"... the rotating-cylinder indications of drop-size distribution are so unreliable that they are of little or no value"

NACA-TN-1904, "Observations of Icing Conditions Encountered in Flight During 1948" ¹

Summary

Meteorological data from flight observations in icing conditions during the first 5 months of 1948 are presented.

Key points

1. The average liquid water content varies over distance traveled in icing.
2. The value of the multicylinder method for determining drop size distributions is questioned.
3. Other flight test instruments are described.

Abstract

Meteorological data from flight observations in icing conditions during the first 5 months of 1948 are presented. A total of 335 measurements of liquid-water content and mean-effective drop diameter were obtained by the multicylinder method in the course of 40 flights in icing conditions covering most of northern United States. Cumulus clouds were predominant during approximately two-thirds of the flights. A continuous record of liquid-water content covering a major portion of the operations was obtained by means of a rotating-disk icing-rate meter. This record was used to investigate the relation between average liquid-water content and the horizontal extent of icing conditions. An analysis of values of maximum drop diameter calculated from the area of drop impingement on a stationary cylinder and corresponding values of mean-effective drop diameter and dropsize distribution as obtained from the rotating cylinders led to the conclusion that the rotating-cylinder indications of drop-size distribution are so unreliable that they are of little or no value. The data indicate that average and maximum values of drop size are significantly greater and more variable near the Pacific coast than in the other parts of the United States included in this investigation.

Discussion

Variation in liquid water content with distance

Data was gathered in icing clouds in flight, with flight paths that were often not straight lines.

It must be emphasized that, due to the small number of cases included in the analysis, the results are not quantitatively accurate. They do, however, give a general idea of the relation between extent and average liquid-water content in icing conditions. Another important factor which affects the validity of these results is the effect of the circling flight paths which were followed in an effort to remain in cumulus clouds. The actual horizontal extent of most of the cumulus clouds investigated was from 1 to 3 miles. The high values of water content recorded for longer distances were due to the practice of circling within clouds or making repeated passes through a single cloud. For this reason, the frequency data for 10 seconds duration (1/2 mile) in cumulus clouds are regarded as representative, the values for 1 minute (3 miles) are subject to some error, but are regarded as approximately representative, while the results for 5 minutes (15 miles) or more are believed to be considerably higher than would be encountered in straight flight.

The data were summarized in a table.

1. An analysis of continuous records of liquid-water content obtained with the rotating-disk icing-rate meter, adjusted by comparison with rotating-cylinder data from previous seasons, yields the following values of maximum liquid-water content averaged over various distances. These values are likely to be encountered once in the course of 1000 flights in icing conditions when it is assumed that 5 percent of the flights encounter cumulus clouds.

The data were shown in Figure 5.

Drop size distributions

Note above "the conclusion that the rotating-cylinder indications of drop-size distribution are so unreliable that they are of little or no value" (!)

This was based on a comparison of maximum drop size a determined by a stationary cylinder to the mean-effective drop diameter and best fit distribution determined by the multicylinder method. The mean-effective drop diameter is noted as "The amount of water in all of the drops greater than the mean-effective diameter is equal to the ".

γ = (maximum drop size from fixed) / (mean-effective drop diameter from rotating multicylinder)

However, as we saw in NACA-RM-A9C09 ², the error in estimating maximum drop size from maximum impingement limit on a fixed cylinder is as large as the error determining the MVD from the multicylinder method, so the maximum drop size determined may not be so reliable of a value to compare with.

In the later NACA-TR-1215 ³ (of which William Lewis was also an author of) a more nuanced view of the drop size distribution determination will be found.

Also note that the determination of the mean-effective drop diameter (MED) is not disputed, only the distribution determination is disputed.

Multicylinder accuracy

Figure 7 essentially reproduces Figure 2 of NACA-RM-A9C09 ², the results of which are largely reproducible as noted in that review.

Appendix A

There is an extensive Appendix A that describes the flight test instruments used.

THE CLOUD INDICATOR (heated cylinder)

It had become apparent during previous years that an instrumental method of supplementing visual observations of the time of entering and leaving clouds, q d the patchy or uniform characteristics of the cloud masses was desirable, The cloud indicator was designed to meet this need. This instrument consists of a heated cylinder 5/8 inch in diameter exposed at right angles to the air stream, with a thermocouple installed to measure the surface temperature at the stagnation point. To provide a continuous surface-temperature record, the thermocouple is connected to a self-balancing potentiometer equipped to provide a continuous ink trace of the variations in temperature. In use, the heating power supplied to the cylinder is adjusted to maintain a surface temperature of from 170 F to 200 F when flying in clear air. Immediately upon entering a cloud, the temperature drops very rapidly, sometimes by as much as 5 F in 1 second. Similarly, a rapid rise of temperature is observed on leaving a cloud. Small areas of clear air within a cloud and variations of cloud density are indicated by irregularities in the temperature trace. Figure 6 is an example of the response of this instrument during passage through various types of clouds. As indicated in the figure, the instrument is more sensitive to liquid-water drops than to snow. Thus, it is possible to identify regions containing liquid water in a continuous snow cloud.

MAXIMUM-DROPSIZE CYLINDER

This device, which is described in reference 2, permits the determination of the angular extent of ice collected on a stationary cylinder 5 inches in diameter. With this information, the diameter of the largest drops present in significant quantity can be calculated. The angle is measured by visual observation of the edge of the ice formation against a scale consisting of white marks spaced at 10' intervals on the surface of the cylinder. When the values of maximum drop diameter from the stationary cylinder observations were compared with corresponding values of mean-effective diameter, as measured by the rotating cylinders, it was noted that in more than half the cases the indicated maximum diameter was less than the mean-effective diameter. ...

THE ROTATING-DISK ICING-RATE METER

The rotating-disk icing-rate meter used by the Ames Aeronautical Laboratory is a modification of the instrument developed by the Massachusetts Institute of Technology, (See references 8 and 9.) The rotating disk, measuring arm, and scraper are essentially the same as in the M.I.T. instrument. However, the magnetic method of measuring the thickness of ice on the edge of the disk has been replaced by a mechanical and optical system. In the Ames instrument, the movement of the measuring arm actuates a mirror which causes the image of a lamp filament to move along a slit. A moving photographic film which passes beneath the slit provides a continuous record of the position of the measuring arm.

Seven pages detail the calibration and theory of operation, including Figure 10.

Conclusions

The following conclusions are drawn from an analysis of flight data presented herein and in previous reports:

1. An analysis of continuous records of liquid-water content obtained with the rotating-disk icing-rate meter, adjusted by comparison with rotating-cylinder data from previous seasons, yields the following values of maximum liquid-water content averaged over various distances. These values are likely to be encountered once in the course of 1000 flights in icing conditions when it is assumed that 5 percent of the flights encounter cumulus clouds.
2. Average and maximum values of cloud-drop diameter are significantly larger and more variable near the Pacific coast than in other parts of the United States included in this investigation.
3. A comparison of data on drop-size the rotating-cylinder method with values of maximum drop diameter as determined from the area of impingement on a stationary cylinder indicates that measurements of drop-size distribution made in flight by the rotating-cylinder method, including those presented herein, are so unreliable that they are of little or no value.

Citations

NACA-TN-1904 cites 9 publications:

• Lewis, William: A Flight Investigation of the Meteorological Conditions Conducive to the Formation of Ice on Airplanes. NACA-TN-1393, 1947.
• Lewis, William, Kline, Dwight B., and Steinmetz, Charles P.: A Further Investigation of the Meteorological Conditions Conducive to Aircraft Icing. NACA-TN-1424, 1947.
• Jones, Alun R., and Lewis, William: Recommended Values of Meteorological Factors to be Considered in the Design of Aircraft Ice-Prevention Equipment. NACA-TN-1855, 1949.
• Jones, Alun R.: An Investigation of a Thermal Ice-Prevention System for a Twin-Engine Transport Airplane. NACA-TR-862, 1946.
• Gumbel, E. J.: On the Frequency of Extreme Values in Meteorological Data. Bulletin of the American Meteorological Society, vol. 23, March 1942, pp. 95-105.
• Peppler, Wilhelm: Unterkuhlte Wasserwolken und Eiswolken. (Supercooled water and ice clouds.) Forschungsund Erfahrungsberichte des Reichswetterdienstes, Ser. B, No. 1, Berlin, 1940. (Brief Summary in Bulletin of the American Meteorological Society. Vol. 29, Nov.1948, p.458.)
• Langmuir, Irving, and Blodgett, Katherine B.: A Mathematical Investigation of Water Droplet Trajectories. Tech. Rep. No. 5418, Air Materiel Command, AAF, Feb. 19, 1946. (Contract No. W-33-038-ac-9151 with General Electric Co.)
• Vonnegut, B., Cunningham, R. M., and Katz, R. E.: Instruments for Measuring Atmospheric Factors Related to Ice Formation on Airplanes. De-Icing Res. Lab., Dept. Meteorology, M.I.T., April 1946.
• Katz, R. E., and Cunningham, R. M.: Aircraft Icing Instruments. Instruments for Measuring Atmospheric Factors Related to Ice Formation on Airplanes - II. Dept. Meteorology, M.I.T., March 1948. (Final Rep. under Air Force Contract No. W-33-038-ac-14165, July 1,. 1945-Dec. 31, 1947.)

NACA-TN-1904 is cited by 10 publications in the NACA Icing Publications Database ⁴:

• Dorsch, Robert G., and Hacker, Paul T.: Photomicrographic Investigation of Spontaneous Freezing Temperatures of Supercooled Water Droplets. NACA-TN-2142, 1950.
• Neel, Carr B., Jr., and Bright, Loren G.: The Effect of Ice Formations on Propeller Performance. NACA-TN-2212, 1950.
• Hacker, Paul T., and Dorsch, Robert G.: A Summary of Meteorological Conditions Associated with Aircraft Icing and a Proposed Method of Selecting Design Criterions for Ice-Protection Equipment. NACA-TN-2569, 1951.
• Perkins, Porter J.: Flight Instrument for Measurement of Liquid-Water Content in Clouds at Temperatures Above and Below Freezing. NACA-RM-E50J12a, 1951.
• Howell, Wallace E.: Comparison of Three Multicylinder Icing Meters and Critique of Multicylinder Method. NACA-TN-2708, 1952.
• Lewis, William, and Bergrun, Norman R.: A Probability Analysis of the Meteorological Factors Conducive to Aircraft Icing in the United States. NACA-TN-2738, 1952.
• Neel, Carr B., Jr., and Steinmetz, Charles P.: The Calculated and Measured Performance Characteristics of a Heated-Wire Liquid-Water-Content Meter for Measuring Icing Severity. NACA-TN-2615, 1952.
• Brun, Rinaldo J., Lewis, William, Perkins, Porter J., and Serafini, John S.: Impingement of Cloud Droplets and Procedure for Measuring Liquid-Water Content and Droplet Sizes in Supercooled Clouds by Rotating Multicylinder Method. NACA-TR-1215, 1955. (Supersedes NACA TN’s 2903, 2904, and NACA-RM-E53D23)
• Perkins, Porter J.: Statistical Survey of Icing Data Measured on Scheduled Airline Flights over the United States and Canada from November 1951 to June 1952. NACA-RM-E55F28a, 1955.
• Gelder, Thomas F., Smyers, William H., Jr., and von Glahn, Uwe H.: Experimental Droplet Impingement on Several Two-Dimensional Airfoils with Thickness Ratios of 6 to 16 Percent. NACA-TN-3839, 1956.
• Lewis, William, and Brun, Rinaldo J.: Impingement of Water Droplets on a Rectangular Half Body in a Two-Dimensional Incompressible Flow Field. NACA-TN-3658, 1956.

Related

NACA-TN-1904 is cited by two NACA publications cited in Appendix C of the icing regulations ⁵.

• Hacker, Paul T., and Dorsch, Robert G.: A Summary of Meteorological Conditions Associated with Aircraft Icing and a Proposed Method of Selecting Design Criterions for Ice-Protection Equipment. NACA-TN-2569, 1951.
• Lewis, William, and Bergrun, Norman R.: A Probability Analysis of the Meteorological Factors Conducive to Aircraft Icing in the United States. NACA-TN-2738, 1952.

See Jeck's ⁶ comments about the relationship of liquid water content and distance noted in NACA-TN-1904.

Notes