ARR Jan. 1943

NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS

1111-1 RI' I NE RE PORT ORIGINALLY ISSUED

January 1943 as Advance Restricted Report

I ICING TESTS OF AIRCRAFT-ENGINE INDUCTION SYSTEMS

By Leo B. Kimball National Bureau of Standards

N AC A :^^

WASHINGTON

NACA WARTIME REPORTS are reprints of papers originally issued to provide rapid distribution of advance research results to an authorized group requiring them for the war effort. They were pre-viously held under a security status but are now unclassified. Some of these reports were not tech-nically edited. All have been reproduced without change in order to expedite general distribution.

W-97

https://ntrs.nasa.gov/search.jsp?R=19930093057 2018-07-03T22:08:03+00:00Z

NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS

ADVANCE RESTRICTED REPORT

ICING TESTS OF AIRCRAFT-ENGINE INDUCTION SYSTEMS

By Leo B. Kimball

SUMMARY

A comprehensive program of icing tests has been conducted on an aircraft-engine induction system con-sisting of a Wright R-1820, G-200 blower section, Holley 1375-P carburetor and adapter, and specially built air scooD. It was the object of this research program to determine the effect of a number of p ossible factors on icing of engine-induction systems. These factors included carburetor-air temerature, air-moisture content, Water-droplet size, trottie opening, mixture ratio, rate of air flow, altitude, and others.

Both fuel evaporation icing and throttle icing were considered in this investigation. No tests were made on impact icing, however. It was found that two of the fac-tors carburetor-air temperature and amount of free water present in the intake air, had . the major effect on icing in the induction system tested.

The rate of ice formation increased rapidly with increase in rate of air flow at constant moisture content, and it is therefore concluded that the rate of icing-is a function of the mass flow, of free water through the in-duction system. Throttle icing was critical at tempera-tures near the freezing point of water but diminished, to a negligible degree at a carburetor-air temperature of 400 F or above.

s/hen there was no free moisture present and the relative humidity of intake air was 100 percent or less, ice formed in the induction system at such a slight rate that neither the rate of air flow nor the mixture ratio was seriously affected. With free moisture present, ice formed in the induction system to a sufficient degree to affect both the rate of air flow and the mixture ratio at carburetor-air temperaturesup to. 60 0 P. Ice did not

form under an y conditions when the carburetor-air temper-ature was 80 0 F or above

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From the results of this research program it is con-cluded that induction-system air scoops should be desigloed to prevent as much water as possible from entering the car-buretor. A brief study has been conducted on this prob-lem. The interior passages of induction systems should be free from protuberances on which ice can adhere.

The investigation described in this report was con-ducted at the National Bureau of Standards in a special laboratory provided with air blowers and refrigeration equipment,

INTRODUCTION

The program of tests described in this report was designed to determine the effect of a number of factors on induction-system Icing, with the view of obtaining data which would serve as a guide for safe winter operation of airplanes and which might also lead to the design of im-proved induction systems from the icing standpoint.

Tests also have been made on methods of eliminating ice formations in induction systems by means of the in-jection of alcohol into the Intake air and b, 7 preheating the air. This research is continuing and it is planned to Issue further reports with results of the de-icing tests.

The present report is limited to a description of the icing tests and a brief study of the possibility of preventing, by design modifications, the ingestion of rain into induction systems. There is also included, a brief appendix describing tests made on several induction-system ice-warning indicators,

The NACA induction-system icing program at the National Bureau of Standards was initiated in January 1941, The project is financed jointly by the Army, the Navy, and the National Advisory Committee for Aeronautics, Support has also been received from the Civil Aeronautics Adminis-t rat ion.

Acknowlement is due to several manufacturers of aircraft and equipment who have supplied engineering personnel and apparatus, and also to several commercial air lines who have furnished equipment and flight-test data on induction-system icing.

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APPARATUS AND METHOD OF OPERATION

The icing tests were carried out in an altitude lab-oratory at the National Bureau of Standards. This labo-ratory is eauiDped with refrigeration machinery located in a separate building consisting of two 25-ton corn-ores-sors, condensers, receivers, and steam-heating units. It was possible to vary the temperature and humidity of the carburetor intake air within wide limits. Two electri-cally driven Nash exhausters located at the end of the exhaust air duct, draw the combustion mixture through the induction system.

The tests described in this report were made in an induction system consisting of a specially built air scoop and a Halley 1375-P carburetor and adapter mounted on a Wright R-1820, G-200 engine. The laboratory induc-tion-system installation was designed to simulate the conditions of flight as closely as possible. The testing apparatus is shown schematically in figure 1. A second schematic sketch, figure 2, has been prepared to show important details of the induction-system installation A photograph showing much of the equipment in the alti-tude chamber is given in figure 3.

A glass window was built into the rear wall of the carburetor adapter and a small electric light placed in--side the adapter. By means of these devices it was pos-sible to observe the ice as it formed in the induction system.* The visual observations of icing proved to be of considerable assistance in analyzing much of the data obtained.

Referring to figure 1, air was drawn into the cold room across the refrigeration coils and through the in-take duet, which contains a straightening grid and water-spray nozzles, and thence through the air scoop into the induction system. The air flow was measured by means of either of two interchangeable flat-plate orifices located in the intake duct as shown in figure l The larger orifice was used on all wide-open-throttle runs, while the smaller orifice was used. for the part-throttle runs.

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Later in the program a glass window was installed in the air scoop, so that ice forming around the fuel nozzle bar could be seen.

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The orifices were made according to A.S,M,E. standards, and were considered accurate within 1 p ercent. The ori-fice differential p ressure was indicated by a water ma-nometer and also by a differential pressure recorder. By means of the latter instrument it was possible to record quite accurately the complex changes of th .e air flow during the icing p rocess. Figure 4, a typical chart from this pressure recorder, illustrates the rapid drop in air flow from the cruising rate of 4000 pounds per hour under severe icing conditions. The various fluctuations in the air-flow curve were caused by ice formations breaking off and re-forming. The initial rate of air flow was con-trolled by bleeding additional air into the outlet duct, thus adjusting the pressure drop across the induction system to a value corresponding to the desired rate of air flow under ice-free conditions.

The temperature of the intke air was automatically controlled by a Brown instrument to an indicated accuracy of ±1/40 F. The air temperature was indicated by two cal-ibrated glass thermometers and recorded by a four-pen re-corder. As a further check on the accuracy of air-temperature measurements, a thermocouple was placed be-tween the two glass thermometers in the air scoop and connected to one of the 12-point recording potentiometers which are shown in figure 5.

Gasoline was supplied into the carburetor by a a pe-cial fuel pump driven by a 1/2-horsepower electric motor. It was possible to pump fuel into the carburetor at rates up to 1600 pounds er hour. The gasoline flow was regu-lated by a combination of two methods. One of these con-

sisted. of bypassing a portion of the fuel back to the supply tanks by means of a manually operated valve. The fuel flow was further regulated by adjusting a pressure valve at the carburetor. For measurement of the rate of gasoline f1ow, one of three calibrated rotarneters of var-ious capacities was used, These instruments are shown in figure 5 at the left of the recording potentiometers. The rate of fuel flow was also indicated and recorded on a differential recording flowmeter. The temperature of the fuel was measured in the pipe lnading to the carbu-retor and was recorded on a chart.

The throttle.. opening was regulated either at the carburetor or by a control handle at the observation station, The latter control is shown in the lower right-hand corner of figure 5.

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The humidity of the intake air was regulated up to the saturation point 'by injection of steam into the in-take air duct a short distance downstream from the cold room, A nozzle bar, located downstream from the straight-ening grid, was employed to add free water above satura-tion to the air in order to simulate rain ingestion Three types of nozzle bars were used for producing water droplets of various sizes. The small nozzles produced a drop let estimated at about 30-. 50 microns in diameter; the medium-size nozzles produced water droplets about 1 mull-meter in size; the large nozzle produced droplets of about 3-millimeter diameter. The grid in the air duct on the upstream side of the water nozzles was installed to straighten the flow of air around the nozzles and to re-duce stratification, The ab€olute moisture content of the intake air was measured by first heating an air sample, which was bypassed through the measuring units until all the water in suspension was vaporized and then measuring the relative humidity of the air at the ele-vated temperature bymeans of wet and dry bulb thermom-eters

Water ingestion into the intake air was measured by either of two rotameters, depending on the rate of in-gestion. For most tests it was found that the tempera-ture of the ingested water could be maintained reasonably close to that of the intake air by passing the water through a coil of tubing wound around the outlet duct leading from the engine blower section, as shown in the left side of figure 3. For some tests involving high values of rain densit" it 'ias found necessary to employ an ice bath for this purDose.

Provisions were made to heat the engine 'blower sec-tion by means of steam in order to simulate flight con-ditions. It was also decided to direct against the car-buretor adapter an air stream to simulate velocity and temperature (100 0 F) that would exist within an engine cowling during flight. This condition was attained by means of an electric fa.n mounted behind the bank of steam coils, as shown in the left side of figure 3.

The temperatures of the metal passages at various p oints in the induction system were measured by means of thermocouples. The locations of a number of these thermo-coup les are indicated in figure 2 and also in figure 7, which shows the interior of the engine blower section. The metal temperatures were recorded by 12-point record-ing potenticmetrs.

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The temperature of the mixture was measured by means of a thermocouple mounted in the rear wail of the carbu-retor adapter and extending into the mixture stream.

The effect of altitude was simulated by the opera-tion of the slide valve in the air-intake duct. Altitude was measured by means of a static tube located in the in-take duct on the downstream side of the slide valve. During each altitude test run the gasoline pressure was adjusted in order to maintain the same differential be-tween the fuel dia phraam chamber and the air entrance to the carburetor that would occur at sea level,

With the thought that vibration of an airplane engine in operation might affect the tendency of ice particles in an induction system to be shaken loose ) a fairly large air vibrator was attached..to the engine blower section. Several icing tests were made to ascertain the effect of such vibration on the icing tendency, No measurable ef-fect of this vibration on the rate of ice formation could be detected from the results of these tests. It was therefore decided not to include the effect of vibration in the testing program either as an icing factor or as a constant test condition

The fuel used in this research program was taken from a. special supply of 50,000 gallons of 73-octane) nonleaded fuel in orc3.er to remove any possible error which might result from the use of different fuels. The volatility of the gasoline was so selected to match as nearly as practicable those of the aircraft fuels now in use. A sample distillation of the gasoline employed is as follows:

Distillation

First drop 5 percent

10 percent 15 percent 20 percent 30 percent 40 percent 50 percent 60 percent 70 percent 80 percent 90 percent 95 percent End point

distilled distilled distilled distilled distilled distilled distilled distilled distilled distilled distilled distilled

Beg F

114.8 140,0 145,4 149.0 154.4 163.4 174.2 183.3 192,2 201.2 12,0

2280 42,0

263.0

7

Distillation (Cont. )

Deg P

Recovery. percent Residue, percent Loss, percent .....

Barometric pressure, millimeters

Vapor pressure, p ounds per s q uare Inch

98.1 .9

1.0

755

68

TESTS AND PROCEDURE

In beginning each test run the air flow was firt adjusted to the desired rate at the proper throttle angle and carburetor pressure drop. When the air temperature had stabilized, fuel was introduced into the air stream at the carburetor. The rate of fuel flow was set by means of the mixture control to give the required fuel-air ratio. Moisture was then added to the intake air in order to reach the desired relative humidity orrate of rain ingestion,

The recording instruments were started during each run with the injection of gasoline, and timing was accom-plished with a stop watch. Observed readings were taken at short-time intervals, according to the rate of fluc-tuation of the air and gasoline flows. When the air flow had either dropped to some predetermined value or had remained unchanged during areasonable period of time the run was concluded. The carburetor was then immediately removed for examination of the ice formation. Photographs were taken of the ice formed in th p induction system dur-ing a number of runs.

As each test run progressed, the presence of ice in the induction system could be detected by the irregulari-ties in the curves of air flow and fuel flow on the re-cording charts caused by small pieces of ice continually breaking off and re-forming,

In an effort to explore thoroughly each factor on icing, a great number of wide range of conditions were made. Of are recorded in table II. The tendency duced in most of these runs is shown in charts.

the effect of test runs over a these tests, 100 toward icing p ro-the various

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The carburetor-air temperature was varied from 325 0 F to 90 0 P. The moisture cntent of the air was varied from 75-percent relative humidity up to saturation -p lus 100-grams--per-cubic-meter rain density. The rates of air flow explored ranged from 2000 to 10,000 pounds per hour, the throttle opening from 150 to 73 0 , and the mixture ratio from 0.060 to 0.120. Tests were made at preosure altitudes from sea level up to 20,000 feet. The operating limits of the mixture ratio for the Wright R-1820 engine were assumed to be from 0.050 to 0,130. No con-sideration was given in the program to the effect of icing on the mixture distribution since the tests were not made on a complete engine.

DISCUSSION OF RESULTS

Ice which forms in an engine-induction system can be classified, under three general types:

1. Atmospheric impact ice - forms on and ahead of

the throttle at temperatures below the free'ing point of water due to the ingestion of snow, sleet, or supercooled water particles which freeze on impact.

2. Throttling ice - forms around the throttle as a

result of the change in kinetic energy of the air caused by passing through this high-velocity section of the car-buretor,

3. Refrigeration ice - forms at and below the fuel

nozzles as a result of cooling the intake air and ingested water by the evaporation of the fuel.

This third type of ice, also known as fuel ice, is encountered more frcauently in flight than either of the other types since it forms at outside air temperatures considerabi r above 330 P. For this reason almost the entire program of tests was devoted to a study of refrig-eration icing. Thus far no tests have been made on impact icing at the National Bureau of Standards. Impact-icing tests have been made by another agency, however. (See reference 1.)

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It appears from comparing the results obtain'ed in this investigation with icing tets made in other labora-tories that the ranges of conditions effecting icing vary somewhat i induction systems made bydifferent manufac-turers. ifl view of this condition, it is believed that the dAta. contained in this report should be of value in the operation of the induction system tested but should be applied with some reserve in connection with other types. Ingonerai however, the manner in which ice formed in the induction system tested and the relative effect of var-ious-factors on the icing tendency should be similar in the Induction system of any modern aircraft engine.

Two factors, carburetor-air tomporaturo and amount of free water in the intake air, were found to have a predominant effect on icing* All other factors considered had a very minor effect. It is possible, however, that some of the icing factors described in this report as minor might have a more appreciable offect on the icing tendency during borderline icing conditions. The two sets of major and minor factors Which influence the occurrence of ice can b grouped as fellows:

Iajor Icing F.ctors

(i) Carbuotor-air temperature

(2) Moisture content of air

Minor icing Factors

(i) Rate of air flow

(2) Throttle opening

(3) Droplet stzo

(4) Fuel-air ratio

(5) Altitude

(6) Metal temp eratures of engine 'blower section

( • 7) Fuel temperature

6, a view of the carburetor adapter after run 9, made at 400 P carburetor-air temp. and rain density of 10 grams/rn3, shows the large amount of ice formed within a period of 20 rain

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The results showed that with free water present in the air when the carburetor-air tem p erature is near the freezing point, refrigeration ice forms very rapidly in the induction system. Figures 15 and 16 indicate that the 2,cne of dangerous icing extends above 50 0 P and that the rate of icing is progressively lessened as higher carburetor-air tem p eratures are reached. At 32 . 5 0 F car-buretor-air temperature and rain density of 10 grams per cubic meter, the air flow was reduced from 4000 to 3000 pounds per hour within 2 minutes. At 50 0 P under similar conditions, the air flow was reduced from 4000 to 3000 pounds per hour in 30 minutes. At temperatures above 500 F, the rate of air flow through the induction system was not noticeably affected even though the rain density of the intake air was maintained at 10 grams per cubic meter. Fluctuations in the mixture ratio due to the pres-ence of ice were observed at temp eratures up to 700 p, however. An examination of the carburetor adapter at the conclusion of a test run made under similar conditi-ons at carburetor-air tem p erature of 80 0 P revealed that no ice remained in the induction system. It is possible of course, that small amounts of ice formed in the adapter at this temperature and were later broken off and carried through the system.

With increase in moisture content of the intake air above the saturation point, refrigeration ice formed very rapidly in the induction system, Somewhat in contrast to the results of previous investigations, it was found that at a moisture content of 100 percent relative humidity or less, the rate of ice accretion was very small and the rate of air flow remained almost constant for periods as long as 30 minutes. It was observed that ice formed and periodically was broken away from the walls of the car-buretor adapter during many of these tests. This action was attributed to the fact that the ice insulated, the walls from the colder mixture stream and because the adapter walls were heated from the outside to a certain extent by a flow of warm air from the fan previously de-scribed.' With increase of moisture content above satura-tion. ice formed progressively faster. At 30-grams-per-- - cubic-meter rain density, it was seen that the rate of air flow was reduced from 4000 to 3000 pounds per hour in 3.5 minutes. When the rain density was increased to the extreme of 100 grams per cubic meter, however, the rate of air flow did not fluctuate from the original cruising power, indicating that the ice forming in the induction

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system was being washedff the walls of the p assages by the great volume of water,. Results of tests made to de-termine the effect of moisture content of the air on the icing tendency are given in figures 17 to 20.

A series of tests was made at a throttle opening of 20 0 , in which the rate of air flow was varied between 2000 and 4000 pounds per hour. The results (figs. 24 and 25) indicated very little difference in icing tendency and a low rate of ice accretion. In another series of tests made at full-throttle opening (figs. 22 and 23) the air flow was varied from 6000 to 10,000 pounds per hour. Refrigeration ice formed very rapidly in the induction system during these latter tests. When the data of both groups of tests were compared together and with other re-sults (figs. 26 to 31) it was evident, however, that as the rate of air flow was increased with constant rain density, the rate of ice accretion increased cuite rap-idly. 1t can be concluded that the rate of ice formation is a function of the total amount of water entering the induction system in-a given period of time.

The results of tests made over a wide range of throt-tle opening .revealed that the amount of throttle opening had no appreciable effect on the rate of ice formation. The data of these tests are given in figures 26 to 31.

In order to determine the possible effect of varying the mixture ratio on the rate of ice formation, tests were made at cruising power and also at full throttle. The results of these tests (figs. 32 and 33) showed, how-ever, that the mixture ratio had little or no effect on the rate of icing.

As might be exp ected, a general examination of all the test data showed that as ice formed in the induction system during each test, the fuel-air ratio was changed. It was noted that below 400 P carburetor-air temperature, the mixture ratio became auite rich and that at 40 0 P or more the mixture ratio usually became lean. These effects were caused to a great extent by ice formations on the fuel nozzle bar. In the case of the higher range of tem-peratures, ice formed on the nozzle bar on the lower side of the fuel-injection holes, thereby reducing the suction at that point and causing the mixture to become lean. At lower temperatures, however, ice formed higher in the in-duction system and often on the nozzle bar above the fuel holes, thereby increasing the suction of the air through the carburetor venturi, enriching the mixture,

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During most of the tests the water introduced into the intake air was the medium-droplet size of about 1-millimeter diameter. A few tests were made with smaller water droplets (about 30 to 50 microns in diam. ) in order to determine the possible effect of water-droplet size on the icing tendency. The small amount of data obtained, however,, was not considered sufficient on which to base a conclusion on the effect of water-droplet size.

In a number of the tests it was found that water particles of colloidal proportions were contained in the air to the extent of 1 or 2 grams per cubic meter at a humidity of less than 100 percent. In these instances no wetting action was observed on the metal surfaces of the air scoop, indicating that the water particles were less than 0.1 micron in diameter.

Very little difference in the rate of ice formation was found in a comparison of tests made at various pre-sure altitudes. These data are given in figures 34 to 36. It is interesting to note in passing that the air-flow curves of runs 62 and 159, made at different temperatures and altitudes, were almost identical.

A series of icing tests was made under propeller load curve conditions. In the first group of runs, in which the rain density was held at 5 grams per cubic meter, the time during which the air flow remained equal to the initial rate increased somewhat with decreasing initial air flow and throttle opening. In each run of the second grouD, in which the rain density was maintained equal to that of run T-1, the air-flow rate remained ap-proximately at the initial rate for substantially equal time periods, apparently as a result of the constant rats of water ingestion. In all of the tests, however, (figs. 37 and 38) the air flow dropped at about equal rate. The results of these tests indicate that variation of propel-ler load curve conditions has little effect on severity of icing. However, the results do supply further evidence of the critical effect of water ingestion.

Tests at constant rain density of 10 grams per cubic, meter with no fuel flowing into the carburetor were made in order to determine the severity of throttle icing in the induction system.. The carburetor-air temperature was varied from 330 to 41 0 P in these tests. Results are shown in figure 39. At 33 0 and 35 0 F, throttle ice formed so rapidly that the rate of air flow was reduced from

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4000 to 2000 pounds per hour in slightly more than 4 min-utes. The range of carburetor-air temperatures in which rapid throttle icing occurred was found to be quite nar-row, however. At 390 F, or above, the air flow was very slightly affected by the formation of throttle ice.

Typical temperatures of various parts of the metal Passages measured by means of thermocouples are given in table I, Early in the program some tests were made in which the engine blower section was heated by means of a steam jet over a range of tem p eratures from 400 F up to 290 0 F. It was found,' however, that in this range the temperature of the carburetor-adapter, walls varied only from 8 0 to 10 0 F, or a change 6f 2 0 F.

It was noticed during some tests in which heated air was not applied to the carburetor adapter, that frost quickly formed on the outside metal walls. As each of these tests proceeded and ice formed within the adapter, the frost gradually melted. This effect can be seen in figures 9 and 10. Melting of the frost was caused by the formation of ice within the adapter which insulated the metal walls from the colder mixture.

Examination of the carburetor adapter after many of the test runs showed that the ice within the adapter was melted away from the metal walls and was sufficiently loose so that it could be easil y removed. It was evi-

dent that the loosened ice would have passed on through the induction system except that it was prevented from doing so by the irregular sha p e of the adapter and blower section and by the various protuberances within the induc-tion system. In view of this phenomenon a special bulle-tin (reference 3) was prepared. This bulletin contained a recommendation that induction systems should be designed so that all protuberances are eliminated.

No tests were made to determine the effect of he the carburetor adapter on the rate of ice formation. It was apparent from the effects described above, however, that the application of heat to the walls of those por-tions of an induction system in which ice usually forms woul& decrease the icing tendency.

*This effect is illustrated in fig. 11, a photograph of carburetor adapter taken after test run 16.

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The temperature of the mixture ranged from 40 0 F at carburetor-air temperature of 80 1 F down to 2 0 P at 32 . 5 0 P carburetor-air temperature. Most of the mixture temperature values were below 32 0 P. It can be seen from figure 40 that the lowest mixture temperature occurred with the most rapid rate of icing. The mixture tempera-tures fluctuated considerably during , the tests and it was therefore not possible to maintain consistently accurate values. These fluctuations in the mixture temperatures were caused by ice forming around the thermocouple bulb and thus insulating it from the mixture stream. A special bulletin (reference 4) has been published to describe this effect, In figure 21 it will be noted that as ice formed in the adapter, the temperatures of the metal walls and the combustion mixture both approached 32 0 F, the mixture being at a slightly lower value.

In general, it was noted in observations made through the glass window of the carburetor adapter that in almost all of the tests made at a high air-moisture content : a considerable quantity of ice formed within the adapter before any appreciable reduction in the air flow could he measured. The air flew then dropped very rapidly, indicat-ing that if this condition was experienced in flight the engine power would be considerably reduced due to ice in the induction system before the pilot had sufficient warning to take preventive measures.

A number of additional tests were made to establish the outer limits of the atmospheric conditions under which ice formed in the induction system tested. The results of these tests of borderline conditions are given in figure 41. The range of conditions under which the air fl-Ow was affected by ice is represented by the double-shaded portion of the chart. Under all conditions out-side of 'the Outer curve no ice was encountered. The intermediate area between the outer. curve and the inner double-shaded portion indicates the range of conditions in which ice was formed in the induction system but did not affect the air flow or fuel flow.

The conditions under which ice formed in the induc-tion system were determined by making visual observations through the glass windows of the fuel nozzle bar and the interior of the adapter. Fluctuations of less than 2 percent in the air flow and fuel flow were considered as not being detrimental to engine operation. Fluctuations greater than 2 percent are indicated in figure 41 by crosses in the lower area,

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As a result of the basic conclusion that rate of ice accretion is a direct function of the total amount of water ingested with the intake air, a brief theoretical study has been male to investigate possible methods of preventing rain from entering induction systems. It is evident that this can be accomolished if, in the design of intake passages, advantage is taken of the inertia of water particles.

An analytical study of the paths of water particles of different sizes in an air stream disturbed by the pres-ence of a circular cylinder is contained in reference 5, It was found that at a velocity of 200 miles per hour the area against which droplets impinged decreased with de-crease in droplet size, The report indicates that water particles 10 microns in diameter, or smaller, will fol-low the path of an air stream regardless of any change in direction, that the paths of particles 10 to 1000 microns in diameter will vary in proportion to their size, while droplets of 1 to 5 millimeters in size have suffi-cient inertia that they can be separated from an air stream of curved path. In reference 2 it is shown that droplets of 1-millimeter diameter can be expected with a rain density of 0,, 27 gram per cubic meter, droplets of 2-millimeter diameter would occur with rain density of 2 grams per cubic meter, and that 3-millimeter droplets would be found in a rain density of 54 grams per cubic meter, This indicates that in the range of average rain-fall, water droplets have sufficient mass so that they can be separated from an air stream because of their inert ia,

It has been found in other investigations that water particles, by virtue of their inertia can be separated from the intake air in an air scoop containing a plenum chamber and rain trap. Separation of water droplets can also be accomplished by greatly changing the path of the intake air. Fi gures 12, 13, and 14 illustrate various possible design applications of this theory, The first of these is a rotatable air scoop which can be reversed in position during rainfall. The other two sketches il-lustrate how the inertia separation principle, together with provisions for preheating the intake air, might 'be applied to intake systems incorporated within engine cowlings. In the system shown in figure 13, the cold-air valve C is closed during precipitation and air is ad-mitted through the alternate Intake D. In the design represented by figure 14, the flapper valve C at the

LI

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lip of the cowl inlet is turned to the vertical position during precipitation so that the air is forced around a 900 turn in entering the intake system. This design has the advantage that a portion of the dynamic ram of the intake air is retained during the o p eration of the alter-nate inlet.

RE00IMENDTI ONS

In view of the critical effect of total rate of rain ingestion on induction-sTstem icing, it is evident that the intake passages of anaircrft engine should be de-signed so that water is prevented as much as possible from entering the carburetor with .the intake air,

In the design of induction systems all possible pro-tuberances should be eliminated, Interior surfaces, if possible, should also have a slight draft in the down- shream direction and should be designed so that ice can- n .--,t be retained by the shape of the induction passages. It is evident that in an induction system having such features, ice would form only to. a slight extent, would then be melted away from the walls due to its own insu-lating effect, and would then pass on through the system.

Based on observations made during this research pro-gram, it appears that by the apnlication of heat to the carburetor adapter, icing could be still further mini-mized. Tests to i;ivestigate this effect more thoroughly are recommended.

From the tests made during this research program it is evident that throttle icing should be considered in the design of carburetors,

Fuel nozzle bars, throttles, and other exposed ele-ments of carburetors should be designed so that a minimum amount of ice will form on them.

In view of the observation made during many of the icing tests that the air flow was not affected until a quite large amount of ice had formed in the carburetor adapter, it appears that the development of a dependable instrument to indicate the presence of ice in induction systems is desirable.

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CONCLUSIONS

The following conclusions should be considered in their entirety only in connection with the induction sys-tem tested.

1. Two factors, carburetor-air temperature and mois-ture content of the intake air, were found to be the major causes of induction-system icing. All other factors con-sidered had a minor or negligible effect on the rate of ice formation for the range of conditions investigated when each of these factors was varied independently with the others held constant,

2. With free moisture present in the intake air, ice formed rapidly in the induction system at carburetor-air tem p eratures u p to 500 P. The mixture ratio was af-fected by the formation of ice at temperatures up to 70 0 P.

3, Ice did not form in the induction system tested under any conditions at a carburetor-air temperature of 80 0 P or above,

4. The rate of ice accretion at an air-moisture con-tent of 100-percent relative humidity or less, with no free moisture present in the air, was so slight that neither the rate of air flow nor the mixture ratio was affected,

5. With constant rain density, the rate of icing was increased in proportion to increase in rate of air flow. From these tests it is concluded that the rate of icing is a function of the total mass flow of rain per hour through the induction system.

6. From the data obtained in this investigation, it is recommended that air scoops of engine-induction sys-tems be designed to prevent as much water as possible from entering the carburetor.

7. The interior passages of induction systems should be free from p rotuberances on which ice can adhere and should also, if p ossible, have a slight draft in the downstream direction.

18

8. The develo pment of a dependable ice-warning in-dicator for induction systems Is-desirable.

National Bureau of Standards, Washington,, , C.

APPENDIX I

TESTS OF ICE-WARNI1G INDICATORS

From the icing tests described in this report, it has been concluded that the develo p ment of a dependable ice-warning indicator for induction systems is desirable It has been pointed out that during almost all of the tests in which ice, formed in the induction system, the rate of air flow, usually taken as an index of the pres-ence of ice, remained almost constant until a large amount of ice had formed in the carburetor adapter. This phenomenon was noted by means of visual observations through the glass window in the rear wall of the adapter. In view of this condition, and also as a result of re-quests from the military services and commercial air-line operators, a number of tests have been made in the Wright

2OO induction-system installation at the National Bureau of Standards in. an effort to develop a satisfactory ice-warning indicator for general use.

One of the chief difficulties in indicating the presence of ice in an induction, system results from the tendency of ice to form at different locations, according to the atmospheric conditions encountered. In the NACA induction-system icing investigation and in tests made by other agencies, it has been found that impact ice forms in the air soop and in the duct leading to the carburetor; throttle ice, 'by definition forms in the vicinity of the carburetor throttle; while refrigeration of fuel-evaporation ice forms on the downstream side of the gasoline-spray nozzles, usually in the carburetor adapter. It was also noticed during the icing tests that at different values of the carburetor-air temperature, ice formed in different locations for each of the three types of ice, it is evident that an effective ice-warning indicator should be capable of detecting ice in the induc-tion system regardless of the point where ice forms,

19

The ice-warning indicators which were tested are listed, as follows:

(1) Anemometer type ice-warning indicator

(2) Cunningham ice-warning indicator

(s) Muter ice-warning indicator

(4) American Airlines ice-warning indicator

(5) National Bureau of Standards ice-warning indicator

All of these instruments with the exception of the ane-mometer type are described in reference 6.

The anemometer unit was installed in the carburetor adapter and mounted on the shaft of a small generator. The flow of air through the induction system caused the rotor to revolve and the resulting electric-current gen-erator was indicated on an ammeter. As ice formed on the rotor, its speed decreased until it finally stopped be-cause of being frozen solidly in the adapter. This device indicated the presence of ice but was considered unsatis-factory because it was subject to fre q uent repair result-ing from damage to the rotor unit by ice and because the rotor in itself greatly increased the formation of ice in the induction system. It became evident in the tests of. this device and in the program of icing tests that all possible obstructions should be eliminated from the interior of an induction system in order to reduce ice accretion.

A number of tests were made on several versions of the Cunningham instrument. When first tested, this de-vice, which indicates the change in a pressure differen-tial in an induction system due to ice accretion, responded only to localized ice formations and for this reason was operative only through a limited range. As the instru-ment was developed further it was found possible to ad-just the sensitivity. It appears that in any further development of this device the range of indication must be further extended so that it will be more effective in varying conditions. In this way the location of.the units in the induction system will not be so 6ritical. One advantage of the Cunningham indicator is its light weight : several ounces per unit, indicating that several

20

of these instruments might be installed within an induc-tion system to indicate the p resence of ice at any of several points where it might occur.

The Muter photoelectric cell-type indicator, which was mountedin thecarburetor adapter, was found to be satisfactory in warning of the presence of ice By re-cessing both the photoelectric cell and the light source in opposite walls of the adapter, the sensitivity of in-dication could be regulated. It was found, that ice par-ticles formed in these two recesses of the walls regard-less of the other points where refrigeration ice was de-posited. As ice formed, the reading of the milliarnmeter gradually increased to a full-scale deflection, indicat-ing that the adapter was being filled with ice. This full-scale reading was usually reached several minutes before any change could be noticed in the rate of air flow.

The American Airlines instrument is a photoelectric cell-type indicator similar to the Muter ice--warning in-dicator, with the principal dif'ferece being that the photoelectric cell has a greater output and the current is read directly on a microammeter. Th i s indicator was considered satisfactory on the basis of the limited tests made. One possible difficulty in the use of his instru-ment is that the sensitive microammeter might not be capable of withstanding the engine vibration to which it would be subjected.

The ice-warning indicator developed by members of the staff of the National ureau of Standards, a tiodified photoelectric type, was found to p rovide satisfactory in-dication of the presence of ice during the few tests made on this instrument. The design of this ice-warning indicator has been turned over to an equipment manufac-turer for possible commercial development

21

REFERENCES

1. Skoglund, Victor J,: Icing of Carburetor Air Induc-tion Systems of Airplanes and Engines. Jour. Aero. Sci. , vol. 8 no. 12, Oct. 1941.

2. Humphreys, W. j.: hysics of the Air. J. B. Lippincott Co., 1920.

3. NACA Induction System Dc-icing Investigation: Some Design Considerations for Induction Systems. Bull. No. 2, Dec. 1941.

4. NACA Induction System Dc-icing Investigation: Engine Icing. Erroneous Adapter Mixture Temperature Read-ings Due to the Insulating Effect of Ice on Thermom-eter Bulbs Located in t'ie Adapter. Bull. No. 1., Sept. 13, 1941.'

5, Kantrowitz, Arthur: Aerodynamic Heating and the Deflection of Drops by an Obstacle in an Air Stream in Relation to Aircraft Icing. T.N. No. 779, NACA, 1940,

6. Chandler, H. C,, Jr.: Survey of Aircraft Anti-icing Equipment. NACA restricted report, Feb. 27, 1942,

The Civil Aeronautics Board is distributing this memorandum.

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system icing testing appara-

tus, (showing cold and hot air intake

ducts, carburetor, engine blower section, Figure 4.- Typical recorded air flow chart.

A-0

NACAFigs. 3,4,5,6

Figure 5.- Pecordin potentiometers,

manometers, mixture andFigure 6.- Straightening grid in inlet throttle manual controls, and gasoline

air duct. rotameters.

Fs. 7,,9,iC

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:

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NACA

6000

5000

• 4000

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300

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Time, Mimife Figure 15.- Effect of varying carburetor-air temperature with initial moisture content

100 percent R.H.+lOg/m3.

6cvx

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300

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Fig. 16

0 /0 eo 30 40 50 60

Figure 16.— Variation oZ oarbu.retor-air temperature with moisture content 100 percent R.N. +lOg/mO and 21.50 throttle angle.

/

_-

Wr,5/it(vp6 -/3

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-

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vR,ii w18, /OOyO+3o/,77'

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= - - - -

.0

500

'300 NO

00 0

L

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0 /0 20 JO ,'O £0 60

- Wi,7

Figure 17.- Effect of rain ingestion at 40 07C.A.T., and 21.50 throttle angle.

Halley Corbui-e/o,-, — frV ;ht G -eoo Rear Section

137JP-.

—— - Legend:

0 Rca., No /5a , fo,sJ,pe Content 32q/m Run No. 184 Mi/upe Content 4'eg/in-'

X Run No. 146 C , Molewe Ccv/ent .57g/n Rv,7 No. Id o, .Moisfc,re Content 7q/rn

V thin No. /8e, /14'oi,¼'r CornLe,n 80911i' ' Rvi' NO // Moisture Co,n'ern 90g/m

'7

— —

:

- —

6000

s000

4000.

3000

c o0o

/000

.10

06

.0

400

300

£00

NACA Jig. 18

0 /0 ZPQ -50 42 60

7me,- Mrnu/e Figure 18. — affect of ezremé rain ingestion a 400 F C.A.T., and 220 throttle angle.

- - -

-

//o //' Cacrb —/3 73 P

.. 0, G ,Qrn / , -I. -8 "

-a,41,7 73

- -

- - - =

,x0

3000

• L'0

/000

Ve

0

"800 Nj

O200

N /00

Iz

NACA

Jig. 19

(1 10 iQ - .iO 60

77i -'it Figure 19. — Effect or rain inge8tlon'at p5°F C.A.T., and 200 throttle angle.

'I

- -£qu/pmei#

No/fey Carh,petor, /37i-F Wright /52O-G2O0 BJon'ep $ect,o,

.

XRcin No 138,1007-Rel Mw

7 ft 40 50

•v .0

400

300

zoo

/co

4000

4 12000-

go Mi

NACA

Fig. 20

- 7'ne - Mrnu/es

Figure 20.— Effeot of rain ingestion at 40°? 0.A.T. 9 and 210 throttle angle.

MESIMMMM

a K IBM r" 'U., 0 0 0 M 0 ol-W,

MEMEMEMSMEMMM

MEMMEMEMMEEM

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0 4 -5 12 16 20 24

Tithe, Ali''t,tes Var,afio, of Mix%re and Adcoi'e,- Te,nperato'res

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NACA

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Fig. 22 \IIIi1IIIIII 1111111111

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17 17' 310 We,-JCC

-

lix Ile

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Rrn W a 6, 800161%r-A7* 27,600

-

VN

-

- -- - ---

(I I" CV ..XI 50 Co°

Time - /Y7ipufg

Figure 22.- Icing at various initial rates of air flow at 35°F C.A.T.. moisture content lOOperoent R.L+ 5g/m 3 and 73° throttle angle.

Fig- 33 -

\ Halley 'rzrbire/or /375-F Wr, 6# 6-20 Reap Sec//an

I . I 7 Legend: - (3Rc, M,24 ,4 C u,gla4r 7mp.

\ xR,,, iVa6 .4,40 O,- CQPbJc..' Air 7èrnp - No. Air Temp.

nGA

.9000

N. 8000

s... 7000

45000

5000 14

4000

3000

2000

/000

.12

.c .10

.08

1000

600

L400

200

U-: /0 130 40 50

Time - /vf/nute.s Figure 23.— Icing at various initial rates of airflow at 40°F C.A.T. moisture content

100% R.H. + 5g/m3 , and 73 ° throttle angle.

NACA Fig. 24

Ho/ley Carbt,re,'or /37.5 F k/9171 /8E'0-6-200 8/owcc Scr,', -

- -------------------.-----------------

-+'-+ +--+-4--• +-+

:IIIZm_iii Legend:

0 Rw' No. 3 400 Q165/hr + Run No. 33. 3000 16s/hc

,',r Flow

A,..- F/ow

r000-44-1-4--4-

-o

_

0 20 40 60 CO /00 120

77me, f/,frwJfes

6000

5000

4000

3000

000

/000

300

400

300

000

/00

rod

Figure 24.- Effect of varying initi1 rate of air flow at 40°F C.A.T., moisture content 100 percent R.H. + 5g/m and 200 throttle angle

NACA

Fig. 25

4,000

.3Oa?

/000

15,00

IM 300

200

/00

• = 0 0 -0 0-0— —0-0 0-0.- —0-0 . -

/0 -

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-

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-4 4-4--

------------------ 3,:

'tLI off IV OU 90 ?me, 1'frnvfes

Figure 25.- Effect of varying initial rate of air flow at 40°F C.A.T., moisture content 100 percent R.R.+ 5g/m3 and 150 throttle angle.

NACA

PIg. 36

600

\

0

600

400

/00

-

-

^//y Corb. -/37S/

w-,4t 6'-2oo& ver

-

-

-

-

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Ai

1 IC..' CU ôQ

Th,'ze -4/i,es

•

Figure 26. Zffect or varyinghrottle angle at 35 0F C.A.T., and moisture content 100 percent R.H.+5g/m

13 I iYo//y Cvr - 75A

-

- - - . '36 ol,£//Thrk -73°

65 'Thpe2'/ - xRrn0Q', 50 0 -

- - - i11 39 * -

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/k2O •/

0

700 O

Soo

10o

3OO

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ral

NACA Fig. 2?

0 /0 20 30 9O 50 60

Time -A1,mites Figure 27.— Effect of varying throt1e angle-at 350F C.A.T., and moisture content

100 percent R.L+ 5g/m.

NAOA - 71g. 28

- - - -

/a IleyC7r6.-/37sE Wr%tG2oo/ow5-ct

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730

-

•

vD------.--:------

6000

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NO

*vo

tr000

/000

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01

600.

30O

tj

0

k N 0O

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T/m c -/W>,wtes Figure 28.— Effect of varying throttle angle at 40°F 0.*.T., and moisture content 100 7

R.H.+5g/nr.

.159

EQpme,,7

Wr94tQaoc&ow1SeCt

- • ,4'crn 8 C S/, 777,0,We -73

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w R,,i c .38

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L?20

/000 -/

0

800

700 )i

600

500

t?O0

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NACA Fig. 29

(1 /C) ea 30 '90 50 60

77;.77

Figure 29.— Effect of varying hrott1e angle at 40°F C.A.T., and moisture content 100 percent R.H. +5g/mO.

NACA

71g. 30

.5V00

4000

3000

z000

/000

sea

400

,300

ex

MMENNOMOMMEME NNEENEEMEMEN

ME--ME ME

IT

ME

ME M MMMM ME

iihJ. ME

m OEMPri

(F öQ 00 /00

Figure 30.- effect of varying throttle angle at 400? O.AT., and moisture content 100 percent R.H.+ 5g/m'.

S

bee M^ft80 W/17

Wc,tG-2oE/wer5ect

75

-/375

- --Ley e,la': - * R4117 04,e cy

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Fig. 31

0 /0 2C 30 40 50 60 77mø

Figure 31.— Effect of varying throttle angle at 40°F O.A.T., and moisture content 100 • • percent +5g/m.

NACA Fig. 32

..

low

a2o

- - - - Eou,10,r,e,7f: -

/ I/alley Ccii.-1375

•,'c -u-- - - - - Wri4too8/owe;5ct - --

I - - 01--- _____ -

Leyeric/ 02__ - - • Ru,5/, •o6,,4RT,

0R 1,7.sI2, 080 N

- - _

-- ----

0 /0 leo Jo 50 oQ

7/-;77e -41rni7's Figure 32.- Effect of varying fuel-air ratio at 40°? C.A.T., InoiBture content 100 per

cent Ft.H.+5g/m3, and 210 throttle angle.

500

vj40°

300 0

1600

" /00

NACA

Fig. 33

C /0 eo . 30 50 60

Figure 33.- Effiot of varying fuel-air ratio at 35°? C.A.?., moisture content 100 percert R.H.+ . 50m° and 730 throttle angle. . . ..

......

Jrm

. ,

•0 •I

700

600.

100.

300 0

/00

Mo

- - - - - - - . -

#a/ky

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- - I- - - -

-. _-_;- - • Rrn ' 51, 090 A/A /Pc't/o X R1117 -105-

ol?un S6 -11S

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Fig. 34

71 o—o

.- _*_ -

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_%tG-oo81awe,-5c1'

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I

I-

=

500

&J/0O

U IC) C) .JC 170 50 60

777

Figure 34.— Effect of varying altitude with 35 0F C.L.T., moisture content 100 percent R.R. +5/ 3, and 730 throttle angle.

1'c1 60

77,77 -1W1,17417$

Figure-35.— Nffeot of varying altitude with 400? C.A.T., moisture content 100 percent R. H.+5g/nr'., and 730 throttle angle.

-

WrhtG-o8/ower5ct 13 zsF

'C --------------___

-- o—G -- -

--___--_---

'9 ------------------

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-

-

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1.

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Q

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NAOA

Fig. 36

-10 50 60

Figure 36.- Effect of varying altitude with 400? O.A.T., moisture content 100 percent R-H-+ 59/m3 l and 350 throttle angle.

Ic, 5C) 60

77me -14i1741tes

El-0-

- -

i%//eyCi-. -/373i -

- Le9e,7a':

37f 77%

6000 9 °761-ct/k, -

5000 /b/A, - ,

400O/1b/

17 Tit , - 3000 /h/r

- • ___ -

- - • L

-I,,

'sxo

JcO

500

r I 300

L/0o

NAOA

Fig. 37

Figure 37.— Effect of propeller load c9ve conditions at 40°F C.A.T., and moisture con,— tent 100 percent R.H. +5g/m

77me -Nin,tes Figure 38.- Effect of propeller load curve conditions at 40°F C.A.T., and moisture

content 100 percent R.H.s 5g/m3.

-

--

- - -

-----------i-----

z

----:— -- /-/o//ey Ccr - /375

- Wr,t?ooB/ower5ect... -

'C

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Lç9ev7a'. -. T/Y, 6000/6/k37SThro/f/e

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/00 7,QeL Munz /09/rn'3

------

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x

U /zD 20 30 'O J0 60

-"4

52OO

3000 ci

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Fig. 38

NACA

FIg. 39

S 1000

3000 0

7oOC

5000

fOO'

6000

2000

- -b

0 Rin,Vo./4 33Cr Th,,o.. A577iWe i4k 4./6/,35;I-arhovreor Air Thm,o,2J'Th,H/eA,'/e

164, 37',Cc1-h're"or Air Temp. ,?JSThrcM/e A,,9/e Run 9. J 6 5, 30rCQrburetor Air Thm,o: , 215' Th,'o#/e ,4n'/e

• i / 4. 3FCarkc,re/orA,,- 7m,o., Thro,We 4j/e

Runt, . 4 39P,r4vrei'or Air Ts ,J5'Thr,H/e An,M r&,,V..I6 7, 40 0PCctr&,retor Air Temp ,Zt3'Th,t,h'i A,j/e

41 OF Carburetor 41 Temo.,2/S'Thro'/e 4v

- -

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-

'C

z

=

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o 2 4 6 8 /0 /4 16 8 20 2 30' iwe, 44,,.

F,9 ure 39. - Ei¼cf of h4ro #14 icing on air "flow af mo,sh're c ontent /00 psrent R. 1/. * /Oj/ni ".

9

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F99. 40'

6000

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20

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mm mmm m R(117 Al,,, 3,?.S Oor,64,, ____

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X Raw MaI0,4S,^,Calh-AIr Temp.

ORun i' ' I Temp

. •

MMMMM

Emmmmm mmmmmmm:- Emmmmmmmmmmom Maw

o.11i #A b, 1.a MMMME

MMMMMMMMME

0 /0 20 30 40 50 60

ThDe, ,ilthw'e.s

F/3Qre 40. - 4'21x furs tem,4oecd,ure (/ic/uat,on,.c dir/,g icrn at r#eiturs corn'eif rn /00 percent R./1. *iOg/ii'.

Fig. 41

1 •

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,

(q

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NC