Judith F. Van Zante

Glenn Research Center, Cleveland, Ohio

Robert F. Ide and Laura E. Steen

Sierra Lobo, Inc., Cleveland, Ohio

Waldo J. Acosta

Jacobs Technology, Cleveland, Ohio

NASA Glenn Icing Research Tunnel: 

2014 Cloud Calibration Procedure and Results

NASA/TM—2014-218392

October 2014
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Judith F. Van Zante

Glenn Research Center, Cleveland, Ohio

Robert F. Ide and Laura E. Steen

Sierra Lobo, Inc., Cleveland, Ohio

Waldo J. Acosta

Jacobs Technology, Cleveland, Ohio

NASA Glenn Icing Research Tunnel: 

2014 Cloud Calibration Procedure and Results

NASA/TM—2014-218392

October 2014

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Acknowledgments

The authors would like to thank NASA’s Aeronautics Test Program for funding this essential effort. Many thanks are also due 

to the other IRT Engineers: Seth Sederholm, Larry Becks, Mark Kubiak and Sean Currie; and the expert and very patient IRT 

Technicians: Joe August, Dave Bremenour, Jason Bryant, Paul Butterfi eld, Bill Magas, Rob Maibauer, Terry Mathes, Shaun McNea, 

Perry Vraja. Also thanks to Colin Bidwell, Mr. LEWICE3D, for running the collection effi ciencies on the multi-wire elements.

Available from

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National Technical Information Service

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

Trade names and trademarks are used in this report for identifi cation 

only. Their usage does not constitute an offi cial endorsement, 

either expressed or implied, by the National Aeronautics and 

Space Administration.

Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2014-218392 1 

NASA Glenn Icing Research Tunnel: 2014 Cloud Calibration 

Procedure and Results 

Judith F. Van Zante 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Robert F. Ide and Laura E. Steen

Sierra Lobo, Inc. 

Cleveland, Ohio 44135 

Waldo J. Acosta 

Jacobs Technology 

Cleveland, Ohio 44135 

Abstract 

The results of the December 2013 to February 2014 Icing Research Tunnel full icing cloud calibration 

are presented. The calibration steps included establishing a uniform cloud and conducting drop size and 

liquid water content calibrations. The goal of the calibration was to develop a uniform cloud, and to 

generate a transfer function from the inputs of air speed, spray bar atomizing air pressure and water 

pressure to the outputs of median volumetric drop diameter and liquid water content. This was done for 

both 14 CFR Parts 25 and 29, Appendix C (‘typical’ icing) and soon-to-be released Appendix O 

(supercooled large drop) conditions.

Nomenclature 

CDP Cloud Droplet Probe, drop sizer, 2 to 50 m 

CIP Cloud Imaging Probe, drop sizer, 15 to 930 m 

DeltaP Spray nozzle Pwat - Pair (psid) 

Dv0.## drop diameter at which ##% of the total volume of water is contained in smaller drops 

FZDZ Freezing Drizzle 

FZRA Freezing Rain 

LWC Liquid Water Content (g/m3

) 

MVD Median Volumetric Diameter (m) 

OAP-230X Optical Array Probe, drop sizer, 15 to 450 m 

Pair Spray nozzle atomizing air pressure (psig) 

Pwat Spray nozzle water pressure (psig) 

SLD Supercooled large drops 

TWC Total Water Content (g/m3

) 

V Calibrated true air speed (velocity) in the test section (kn) 

m micrometer 
NASA/TM—2014-218392 2 

Introduction 

A calibration of the NASA Glenn Research Center Icing Research Tunnel (IRT) was performed 

between December 2013 and February 2014. The first step in this calibration process was to complete 

water flow coefficient tests of new spray nozzles. Once new nozzles were chosen, cloud calibration 

commenced with establishing a uniform cloud and calibrating the drop size and liquid water content. 

Transfer functions were developed to relate the inputs of air speed, spray bar atomizing air pressure and 

water pressure to the outputs of median volumetric drop diameter and liquid water content. This was done 

for both 14 CFR Parts 25 and 29, Appendix C (Refs. 1 and 2) (‘typical’ icing) and soon-to-be released 

Appendix O (supercooled large drop) conditions. The previous cloud calibration report was released in 

2012, following the 2011 IRT heat exchanger and refrigeration plant upgrades (Ref. 3). 

Facility Description 

The IRT is a closed-loop refrigerated wind tunnel that simulates flight through an icing cloud. A plan 

view of the facility is shown Figure 1. A 5000-hp electric motor drives the 24-ft fan made of wood from 

Sitka spruce. The calibrated speed range in the test section ranges from 50 to 325 kn. The fan drives air 

through expanding turning vanes in “C-Corner”, and into the face of the staggered heat exchanger. There, 

the air gets chilled/warmed within a temperature range of 20 C total to –40 C static. Twenty-four RTDs 

distributed on the D-Corner contracting turning vanes measure the total temperature. Several are visible in 

the inset picture. The heat exchanger is 26 ft high and 50 ft wide. 

Downstream of the D-Corner contracting turning vanes are 10 rows of spray bars with two different 

air-atomizing nozzle types: Mod1 (lower water flow rates) and Standard (higher water flow rates). Each 

bar has 55 nozzle positions that contain either a Mod1 nozzle, a Standard nozzle or a plug. Each nozzle 

location is fed from two water manifolds through remotely controlled solenoid valves. It is possible to 

turn on only the Mod1 nozzles, only the Standard nozzles or both (with the same air pressure). The struts 

mounted vertically between spraybars that were required to help mix the cloud in 2006 are still necessary 

(Ref. 4). 

The contraction area ratio into the test section is 14:1. The test section itself is 20-ft long by 6-ft high 

by 9-ft wide. The center of the test section is 44 ft from the spraybars. From the test section, the cloud 

flows into the diffuser toward A-Corner, and on around into B-Corner and into the fan. 

Facility Change Notes 

In preparation for the new icing cloud calibration, the Mod1 nozzle water tubes were replaced with 

new versions. A test stand was developed to perform water flow coefficient calibrations so that the new 

tubes could be tested and a set of nozzles could be selected with closely matching flow coefficients. 

A set of nozzles with flow coefficients within 3 percent of the average were installed and used for this 

calibration effort. The plan had also been to replace the Standard nozzle water tubes as well. However, the 

flow coefficients for the new batch of Standard tubes were found to be 20 percent higher than the 

previous set. This would have increased the operating envelope gap between the Mod1 and Standard 

nozzles. The Standard tubes therefore were not replaced. 

Icing Cloud Uniformity 

Before the cloud characteristics of MVD and LWC can be determined, a uniform cloud must be 

established. This is determined by identifying which of the 550 possible nozzle locations should spray 

Mod1, which should spray Standard nozzles, and which should be plugged. 

 
NASA/TM—2014-218392 3 

The diagnostic device is a 66 ft grid. This grid extends floor to ceiling, and covers the central 6-ft of 

the 9-ft span. The grid mesh is 66 in. Mesh elements are 2-in. deep with a flat 1/8-in. face for ice 

accretion. Digital calipers are used to measure the ice thickness accreted at the center mesh point of the 

vertical elements. An image of a technician measuring ice on the grid is shown in Figure 2. 

The first step in developing a uniform cloud is to establish a nozzle transfer map. Single rows (i.e., 

spray bars) and columns of nozzles are sprayed to observe where ice accretes on the grid. This transfer 

map aids in the optimization of nozzle locations to produce a uniform cloud, and in the identification of 

problematic nozzles during routine testing. 

To start the uniformity calibration, the previous January 2012 Standard nozzle pattern was loaded 

(Ref. 3), and still exhibited good uniformity over all airspeeds and drop sizes. A representative Standard 

nozzle uniformity pattern is shown in Figure 3; the legend indicates a ratio of the local LWC measured on 

the grid compared to the average of the central twelve values. The light green color shows most of the 

map is within 10 percent. 

The Mod1 nozzles were installed in a somewhat uniform pattern. Nozzles positions and number of 

nozzles were adjusted until a uniform icing cloud was developed on the icing grid. Since the majority of 

the IRT’s models are airfoils mounted vertically and centered in the test section, greater care was taken to 

make the spanwise central –12 to 12 in. as uniform as possible. The Mod1 pattern was finalized with 

88 spraying nozzles and 2 air-only nozzles. The LWC uniformity plots are shown in Figure 4 for four 

airspeeds at a drop size of 20 m. Similar, but not shown, are the uniformity plots as a function of MVD. 

These can be made available to customers upon request. 

Uniformity for a large drop case, MVD = 138 m, is shown in Figure 5. Clearly, the vertical extent of 

the cloud is smaller than those in Figure 4, but the large drop uniformity is significantly improved over 

previous calibrations. 

Drop Size Calibration 

Drop Size Data Acquisition 

In order to measure a complete drop size distribution, three instruments are sometimes needed. For 

the January 2014 drop-size calibration, these were the Cloud Droplet Probe, CDP (2 to 50 m), Optical 

Array Probe, OAP-230X (15 to 450 m) and Cloud Imaging Probe—Grayscale, CIP-GS or simply CIP in 

this document (15 to 930 m). The IRT’s OAP-230Y for the 50 to 1500 m range was not functional at 

the time of testing. Figure 6 shows these probes mounted in the IRT test section. The OAP-230X was 

made by the Particle Measurement Systems, Inc. and is no longer being manufactured. The CDP and CIP 

were made by Droplet Measurement Technologies, Inc. (Boulder, CO). Through extensive testing in the 

IRT, it has been found that both these probes require considerable tuning to obtain reasonable data. 

The CDP measures drop sizes between 2 to 50 m in diameter using the Mie-scattering theory for 

forward-scattered light intensity. When a drop passes through the beam path of the CDP laser, it scatters 

light in all directions, and the forward-scattered light intensity is recorded by the probe and used to sort 

the drop size into one of 30 size bins. Since the CDP measures the smallest drop sizes of the three probes 

and since all spray conditions in the IRT contain small drops, data from the CDP is collected and used for 

the full range of spray conditions. Spray conditions ranged over the following: air pressures, Pair = 10 to 

60 psig; delta pressures (water pressure minus air pressure), DeltaP = 5 to 250 psid for the Mod1 nozzles 

or 5 to 150 psid for the Standard nozzles. 

Both the OAP-230X and the CIP measure drop size using a shadowing technique. Both probes have 

an array of photodiodes that are illuminated by a collimated laser beam. When a drop passes through the 

laser beam, the diodes that are shadowed to a certain threshold define the drop size. Data were taken with 

the OAP-230X for all spray conditions that produce a median volumetric diameter (MVD) greater than 

18 m. For this test, CIP usage was limited to larger drop conditions, Pair  8 psig. 
NASA/TM—2014-218392 4 

Data Processing of Drop Size Distributions 

An example drop size distribution is shown in Figure 7. The number density is the number of drops 

that are recorded in each bin normalized by its sample volume for that drop size and the bin width. The 

squares show the size distribution as measured by the CDP, and the triangles show the size distribution as 

measured by the OAP-230X. Note that the first three bins of the OAP-230X (the solid-black triangles) 

overlap with the CDP; these three OAP bins are not used in the drop size calculations. 

The MVD is used to characterize the drop size distribution. This is the value at which half of the 

water volume is contained in smaller (or larger) drops; MVD is also referred to as Dv0.5. 

Correspondingly, Dv0.9 is the diameter at which 90 percent of the volume is contained in smaller drops. 

Normalized cumulative volume distributions for the IRT are shown in Figure 8. Bin volumes are plotted 

cumulatively, such that each data point represents the amount of water contained in all smaller diameters, 

normalized by the total volume contained in all bins. Figure 8(a) shows cumulative volume distributions 

for MVD values less than or equal to 50 m and Figure 8(b) shows cumulative volume distributions for 

MVD greater than 50 m. 

Drop Size Equations 

The MVD curve fit equations were determined by inputting the measured Pair, DeltaP and MVD into 

the curve fit generator TableCurve (Systat Software Inc.). The equations that were generated, while quite 

complex, fit the majority of the data within 10 percent, which is the typical target for the IRT. Different 

curve fits were generated for the Standard and Mod1 nozzles. 

The Standard MVD curve fit equation is, 

   

1 e *      Pair f * Pair g * Pair h * ln DeltaP

a b * Pair c * Pair d * ln DeltaP MVD 2 3

2

Standard

	 	 	 	

	 	 	 
 (1) 

where a = 19.034, b = 0.0927, c = 0.00150, d = –3.945, e = 0.0474, f = –0.000669, g = 0.00000497, 

h = –0.438. 

The Mod1 MVD curve fit equation is 

       c e c e MVDMod1 
 a 	 b * Pair 	 d * DeltaP 	 f * Pair * DeltaP (2) 

where a = 12.178, b = 267.014, c = –2.184, d = 0.00112, e = 1.362, f = 22.742. 

Figure 9 and Figure 10 summarize these MVD curve fits for Standard and Mod1 nozzles, 

respectively. In Figure 9(a) and Figure 10(a), the curve fit lines are plotted as a function of DeltaP for 

each calibrated Pair line. Measured MVDs are plotted against the respective curve fits for two Pair lines 

in each plot. Figure 9(b) and Figure 10(b) show how the curve fit values from the above equations 

compare to the measured values for all Standard and Mod1 conditions. The 1:1 line, as well as 

10 percent lines are shown for reference. These plots show that the curve fits for the vast majority of the 

data points are within the IRT’s typical targeted accuracy of 10 percent. 

SLD in the IRT 

A significant, current question is the IRT’s ability to produce supercooled large drops or SLD. Larger 

drops are achievable by reducing the spray nozzle atomizing air pressure so there is less break up of the 

water stream. Operating the Mod1 nozzles between 2  Pair  8 psig is referred to as “SLD” conditions in 

the IRT. To be clear, this short-hand phrase is mostly, but not fully technically accurate. 

These SLD conditions were measured with the CDP and both the OAP-230X and CIP. Since the 

OAP-230X cannot measure drops larger than 450 m, for the IRT nozzles its effective MVD limit is 
NASA/TM—2014-218392 5 

175 m. Previously, the CIP had been able to extend the range and returned values very similar to the 

OAP-230Y-combined distributions. During this calibration, however, there was a significant problem 

with the larger drops splashing or running off the CIP probe tips into the measurement volume. Until 

these difficulties can be overcome, the calibrated MVD range is limited to 175 m. The MVD reported is 

the average of the CDP + OAP-230X and CDP + CIP that agreed within 10 percent. Note that the IRT has 

not lost the ability to produce its largest MVDs (historically up to 230 m), only the ability to accurately 

characterize their diameter. 

The measured Pair, DeltaP and averaged MVD values were put into the curve fit generator 

TableCurve (Systat Software Inc.). The SLD MVD (Mod1, 2  Pair  8 psig) curve fit equation is 

  Pair

DeltaP

e * DeltaP f * Pair

d

c * DeltaP

Pair

b MVD a

2

SLD 2 
 	 	 	 	 	 (3) 

where a = 6.5846, b = –92.6377, c = –1.7970, d = 341.4440, e = 0.007341, and f = 23.2670. 

Figure 11 summarizes the MVD curve fits for the SLD operating range. Figure 11(a) shows the curve 

fit lines as a function of DeltaP for each Pair line. Each of the measured MVDs are also plotted against 

their respective curve fits for all Pair lines. Figure 11(b) shows the MVD goodness of fit, plotting how the 

curve fit values (per Eq. (3)) compare to the measured MVD. The 1:1 line, as well as 10 percent lines 

are shown for reference. These plots show that the curve fit for these data points is well within the IRT’s 

typical targeted accuracy of 20 percent for SLD conditions. 

With the difficulty in measuring the largest MVDs, comparisons of the IRT distributions to the FAA 

Appendix O (Ref. 5) requirements are limited to freezing drizzle (FZDZ) conditions. These are seen in 

Figure 12 for FZDZ, MVD  40 m and FZDZ, MVD  40 m. IRT distributions are selected that match 

the MVD (Dv0.5), or Dv0.98. Matching Dv0.9 (or similarly, Dv0.95 or Dv0.98) rather than the MVD 

would better assure that the effects of the larger drops in the distribution are captured. 

Also recognize that FZDZ, MVD  40 m conditions can be met with the more normal operation of 

Pair  10 psig for the Mod1 nozzles. This is due to the fact that the IRT nozzles have long “tails”; that is, 

the largest drops produced for a spray condition are typically three to six times the MVD value. 

Another concern with producing large drops in any facility is the ability to supercool them. A 

numerical and experimental study was conducted for and in the IRT (Ref. 6). Though the number of cases 

was limited, it found that 160 m MVD drops (max drop size approximately 400 m) at V = 195 mph 

were 1.9 F warmer than the static air temperature in the test section. As drop size increases, it can be 

expected that the difference between the drop temperature and the static air temperature will increase. 

Liquid Water Content Calibration 

Liquid Water Content Data Acquisition 

A multi-wire instrument manufactured by Science Engineering Associates’s (SEA; Mansfield 

Hollow, CT) was used to generate the liquid water content calibration (LWC). A picture of this 

instrument is shown in Figure 13. There are four elements within the anti-iced probe head. The three 

vertical elements (perpendicular to the flow) are a 0.5-mm wire, 2-mm cylindrical tube, and a 2-mm halfpipe element. The wire and cylindrical tube primarily measure LWC while the half pipe captures both 

LWC and ice particles (if present). The half-pipe measurement is therefore called total water content 

(TWC). The fourth element is a compensation wire which is placed behind the center element and parallel 

to the flow. It is designed to stay dry so that it tracks air temperature, air pressure and airspeed effects 

only. The four heated elements are held at a constant temperature of 140 C. The power required to 

maintain this temperature is proportional to the amount of water each element must evaporate. SEA 

defines the LWC calculation as (Ref. 7): 
NASA/TM—2014-218392 6 


    evap evap amb sense sense

sense,wet

3

L T T * V * L *W

C* P

m

g LWC

	 

 




 



 (4) 

where C = 2.389*105

, Psense,wet = Psense,total – Pcomp = wet power (W) measured by the sense element, 

Levap = latent heat of evaporation (cal/g), Tevap = evaporative temperature (C), Tamb = ambient static 

temperature (C), Lsense and Wsense = length and width respectively of the sense element (mm) and 

V = true airspeed (m/s). Psense,total is the total power measured by the element, Pcomp is the power measured 

by the compensation element. 

The half-pipe is the primary sensor used in the IRT for LWC measurements. A collection efficiency 

correction for this element was determined and applied for analysis. The collection efficiency is a 

function of airspeed, drop size and geometry. For this design, Eb was calculated by the FWG twodimensional particle trajectory code (Ref. 8). 

Liquid Water Content Equations 

The data from the multi-wire were compiled to build the LWC curve fits, correlating liquid water 

content to Pair, DeltaP, and V. As described in the 2006 IRT calibration report (Ref. 4), the liquid water 

content calibration is a function of the form: 

  V

DeltaP LWC 
 K V, Pair * (5) 

where K is a function of velocity and Pair. The first step of fitting the LWC curve is to determine the 

function K for both parameters, “Kv” and “Ka”. To do this, measurements were made from V = 50 to 

325 kn while Pair and DeltaP (and MVD) are held constant, defining Kv. Ka is defined by holding 

velocity and MVD constant while making measurements from Pair = 10 to 60 psig. K is calculated for 

each of these measured values based on the above relationship:    	

. Figure 14 shows 

the linear relationships that were found when K was correlated to velocity and Pair for both Mod1 and 

Standard nozzles. These linear fits are combined to determine the planar function K. An additional 

correction factor was added after it became apparent that even with this curve fit optimized, there was a 

clear correlation of LWC to MVD for both the Mod1 and Standard nozzle sets. From this, the expected 

curve is of the following form: 

  V

DeltaP * d

MVD LWC a * V b * Pair c *

e

Mod1 



 



 
 	 	 (6) 

where a is the slope of the Kv versus V line, b is the slope of the Ka v Pair line, d is a basis MVD and c 

and e are constants. 

A MATLAB code was written that would optimize a curve fit in this specific format. For the Mod1 

nozzles, the curve fit predicted the measured values within ±10 percent, as shown in Figure 15(a). The 

LWC curve fit equation for Mod1 nozzles in Appendix C conditions follows the above form, with 

a = 0.02989, b = –0.050414, c = 9.519, d = 21.6256, e = 0.18306. 

 
NASA/TM—2014-218392 7 

The Standard LWC curve fit is similar, but with a DeltaP correction that was found to improve the fit 

of the curve. The LWC curve fit for Standard nozzles is 

  V

DeltaP 1 * d

MVD LWC a * V b * Pair c *

e

Standard

 



 



 
 	 	 (7) 

where a = 0.156, b = –0.25, c = 44, d = 22, e = 0.17. The goodness of fit for this equation is shown in 

Figure 15(b), which plots the predicted curve fit values against the measured values. 

The LWC curve fit equations for SLD conditions (Mod1, 2  Pair  8 psig) were determined by 

inputting the measured Pair, DeltaP and LWC into the curve fit generator TableCurve (Systat Software 

Inc.). Because of the parameter limitations of the software, one curve was created for the basis velocity 

value, and then a velocity correction was applied so the equation would account for the velocity 

dependency of LWC. 

The LWC curve fit for SLD conditions is 

m

2

2

3

2

2 SLD

k

V * PairDeltaP i * PairDeltaP h *

Pair

g

Pair

DeltaP

e * DeltaP f * Pair

d

c * DeltaP

Pair

b LWC a





 













	 	





 
 	 	 	 	 	 	

(8) 

where a = 0.1871, b = –0.2469, c = 0.00173, d = –0.0396, e = 0.000107, f = 0.1058, g = 0.6597, 

h = –0.000713, i = –0.0830, k = 148, and m = –0.75. The goodness of fit for this equation is shown in 

Figure 15(c), which plots the predicted curve fit values against the measured values. 

Figure 15 shows that the curve fits agree with the majority of the data within 10 percent, even for 

SLD conditions, which have a target accuracy of 20 percent. 

Icing Cloud Operating Ranges 

The IRT’s icing envelopes for both the Mod1 and Standard nozzles are compared to the FAA 

Appendix C icing criteria in Figure 16. The airspeed of 225 kn is selected. At lower air speeds, the curves 

shift up to higher LWC, at higher air speeds they shift down. While the commonly requested data points 

MVD = 20 m, LWC = 0.5 and 1 g/m3

 are easily achieved, another requested point, MVD = 40 m, 

LWC = 0.07 g/m3

 will likely never be achieved with the current nozzle design. The operating envelopes 

for the SLD conditions of Mod1 at 2  Pair 8 psig are shown in Figure 17. The maximum and minimum 

calibrated velocities are plotted. Also indicated are the range of temperature-dependent maximum LWC 

values for each freezing drizzle environmental condition of Appendix O (per Fig. 1 in Ref. 9). 

Conclusion 

The procedures and results of the full cloud calibration that was conducted in NASA Glenn’s Icing 

Research Tunnel from December 2013 to February 2014 have been described. The calibration followed 

the installation of a new set of Mod1 nozzles that had a tighter range of water flow coefficients. 

Uniform icing clouds were established with both the Standard and Mod1 nozzles. The MVD and 

LWC curve fits for both the Standard and Mod1 nozzle sets are within the ±10 percent targets. For SLD 

conditions, the calibrated MVD range is limited to 175 m, until such time that the difficulties in 

measuring the largest drops the IRT can produce can be overcome. The SLD MVD and LWC curve fits 

were well within the 20 percent targets, coming in around 10 percent. 
NASA/TM—2014-218392 8 

Figure 1.—A plan view schematic of the Icing Research Tunnel. Inset: View of the staggered heat exchanger and 

inner D-Corner turning vanes, flow is left to right. 

Figure 2.—A technician measures the thickness of ice accreted on the grid. 
NASA/TM—2014-218392 9 

Figure 3.—LWC uniformity contour plot for the standard nozzles, V = 150 kn, 

MVD = 20 m, Pair = 15 psig. 

69

63

57

51

45

39

33

27

21

15

9

3

-36 -30 -24 -18 -12 -6 0 6 12 18 24 30 36

Height from floor (in)

Distance from tunnel center (in)

0.50.6 0.60.7 0.70.8 0.80.9 0.91

11.1 1.11.2 1.21.3 1.31.4 1.41.5
NASA/TM—2014-218392 10 

Figure 4.—Mod1 nozzle LWC uniformity plots for several airspeeds, MVD = 20 m, Pair = 20 psig, 

V = (a) 100 kn, (b) 150 kn, (c) 200 kn, (d) 250 kn (Axes same as Figure 3). 
NASA/TM—2014-218392 11 

Figure 5.—LWC uniformity contour plot for an SLD case, V = 150 kn, 

MVD = 138 m, Pair = 3 psig. (Axes same as Figure 3). 

 (a) (b) (c) 

Figure 6.—Images of drop-sizing probes used for calibration in the IRT: (a) Cloud Droplet Probe (CDP), 

(b) Optical Array Probe (OAP-230X), (c) Cloud Imaging Probe-Grayscale (CIP-GS). 

69

63

57

51

45

39

33

27

21

15

9

3

-36 -30 -24 -18 -12 -6 0 6 12 18 24 30 36

0.5-0.6 0.6-0.7 0.7-0.8 0.8-0.9 0.9-1

1-1.1 1.1-1.2 1.2-1.3 1.3-1.4 1.4-1.5
NASA/TM—2014-218392 12 

Figure 7.—Drop size distribution presented as number density 

versus drop diameter, measured by the CDP + OAP-230X, 

MVD = 25 m. 

 (a) (b) 

Figure 8.—Cumulative volume distributions: (a) MVD  50 m with Dv0.5 (= MVD) and Dv0.9 indicated, (b) MVD  50 m. 

1E-6

1E-5

1E-4

1E-3

1E-2

1E-1

1E+0

1E+1

1E+2

1 10 100 1000

Number Density (#/cm3/m)

Drop Diameter (m)

CDP

OAP

Dv0.9 

Dv0.5 
NASA/TM—2014-218392 13 

(a) (b)

Figure 9.—Drop size calibration curves for the Standard Nozzles: (a) MVD versus DeltaP for each (labeled) Pair line. 

The curves are the fit. The measured MVD for Pair = 10 and 40 psig lines are also plotted as data points. 

(b) Curve fit versus measured MVD for all Standard-nozzle calibration points. 

(a) (b) 

Figure 10.—Drop size calibration curves for the Mod1 Nozzles: (a) MVD versus DeltaP for each (labeled) Pair line. 

The curves are the fit. The measured MVD for Pair = 10 and 40 psig lines are also plotted as data points. (b) Curve 

fit versus measured MVD for all Mod1-nozzle calibration points. 
NASA/TM—2014-218392 14 

(a) (b) 

Figure 11.—Drop size calibration curves for SLD conditions (Mod1 nozzles, Pair  8 psig): (a) MVD versus DeltaP 

for each (labeled) Pair line. The curves are the fit. The measured MVD for each Pair line are also plotted as data 

points. (b) Curve fit versus measured MVD for all SLD calibration points. 

(a) (b) 

Figure 12.—Comparison of IRT distributions to proposed FAA Appendix O conditions for FZDZ: (a) MVD  40 m, 

(b) MVD  40 m. 
NASA/TM—2014-218392 15 

Figure 13.—The Multi-wire liquid 

water content instrument 

mounted in the icing tunnel. 

(a) (b) 

Figure 14.—LWC parameters, K, for the Standard and Mod1 nozzles sets showing the effect of (a) Kv versus 

airspeed, and (b) Ka versus Pair. 
NASA/TM—2014-218392 16 

(a) (b) 

(c) 

Figure 15.—A comparison of the curve fit versus measured LWC at all calibrated conditions for (a) Mod1 nozzles, 

(b) Standard nozzles, (c) SLD conditions. 
NASA/TM—2014-218392 17 

Figure 16.—Comparison of the IRT operating envelopes, LWC versus MVD, to the FAA 

Icing Certification criteria for an airspeed of 225 kn. The FAA Appendix C envelopes are 

shaded and indicated in black. The Mod1 nozzles are in red and the Standard nozzles 

are in dashed blue. The Mod1 and Standard nozzles can be combined under limited 

circumstances to produce higher LWC, shown in green. 

Figure 17.—The IRT SLD operating envelopes at V = 100 and 250 kn. (Pair between 2.0 and 

8.0 psig.) For reference, the Appendix O max LWC ranges are also shown for each FZDZ 

condition, per Figure 1 of Reference 9. 
NASA/TM—2014-218392 18 

References 

1. CFR 14, Part 25, Appendix C, “Atmospheric Icing Conditions,” http://www.ecfr.gov/cgi-bin/textidx?SID=90a70d57d146d0a7dfecb6d394f2c2d0&node=14:1.0.1.3.11&rgn=div5#14:1.0.1.3.11.9.201.

2.19 accessed on 3/18/2014, updated annually. 

2. Jeck, Richard K., “Icing Design Envelopes (14 CFR Parts 25 and 29, Appendix C) Converted to a 

Distance-Based Format,” DOT/FAA/AR-00/30, Apr 2002. 

3. Van Zante, Judith F., Ide, Robert F., Steen, Laura E., “NASA Glenn Icing Research Tunnel: 2012 

Cloud Calibration Procedure and Results,” 4th AIAA Atmospheric and Space Environments 

Conference, New Orleans, Jun 2012. 

4. Ide, Robert. F., Sheldon, David W., “2006 Icing Cloud Calibration of the NASA Glenn Icing 

Research Tunnel,” NASA/TM—2008-215177, May 2008. 

5. FAA FAR 14 CFR Parts 25 and 33, “Airplane and Engine Certification Requirements in Supercooled 

Large Drop, Mixed Phase, and Ice Crystal Icing Conditions,” Federal Register / Vol. 75, No. 124 

/Tuesday, June 29, 2010/Proposed Rules. 

6. Miller, Dean R., Addy, Harold E., Ide, Robert F., “A Study of Large Droplet Ice Accretions in the 

NASA Glenn IRT at Near-Freezing Conditions,” NASA/TM—1996-107142/REV1, ARL–MR–294, 

AIAA–96–0934, June 2005. 

7. Science Engineering Associates, “WCM-2000 Users Guide,” http://www.scieng.com/ Oct 6, 2010. 

8. Frost, W., “Two-Dimensional Particle Trajectory Computer Program,” Interim Report for Contract 

NAS3–22448, Mar. 1982. 

9. Cober, S. Bernstein, B., Jeck, R., Hill, E., Isaac, G., Riley, J., and Shah, A., “Data and Analysis for 

the Development of an Engineering Standard for Supercooled Large Drop Conditions,” 

DOT/FAA/AR-09/10, Mar 2009.