David N. Anderson

Ohio Aerospace Institute, Brook Park, Ohio

Galdemir C. Botura

B.F. Goodrich Aerospace, Uniontown, Ohio

Andy P. Broeren

University of Illinois at Urbana-Champaign, Urbana, Illinois

A Study of Scaling for Intercycle

Ice Accretion Tests

NASA/CR—2003-211825

June 2003

AIAA–2001–0834
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David N. Anderson

Ohio Aerospace Institute, Brook Park, Ohio

Galdemir C. Botura

B.F. Goodrich Aerospace, Uniontown, Ohio

Andy P. Broeren

University of Illinois at Urbana-Champaign, Urbana, Illinois

A Study of Scaling for Intercycle

Ice Accretion Tests

NASA/CR—2003-211825

June 2003

National Aeronautics and

Space Administration

Glenn Research Center

Prepared under Cooperative Agreement NCC3–884

Prepared for the

39th Aerospace Sciences Meeting and Exhibit

sponsored by the American Institute of Aeronautics and Astronautics

Reno, Nevada, January 8–11, 2001

AIAA–2001–0834
Acknowledgments

The study reported here was jointly supported by the FAA, NASA, and B.F. Goodrich. The authors wish to thank

Jim Riley and Gene Hill of the FAA, Gene Addy of NASA, and Dave Sweet of B.F. Goodrich for their

guidance during the planning of the tests and for their support and direction during the course

of the study. In addition, Tim Hawk of NASA, Manny Rios and Chris Dumont of the FAA,

and Pete Brown, Jerry Hansard, and Jason McHood of B.F. Goodrich provided

invaluable assistance during the execution of the tests.

Available from

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22100

This report contains preliminary

findings, subject to revision as

analysis proceeds.

The Aerospace Propulsion and Power Program at

NASA Glenn Research Center sponsored this work.

Available electronically at http://gltrs.grc.nasa.gov
NASA/CR2003-211825 1 

Abstract

The Ruff method with matched scale and reference 

velocity was used to determine appropriate 1⁄2-scale test 

conditions to simulate a full-size icing encounter for an 

NACA 23012 wing section protected with a pneumatic 

boot de-icing system. Intercycle ice accretions were 

recorded on a 36-in.-chord model used to represent 1⁄2 

scale and compared with a hybrid reference model (fullsize leading-edge and truncated aft section) 

representing a 72-in.-chord full-size airfoil. The 

intercycle ice thickness and extent of icing for the scale 

tests generally compared well with those from the 

reference model. However, the scale tests did not 

reproduce the location and number of feather rows seen 

in the reference tests aft of the main ice shape. Many of 

the differences observed were believed to result from 

not scaling the pneumatic boot design along with the 

model size for these tests. 

Nomenclature

Ac Accumulation parameter, dimensionless 

b Relative heat factor, dimensionless 

c Model chord, in. 

cp Specific heat of air, Btu/lbm °R 

cp,ws Specific heat of water at the surface 

temperature, Btu/lbm °R 

hc Convective heat transfer coefficient, 

Btu/hr ft2

 °R 

hG Gas-phase mass transfer coefficient, 

lbm /hr ft2

k Thermal conductivity of air, Btu/hr ft °R 

K0 Modified inertia parameter, dimensionless 

LWC Cloud liquid-water content, g/m3

MVD Water droplet median volume diameter, μm 

n Freezing fraction, dimensionless 

p Static pressure, psi 

pw Vapor pressure of water in the atmosphere, psi 

pww Vapor pressure of water over liquid water, psi 

Pr Prandtl number, dimensionless 

r Recovery factor, dimensionless 

rle Leading-edge radius of airfoil, in. 

Re Reynolds number of model, dimensionless 

S Chordwise distance from leading edge, in. 

tf Freezing temperature of water, 0°F 

ts Surface temperature, °F 

tst Static temperature, °F 

V Air velocity, mph 

α Angle of attack, ° 

β Local collection efficiency, dimensionless 

β0 Collection efficiency at stagnation line, 

dimensionless 

θ Air energy transfer terms in energy balance, °F 

κ Constant exponent in equation (1) 

Λf Latent heat of freezing, Btu/lbm 

Λv Latent heat of vaporization, Btu/lbm 

ρι Ice density, lbm/ft3

τ Accretion time, min 

φ Droplet energy transfer terms in energy 

balance, °F 

Subscripts 

R Reference conditions 

S Scale conditions 

Introduction

To minimize test-section blockage in icing tunnels, it is 

often necessary to scale the test model. Two 

approaches to scaling are: size scaling, in which the 

entire model is geometrically reduced in size while 

maintaining the same non-dimensional shape, and 

 * Senior Research Associate, Member AIAA 

 † Senior Product Engineer, R&D, De-icing and 

Specialty Systems Division, Member AIAA 

 ‡ Post-Doctoral Research Associate, Department of 

Aeronautical and Astronautical Engineering, Member 

AIAA

A STUDY OF SCALING FOR INTERCYCLE ICE ACCRETION TESTS 

David N. Anderson* 

Ohio Aerospace Institute 

Brook Park, Ohio 44142 

Galdemir C. Botura†

BFGoodrich Aerospace 

Uniontown, Ohio 44685 

Andy P. Broeren‡

University of Illinois at Urbana-Champaign 

Urbana, Illinois 60680 
NASA/CR2003-211825 2 

hybrid scaling, in which a full-size leading edge is 

attached to a truncated after body. 

For size scaling, methods are required to determine 

scaled test conditions that will simulate the full-scale 

icing encounter and produce the same non-dimensional 

ice shape. Similarity parameters1, 2 have been identified 

which best describe the important phenomena of icing 

physics, including the flowfield approaching and 

around the model, the droplet trajectories, the quantity 

of ice accumulation, and the surface heat balance. Each 

of the commonly used scaling parameters will be 

described below. 

For hybrid scaling, the after body is designed so that the 

flow around the leading edge and the droplet 

impingement characteristics reproduce those of the fullscale airfoil. Consequently, the test conditions used for 

the hybrid-scaled test simply match the full-size 

(reference) conditions. The hybrid airfoil is more 

attractive for testing than the full-size model since the 

actual chord length is less than the equivalent full-scale 

airfoil. This reduces potential problems associated with 

model blockage, mounting, weight, fabrication cost, 

etc. 

The pneumatic boot de-icing system works by 

periodically inflating tubes made of an elastic material 

to break and shed the intercycle ice accreted since the 

last activation. To be successful, scaled tests should 

provide a good simulation (relative to the airfoil size) of 

the intercycle ice quantity, location and size of 

protrusions and icing limits for given icing situations. 

In the present series of tests, scaling was applied for 

both size scaling and hybrid scaling based on a full-size 

model of an NACA 23012 airfoil with a 72-in. chord. 

Tests were made in the BFGoodrich Aerospace Deicing and Specialty Systems Division (DSSD) Icing 

Wind Tunnel (IWT). The full-size airfoil would have 

been too large to be tested in this facility, so the hybridscaled model with a full-size leading edge served as the 

reference model. The scale test used a 36-in.-chord 

NACA 23012 model with the scaled test conditions 

determined from the reference values by applying the 

Ruff scaling method using constant velocity. Both size 

scaling and hybrid scaling methods will be described in 

this paper along with model descriptions and 

experimental ice shapes recorded in the tests. 

Size Scaling

Scaling Parameters

One goal in testing models with the size reduced from 

the desired reference dimensions, is to produce accreted 

ice-shapes whose coordinates are non-dimensionally 

the same as would have been obtained with the 

reference test. To do this, similarity of the physics of 

ice accretion has to be assured in these areas: the 

flowfield around the model, droplet trajectories, total 

water catch and, for glaze ice, heat balance on the 

surface. Similarity of water-film phenomena may also 

need to be satisfied for glaze ice,3

 but these will not be 

discussed here. 

The flowfield can be simulated by using a model that is 

dimensionally similar to the full-scale (reference) 

article, by using the same angle of attack and by 

matching scale and reference values of Re and M. For 

icing encounters, the speeds involved are usually low 

enough that M should have little effect and is neglected. 

Re is usually ignored as well by arguing that any ice 

accretion will trip the boundary layer and the flow will 

then be independent of Re. 

Similarity of droplet trajectories and, therefore, droplet 

collection efficiencies, can be obtained by matching the 

modified inertia parameter, K0, of Langmuir and 

Blodgett.4

 An exact determination of K0 is somewhat 

involved, but it can be approximated using: 

where κ is a constant and rle is the leading-edge radius 

of the model. For the NACA 23012, rle = 0.0158c. The 

use of 2rle as the length scale in eq. (1) instead of chord 

provides consistency between airfoil and cylinder 

studies (much of the Langmuir and Blodgett analysis 

was for cylinders). Note also that this equation can 

give different values of K0 for different airfoil forms 

even if the chord is the same. Eq. (1) results from using 

a straight line to approximate the droplet drag vs droplet 

Reynolds number curve over a portion of the droplet 

Reynolds number range. This approximation was used 

to develop equations for icing scaling methods as early 

as 1955.5

 κ is the slope of this line, and a value of κ = 

0.38 provides modified inertia parameters sufficiently 

close to values found from more involved procedures. 

Equating the scale and reference values of K0 gives a 

practical expression that relates scaled conditions to 

reference conditions to satisfy droplet trajectory 

similarity: 

The collection efficiency at the stagnation line, β0, is 

uniquely related to K0 by the expression given by 

( )

2 1

constant

2

- -

0

le

MVD V K = r p

κ κ

κ (1)

(2) 

0.62 .023 0.38

SS S S le

R R le R R

MVD V r p = MVD V r p

−                      
NASA/CR2003-211825 3 

Langmuir and Blodgett: 

When the stagnation-line collection efficiency matches 

for two geometrically-similar models, so does the 

collection efficiency everywhere on the model. 

Similarity of ice accumulation results from a match of 

the accumulation parameter, Ac: 

Finally, the energy balance at the surface can be written 

following Messinger6

 in the form, 

where n is the freezing fraction; φ, the water energy 

transfer parameter; θ, the air energy transfer parameter; 

and b, the relative heat factor of Tribus, et al.7

 The 

parameters φ, θ and b are defined in the following 

equations. 

The convective heat-transfer coefficient, hc, in eq. (8) is 

found from 

Here Re is based on air properties, on the free-stream 

velocity, V, and on a length of twice the leading-edge 

radius, rle. 

Ruff Scaling Method

Using combinations of the parameters K0, Ac, n, φ, and 

θ, Ruff2 performed scaling tests in the AEDC R-1D 

icing tunnel. He found that the scaled ice shapes agreed 

best with reference shapes when all five of these scaling 

parameters were matched to the reference values. This 

approach to scaling is known as the Ruff (or AEDC) 

method. Because the R-1D facility allows altitude 

simulation, Ruff’s work included the calculation of the 

scale test-section pressure in addition to tst, MVD, LWC

and time. With only five scaling parameters to 

establish these five test conditions, the sixth test 

condition, scale velocity, was selected arbitrarily. The 

Ruff method has also been used in sea-level tunnels in a 

modified form in which θ is ignored and just K0, Ac, n, 

and φ are matched, with the scale velocity chosen by 

the user. 

In Ruff’s experiments, the scale velocity was often 

simply equated with the reference value. Tests at 

NASA3,8 with large glaze-ice accretions for n < 0.6 

have shown that when the Ruff method with a constant 

scale velocity was used to determine scale test 

conditions, the quantity of ice accreted and accretion 

limits were properly simulated, but features such as 

horn angle and location were not exactly the same as 

for the reference ice shape. The best similarity of 

scaled and reference ice shapes was achieved when the 

Ruff method was used with an additional similarity 

parameter, such as the Reynolds number or Weber 

number, to determine the scale velocity. The use of 

either of these parameters results in a scale velocity 

higher than the reference value. For the present study 

of intercycle ice, the short times between boot 

activation resulted in small accretions for which 

features like horns do not have time to develop. Thus, 

the matching of scale and reference velocities was 

judged to be an adequate approach. 

Note from equation (6) that matching φS and φR, with VS

= VR, gives tst,S = tst,R. For sea-level tunnels, when 

velocity is matched, the static pressure will also match, 

and equation (2) reduces to 

Once the droplet size, MVDS, was determined from eq. 

(10), by solving nS = nR using eqs. (5–9), the liquidwater content, LWCS, was found. Finally, eq. (4) gave 

the scale accretion time from by setting AcS = AcR. 

Hybrid Scaling

Description of Hybrid Airfoil Design Method

The term “hybrid” is used to describe the airfoil that has 

a nose (or leading-edge) section with full-size airfoil 

coordinates and a specially designed, truncated, 

n

c

b

p ws

f

= + 



 



 ,

Λ φ θ

(5) 

2 c

le i

LWC V A rτ

ρ = (4) 

φ =−− t t

V

c f st

p ws

2

2 ,

(6) 

θ = −−















 +

 −



 

 tt r  V

c

h

h

p p

p

s st

p

G

c

ww w v

2

2

Λ (7) 

b =

LWC V c

h

0 p,ws

c

β (8) 

0.62

le S

S R

le R

r MVD MVD r

  =    

(10)

( )

( )

0.84

0

0 0.84

0

1.4 1/ 8

1 1.4 1/ 8

K

K

β − =

+ −

 (3)

le

. .

c r

. Re Pr k h

2

1 14 5 4

= (9)
NASA/CR2003-211825 4 

afterbody. The basic hybrid design method uses an 

iterative process to update the contour of the after body 

until the droplet impingement characteristics and 

pressure distribution of the common nose section match 

those of the full-size airfoil. The extent of the common 

nose section for the present tests is illustrated in figure 

1 as an example. 

The design routine incorporates several analysis tools. 

For example, for the present study the inviscid 

flowfield about the airfoil was calculated using the 

Eppler code.9

 The droplet trajectories and resulting 

impingement characteristics were calculated using the 

AIRDROP code.10 The results from both of these codes 

are known to compare very well with experiments. 

While the design process was computerized, it did rely 

on the experience of the designer to make judgments 

about how well the pressure distributions and droplet 

impingement characteristics over the common nose 

section of the two airfoils matched. Saeed11 and Saeed, 

Selig and Bragg12,13,14 describe the method in detail and 

include several illustrative examples. 

For this study, the chord length of the hybrid airfoil (36 

inches) was 1⁄2 the desired full-scale NACA 23012 

airfoil chord length. The design process used a 40-μm 

water droplet, with the results verified for other sizes. 

The agreement in impingement curves between the 

hybrid and full-scale airfoils was generally very good 

when the droplet size was less than the design value. 

This was true because as droplet size is reduced, the 

droplet inertia is also reduced and particles become 

more responsive to the flowfield. Since the hybrid 

airfoil was meant to simulate the full-scale airfoil over a 

0 – 4° angle of attack range, the design procedure used 

an angle of attack of two degrees. As shown in figure 

2(a), the local impingement efficiencies, β, compared 

very well for this case. 

A simple trailing-edge flap was added to achieve better 

conformity with full-scale at the off-design angles of 

attack, 0 and 4°. The required flap deflection for 0° 

angle of attack was –3.5° (upward). With this flap 

setting at 0°, the agreement between the hybrid and 

full-scale collection efficiency was as good over the 

entire surface as that shown in figure 2(a). 

The results for the 4° case are shown in Fig. 2(b), with 

a 2° flap deflection (downward). Again, the agreement 

was very good everywhere, except for the lower surface 

impingement limit. The reason for this disparity is that 

Figure 2. Comparison of the Local Collection 

Efficiency Between Full-Scale and Hybrid Airfoils. 

MVD, 40 μm. 

(a) Angle of Attack, 2°. 

s/c

-0.08 -0.06 -0.04 -0.02 0.00 0.02 0.04 0.06 0.08

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8 Full-Scale

Hybrid

β

(b) Angle of Attack, 4°; Hybrid Flap Deflection, 2°. 

s/c

-0.20 -0.15 -0.10 -0.05 0.00 0.05

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8 Full-Scale

Hybrid

β

NACA 23012

Hybrid

Figure 1. Comparison of Full-Scale NACA 23012 

Airfoil with the Hybrid. 
NASA/CR2003-211825 5 

the lower-surface limit of the common nose section was 

not far enough aft to simulate the full-scale 

impingement. The lower-surface limit of the nose 

section could have been moved farther aft, but this 

would have resulted in a much thicker airfoil, which 

was undesirable (see fig. 1). The difference in the 

impingement characteristics shown in figure 2(b) is not 

considered critical since β was small (less than 0.1) in 

this region. It should be noted that while maintaining 

the proper flap setting is important to match the droplet 

impingement, it might not be critical in determining the 

final ice shape. This effect is discussed in more detail 

in reference 14. 

In addition to assisting in the after-body design, the 

droplet impingement curves were also used to 

determine the de-icing boot coverage. The approximate 

limits were 5% chord on the upper surface and 9% on 

the lower surface. (Note that these figures are to the 

edge of the boot rubberthe active portion of the boot 

was about one inch upstream of these locations.) For 

convenience, the upper surface limit of the common 

nose section was shifted aft to 7.5% chord and the 

lower-surface limit was shifted aft to 10% chord. 

Scaling Tests

Two series of tests were performed in the BFGoodrich 

DSSD IWT to compare results of the Ruff scaling 

method with the hybrid method for intercycle ice. In 

the first, a 36-in.-chord NACA 23012 airfoil was used 

to represent a half-scale model. The second series of 

tests used a hybrid scaled model to represent the 

reference, or full-scale, 72-in.-chord NACA 23012 

wing section. 

Test Facility

The IWT (fig. 3) is an atmosphericpressure, closed-loop refrigerated tunnel 

measuring 40 ft by 70 ft overall. It has an 

external 200-hp electric motor driving a 

79-in.-diameter axial fan to provide wind 

velocity, a 70-ton-capacity refrigeration 

system for cooling, and two 75-hp air 

compressors dedicated to supply air for 

the atomizing nozzles. A honeycomb 

flow straightener is mounted upstream of 

the spray bars. There are seven spray 

bars, heated to prevent water freeze out, 

with NASA-type spray nozzles to produce 

the icing cloud. The test section is 22 in. 

wide, 44 in. high, and 60 in. long. Models 

are typically mounted horizontally 

between 1-inch-thick aluminum turning 

plates 30 in. in diameter; these plates can 

be rotated 360°, even with the tunnel in operation. 

Access to the model can be attained through two hinged 

side windows with heated glass panels and a 52-in. 

section of floor which hinges down to allow full-width 

access. The bottom door and the ceiling also have 

heated windows, and unheated acrylic panels can be 

installed in the turning plates for additional viewing if 

necessary. The test section is vented to ambient static 

pressure for baseline pressure measurements. 

The IWT can run at speeds from 30 to 230 mph, though 

top speed is limited by temperature, spray time, and 

percent blockage of the test model. Velocity is 

measured with a pitot tube located at the entrance to the 

test section, and is density-corrected for a “true” 

velocity reading. Temperature in the tunnel is 

microprocessor-controlled from –22 ° to +32 °F. It can 

be held within ±1 °F of setpoint through most of this 

range. Spray conditions can be varied from about 

0.1 g/m3

 to over 3.0 g/m3, with droplet sizes from 14 to 

over 40 microns, depending on velocity and number of 

nozzles used. 

A 4-ft-by-12-ft-by-8-ft cold room, attached to the test 

section, is available for uses such as casting of ice 

structures and determining ice adhesion values. The 

cold room can provide temperatures as low as –65 °F. 

Scale Model

The 36-in.-chord NACA 23012 airfoil used here to 

represent a scale model is shown in figure 4. It had a 

removable leading edge and main body manufactured 

in aluminum alloy. The leading edge had a nominal 

recess to allow a flush installation of the pneumatic deicer type 29 S, constructed of 0.085-in.-thick (nominal) 

rubber and fabric blankets. The de-icer contained five 

Figure 3. BFGoodrich Aerospace De-icing and Specialty Systems 

Division Icing Wind Tunnel (IWT). 

Heat Exchanger

Honeycomb

Flow

Straightener

7 Spray Bars

22" x 44"

Test Section

Axial Fan

140,000 CFM

200-hp

Electric Motor

Cold Room
NASA/CR2003-211825 6 

spanwise de-icing tubes that inflated during 

activation. The nominal operating pressure of 

the pneumatic system was 18 psig. 

There are currently no guidelines dealing with 

boot design or operation for scale tests. 

Consequently, the same conventional boot 

tube size was used for both the reference 

(hybrid) and scale models. Boot inflation and 

deflation rate characteristics were equivalent 

for all tests, as well, although times between 

boot cycles were scaled in the same ratio as 

the total accretion times as given by the Ruff 

scaling equations. A thorough scaling study 

would consider scaling of boot tube sizes and 

operating characteristics in addition to scaling 

model size. 

Hybrid Model 

The hybrid model also had a chord of 36 in., 

but the leading edge portion had the same 

coordinates as a 72-in. NACA 23012 airfoil. 

It was designed with a removable leading 

edge (Fig. 5) manufactured in fiberglass with 

three internal ribs to improve the airfoil 

stiffness. The main body was manufactured 

in aluminum alloy with a moveable trailing 

edge to allow adjustment to the pressure 

distribution on the leading edge as previously 

described. The pneumatic deicer was again 

the 29 S type integrated into the leading edge 

as described above, but for the hybrid model, 

ten spanwise de-icing tubes were used. The 

nominal operating pressure for the pneumatic 

de-icer was 20 psig. 

Both models were mounted horizontally 

midway between the IWT ceiling and floor. 

De-icing System

For both tests, two pneumatic de-icing boots 

were used, with one on the model and one 

outside the tunnel. This arrangement made it 

possible to reproduce the inflation rate of a 

typical airplane de-icer. Supply pressure to 

inflate the de-icers was obtained from the 

facility air compressors. The system timer 

controlled the pressurized air to inflate both 

of the de-icers simultaneously. A pressure 

transducer was installed close to the de-icer to provide 

continuous monitoring of the inlet pressure. When the 

solenoids were de-energized, an ejector flow control 

valve provided the vacuum necessary to keep the deicers deflated. 

The inflation-deflation process was controlled 

electronically and was the same for both models. The 

boot cycle time (the time between activations) was 

adjusted for each icing condition to provide a range of 

intercycle ice accretion sizes. For the scale tests, the 

boot cycle time was scaled from the reference values in 

Figure 4. 36-in.-Chord Model of NACA 23012 Wing Section. 

36.00"

Boot Edge

1.00" Tube

1.25" Tubes

1.00" Tubes

Inactive

 

2.9"

1.3"

Inactive

Boot Edge

Figure 5. Hybrid Model with Full-Size (72-in.-Chord) NACA 

23012 Leading Edge and Truncated After Body. 

36.00"

3

1.00"

Tubes

2

1.25"

Tubes

Boot Edge

5.7" Inactive

Boot Edge

Inactive

2.5"

5 1.00" Tubes
NASA/CR2003-211825 7 

the same proportion as the total accretion time. These 

times were determined by the Ruff scaling method to 

satisfy Ac,S = Ac,R, with Ac defined by eq. (4). 

Test Procedure

Prior to each spray, the tunnel airspeed and temperature 

were stabilized at the desired test values. The spray 

was then initiated. LWC and MVD reached steady-state 

within seconds of turning on the nozzles. For each test, 

the boot was activated to inflate and deflate a few 

seconds after the start of spray to simulate a justcompleted cycle. Then it was cycled 2 or 3 more times 

during the spray at the intervals indicated in Table I 

(“Boot Cycle”) to achieve a steady-state with regard to 

any residual left after multiple de-icer activations. 

Finally, just before the next scheduled de-icer cycle the 

spray was shut off to preserve the largest accretion of 

intercycle ice. The tunnel fan was then stopped, 

photographs taken of the ice, the cold-room hatch was 

opened, and the tunnel entered to make a tracing of the 

ice shape. One, or sometimes two, span-wise location 

was identified as being typical, or having particularly 

pronounced features. A thin aluminum plate was 

heated and applied normal to the model surface at the 

selected locations to melt a chord-wise groove into the 

ice down to the airfoil surface. A cardboard template 

from which the shape of the airfoil leading edge region 

was cut was then placed into this groove to fit snugly 

against the clean surface of the wing. Finally, the 2-

dimensional profiles of the ice-shape features were 

traced onto these templates with a pencil. 

The ice was then cleaned from the model, the hatch 

closed, and the tunnel, spray bars and deicer controller 

set to run the next set of conditions. 

Ice-shape tracings obtained during the tests were later 

digitized using automatic line-tracing software. The 

coordinates were then normalized with respect to the 

representative chords shown in Table I to permit direct 

comparison of the reference and scale results. 

Test Conditions

Table I lists the test conditions. These test conditions 

were adjusted for the icing scaling study and some do 

not reflect conditions within the FAR 25 Appendix C 

icing envelope. For example, for runs 54/4S, 7/2, 7/3, 

and 8/3, the combination of temperature, MVD, LWC

and duration are outside of (more severe than) the icing 

envelope. Corresponding reference and scale tests are 

grouped together in the table. The chords listed are the 

values consistent with the leading-edge size, which is 

also the actual model chord for the scale model. For the 

hybrid model, the value listed in the table is not the true 

chord of the test model, but rather that which a full 

NACA 23012 airfoil would have (72 in.) if it had the 

same leading edge section as the hybrid model. 

Angles of attack were 0 and 4° and static temperatures 

were 14 and 21 °F. Except for one pair of tests at 

175 mph, all tests were made with an airspeed of 200 

mph. Reference MVD’s were 20 and 31 μm. Reference 

LWC’s covered the range of 0.32 to 1.56 g/m3

. Total 

spray times for the reference case were 8.6 to 34.5 min. 

Run Date Model c, 

in 

α, 

° 

tst,

°F 

ttot,

°F 

V, 

mph 

MVD,

μm 

LWC,

g/m3 τ,

min

Boot

Cycle,

sec 

K0 Ac n b φ, 

°F 

θ, 

°F

7/5R 3-17-00 Ref 72 0 14.0 21.2 200.0 20 0.45 12.4 181 0.73 0.14 0.56 0.32 16.3 20.5

8/2 3-17-00 Ref 72 0 14.0 21.2 200.0 20 0.45 12.4 181 0.73 0.14 0.56 0.32 16.3 20.5

54/4S 12-9-99 Scale 36 0 14.0 21.2 200.0 13 0.64 4.4 64 0.74 0.14 0.56 0.32 16.3 20.5

8/4 3-15-00 Ref 72 0 14.0 21.2 200.0 31 0.32 34.5 509 1.40 0.27 0.60 0.30 16.3 20.6

1/2 11-30-99 Scale 36 0 14.0 21.2 200.0 20 0.45 12.2 180 1.41 0.27 0.60 0.30 16.3 20.6

7/2 3-15-00 Ref 72 0 21.0 26.5 175.0 31 1.56 8.6 170 1.29 0.39 0.13 1.32 9.7 12.0

45/46 12-8-99 Scale 36 0 21.0 26.5 175.0 20 2.20 3.0 60 1.29 0.39 0.13 1.32 9.7 12.1

7/3 3-15-00 Ref 72 4 14.0 21.2 200.0 31 1.38 8.6 170 1.39 0.39 0.23 1.29 16.3 20.5

42/44 12-8-99 Scale 36 4 14.0 21.2 200.0 20 1.95 3.1 60 1.40 0.39 0.23 1.29 16.3 20.5

8/3 3-14-00 Ref 72 4 21.0 28.2 200.0 31 0.46 34.3 509 1.39 0.40 0.23 0.43 9.3 10.3

7/5 12-1-99 Scale 36 4 21.0 28.2 200.0 20 0.65 12.1 180 1.40 0.39 0.23 0.43 9.3 10.3

Table I. FAA/BFG Intercycle Ice Tests 

Scaling Conditions and Similarity Parameters 
NASA/CR2003-211825 8 

De-icing boot cycle times for the reference case were 

170 to 509 sec. Table I also gives the similarity 

parameters that corresponded with the test conditions 

for each run. The accumulation parameter given is that 

based on the boot cycle time. 

Results

The intercycle ice that formed on each model exhibited 

primarily two types of features: (1) an ice cap or main 

ice structure that accreted at the stagnation line and 

extended some distance aft on both upper and lower 

surfaces and (2) feathers that formed at minor 

discontinuities in the model surface aft of the main ice. 

Much of the feather growth began in continuous spanwise rows at the nearly imperceptible stitch lines 

between the active tubes of the de-icing boots. When 

these in-line feathers had grown high enough, shedding 

was observed now and then along portions of the 

original growth line with new feathers immediately 

starting to form where the old had been. Thus, the final 

appearance was that of a number of discrete span-wise 

segments of ice structures with varying heights above 

the model surface. In the two-dimensional ice profile 

tracings, these feather rows had the appearance of 

protuberances whose size and location often depended 

on the span-wise location at which the tracing was 

made. 

To determine the variability of intercycle ice accretion, 

some of the tests were repeated. Figure 6 is a 

comparison of ice shapes from two repeated tests with 

the hybrid model. On this and subsequent plots, the 

upper and lower extents of the active portions of the deicing boots are marked. The boots covered the same 

portion of the leading edge of both models, relative to 

the chord corresponding with each model’s leadingedge size. Tracings of the ice were taken at locations 

felt to capture the most prominent features; 

consequently, centerline positions along the span were 

not always recorded. In the case of the two results 

shown in figure 6, run 7/5R was traced at the center, 

while 8/2 was traced 2 in left of center. The two 

tracings showed features of similar size and chord-wise 

location and generally agreed well. 

The random nature of some features due to feather 

growth and shedding can best be illustrated by looking 

at variations along the span of the model. The 

examples in figure 7 were also taken from the hybridmodel tests. Figure 7(a) presents shapes from one of 

the tests with the longest time between boot firings, 509 

sec. Significant features of the ice were able to form in 

this time. In figure 7(a), the size and shape of the main 

ice cap can be seen to have been consistent at the two 

locations, while the size and location of the feathers aft 

of the main shape were not exactly duplicated. 

Similarly, for the example given in figure 7(b) the 

leading-edge accretion and extent of icing were 

repeated for the two span-wise locations, while the 

sizes and locations of feather structures were not. Thus, 

while the main ice shapes repeated well from run to run 

Figure 6. Ice-Shape Repeatability with Hybrid Model. 

AOA, 0°; tst, 14 °F; V, 200 mph; MVD, 20 μm; LWC, 

0.45 g/m3

; Time Between De-Icer Cycles, 180 s. 

(a) AOA, 0°; tst, 14 °F; V, 200 mph; MVD, 31 μm; 

LWC, 0.32 g/m3

; Time Between De-Icer Cycles, 509 s. 

Figure 7. Ice-Shape Variation Along the Span. 
NASA/CR2003-211825 9 

for a given model, protuberances aft of the main shape 

were typically random in size and location, primarily 

due to feather shedding and regrowth. 

The two models used in this study were designed with 

objectives other than scaling. Consequently, the same 

boot tube size was used for both the reference and scale 

model. Because the hybrid (reference) model contained 

twice as many deicer tubes as the scale model (see 

figures 4 and 5), the reference model produced more 

rows of feathers over the boot than did the scale model. 

Thus, the scale tests could not be expected to simulate 

the location of these feather rows adequately. 

Figure 8 gives examples of the scaling results. Figures 

8(a), (b) and (c) show results obtained at 0° AOA, 

while 8(d) and (e) are for 4° AOA. In general, the scale 

test accurately simulated the thickness of the leadingedge ice and chord-wise extent of icing. Successful 

scaling of these two characteristics indicates that the 

accumulation parameter, Ac, and modified inertia 

parameter, K0, were properly matched in determining 

the scale conditions. 

In figure 8(a), it can be seen that the scale test produced 

ice without some of the feather structures seen in the 

reference. From the above discussion, this result is not 

surprising. However, even in the stagnation region, the 

scaled ice lacked some of the reference-ice 

protuberances, giving the scaled ice a generally 

smoother appearance. The reasons for this smoother 

ice are not understood at this time, but the possible 

effect of the boot inflation-deflation characteristics 

should be considered. A rigorous scaling test would 

(b) AOA, 4°; tst, 14 °F; V, 200 mph; MVD, 31 μm; 

LWC, 1.38 g/m3

; Time Between De-Icer Cycles, 170 s. 

Figure 7 (concluded). 

(a) Ice Accretions for Runs 7/5R (Reference) and 

54/4S (Scale). AOA, 0°. 

(b) Ice Accretion for Runs 8/4 (Reference) and 1/2 

(Scale). AOA, 0°. 

Figure 8. Comparison of Reference and Scale Ice 

Shapes. NACA 23012 Airfoil; Reference Chord, 

72 in.; Scale Chord, 36 in. Test Conditions for each 

Run Listed in Table I. 
NASA/CR2003-211825 10 

include scale and reference model designs for which all 

physical features, including boot tube sizes, have the 

correct scale, and boot inflation, dwell and deflation 

times would also be scaled. For the present tests, this 

rigor was not applied. 

The results shown in figure 8(b) give an example for 

which both the size and approximate position of the 

feather structures of the reference were simulated by the 

scale test. However, the small horns at the sides of the 

main ice shape in the reference test were missing from 

the scale results, so the match of shapes is not exact. 

For the examples in figure 8(c) the scale and reference 

shapes generally matched in thickness over the active 

boot surface, while the scaled shape showed fewer rows 

of feathers. The aft edge of the inactive boot material 

joined the model skin at x/c = 0.05 on the upper surface 

of the reference model, and the profile of a row of large 

feathers can be seen at this location. This joint was at 

x/c = 0.08 for the scale model, and small feathers were 

formed forward of this surface discontinuity as well. 

At 4° AOA (figs. 8(d) and (e)) the lower-surface 

accretion limit for the 1⁄2-scale model extended farther 

aft than for the reference (hybrid) model. It is 

interesting to note from figure 2(b) that the lowersurface impingement limit for a full-size 72-in.-chord 

NACA 23012 airfoil was also predicted to be farther aft 

than that for the hybrid model. Thus, for the 4° AOA 

tests the hybrid model aft of the lower-surface active 

boot region may not have adequately represented a fullsize reference model, and ice-accretion characteristics 

cannot be meaningfully compared for the two test 

models in that region. 

As with the 0° AOA tests, the scale tests showed fewer 

rows of feathers than the reference tests, and the main 

ice was smoother for the scale test. The scale tests did 

simulate the sizes of the feather structures reasonably 

well. Figures 8(d) and (e) show rows of feathers near 

(e) Ice Accretions for Runs 8/3 (Reference) and 7/5 

(Scale). AOA, 4°. 

Figure 8. (concluded). 

(c) Ice Accretions for Runs 7/2 (Reference) and 45/46 

(Scale). AOA, 0°. 

(d) Ice Accretion for Runs 7/3 (Reference) and 42/44 

(Scale). AOA, 4°. 

Figure 8. (con’t.) 
NASA/CR2003-211825 11 

the lower-surface boot termination for both the 

reference (x/c = 0.09) and scale (x/c = 0.12) models. 

Because the boots extended farther relative to the chord 

for the scale model, the feathers formed for the two 

models were understandably in different positions. 

Summary and Conclusions 

The Ruff method with matched scale and reference 

velocity was used to determine appropriate 1⁄2-scale test 

conditions to simulate a full-size icing encounter for an 

NACA 23012 wing section protected with a pneumatic 

boot de-icing system. Intercycle ice accretions were 

recorded on a 36-in.-chord model that represented 

1⁄2 scale and compared with those on a reference model 

(a hybrid with a full-size leading-edge and truncated aft 

section) representing a full-size 72-in.-chord airfoil. 

Relative to the chord, the intercycle ice thickness and 

extent of icing for the 1⁄2-scale tests generally compared 

well with those from the reference model. However, 

the scale tests typically produced ice with fewer rows of 

feathers. These results appear to be due to the use of 

twice the number of de-icing boot tubes for the 

reference model than for the scale. Smoother ice also 

resulted for the scale model in the stagnation region. 

While the reason for this difference is not understood at 

this time, it may be the result of not scaling the boot 

inflation, dwell and deflation times along with the 

model size. 

Although the tests reported here did not demonstrate 

fully successful scaling, the differences between the 

scale and reference results appeared to be linked 

primarily to the differences in the relative sizes of the 

pneumatic tubes used for the two models. Further 

scaling studies are needed in which boot tube sizes and 

operating characteristics are both scaled with model 

size. It is likely that with properly scaled ice-protection 

systems, scale modelsusing either full geometric 

scaling or hybrid scalingcan be used to predict 

intercycle ice accretion characteristics of larger models. 

References

1

 Charpin, Francois and Fasso, Guy, “Icing Testing in 

the Large Modane Wind Tunnel on Full Scale and 

Reduced Scale Models,” L’Aeronautique et 

l’Astronautique, no. 38, 1972. English translation 

published as NASA TM–75373, March 1979. 

2

 Ruff, G.A.: “Analysis and Verification of the Icing 

Scaling Equations,” AEDC–TR–85–30, vol. 1 (Rev), 

March 1986. 

3

 Anderson, David N. and Ruff, G.A., “Evaluation of 

Methods to Select Scale Velocities in Icing Scaling 

Tests,” AIAA–99–0244, January 1999. 

4

 Langmuir, Irving and Blodgett, Katharine B.: “A 

Mathematical Investigation of Water Droplet 

Trajectories,” Army Air Forces Technical Report No. 

5418, February 1946. 

5

 Sibley, E.J. and Smith, R.E., “Model Testing in an 

Icing Wind Tunnel,” Lockheed Aircraft Corporation, 

California Division, Report LR10981, October 1955. 

6

 Messinger, B.L., “Equilibrium Temperature of an 

Unheated Icing Surface as a Function of Airspeed,” J. 

Aeron. Sci. vol. 20 no. 1, January 1953, pp. 29–42. 

7

 Tribus, Myron, Young, G.B.W. and Boelter, L.M.K., 

“Analysis of Heat Transfer Over a Small Cylinder in 

Icing Conditions on Mount Washington,” Trans. ASME

vol. 70, November 1948, pp. 971–976. 

8

 Anderson, David N., “Effect of Velocity in Icing 

Scaling Tests,” AIAA–2000–0236, January 2000. 

9

 Eppler, R., “Direct Calculation of Airfoils from 

Pressure Distribution,” NASA TT F–15, 417, March 

1974. (Translated from Ingenieur-Archive, vol. 25, no. 

1, 1957, pp. 32–57). 

10 Bragg, M.B., “Rime Ice Accretion and Its Effect on 

Airfoil Performance,” Ph.D. Dissertation, Dept. of 

Aeronautical and Astronautical Engineering, Ohio State 

Univ., Columbus, OH, 1981; also NASA CR–165599, 

March 1982. 

11 Saeed, F., “Hybrid Airfoil Design Methods for FullScale Ice Accretion Simulation,” Ph.D. Dissertation, 

Dept. of Aeronautical and Astronautical Engineering, 

Univ. of Illinois at Urbana—Champaign, Urbana, IL, 

Jan. 1999. 

12 Saeed, F., Selig, M.S., and Bragg, M. B., “Design of 

Subscale Airfoils with Full-Scale Leading-Edges for Ice 

Accretion Testing,” J.Aircraft, vol. 34, no. 1, 1997, 

pp. 94–100. 

13 Saeed, F., Selig, M.S., and Bragg, M. B., “Hybrid 

Airfoil Design Method to Simulate Full-Scale Ice 

Accretion Throughout a Given α Range,” J. Aircraft, 

Vol. 35, No. 2, 1998, pp. 233–239. 

14 Saeed, F., Selig, M.S., and Bragg, M. B., “Hybrid 

Airfoil Design Procedure Validation for Full-Scale Ice 

Accretion Simulation,” J. Aircraft, vol. 36, no. 5, 1999, 

pp. 769–776. 
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NCC3–884

17

A Study of Scaling for Intercycle Ice Accretion Tests

David N. Anderson, Galdemir C. Botura, and Andy P. Broeren

Aircraft safety; Aircraft icing; Ice-protection systems; Icing scaling

Unclassified -Unlimited

Subject Category: 02 Distribution: Nonstandard

Ohio Aerospace Institute

22800 Cedar Point Road

Brook Park, Ohio 44142

Prepared for the 39th Aerospace Sciences Meeting and Exhibit sponsored by the American Institute of Aeronautics and

Astronautics, Reno, Nevada, January 8–11, 2001. David N. Anderson, Ohio Aerospace Institute, Brook Park, Ohio;

Galdemir C. Botura, B.F. Goodrich Aerospace, Uniontown, Ohio; and Andy P. Broeren, University of Illinois at UrbanaChampaign, Urbana, Illinois. Project Manager, Thomas H. Bond, Turbomachinery and Propulsion Systems Division,

NASA Glenn Research Center, organization code 5840, 216–433–3900.

The Ruff method with matched scale and reference velocity was used to determine appropriate 1/2-scale test conditions

to simulate a full-size icing encounter for an NACA 23012 wing section protected with a pneumatic boot deicing system.

Intercycle ice accretions were recorded on a 36-in-chord model used to represent 1/2-scale and compared with a hybrid

reference model (full-size leading-edge and truncated aft section) representing a 72-in-chord full-size airfoil. The

intercycle ice thickness and extent of icing for the scale tests generally compared well with those from the reference

model. However, the scale tests did not reproduce the location and number of feather rows seen in the reference tests aft

of the main ice shape. Many of the differences observed were believed to result from not scaling the pneumatic boot

design along with the model size for these tests.