NASA Glenn Research Center for real time observation of the ice accretion process was extended to 

obtain the measurements. The test conditions were within the Appendix C envelope. 

For the experiment in the IRT, time sequence close-up photographic data was obtained from a single 

camera positioned on the ceiling of the tunnel. For the experiment in the IWT, the smaller size of the test 

section permitted positioning three cameras closer to the airfoil: one on the ceiling of the tunnel and two 

from the side of the test section. In each experiment, additional close-up photographic data was obtained 

of the final ice accretion at the end of each run. The time sequence photographic data was taken at 2-sec 

intervals during each run for 120 sec. Before each run, a printed grid was photographed on the leading 

edge of the airfoil to be used as the image measurement scale. The raw data was studied frame by frame 

with digital imaging software and combined with the picture of the grid to obtain the measurement of the 

critical distance. Digital movies were also created to observe the dynamic elements in the process. 

The experiments provided insights into how the critical distance develops during the first 2 min of the 

ice accretion process and allowed its measurement at different icing conditions. The database of 

measurements and the observations will help in the development of models that can predict the type of ice 

accretion that will form on swept wings. 

A. Icing Research Tunnel 

The IRT is a closed-loop refrigerated wind tunnel with a test section 1.8 m (6 ft) high, 2.7 m (9 ft) 

wide and 6.0 m (20 ft) long. The total air temperature in the test section can be varied between –29 C 

(–20 F) and 1 C (33 F) within ±0.5 C (±1 F). Velocities up to 160 m/s (350 mph) can be obtained 

with a blockage of 5 percent in the test section. A spray system allows control of the liquid water content 

(LWC) between 0.2 to 3.0 g/m3

. The spray nozzles provide droplet median volume diameters (MVD) 

from 14 to 50 m at Appendix C conditions and from 50 to 200 m at SLD conditions. 

B. Goodrich Icing Wind Tunnel 

The Goodrich Icing Wind Tunnel (IWT) is a closed-loop refrigerated wind tunnel with a test section 

0.6 m (1.8 ft) wide, 1.1 m (3.7 ft) high and 1.5 m (5 ft) long. The total air temperature in the test section 

can be varied between –30 C (–22 F) and 0 C (32 F) within ±0.5 C (±1 F). Velocities from 13 m/s 

(30 mph) up to 103 m/s (230 mph) can be obtained. A spray system allows control of the liquid water 

content (LWC) between 0.1 to 3.0 g/m3

. The spray nozzles provide droplet median volume diameters 

(MVD) from 14 to 40 m at Appendix C conditions. 
NASA/TM—2011-216966 3 

C. Model 

The model used in all the experiments was a wooden NACA 0012 swept wing tip. The airfoil has a 

0.381 m (15 in.) chord measured normal to the leading edge, and a 0.609 m (24 in.) span. It is mounted in 

the tunnel on a stand that allows pivoting of the airfoil to sweep angles of 0 to 45 at 5 increments. At 

sweep angles larger than 30 a small extension was fitted at the base of the airfoil to improve the airflow. 

A grid was painted on the surface of the airfoil to allow identification of the flow direction on the 

photographic data and for placement of the printed grid used for measurement of the critical distance. 

D. Model Position in the IRT Test Section 

The model was mounted vertically in the test section at zero angle-of-attack (Fig. 1). The base of the 

model rested on a metallic table bolted to the tunnel floor. This is the standard position of the model in the 

test section where the area of observation on the leading edge is about 36 in. from the tunnel ceiling. Two 

sweep angles were used during the test: 45 and 15. 

E. Model Position in IWT Test Section 

The model was mounted vertically in the test section at a 30 sweep angle and at zero angle-of-attack 

(Fig. 2). The base of the model was bolted to the tunnel floor. The position of the model allowed 

observation of the leading edge from a 65 angle between the lens of the camera positioned on the ceiling 

and the attachment line direction on the airfoil. This angle was found to be the optimal through trial and 

error. 

F. Test Matrix—IRT 

Table 1 lists the test matrix for the IRT test. The table includes the run number and the icing 

conditions. The wooden NACA 0012 swept wing tip airfoil was used. The sweep angles tested were 15

and 45. A baseline icing condition was used: V = 150 mph, Ttotal = 25 F, LWC = 0.75 g/m3

, MVD = 

20 m. Starting from the baseline condition, runs were conducted to study the effect of changes in 

velocity, liquid water content, drop size and total temperature at each sweep angle. At 15 sweep angle 

tunnel time limitations restricted the study of each effect to four runs but they covered the same overall 

range as the runs conducted at 45 sweep angle. 

G. Test Matrix—IWT 

Tables 2 and 3 show the test conditions for the test entries in the Goodrich Icing Wind Tunnel. The 

table includes the run number and the icing conditions. The wooden NACA 0012 swept wing tip airfoil 

was used. The airfoil was set at a 30 sweep angle for the entire experiment. A baseline icing condition 

was used: V = 150 mph, Ttotal = 25 F, LWC = 0.75 g/m3

, MVD = 20 m. The effect of changes in 

velocity, drop size, liquid water content and total temperature was studied. 

H. Test Procedure—IRT 

The TSIT system used a single camera located on the ceiling of the tunnel. Before each run, the 

camera focus was set on the area of observation. A grid was placed on the leading edge of the airfoil and a 

picture of it was taken and then removed. The TSIT system was activated at the start of each run when the 

tunnel was brought to the target velocity and total temperature and the tunnel spray system was started. 

During the run, the camera took time sequence close-up pictures at 2-sec intervals for 120 sec. A flash 

system was used for each picture. After the TSIT system stopped recording images, the tunnel was kept 

running for additional ice accretion time to obtain a well defined ice shape at the end of the run. Tunnel 
NASA/TM—2011-216966 4 

parameters were recorded using the NASA-Glenn Escort data acquisition system. Once the target ice 

accretion time was reached, the tunnel was brought to idle. After entering the tunnel, a measuring tape 

was placed around the ice shape, and close-up photographic data was taken in a predetermined sequence 

and location. A single cut was made at a predetermined location in the ice accretion to prepare for taking 

an ice shape tracing. A cardboard template was placed in the cut and a tracing of the ice shape was made. 

Following this, general observations on the ice accretion types and formations were recorded. The airfoil 

was cleaned before the next run. 

I. Test Procedure—IWT 

The TSIT system used a camera located on the ceiling of the tunnel and two cameras from the side of 

the test section (Fig. 3). One of the side cameras pointed perpendicularly to the leading edge of the airfoil, 

the other pointed at an angle. Before each run, the TSIT cameras’ focus was set on the area of observation 

and a picture of the grid was taken. After the tunnel had reached the target velocity and total temperature, 

the TSIT system was activated either in synchronization with the tunnel spray or at a delayed time. 

During the run, the cameras took time sequence close-up pictures at 2-sec intervals for 120 sec. After the 

TSIT system stopped taking images, the tunnel was kept running for additional ice accretion time to 

obtain a well defined ice shape at the end of the run. Tunnel parameters were recorded using the IWT data 

acquisition system. Once the target ice accretion time was reached the tunnel was brought to idle, a 

measuring tape was placed around the ice shape, and close-up photographic data was taken in a 

predetermined sequence and location. No ice shape tracings were taken during the IWT test. The airfoil 

was then cleaned before the next run. 

J. Measurement Technique 

Before each run, a picture of the grid was taken with the TSIT system. The grid picture provided the 

scale to measure the critical distance. To take the measurement, an ice accretion image was selected from 

the sixty taken by the TSIT during the run. The white tone from the grid image was removed using 

imaging processing software. The modified grid image and the ice accretion image were combined and 

the grid was used to measure the distance from the attachment line where the feathers begin forming if 

they are present. Figure 4 shows the technique. 

II. Results 

The growth of roughness elements into feathers is a glaze or rime ice freezing process depending on 

the local icing conditions at the element location. An energy balance can be adapted to interpret 

qualitatively if a change in the icing conditions will be favorable for the element to develop into a feather. 

In Section A, a simple energy balance is presented. It will be used in the interpretation of the data in 

subsequent sections. 

A. Simple Energy Balance to Interpret the Data 

Figure 5 shows the energy balance at a roughness element assuming glaze ice conditions. The energy 

balance is based on the one used in scaling (Ref. 8) and icing codes (Ref. 9). The heat gained by the 

surface of the element per unit area per unit time is the heat gained by the surface from the kinetic energy 

of water droplets striking the surface (qk), plus the heat gained by the surface due to the release of latent 

heat of fusion from freezing water (qf). The energy per unit area per unit time lost by the surface is the 

heat loss from the surface by convection (qc), plus the heat loss from the surface to raise the temperature 

of the impinging liquid to the freezing point (qw), plus the heat losses by evaporation (qe), sublimation (qs) 

and radiation (qr). Conduction heat transfer at the base of the element is assumed to be zero since ice is a 
NASA/TM—2011-216966 5 

good insulator. The total energy gained or lost by an element per unit time is the above heat fluxes times 

the area of the element. 

For the purpose of interpreting the data, the effect of changing icing conditions was assumed to 

depend on only two terms in the heat balance. Glaze ice conditions were also assumed. For the heat 

gained, the kinetic energy of the water droplets term, qk, was kept. For the heat loss, only heat loss by 

convection, qc, was kept. 

The heat gained from the kinetic energy of the water droplets is given by: 

Qin = qk  A = LWCV, (V2/2) A (1) 

where LWCV is the incoming mass flux, A is the area of the element, V is the air velocity, LWC is 

the liquid water content of the cloud, and  is the collection efficiency at the location of the element. The 

effect of changing the icing conditions will be to increase or decrease the heat gained, Qin, depending on 

the effect that the condition change has on the velocity (V), the liquid water content of the cloud (LWC), 

or on the collection efficiency (). 

For a given sweep angle , the heat gain is given by: 

 Qin,  = LWC[Vcos()] [(Vcos())2/2] A 

where the term  accounts for the dependence of the local collection efficiency with the sweep angle and 

any shadowing of areas of the element from the incoming droplets due to the direction of the velocity. 

The heat loss by convection is given by: 

Qout = qcA = hc,avg  (ts - ttot) A (2) 

where hc,avg is the average convection heat transfer coefficient over the element, ts is the surface 

temperature (32 F for glaze ice) and ttot is the total temperature of the air (assuming the recovery factor, r, 

to be equal to one). The average heat transfer coefficient over the roughness element, hc,avg, is proportional 

to a power of the velocity. The effect of changing icing conditions will be to increase or decrease Qout

depending on the effect that the condition change has on the total temperature of the air, ttot, and the 

average heat transfer coefficient hc,avg through any change in the local velocity of the air. The only icing 

cloud parameter not a part of Equations (1) and (2) is the mean drop diameter (MVD). The effect of 

changing the MVD in the icing conditions is felt through . Increasing the MVD will increase  at a given 

location. 

For a given sweep angle , the heat loss is given by: 

 Qout,  = hc,avg,   (ts - ttot) A 

where hc,avg,  indicates the dependence of the average heat transfer coefficient with the sweep angle 

through the effect of the sweep angle on the local velocity. For roughness elements along the attachment 

line, that velocity is given by Vsin(

In the analysis of the data, the effect of icing conditions on Equations (1) and (2) is considered. 

Figure 6 illustrates the effect of the heat balance in the formation of the critical distance. The drawing 

shows roughness elements and feathers forming on the leading edge of a swept wing. The critical distance 

is indicated with an arrow drawn from the attachment line to where the feathers begin forming. Feathers 

have developed from roughness elements where their energy balance is Qout  Qin. Roughness elements do 

not developed into feathers where the energy balance is Qout  Qin. The drawing also helps to illustrate 

that for the critical distance to decrease, the ice accretion has to be formed for icing conditions where the 

heat balance on the roughness element (or elements) to the left of the first feather is changed so that Qout 

Qin. Correspondingly, to have an ice accretion with a larger critical distance, the heat balance for the 

roughness elements that formed feathers needs to be changed by the new icing conditions so that Qout 

Qin. 
NASA/TM—2011-216966 6 

B. Baseline Condition 

A baseline condition of  = 45, V = 150 mph, Ttot = 25 F, LWC = 0.75 g/m3, MVD = 20 m was 

used as the starting point in the critical distance measurements. This baseline condition has been used in 

past experiments (Refs. 1 to 8). In past experiments the ice accretion at this baseline condition was 

observed to be a complete scallop (dcr = 0). In the analysis of the data, the measured critical distances and 

ice accretion shapes were compared against the baseline condition and interpreted using the simple energy 

balance explained in Section A. 

C. Liquid Water Content (LWC) Effect 

Figure 7 shows the effect of changing the LWC for the baseline condition at 15 and 45 sweep 

angles. For 45 at LWCs of 0.5 and 0.6 g/m3

 the critical distance is zero. As the LWC is increased, the 

critical distance increases monotonically. For 15 sweep angle only four LWC cases were tested but they 

cover the same range as for 45, from 0.5 to 1.25 g/m3

. The values of the critical distance for the 15

sweep angle case are larger than for the 45 case. The behavior of the critical distance with LWC is 

consistent with the energy balance from Equations (1) and (2). As the LWC is increased, the heat gained 

by the roughness elements increases while the heat loss remains the same. The increase in heat gained is 

due to the increase in the kinetic energy of the water droplets arriving on the element. The increase in 

kinetic energy is due to the increase in the mass flux. The roughness elements closer to the attachment 

line have higher values of the local collection efficiency and therefore they are the last to develop into 

feathers as the LWC is increased from 0.5 g/m3

. 

The larger values of dcr for the 15 sweep angle versus the 45 can be interpreted in the context of the 

heat balance. An increase in the sweep angle for the same icing conditions will decrease the heat gained 

by the roughness elements while increasing the heat lost. The heat gained by the elements decreases 

because the local collection efficiency decreases as the sweep angle is increased, and also because the 

component of the velocity perpendicular to the leading edge decreases as the cosine of the sweep angle. 

The heat loss will increase because the heat transfer coefficient gets larger as the sweep angle increases 

because of the larger local velocity. 

D. Total Temperature (Ttot) Effect 

Figure 8 shows the effect of changing the total temperature for the baseline condition at 15 and 45

sweep angles. For 15 at a total temperature of 28 F the value of the critical distance is 25 mm. As the 

temperature is decreased the critical distance decreases reaching a value of about 5 mm at 16 F. It 

remains close to 5 mm for lower temperatures of 14 and 12 F. For 45 sweep angle the critical distance 

at 28 F is 4.3 mm and lowering the temperature reduces the critical distance to a value of zero. The 

behavior of the critical distance with temperature can be interpreted using the heat balance from 

Equations (1) and (2). Lowering the total temperature increases the heat loss from the element without 

changing the heat gained. As the total temperature is lowered, the critical distance decreases. A similar 

trend is observed for the 30 sweep angle case as shown in Figure 9 for 150 and 200 mph. 

E. Mean Value Diameter (MVD) Effect 

Figure 10 shows the effect of changing the MVD for the baseline condition at 15 and 45 sweep 

angles. For 45 at a MVD of 14 m the value of the critical distance is zero. As the MVD is increased the 

critical distance increases monotonically reaching a value of 11.1 mm at an MVD of 50 m. The increase 

in the critical distance with MVD can be interpreted with the energy balance. Increasing the MVD will 

decrease the critical distance because as the MVD increases, the heat gained by the element increases 

while the heat loss remains the same. The heat gained by the roughness element increases because the 
NASA/TM—2011-216966 7 

local collection efficiency gets larger as the MVD increases. Only four cases were tested for 15 sweep 

angle. They ranged from 20 to 50 m. The behavior of the critical distance with MVD for 15 sweep 

angle followed the same pattern observed for the 45 sweep angle, the pattern expected from the heat 

balance. Figure 11 shows the effect of changing the MVD at 30 sweep angle. The trend is the same as 

for the 15 and 45 cases but the data shows large scattering above a MVD of 25 m. 

Figure 12 shows the effect of total temperature on the critical distance at MVDs of 20, 25, and 30 m. 

In all three cases as the total temperature is lowered the critical distance decreases. At a given total 

temperature the valued of the critical distance increases with drop size as observed before. The decrease 

in the critical distance at a given MVD value is due to the increase in the heat loss as the temperature is 

decreased. 

F. Velocity Effect 

The effect of velocity on the critical distance is more complex and a pattern harder to predict. This 

can be observed in the energy balance. As the velocity is increased, the heat gained by the element will 

increase but there is also an increase in the heat lost by the element because increasing the air velocity 

will increase the local velocity on the element affecting the heat transfer coefficient over the roughness 

elements. The final effect will depend on how Qin and Qout are changed. From the energy balance used it 

is not possible to predict the direction in which the critical distance is going to move when the velocity is 

changed. 

Figure 13 shows the effect of changing the velocity for the baseline condition at 15 and 45 sweep 

angles. For 45 between 100 and 120 mph the value of the critical distance is zero. From 130 to 200 mph 

the value of the critical distance increases to about 5 mm and remains there. For the 15 sweep angle only 

four cases were tested in a range of velocities from 130 to 200 mph, and the critical distance decreases as 

the velocity is increased. The change in the critical distance is from about 19 to 11.8 mm. Figure 14 

shows the effect of changing the velocity at 30 sweep angle. The value of the critical distance remains 

between 4 and 10 mm when the velocity goes from 100 to 200 mph. 

G. Sweep Angle Effect 

The energy balance can be used to anticipate the effect of increasing the sweep angle. As the sweep 

angle is increased at a given icing condition, the heat gained by an element will decrease because the local 

collection efficiency and the value of the component of the velocity perpendicular to the leading edge of 

the airfoil will decrease. On the other hand, the heat loss will increase because the heat transfer coefficient 

over the roughness element will increase due to the increase in the local velocity. According to the simple 

heat balance used to interpret the data, the critical distance at a given icing condition should decrease as 

the sweep angle increases. This is observed in Figures 7, 8, 10, and 13 where the value of the critical 

distance for the 15 sweep angle case is always larger than for 45. 

III. Discussion 

The measurements of the critical distance conducted in three separate experiments provided 

quantitative information on how the critical distance changes with icing conditions. The critical distance 

was found to increase with liquid water content and drop size, and to decrease with total temperature. 

Increasing the sweep angle decreased the critical distance. The response of the critical distance to changes 

in velocity was more complex since any change in the velocity modifies at the same time the heat gain 

and the heat loss terms in the energy balance. 

The simple energy balance from Equations (1) and (2) helped to interpret the response of the critical 

distance to parametric changes in the liquid water content, drop size, total temperature, velocity and 

sweep angle. Although only two terms in the energy balance are used, the way that the energy balance 
NASA/TM—2011-216966 8 

interprets the trends in the experimental data gives confidence in its applicability. The energy balance can 

be used to understand results from past experiments that could not interpreted at the time such as why the 

formation of scallops is so strongly influenced by the local sweep angle. In previous experiments on the 

formation of ice accretions on swept wing (Refs. 1 to 8) it was observed that when the airfoil was set at 

15 sweep angle and no scallops formed on the leading edge along the span of the airfoil at a given icing 

condition, the change in the local sweep angle on the end cap of the airfoil, allowed the formation (on the 

end cap) of well defined scallops made of feathers that had grown from roughness elements. The heat 

balance explains this as follows: increases in local sweep angle decrease the heat gain because of the 

decrease in the value of the component of the velocity perpendicular to the leading edge and the decrease 

in the local collection efficiency, while at the same time the heat loss is increased because of the increase 

in the heat transfer coefficient due to the increase in local velocity at a given location. The same reasoning 

explains why the critical distance decreases at a given icing condition when the airfoil sweep angle is 

increased. 

Although a good number of measurements of the critical distance against icing conditions were 

obtained, additional data is needed to extend the database to a wider range of icing conditions in 

Appendix C and SLD conditions. The experience gained in the three experiments will help to improve the 

accuracy of the measurement technique. The results obtained so far indicate the need to run the TSIT 

system for times longer than 120 sec, up to 300 sec. In the past the TSIT system could not be operated at 

longer than 120 sec at 2 frames per second. This technical limitation has been removed and in future 

experiments the system will be operated for longer ice accretion times. 

The work presented in this paper is the first step toward the development of a database of critical 

distance measurements against icing conditions. It is hoped that those measurements will provide clues 

and direction on how to predict the critical distance for a given airfoil geometry at given icing conditions. 

The experimental measurements of the critical distance have indicated the usefulness of the energy 

balance from Equations (1) and (2). The heat balance will be used to plan additional measurements of the 

critical distance and to begin numerical calculations that when compared to the experimental data may 

provide additional guidance on how to develop a model that allows the critical distance prediction, or at 

least a prediction of when complex scalloped ice shapes will form. 

References 

1. Vargas, M., “Ice Accretion on Swept Wings at Glaze Ice Conditions,” Ph.D. Thesis, Case Western 

Reserve University, Cleveland, Ohio, May 1998. 

2. Vargas, M. and Reshotko, E., “Physical Mechanisms of Glaze Ice Scallop Formations on Swept 

Wings,” AIAA Paper 98-0491 and NASA/TM—1998-206616, Jan. 1998. 

3. Vargas, M. and Reshotko, E., “Parametric Experimental Study of the Formation of Glaze Ice Shapes 

on Swept Wings,” AIAA Paper 99-0094, Jan. 1999. 

4. Vargas, M. and Reshotko, E., “LWC and Temperature Effects on Ice Accretion Formation on Swept 

Wings at Glaze Ice Conditions,” AIAA Paper 2000-00483, Jan. 2000. 

5. Vargas, M., Giriunas, J. and Ratvasky, T., “Ice Accretion Formations on a NACA 0012 Swept Wing 

Tip in Natural Icing Conditions,” AIAA Paper 2002-0244, Jan. 2002. 

6. Vargas, M., “Current Experimental Basis for Modeling Ice Accretions on Swept Wings,” AIAA 

Paper 2005-5188, June 2005. 

7. Vargas, M., “Swept Wing Icing Physics Studies at NASA Glenn Research Center 1990-2006,” SAE 

Paper 2007-01-3332, September 2007. 

8. Anderson, D.N., “Manual of Scaling Methods,” NASA/CR—2004-212875, March 2004. 

9. Wright, W.B., “Users Manual for the Improved NASA Lewis Ice Accretion Code, LEWICE 1.6,” 

NASA CR 198355, June 1995. 
NASA/TM—2011-216966 9 

TABLE 1.—IRT TEST, WOODEN NACA 0012

Run number Sweep 

angle 

Velocity, 

mph 

Total 

temperature, 

F 

LWC, 

g/m3

MVD, 

m 

Ice accretion 

time, 

min 

051809.01 Baseline 45 150 25 1.00 20 3.0 

051809.02 45 150 25 0.75 20 3.0 

 

 Velocity 

effect 

 

051809.03 45 100 25 0.75 20 3.0 

051809.04 45 100 25 0.75 20 9.7 

051809.05 45 110 25 0.75 20 8.9 

051809.06 45 120 25 0.75 20 8.1 

051809.07 45 130 25 0.75 20 7.5 

051809.08 45 140 25 0.75 20 7.0 

051809.09 45 150 25 0.75 20 6.5 

 

051909.01 45 160 25 0.75 20 6.1 

051909.02 45 170 25 0.75 20 5.7 

051909.03 45 180 25 0.75 20 5.4 

051909.04 45 190 25 0.75 20 5.1 

051909.05 45 200 25 0.75 20 4.9 

 

 LWC effect 

051909.06 45 150 25 0.50 20 9.8 

051909.07 45 150 25 1.00 20 4.9 

051909.08 45 150 25 1.25 20 3.9 

051909.09 45 150 25 0.60 20 8.1 

051909.10 45 150 25 0.70 20 7.0 

 

 LWC effect 

052009.01 45 150 25 0.80 20 6.1 

052009.02 45 150 25 0.80 20 6.1 

052009.03 45 150 25 0.90 20 5.4 

052009.04 45 150 25 1.10 20 4.4 

052009.05 45 150 25 1.20 20 4.1 

 

 MVD effect 

052009.06 45 150 25 0.75 14 6.5 

052009.07 45 150 25 0.75 18 6.5 

052009.08 45 150 25 0.75 22 6.5 

052009.09 45 150 25 0.75 26 6.5 

052009.10 45 150 25 0.75 30 6.5 

052009.11 45 150 25 0.75 34 6.5 

052009.12 45 150 25 0.75 38 6.5 

052009.13 45 150 25 0.75 42 6.5 
NASA/TM—2011-216966 10 

TABLE 1.—Concluded. 

Run number Sweep 

angle 

Velocity, 

mph 

Total 

temperature, 

F 

LWC, 

g/m3

MVD, 

m 

Ice accretion 

time, 

min 

 Sweep 

angle/temp 

effect 

 

 

052109.01 45 150.00 10 0.75 20.00 6.5 

052109.02 45 150.00 15 0.75 20.00 6.5 

052109.03 45 150.00 20 0.75 20.00 6.5 

052109.04 45 150.00 30 0.75 20.00 6.5 

 

 Velocity effect 

052109.05 15 200 25 0.75 20 2.6 

052109.06 15 175 25 0.75 20 3.0 

052109.07 15 150 25 0.75 20 3.5 

052109.08 15 125 25 0.75 20 4.2 

 

 LWC effect 

052109.09 15 150 25 0.50 20 5.2 

052109.10 15 150 25 0.75 20 3.5 

052109.11 15 150 25 1.00 20 2.6 

052109.12 15 150 25 1.25 20 2.1 

 

 MVD effect 

052209.01 15 150 25 0.75 20 3.5 

052209.02 15 150 25 0.75 20 3.5 

052209.03 15 150 25 0.75 30 3.5 

052209.04 15 150 25 0.75 40 3.5 

052209.05 15 150 25 0.75 50 3.5 

 

 Sweep 

angle/temp 

effect 

 

 

052209.06 15 150 30 0.75 20 3.5 

052209.07 15 150 28 0.75 20 3.5 

052209.08 15 150 26 0.75 20 3.5 

052209.09 15 150 24 0.75 20 3.5 

052209.10 15 150 20 0.75 20 3.5 

052209.11 15 150 18 0.75 20 3.5 

052209.12 15 150 16 0.75 20 3.5 

052209.13 15 150 14 0.75 20 3.5 

052209.14 15 150 12 0.75 20 3.5 
NASA/TM—2011-216966 11 

TABLE 2.—IWT TEST, WOODEN NACA 0012 

Run number Sweep angle Velocity, 

mph 

Total 

temperature, 

F 

LWC, 

g/m3

MVD, 

m 

Ice accretion 

time, 

min 

022309.01 30 150 15 1.00 20 120 

022309.02 30 150 25 1.00 20 120 

022309.03 30 150 25 1.00 20 600 

022309.04 30 150 25 1.00 20 300 sec 

 

022409.01 30 150 25 1.00 20 180 sec

022409.02 30 150 28 1.00 20 180 sec

022409.03 30 150 26 1.00 20 180 sec

022409.04 30 150 24 1.00 20 180 sec

022409.05 30 150 22 1.00 20 180 sec

022409.06 30 150 20 1.00 20 180 sec

022409.07 30 150 18 1.00 20 180 sec

022409.08 30 150 16 1.00 20 180 sec

022409.09 30 150 14 1.00 20 180 sec

022409.10 30 150 12 1.00 20 180 sec

 Velocity effect Velocity effect

 

022509.01 30 100 25 0.75 20 3.0 min

022509.02 30 110 25 0.75 20 9.0 min

022509.03 30 120 25 0.75 20 8.5 min

022509.04 30 130 25 0.75 20 8.0 min

022509.05 30 140 25 0.75 20 7.5 min

022509.06 30 150 25 0.75 20 7.0 min

022509.07 30 160 25 0.75 20 6.5 min

022509.08 30 170 25 0.75 20 6.0 min

022509.09 30 180 25 0.75 20 5.5 min

022509.10 30 190 25 0.75 20 5.0 min

022509.11 30 200 25 0.75 20 4.5 min

022509.12 30 100 25 0.75 20 9.5 min

 

LWC effect 

022609.01 30 150 25 0.5 20 10 min

022609.02 30 150 25 0.5 20 6 min

022609.03 30 150 25 0.75 20 4 min

022609.04 30 150 25 1.0 20 3 min

022609.05 30 150 25 1.0 20 3 min

022609.06 30 150 25 1.2 20 2.5 min

022609.07 30 150 25 1.4 20 2 min

022609.08 30 150 25 1.6 20 2 min

022609.09 30 150 25 1.8 21 2 sec

022609.10 30 150 26 1.00 20 180 sec 

022609.11 30 150 26 1.00 20 180 sec 

022609.12 30 150 28 1.00 20 180 sec 
NASA/TM—2011-216966 12 

TABLE 2.—Concluded. 

Run number Sweep angle Velocity, 

mph 

Total 

temperature, 

F 

LWC, 

g/m3

MVD, 

m 

Ice accretion 

time, 

min 

MVD effect 

 

022709.01 30 150 25 0.75 10 6 min

022709.02 30 150 25 0.75 18 6 min

022709.03 30 150 25 0.75 20 6 min

022709.04 30 150 25 0.75 22 6 min

022709.05 30 150 25 0.75 24 6 min

022709.06 30 150 25 0.75 26 6 min

022709.07 30 150 25 0.75 28 6 min

022709.08 30 150 25 0.75 30 6 min

022709.09 30 150 25 0.75 32 6 min

022709.10 30 150 25 0.75 34 6 min

022709.11 30 150 25 0.75 36 6 min
NASA/TM—2011-216966 13 

TABLE 3.—IWT TEST, WOODEN NACA 0012 

Run number Sweep angle Velocity, 

mph 

Total 

temperature, 

F 

LWC, 

g/m3

MVD, 

m 

Ice accretion 

time, 

min 

LWC/Temp 

effect 

 

 

040609.01 30 150 28 0.50 20 180 sec

040609.02 30 150 26 0.50 20 180 sec

040609.03 30 150 24 0.50 20 180 sec

040609.04 30 150 22 0.50 20 180 sec

040609.05 30 150 20 0.50 20 360 sec

040609.06 30 150 18 0.50 20 360 sec

040609.07 30 150 16 0.50 20 360 sec

 

040709.01 30 150 28 0.75 20 240 sec

040709.02 30 150 26 0.75 20 240 sec

040709.03 30 150 24 0.75 20 240 sec

040709.04 30 150 22 0.75 20 240 sec

040709.05 30 150 20 0.75 20 240 sec

040709.06 30 150 18 0.75 20 240 sec

040709.07 30 150 16 0.75 20 240 sec

 

Velocity/Temp 

effect 

 

040709.08 30 200 28 0.75 20 180 sec

040709.09 30 200 26 0.75 20 180 sec

040709.10 30 200 24 0.75 20 180 sec

040709.11 30 200 22 0.75 20 180 sec

040709.12 30 200 20 0.75 20 180 sec

040709.13 30 200 18 0.75 20 180 sec

040709.14 30 200 16 0.75 20 180 sec

040709.15 30 100 16 0.75 20 360 sec

040709.16 30 100 18 0.75 20 360 sec

 

040809.01 30 100 20 0.75 20 360 sec

040809.02 30 100 22 0.75 20 360 sec

040809.03 30 100 24 0.75 20 360 sec

040809.04 30 100 26 0.75 20 360 sec

MVD/Tem 

effect 

 

 

040809.05 30 150 28 0.75 25 240 sec

040809.06 30 150 28 0.75 25 240 sec

040809.07 30 150 26 0.75 25 240 sec

040809.08 30 150 24 0.75 25 240 sec

040809.09 30 150 22 0.75 25 240 sec

040809.10 30 150 20 0.75 25 240 sec
NASA/TM—2011-216966 14 

 

TABLE 3.—Concluded. 

Run number Sweep angle Velocity, 

mph 

Total 

temperature, 

F 

LWC, 

g/m3

MVD, 

m 

Ice accretion 

time, 

min 

Velocity/Temp 

effect 

 

040909.01 30 100 28 0.75 20 360 sec

 

MVD/Tem 

effect 

 

040909.02 30 150 28 0.75 30 240 sec

040909.03 30 150 26 0.75 30 240 sec

040909.04 30 150 24 0.75 30 240 sec

040909.05 30 150 22 0.75 30 240 sec

040909.06 30 150 20 0.75 30 240 sec

040909.07 30 150 18 0.75 30 240 sec

 

040809.08 30 150 18 0.75 25 240 sec

 

040909.09 30 150 16 0.75 30 240 sec

 

040809.10 30 150 16 0.75 25 240 sec

 

LWC/Temp 

effect 

 

040909.11 30 150 16 1.50 20 120 sec

040909.12 30 150 18 1.50 20 120 sec

040909.13 30 150 20 1.50 20 120 sec

040909.14 30 150 22 1.50 20 120 sec

040909.15 30 150 24 1.50 20 120 sec

040909.16 30 150 26 1.50 20 120 sec

040909.17 30 150 28 1.50 20 120 sec
NASA/TM—2011-216966 15 

 

Figure 1.—Wooden NACA 0012 swept wing tip 

in the IRT test section. 

Figure 2.—Wooden NACA 0012 swept 

wing tip in the Goodrich Icing Wind 

Tunnel test section. 

Figure 3.—Experiment set-up in the Goodrich Icing Wind Tunnel. The 

camera on the ceiling of the tunnel is pointing at a 65 angle with 

respect to the leading edge of the airfoil. 
NASA/TM—2011-216966 16 

Figure 4.—Measurement technique. The picture of the grid and the image are combined 

to extract the measurement of the critical distance. The red lines indicate the location 

where the feathers begin forming. 
NASA/TM—2011-216966 17 

(a) (b)

Figure 5.—(a) Energy balance on a single roughness element; (b) The approximated energy balance used to 

interpret the data. 

Qin = (qk + qf) A 

Qout = (qc + qe + 

 qs + qr + qw)A 

Qconduction 

Qin = LWCV, (V2/2) A Qout = hc,avg  (ts - ttot) A 

Figure 6.—Energy balance in the zone where 

roughness elements develop into feathers 

and in the zone where they do not.

Attachment

Attachment

dcr

Feathers 

Qin  Qout 

Roughness 

Elements 

Qin  Qout 

Line 

Line 

Roughness 

Elements 
NASA/TM—2011-216966 18 

Liquid Water Content (LWC)

g/m3

0.0 1.0 0.2 0.4 0.6 0.8 1.2 1.4

Critical

Distance

dcr

mm

2

4

6

8

12

14

16

18

22

24

26

28

0

10

20

30

45o sweep angle 

15o sweep angle

Figure 7.—Change in critical distance with liquid water content for 15 and 45 sweep angles, 

Wooden NACA 0012, V = 150 mph, Ttotal = 25 F, MVD = 20 m. 

Total Temperature

 o

F

5 10 15 20 25 30

Critical 

Distance

dcr

mm

2.0

4.0

6.0

8.0

12.0

14.0

16.0

18.0

22.0

24.0

26.0

28.0

0.0

10.0

20.0

30.0

45o sweep angle 

15o sweep angle 

Figure 8.—Change in critical distance with total temperature for 15 and 45 sweep angles, Wooden 

NACA 0012, V = 150 mph, LWC = 0.75 g/m3

, MVD = 20 m. 
NASA/TM—2011-216966 19 

Drop Size (MVD) 

m

10 15 20 25 30 35 40 45 50

Critical

Distance

dcr

mm

2

4

6

8

12

14

16

18

22

24

26

0

10

20

45o sweep angle

15o sweep angle

Figure 10.—Change in critical distance with drop size for 15 and 45 sweep angles, Wooden 

NACA 0012, V = 150 mph, Ttotal = 25 F, LWC = 0.75 g/m3

. 

Total Temperature

 o

F

5 10 15 20 25 30

Critical 

Distance

dcr

mm

2.0

4.0

6.0

8.0

12.0

14.0

16.0

18.0

22.0

24.0

26.0

28.0

0.0

10.0

20.0

30.0

30o sweep angle, V=150 mph 

30o sweep angle, V=200mph 

Figure 9.—Change in critical distance with total temperature for 30 sweep angles at velocities of 

150 and 200 mph, Wooden NACA 0012, LWC = 0.75 g/m3

, MVD = 20 m. 
NASA/TM—2011-216966 20 

Drop Size (MVD) 

m

10 15 20 25 30 35 40 45 50

Critical

Distance

dcr

mm

2

4

6

8

12

14

16

18

22

24

26

0

10

20

30o sweep angle

Figure 11.—Change in critical distance with drop size for 30 sweep angle, Wooden NACA 0012, 

V = 150 mph, Ttotal = 25 F, LWC = 0.75 g/m3

.

Total Temperature o

F

10 15 20 25 30

Critical

Distance

dcr

mm

2

4

6

8

12

14

16

18

0

10

20

MVD 20 - 30o sweep angle

MVD 25 - 30o sweep angle

MVD 30 - 30o sweep angle

Figure 12.—Change in critical distance with total temperature for 30 sweep angles at MVDs of 

20, 25, and 30 mm, Wooden NACA 0012, V = 150 mph, LWC = 0.75 g/m3

. 

 
NASA/TM—2011-216966 21 

Velocity

mph

100 120 140 160 180 200 220

Critical

 Distance

mm

0

5

10

15

20

25

30

45o sweep angle

15o sweep angle

Figure 13.—Change in critical distance with velocity for 15 and 45 sweep angles, Wooden 

NACA 0012, Ttotal = 25 F, LWC = 0.75 g/m3

, MVD = 20 m.

Velocity

mph

100 120 140 160 180 200 220

Critical

 Distance

mm

0

5

10

15

20

25

30

30o sweep angle

Figure 14.—Change in critical distance with velocity for 30 sweep angle, Wooden NACA 0012, 

Ttotal = 25 F, LWC = 0.75 g/m3

, MVD = 20 m. 
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Measurement of the Critical Distance Parameter Against Icing Conditions on a NACA 0012 

Swept Wing Tip 

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Vargas, Mario; Kreeger, Richard, E. 

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John H. Glenn Research Center at Lewis Field 

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14. ABSTRACT

This work presents the results of three experiments, one conducted in the Icing Research Tunnel (IRT) at NASA Glenn Research Center and 

two in the Goodrich Icing Wind Tunnel (IWT). The experiments were designed to measure the critical distance parameter on a NACA 0012 

Swept Wing Tip at sweep angles of 45°, 30°, and 15°. A time sequence imaging technique (TSIT) was used to obtain real time close-up 

imaging data during the first 2 min of the ice accretion formation. The time sequence photographic data was used to measure the critical 

distance at each icing condition and to study how it develops in real time. The effect on the critical distance of liquid water content, drop 

size, total temperature, and velocity was studied. The results were interpreted using a simple energy balance on a roughness element.

15. SUBJECT TERMS

Ice accretion; Swept wings; Scallop formations 

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