NATIONAL BUREAU OF STANDARDS REPORT

5260

PROGRESS REPORT FOR. CALENDAR YEAR, 1956

AIR FILTER SYSTEMS FOR ARMY AIRCRAFT

Report to

Engineering and Development Branch

Office of the Chief of Transportation

Department of the Amy

Washington, D. C.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

on

C. W. Coblentz

W. F. Goddard
THE NATIONAL BUKEAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of Standards are set forth in the Act of Congress, March

3, 1901, as amended h) Congress in Public Law 619, 1950. These include the development and

maintenance of the national standards of measurement and the provision of means and methods

for making measurements consistent with these standards; the determination of physical constants and properties of materials; the development of methods and instruments for testing materials,

devices, and structures; advisor) services to Government Agencies on scientific and technical

problems; invention and development of devices to serve special needs of the Government; and the

development of standard practices, codes, and specifications. The work includes basic and applied

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various consultation and information services. A major portion of the Bureau’s work is performed

for other Government Agencies, particularly the Department of Defense and the Atomic Energy

Commission. The scope of activities is suggested by the listing of divisions and sections on the

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Reports and Publications

The results of the Bureau’s work take the form of either actual equipment and devices or published papers and reports. Reports are issued to the sponsoring agency of a particular project or program. Published papers appear either in the Bureau’s own series of publications or in the

journals of professional and scientific societies. The Bureau itself publishes three monthly periodicals, available from the Government Printing Office: The Journal of Research, which presents

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Information on the Bureau’s publications can be found in NBS Circular 160, Publications of

the National Bureau of Standards ($1.25) and its Supplement ($0.75), available from the Superintendent of Documents, Government Printing Office, Washington 25, D. C.

Inquiries regarding the Bureau’s reports should be addressed to the Office of Technical Information, National Bureau of Standards, Washington 25, D. C.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1000-20-4838 April 25, 1957 5260

PROGRESS REPORT FOR CALENDAR YEAR, 1956

on

AIR FILTER SYSTEMS FOR ARMY AIRCRAFT

by

C. W. Coblentz

W. F. Goddard

Air Conditioning, Heating, and

Refrigeration Section

Building Technology Division

to

Engineering and Development Branch

Office of the Chief of Transportation

Department of the Army

Washington, D. C.

IMPORTANT NOTICE

NATIONAL BUREAU OF ST/ intended for use within the I to additional evaluation and r listing of this Report, either i the Office of the Director, Na

however, by the Government

to reproduce additional copie

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

rogress accounting documents

mally published it is subjected

reproduction, or open-literature

iion is obtained in writing from Such permission is not needed,

prepared if that agency wishes

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

PROGRESS REPORT FOR CALENDAR YEAR, 1956

on

AIR FILTER SYSTEMS FOR ARMY AIRCRAFT

by

C. Wo Coblentz and W, F. Goddard

to

Engineering and Development Branch

Office of the Chief of Transportation

Department of the Army

Washington, D„ C.

Abstract

A new method for conducting gravimetric performance tests on air cleaners is described and

examples of the precision and consistency of the

method are reported. The development and laboratory tests of a deflect ing-vane type prefilter for

the L-19 aircraft are described and its performanc

evaluated. Development of a modified oil-bath air

cleaner was undertaken during the year and comparisons between a proprietary model and an expert

mental modification are made. Various cloths and

a few foam rubber materials were investigated as

dry filter media. A summary of the performance of

all proprietary and experimental air cleaners

investigated since the inception of this project

is reported. The results of measurements of dust

concentration around a hovering helicopter are

reported and the size analysis of dust samples

collected during these tests at Fort Eustis is

tabulated. Recommendations are made as to the

types of filters most suited to the conditions

encountered on improved airfields, whereas different types are recommended for use in dusty

areas.
.

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I. DEVELOPMENT AND EVALUATION OF A

NEW AIR CLEANER TEST METHOD

The commonly used method for determining the efficiency of

induction air cleaners for internal combustion engines is

described in U. S. Army Specification #90.21. This method uses,

as an absolute filter,, two layers of heavy cotton flannel downstream from the air cleaner under test to arrest all the dust

that has passed the air cleaner., The weight increase of the

absolute filters is determined by weighing them before and after

the test with a drying period of six hours at about 230 F preceding each weighing. The size of the filter used is related to

the air flow rate at which the air cleaner is being tested,, and

for air flow rates in the range from 250 cfm to 500 cfm, a 30 - inch diameter is required.

An alternate to this method was sought because the necessity

for drying the flannel filter material extended a single test to

more than one working day and because the absolute filters

required for specimens with air flow rates in excess of 500 cfm

were large and cumbersome.

An investigation of materials that could be used as absolute filters led to a glass fiber paper, produced at the

National Bureau of Standards, which is practically non-hygroscopic

and which has fibers as small as 0 o 3 micron in diameter. This

material had been tested by the Atomic Energy Commission and

found to retain more than 99-99 percent of all solid particles

of 0.3 micron size and larger. This glass fiber paper has an

air flow resistance of about 14 in. W,G 0 at 4l feet per minute

face velocity, and it breaks easily when bent. Since it was not

adaptable for use as a full flow filter for flow rates applicable to aircraft or automotive engines, a method was developed

in which this paper was utilized as an absolute filter for small

samples of air drawn simultaneously from upstream and downstream of the air cleaner under test.

Isokinetic sampling is regarded as desirable for the air

flow rates and dust particle sizes encountered in this field of

application. Isokinetic sampling consists of drawing samples

- 3

-

from a stream of air such that the velocities inside and outside the sampling nozzle are equal at or near the tip. The

intent is to avoid error due to inertia, particularly critical

for the larger particles,, For this work, sampling rates in

the range from 0.5 to 1.5 cubic feet per minute were used.

Sampling nozzles of four different sizes were provided and

the center of the test duct was used as the s&rnpling station.

It was found that a three-inch disc of the glass fiber

paper could withstand the pressure of an air flow rate of two

cfim without rupture with a moderate dust accumulation on its

surface. These three-inch discs are clamped into aluminum

frames and are weighed on a laboratory balance before and after

each test with an accuracy of ±0.1 mg. After passing through

the glass fiber papers, the air samples from upstream and downstream of the air cleaner are measured with two identical orifice flow meters. The manometers for these flow meters are

mounted on opposite sides of a graduated scale to facilitate

the adjustment for equal air flow rates. By maintaining a

fixed and equal vacuum on the downstream side of both orifices,

the flow meters could be calibrated in terms of standard air

flow rates with a gas meter, independent of the flow resistances of the samplers and the air pressures at the nozzles

inlets

The filtering efficiency is determined with the formula:

E

U - D

U

x 100

where E = efficiency of air cleaner, percent

U = weight increase of upstream sampler

D = weight increase of downstream sampler

Using this method, good reproducibility in test results

was obtained with a variety of air cleaners. It will be noted

that an error of ±10 percent in the determination of either

the upstream or downstream sampler gain will produce an error

in the efficiency of only ±0.5 percent for a filter with an

efficiency of 95 percent.
,*
- 4-

The validity of the observed weight increases of the samplers can easily be ascertained by operating the complete

apparatus without any air cleaner mounted in the test position.

The dust collected by both samplers, then,, should be the same

and equal to the weight of the dust introduced into the duct

multiplied by the ratio of the sampling air flow rate to the

total air flow rate.

The Dust Injection Apparatus

In order to obtain efficiency values of adequate accuracy,

a dust injection apparatus was designed and constructed that

provided the following features:

1, Infinite adjustment of the dust feed rate, to furnish

the desired dust concentration at any air flow rate;

2. Constant dust feed rate for the duration of each test,"

3 . Uniform injection of the dust during the entire test

period without separating the fine and coarse particlesj

4 , Dispersion of dust agglomerations.

Variable dust feed rate was attained by mounting a turntable on a, variable speed transmission, whose speed ratio could

be infinitely varied to provide turntable speeds from zero to

nine rpm. The turntable was provided with a concentric groove

Into which the dust was supplied from a small hopper, filling

the groove to a constant, predetermined level, A high pressure

aspirator removed the dust from the groove and injected it

as a. high velocity air stream into the tdst'duct. This air ^ stream impinged against a baffle in the center of the test duct

to break up any dust agglomerations that may have passed the

aspirator and to promote thorough mixing of the dust and air.

Performance on Four Different Types of Air Cleaners

a. Observations with a paper Air Cleaner

Filtering efficiencies determined with this new test

method on four specimens of a paper filter, using successively

classified air cleaner test dust "fine” and a mixture of 50

percent each "fine" and "coarse,” are shown in Table 1,
*

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TABLE 1

Filtering Efficiency of Paper Air Cleaners

Filter No, 1 2 3 4

Fine dust 98 „ 4 -- 98.4

Mixed dust 97.8 97 »

-- 99=1

The tests with "fine" dust were made with specimens #1

and #3 new and showed perfect agreement. The test results in

specimens 1 , 2, and 4 with "mixed" dust are not in as good

agreement with each other. Examination of the filters after

the tests revealed that the rim of specimen 2 was damaged and

the seal of specimen 1 was probably damaged by rapping the element to remove the "fine" dust accumulated during the first

test with this specimen. Specimen 4 showed a slightly higher

efficiency with "mixed" dust than either specimens 1 or 3 with

"fine" dust. Presumably the damage to filters 1 and 2 accounted

for the lower efficiency with mixed than with fine dust in

specimens 1 and 3 . Usually, higher efficiencies are observed

with mixed dust than with fine dust on this type filter

b. Observations with a Block-Type Aircraft Air Cleaner

Consistent test results were obtained with a block-type

aircraft air cleaner using both "fine" and "coarse" dust and

also a mixture of 50 percent of each. Two runs were made with

the same type of dust and then the filter was cleaned and oiled

for the next run with the next coarser grade of dust. Table 2

shows how the efficiency of the filter increased with the use

of coarser dust and it also shows the increase of efficiency

of each second run caused by the accumulated dust loads.

TABLE 2

Efficiency of a Block-Type Aircraft Air Cleaner

percent

Test Dust 1 . Run 2 . Run

Fine

Mixed

Coarse

96.7

97.8

97.9

98.1

98.7
.* •
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c. Observations with a Cyclone-Type Air Cleaner

As part of the performance test of a cyclone., selfcleaning air cleaner, the effect of a variation of the bleedoff rate on the cleaning efficiency was determined. The following Table 3 shows the values obtained.

TABLE 3

Effect of Bleed-Off Rate on the Efficiency of a

Self-Cleaning Cyclone Cleaner

Bleed-Off Rate,, % Efficiency, %

5 88.9

7 -5 90.8

10 91 O o

15 91 o 5

20 93.0

It will be noted that a change of bleed-off rate between

7.5 percent and 15 percent produced a change in the efficiency

of only 0.7 percent. However, deviations beyond this range

of bleed-off rates in both directions caused appreciable

changes in the cleaning efficiency. The bleed-off rate recommended by the manufacturer was 10 percent.

d. Observations with an Oil-Bath Air Cleaner

The results of 40 consecutive test runs with an oil bath

air cleaner are shown graphically in Fig. 1 . During each test

run 50 grams of dust were introduced into the air stream and

the air flow rate and dust concentration were maintained constant. The dust was alternated between fine, half and half,

and coarse during successive tests. The curves of least mean

distances show some increase in filtering efficiency with

increasing dust load. The average deviation of the points of

observation from the lines of least mean distances corresponds

to 0.25 percent efficiency and there is one point out of the

forty which deviates more than 1.0 percent.
»

I
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Comparison of the Absolute Filter Cloth and the Sampling Methods

The performances of the absolute filter described in U„ S.

Army Specification #90.21 and the glass fiber paper developed

at the National Bureau of Standards were compared by conducting

tests with both in the test duct,, simultaneously.

The absolute filter consisted of two layers of ten-ounce

Canton flannel installed in a circular frame and backed up by a

16-mesh wire screen. The free inside diameter of the filter was

16 inches^ and the air flow rate used was 200 cfm, well within

the 50 to 250 cfm limit called for in the U. S. Army specification for this size absolute filter. An insulated box, electrically heated to 230 F ±1/4 F, was used to dry the filter

material. The filter cloths were suspended on wires passing

freely through the top of the box to a balance and thus could

be weighed after drying without opening the box. The weight of

the cloths was determined to the nearest 0.01 g.

The results of two series of three tests each made with this

arrangement are shown in Table 4. A different specimen air

cleaner of the same type was used for each series of three

testSj and 20 g of the same classified test dust was introduced

into the test apparatus during each test run. It will be noted

that new absolute filters were used for the first three tests

whereas the cloths used for the second three runs had been used

previously and cleaned by simply shaking out the dust. The

pressure drops of the absolute filters with new cloths averaged

15.7 cm W„G, initially and 29.1 finally; for those with used

cloths the corresponding pressure drops were 18.6 and 34.8 cm

W.G. The final dust load on the absolute filters was about

1.2 g/sq ft,, at which load the absolute filter pressure drop

taxed the suction pressure capacity of the exhaust blower at

200 cfm. The total weight of the two layers of ten-ounce cloth

was about 105 g and it was determined that a new cloth lost

approximately 0.10 g when dried a second time without being

exposed to any more dust. This is about 6 percent of its practical dust holding capacity.
.* •

'
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TABLE 4

Comparative Performance of Two Absolute Filters

Test 1 2 3 4 5 6

Air Cleaner

Ident

1 1 1 2 2 2

Dond.of Abs.

Filter Element new used second time

Dust Concentration mg/cu ft 13.3 12.3 14.

14.9 15.9 16.2

Pressure

Drop, Air

Cleaner, Start cm W . G o 1.8 2.6 3.5 1 .

2.2 3.4

Pressure Drop,,

Air Cleaner, End cm W „ G . 2.7 3.5 6.5 2.4 3.6 7.6

Pressure

Drop, Abs

Filter, Start cm W.G. 15.3 16.2 15.5 18.4 18.8 18.6

Pressure Drop,

Abs . Filter, End cm. W . G. 29.7 29.8 27.8 34.4 36.4 33.7

Sampler Gain,

Upstream mg 78.4 78.4 80.3 76.4 72.6 72.0

Sampler Gain,

Downstream mg 6 . l 8.1 9.1 7.9 8.1 6.9

Abs. Filter Gain g l.4l 1.44 1.75 1.91 1.99 1.87

Efficiency

^sampler % 92 o 2 89o7 88.7 89.7 88.8 90 .

Efficiency

^a.bs .filter % 93 o 0 92.8 91.3 90.5 90.0 90.7

Ea - E S % 0.8 3.1 2.6 0.8 1.2 0.3
-
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The dust retaining efficiency of the flannel filter cloth

itself was determined by installing glass fiber samplers on its

upstream and downstream sides and introducing the dust into an

air cleaner of the same type used for the other tests. Seven

test runs were made in which the filter paper on the upstream

of the absolute filter and downstream of the air cleaner was

changed each time, whereas that in the sampler downstream of

the absolute filter was used during all seven runs. The downstream sampler showed a total weight gain of 2.6 mg; and the

total weight gain of the seven upstream samplers was 43 . 9 mg,

which indicated an efficiency of the filter cloth of

(l - X 100 = 94 .ofo

43.9

This appears reasonable considering that the dust particles

which pass«dthe air cleaner were of a very much smaller mean

size than those introduced into it.

The average efficiency determined with the absolute filter method for the six tests in Table 4 is 91.4 percent. The

total dust introduced was 120 g, and the total weight gain of

the absolute filters was 10.37 g lor these six runs. Assuming

that 94.0 percent was the true efficiency of the filter cloth,,

the actual amount of dust which would have been retained if the

cloth had been an absolute filter is then

10.37 = 11.03 g

0.940

This would reduce the efficiency determined with the absolute

filter method from 91.4 percent to 90.8 percent. Upon applying

a correction for the small amount of dust drawn by the samplers

from the test duct upstream of the absolute filter (0.4 percent of that fed); the average absolute filter method efficiency

is reduced to 90.4 percent. This value is still 0.5 percent

higher than the average efficiency of 89 .9 percent determined

by sampling method and the glass fiber paper for the same

tests
.
- 10 -

There was some inconsistency in the efficiency values

obtained by both methods as shown in Table 4 . The efficiency

values observed by the sampling method ranged from 0.3 to 3 °

percent lower than those observed with the absolute filter cloth

indicating that the glass fiber paper arrested more of the fine

dust particles passing through the test filter than did the

flannel cloth filter. Less time was required to make an efficiency determination by the sampling method, and the test results

observed with four types of air cleaners were quite consistent,

as shown earlier in this report.

II. DEVELOPMENT AND TEST RESULTS OF A PREFILTER

FOR THE L-19 AIRCRAFT

Prefilters used for engine air cleaners are usually of

either the cyclone or the deflection type. The first type

separates the heavier dust particles from the air stream by

drawing the dust-laden air thrugh a "turbulator" which imparts

a spinning motion to the air. The deflector-type abruptly

changes the direction of the air flow by means of a series of

baffles. Both utilize the inertial forces to separate the

larger dust particles from the air stream. It is believed that

these prefilters break up a portion of the large particles by

impact on. the walls with a resulting increase of small particles that may reduce the efficiency of the air cleaner.

Earlier investigations at other laboratories indicated that

although the service interval of some air cleaners can be

lengthened by using a prefilter, the filtering efficiency is

decreased. For example, unpublished reports of laboratory

tests made at the Cadillac Motor Car Division of the General

Motors Corporation showed that an oil bath air cleaner, with

an efficiency of 99*5 percent when used alone, was able to arrest

only 98.8 percent of the dust in conjunction with a precleaner.

The efficiency of the precleaner alone was 67 percent and, consequently, the service period of the oil bath air cleaner could

presumably be lengthened three times. Since lengthening of

the service period appeared very desirable for military aircraft induction air cleaners, it was decided that prefilters

would be investigated even though their use might decrease the

filtering efficiency of the combination.
**

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Conversations with service supervisors and representatives

of the engine manufacturers have confirmed the opinion expressed

in the report on the performance test of the Air-Maze aircraft

filter (NBS Report 4569) that the light weight of that filter

combined with its small size, reasonable pressure loss and good

filtering efficiency make this type extremely attractive for use

in small aircraft; since there is practically no correlation

between the operating time of the aircraft and the service

interval of the filter, the establishment of an effective service regulation appeared extremely difficult. Under favorable

conditions, the filter may not be loaded with dust in 100 hours

flight time, whereas it may be loaded to capacity in only 20

minutes when operating in a very heavy dust concentration.

Preliminary tests showed that the pressure drop of the AirMaze filter increased in proportion to the weight of the accumulated dust load regardless of the particle size, whereas paper

filter elements showed that the larger dust particles affected

the pressure drop much less than small particles. For example,

a Fram paper filter could be loaded with seven times as much

coarse dust gathered at Fort Eustis, than with A.C, Spark Plug

t'est dust "fine" for the same pressure drop. The Air-Maze

filter also showed little difference in efficiency when operate!

with coarse and fine dusts.

An experimental prefilter was designed for use on the L-19

aircraft in which the ram effect of the air stream produced by

the propeller or the movement of the aircraft would deflect the

major portion of the dust from the air cleaners. The pitch of

the deflecting vanes of this experimental model could be

changed to determine the angle at which prefilter pressure drop

and efficiency were optimal. The ram air pressure obtainable

with the laboratory blowers was less than 0„5 in. W.G,, equivalent to the velocity pressure of a32mH& per hour wind, whereas the velocity of air just behind the propeller would be on

the order of 200 miles per hour producing a ram pressure of

about 19 in. W.G. Although the effectiveness of the prefiiter

increases with the ram air pressure, it was found that even

with this low ram pressure, the amount of dust that could be

introduced was quadrupled with the addition of the prefilter

-12

to the Air-Maze air cleaner for the same pressure drop. The

effect of the prefilter is shown in Figure 2 where the left

curve presents the pressure drop without a prefilter and the

right one with a prefilter. It will be noted that the pressure

drop of the new air cleaner was 0.7 in. W.G. less without a

prefilter than with a prefilter but that with a dust load of 20

g the pressure drops became equal. At higher dust loads, the

pressure drop of the air cleaner increased at a much higher

rate when the precleaner was not used.

A series of tests was made with an Air-Maze filter in

which it was loaded alternately with and without a prefilter

with increments of 20 g of dust. Table 5 shows the results

of this series. It indicates that the increase in pressure

drop during each run made with the prefilter installed was

lower than that of the preceding run without a prefilter,

although the overall pressure loss increased with the accumulated dust load.

TABLE 5

Comparison of the Incremental Loading Rate and Pressure

Drop of an Air-Maze Filter with and without a Prefilter

Test No. 1 2 3 4 5 6 7

Prefilter Used no yes no yes no yes no

Cumulative Dust

Load, g 20 40 60 80 100 120 j 140

Pressure Drop,

Start, in. W.G

,

2.09 3.15 2.83 4.80 5.31 7.44 9.50

Increase of

Pressure Drop,

in. W.G„ 1.11 0.67 1.46 1.14 2.60 2.32 6 . 06

Weight Increase

of Filter, g 18.3 9.0 17.7 8.5 17.3 7-2 18 .O

Filtering

Efficiency, % 96.0 96 .

97 0 2 97.7 98.0 98.7 99.2
. *
13-

The filtering efficiency increased steadily with dust load

regardless of whether or not a prefilter was used

As stated above,, it can be assumed that the -effectiveness

of the precleaner increases with the ram air velocity and,, also,

if coarser dust is involved,, as will be the case when ground

sand is thrown up into the air induction system by the propeller. Under laboratory conditions it was found that the air

cleaner and precleaner combined could accommodate 168 grams of

dust for a pressure drop of 10 in„ W„G„, whereas the air filter

alone could accommodate only 50 grams of dust for the same

pressure drop. This shows a ratio of about 3*4 to 1 in dust

handling capacity in favor of the precleaner.

After consultation with representatives of the Office of

the Chief of Transportation,, five prototype prefilters were

built of light weight metal for flight test on the L- 19 . Figures 3 and 4 show photographs of these prefilters which

weigh 17 oz each and can be attached to the clevis pins which

hold the Air-Maze air cleaner without any modification or

special tools.

These prefilters were supplied to the sponsor in June,

1956 ^ for flight tests.

III. PERFORMANCE OF SOME EXPERIMENTAL

OIL BATH AIR CLEANERS

The Donaldson oil bath air cleaner had been tested previously and the findings were reported to the Office of the

Chief of Transportation in NBS Report 4620. The specimen was

later disassembled and it was found that oily dust deposits had

clogged considerable portions of several screens. This observation led to the assumption that the scrubbing effect of the

oil had not adequately reached those parts on which dust had

accumulated. The beneficial effect of the screens and the

Inverted cone over the oil sump was considered doubtful, so it

was decided to remove all these parts and to operate the air

cleaner without them.
,• .
-14-

The air cleaner was then tested at an air flow rate of

315 cfm, the -same as that used for the former tests. Pig. 1

shows the efficiencies obtained when the modified filter was

loaded -with dust using different particle sizes for alternate

tests. These three curves are also shown as the dotted lines

in the lower family of curves in Pig. 5, with the individual

points of observation omitted. The three solid lines in the

lower part of Fig, 5 were takqpi from NBS Report 4620,, Pig. 1,

to show how the filtering efficiency was affected by the removal

of certain components from the ’filter. For all three dust

size ranges the efficiency of the modified filter was about 1 %

lower than the original at the beginning of the test; but the

efficiency of the modified filter increased steadily with the

dust load whereas the efficiency of the original filter

decreased with increasing dust load.

More important than the effect on the efficiency was the

effect of the modification on the pressure drop and dust holding

capacity. The pressure drop of the filter with clean oil

decreased from J ,.87 in. W.G. to 4„38 in. W.G. as a result of

the modification.

The pressure drop of the original filter went up to 9° 45

in. W.G. with a dust load of 1380 grams., whereas, after the

modification, a dust load of 2000 grams produced a pressure

drop of only 5-90 in. W.G. The upper pair of curves in Pig. 5

show the increase of pressure drop in the two tests. These

findings indicated that it should be possible to develop an

oil bath air cleaner for small aircraft with satisfactory

weight and filtering efficiency which would have a sufficient

dust holding capacity to permit service intervals of more than

one hundred hours.

An experimental oil bath air cleaner was designed and built

and operated with various modifications. It has been found

that a filtering efficiency of about 97% can be reached with

a mixture of 50$ each A.C. Spar! Plug dust "coarse" and "fine"

at a pressure drop of less than 4 in. W.G. Several media,

like steel wool and wood excelsior, have been found to be

nearly equally effective as oil separators and present investigations are concerned with the determination of the oil

supply rate which will allow the maximum air flow rate without
»'*

'
-15

an oil pull-over and which will not reduce the filtering

efficiency. Tests are also being conducted with knitted wire

mesh as an oil separator 0 Pig. 6 is a photograph of an experimental oil bath air cleaner made of transparent material to

permit observation of the flow of oil during operation.

Synthetic felts and fabrics were found not to be feasible

as an oil separating medium because the wick effect of these

materials did not let the oil drain back into the sump. Several

of these materials with different thicknesses and density have

been investigated for their usefulness as filter media for

oiled impingement filters. Although the filtering efficiency

of most of these materials was found to be excellent when

saturated with oil, it was observed that the dust did not penetrate into the material but accumulated on the surface only.

This so reduced the dust holding capacity of the materials

that they were not considered useful as filter media for oil

bath cleaners.

IV. PERFORMANCE OP CLOTH AS A DRY FILTER MEDIUM

Investigations were made of various cloths as dry filter

media for air cleaners. Preliminary tests showed that the

filtering efficiency was satisfactory with almost all tightly

woven materials. However,, unless there was a heavy nap on the

inlet side of the cloth it was found that the dust particles

would build up a thin and dense layer on the surface of the

filter which caused the pressure drop to increase very fast

with a relatively small dust load. Prom these findings it was

deducted that an acceptable filter cloth should have a heavy

napping on one side and a dense woven body.

Table 6 represents a summary of the performance tests

made on nine knitted or woven fabrics. Pour of these materials

were commercial cloths and the others were made especially for

these tests by the North Carolina School of Textiles.

The highest filtering efficiency observed was 98' percent

with "Skinner Suhback" which Is- a light fleeced material with

a; tightly woven silk backing. The pressure drop, however,

increased beyond an acceptable value with a -small dust IPad

which Is believed due to the -light fleece and the dense

backing.

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-18-

The orlon fleece, on the other hand, had a rather loose

backing which reduced the efficiency but the long fleece increased

the dust holding capacity considerably. It will be noted that

the pressure drop of the clean material increased almost

linearly with the air flow rate whereas the efficiency for

the first 20-gram dust load decreased with increasing air flow

rate. The filtering efficiency increased with accumulated dust

load. At 200 ft/min face velocity the efficiency for the first

20-gram dust load was only 54 percent but the average efficiency

for an 80-gram dust load was 82 percent and equalled that determined for the first 20-gram dust load at 100 ft/min face

velocity.

The performance of a knitted dyne 11 cloth with a cotton

nap which was the first sample made by the Textile School was

similar to that of the orlon fleece. A 9 oz/sq yd cotton flannel showed about the same efficiency but its pressure drop

increased much more rapidly due to lack of sufficient napping.

The loose knitted cotton fabric used for sweat pants had a filtering efficiency of 94 percent at 50 ft/min face velocity,, but

the efficiency decreased considerably at higher air flow rates.

Based upon this information four more napped fabrics were

procured from the North Carolina School for Textiles which

weighed close to 13 oz/sq yd. Preliminary results showed that

these materials could be loaded with 120 grams of dust per

square foot and permitted face velocities of 150 ft/in. without sacrificing filtering efficiency.

Further tests are scheduled to determine whether any one

of these cloths can be recommended for actual use in an aircraft air cleaner. Also the mode of arranging the cloth in the

air cleaner to obtain the minimum of weight and space requirement and the simplest accessibility for cleaning needs further

study. Present data indicate that a dry cloth air cleaner for

the H-13 aircraft will need about 2 sq ft of cloth and would

have a dust holding capacity of 240 grams, compared to 45 grams

to 60 grams observed for the Air-Maze aircraft filter.

- 19 -

V, SUMMARY OF PERFORMANCE TESTS ON SEVERAL

TYPES OF AIR CLEANERS

A summary of the performance of the principal air cleaner

types and materials tested is contained in Table 7° Some of

these air cleaners such as the Fram paper filter, the Air-Maze

P-1 aircraft type, the Farr Rotonamic, and the Donaldson oil

bath air cleaner, have been described in detail in previous

reports

The Purolator #51845 paper filter with a design air flow

rate of 300 cfm was tested with the corresponding Fram Model

AX-13638 o Both filters had the same dimensions and their performance was found to be almost identical 0 The dust holding

capacity of 45 g was about equal to that of the Air-Maze filter,

whereas their filtering efficiency was slightly lower,, All

these filters are satisfactory if they are serviced properly,

but their low dust holding capacity reduces their value considerably when operated in very dusty surroundings.

Rotating Air Cleaners

The experiments with rotating air cleaners as previously

reported have been expanded and the dffect of air flow rate, dust

concentration and particle size range was further investigated.

Efficiencies up to 92 percent were attained with A„C. ’'coarse”

dust „

A modification of this type of cleaner was designed and

built that did not require an aspirator to produce the air

bleed-off. This was accomplished by mounting two blower wheels

on one axis and operating one wheel as a blower and the ether

one as the dust separator, Fig 0 ,Jis a photograph of this

apparatus which was driven by a variable speed motor. The

scrolls or housings around the two wheels expand in opposite

directions such that the air pressure from the blower is used

to push the air through the dust-separating wheel and remove

the dust from the bleed-off slot. The funnel attached on the

bleed-off slct is on the lower part of the right housing and the

bleed-off air flow produced by an outside blower was adjusted
.*
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- 21 -

so that the pressure in the funnel was equal to atmospheric pressure, simulating a free bleed-off air flow from the dust separator. The bleed-off air was drawn through a filter bag in the square

can, and the efficiency of the apparatus was determined from the

weight increase of the filter bag and the weight of the total dust

introduced. An exhaust blower was used downstream of the air

cleaner to overcome the pressure drop of the orifice flow meter

used for measuring the air flow rate. The pressure at the outlet

flange of the dust separator was maintained at between ±1 in. W.G,

Table 8 below shows the performance of this apparatus operating at

speeds from 1200 rpm to 2800 rpm and air flow rates of 100 cfm and

150 cfm for both A.C. test dust "coarse" and "fine."

TABLE 8

Efficiency of Rotating Air Cleaner

Air Flow Rate, cfm

Speed of Separator 100 150

rpm Fine Coarse Fine Coarse

1200 57-9 -- 69 0 6 85 0 2

1600 68 0 2 89.1 71.7 86 0

1800 -- 89.3 68 .

88.2

2000 64.9 86 .

70.1 82.7

2400 57-5 89.3 67-7 90.7

2800 67 0 4 88.0 75 08 88.3

It will be noted that peak efficiency of 90.percent was

observed with "coarse" dust at 2400 rpm and 150 cfm air flow rate.

The efficiency with "fine dust increased from 57 •percent at 100

cfm and 1200 rpm to 75-8 percent at 150 cfm air flow rate and 2800

rpm blower speed. Although the efficiency of this air cleaner was

found to increase about three percent when a 32-mesh wire screen

was installed inside the separating wheel, further work on this

type of device was discontinued „ The development of special separator blades to obtain a higher efficiency would have required considerable expense which appeared unwarranted at this time.

The power requirement of a rotating air cleaner is estimated

at 0.1 hp per 100 cfm air flow rate. Future development of such

air cleaners, possibly driven by an engine exhaust turbine.
.

'
-22

appears premising since they would produce a better volumetric

efficiency of the aircraft engine because the full ram pressure

of the air could be utilized without any loss in the air cleaner,

Winslow Air Cleaner

A special purpose aircraft air cleaner manufactured by the

Winslow Engineering Company of Oakland, California, which consisted of several knitted socks, was tested,. The socks were spread

open by helical springs and the air flowed from the outside to the

inside „ The socks were oiled but the material was so light that

the small amount of oil that adhered to the fibers did not cause a

build-up of accumulated dust. This appeared to be the reason for

an observed decrease of the efficiency from 87 percent with a clean

sock to 57 percent after 120 g dust had been introduced into one of

the four socks of the test specimen.

Based on the experience obtained in testing different cloth fil

ters it now appears that the efficiency of the Winslow type could

be considerably improved by using a much heavier material. The

dimensions of the filter tested were about 30x24x7 inches and its

weight 10 pounds. It was not considered feasible for use in military aircraft in its present form.

Cyclonic Air Cleaners

The use of self-cleaning cyclone-type air cleaners appeared as

a solution to the service problem and two cyclonic models were

tested. The Parr Rotonamlc showed an efficiency of 92.6 percent with

a pressure drop of 8.4 in. W.G. and an experimental model built by

the Thermix Corporation, of Greenwich, Connecticut, had an efficiency

of 83.2 percent and a pressure drop of 7-6 in. W.G.

Tests were also made with an experimental cyclone built in this

laboratory but they were discontinued after it appeared impossible

to obtain an efficiency of 95 percent with a mixture of 50 percent

each ’’fine” and "coarse" dust with a pressure drop of less than

10 in. W.G.

It is probable that the combination of weight and pressure drop

of the cyclonic air cleaner would result in a loss in useful power

of an aircraft engine on the order of 3 percent. However, this

penalty might be justified in very dusty atmosphere to prevent premature destruction of engines from cylinder wear.
. * .
Foam Rubber Air Cleaners and Miscellaneous Materials

Latex foam, 3/8 inch thick., showed initially an excellent

filtering efficiency and dust holding capacity. However, after

operating this filter for about one hour, cracks appeared in the

material which reduced the filtering efficiency considerably.

It was learned, thereafter, that these cracks were the result

of ozone in the air and that latex foam would be destroyed by

the effect of ozone.

Subsequently, all known manufacturers were asked to supply

samples of their synthetic foam which were known not to be

affected by ozone. A considerable number of samples was received

and tested, but none had an air flow resistance that was low

enough to be considered as a suitable material for air cleaners.,

In the course of these investigations the B. F. Goodrich

Corporation supplied several samples of a neoprene coated felt

of wool and horsehair. When oiled, this material showed about

the same air flow resistance and efficiency as the Air-Maze air

cleaner but almost 2 1/2 times as much dust holding capacity.

This very promising result was obtained with one out of a group

of eight samples. When additional quantities of this particular

construction were requested, the manufacturer was unable to

furnish any of a quality equal to the one with which the good

results were obtained. Therefore, further tests on this material

were abandoned.

Test with a white plastic felt showed a very low filtering

efficiency when used dry, but when oiled, it could be operated

at a face velocity of 1000 ft/min , a.bout equal to the AirMaze filter. It had a good efficiency, but the dust holding

capacity was little better than half of that of the Air-Maze

filter at equal pressure loss. This material did not appear

promising for use in air cleaners.

VI. OBSERVATIONS MADE IN FIELD TESTS AT

FORT EUSTIS, VIRGINIA

An examination was made of a sample of dust taken from the

paper air cleaner on a H-13 helicopter which had been operated in

the Fort Eustis area. Four micro-photographs were taken of this
.* •
-24-

dust and are shown in Fig. 8 to Fig. 11. Each division of the

scale in the center of the photographs represents a distance of

16 2/3 microns (0„ 00066 inch). The particles range in size

frorn ...the limit of resolution of the microscope to few in excess

of 150 micron size. It appears that the large dark area on the

upper left side of Fig. 8 is an agglomeration of smaller particles and that the light and transparent particles consist of

quartz sand whereas the dark particles are composed of some

decrepitated material, most likely to be clay. The broad dark

line in the center of Fig. 9 as well as the curved line near

the center of Fig. 10 are organic fibers of different origin

Fig. 11 shows numerous quartz particles in the 20 to 50-

micron. size range to which some clay is bonded; there also is

a clay particle with a small chip of quartz adhering to it,

about 1 inch below the center and 1/4 inch left.

A microscopic count was made of the dust particles in

seven pa.rticle size ranges, the percentage weight of each size

was determined and summarized in Table 9 below.

TABLE 9

Size Range Distribution of Dust Collected in an

Air Cleaner of an H- 1 3 Helicopter in Fort Eustis

Size Range, microns Percent, weight

0-5

5-10

10-20

20-40

40-80

80-200

200 -h

1.2

1.2

5 = 4

4.2

19»6

34 . 6

32 08

The grain size distribution of the dust by weight was computed from the counting of particles of each size range per

unit area. Random samples of the dust were spread on micoscopic slides with an a.dhesive made of Canadian balsam and

Castor oil. These slides were then mounted on a "micro viewer"

which was divided into 150 equal squares. Three different magnifications were used to count the different particle sizes.
’

‘

'
-25-

The size of one square on the viewer corresponded to 22 microns

for counting the 5 to 10- and 10 to 20-micron particles, a smaller

magnification that produced 100-micron length of each side of

the square was used for the 20 to 40- and 40 to 80-micron particle range and a still smaller magnification, for the dust

particle range of 80 to 200 microns and larger than 200 microns

had a 220-micron length of each side of the square.

The number of particles of each of the six size ranges was

counted on at least three complete fields on the viewer. If

less than 20 particles of any size range had been counted in

three fields further fields were counted until 20 particles of

that size range were found, but not more than ten fields were

examined for any one particle size range.

If the number of particles for one size range thus counted

was A, the number of total squares examined was B and the length

of the side of each square on the viewer corresponded to C

microns, then the number N of particles of that size range

determined per unit i#

N = B x C2

No counting of the particles in the to 5-micron size

range was made as the percentage weight of this range was

expected to be small and a reasonably accurate count would have

been extremely difficult. The assumption was made that the

weight in this size range was approximately equal to that of

the 5 to 10-micron size range.

The mean particle size for each of the six size ranges

was assumed to be the arithmetic mean of the smallest and

largest particles, i.e., the mean particle in the 40 to 80-

micron size range was considered to be 60 microns and that of

the particles larger than. 200 microns was estimated at 250

microns. By multiplying the number of particles determined for

each size range per unit with the cube of the mean size, the

ratio of weight of the particles in each size range was determined. Assuming that the weight of the O to 5-micron particles equals that of the 5 to 10-micron particles, the

percentage weight for each particle size range could then be

calculated

- 26 -

Several tests were made to obtain information regarding the

dust concentration on or near a helicopter hovering low above

ground. These data cannot be considered comprehensive, but are

indicative of the conditions at a certain time at Port Eustis.

The dust concentrations determined at three-feet and sixfeet elevation at different distances downstream from the helicopter with a wind of approximately ten mph are shown in Table

10 below.

TABLE 10

Dust Concentration near a Helipopter

Distance from Dust Concentration, mg/cu ft

Rotor Axis,, ft Elevation,, 3 ft Elevation,, 6 ft

20 - 8.0

30 12.3 6 0 0

40 14 .

3.3

70 17.1 _

It appears that the concentration at the six-foot elevation

decreased within a short distance, whereas at three feet above

ground the dust concentration increased as far as 70 feet away

from the rotor axis.

The range of visibility depends not only on the dust concentration but also to a large degree on the particle size, A

wide scattering of the observed data from different tests was,

therefore, expected. It is believed that the most indicative

observations of the visibility were obtained behind an L-19

aircraft, about three feet above the ground. In these tests,

the visibility decreased from 600 feet at a 15.0 mg/cu ft dust

concentration to 25 feet at a 51.7 mg/cu ft concentration.

An attempt was made to determine the air flow pattern around

a landing helicopter by putting a strip of flour two feet wide

and 30 feet long on the ground. Two tests were made with layers

“27

of flour of different thickness, but both times the flour was

blown away when the helicopter was still too high above ground

and no distinct flow pattern could be noticed „ The determination of the air flow pattern underneath the moving rotors was

tried when the helicopter was idling on the ground by blowing

flour through a 20-foot long, 5/16- inch diameter pipe to a

place near the rotor 0 It was found that the air velocity produced by the rotor was too high and the flour was diffused at

such a rate that it was not at all visible.

CONCLUSIONS

This study of the performance characteristics of various

proprietary and experimental air cleaners suggests that their

application to small aircraft and helicopters should be divided

into two categories; namely., those for aircraft operating from

relatively dust free landing fields only., and those which operate under any possible conditions. It also appears to be desirable that the dust holding capacity of the air cleaner be such

that its service period can coincide with the major service

period of the aircraft itself.

For aircraft that operate only on improved landing fields

with rather low dust concentrations , air filters such as the AirMaze., Fram, and Purolator units described in this report provide

a high cleaning efficiency with a simple., lightweighty and inexpensive device. Because the accumulation of dust on the filter

in such areas would be slew, their relatively low dust-holding

capacity would not be a serious disadvantage. If necessary, the

deflecting-vane type of prefilter described in this report could

be used with the Air-Maze filter to increase the service interval

three to four times. The addition of a prefilter would probably

avoid the occasional need, of by-passing the Air-Maze filter

because of excessive dust load and high pressure drop. The

deflecting-vane type prefilter removes only the larger particles

and therefore would not appreciably extend the service interval

of paper-type filters because their pressure drop is affected

principally by small particles only.

On aircraft operating on unimproved airfields or in dusty

areas., air filters such as the Air-Maze, Fram, and Purolator

types could accumulate enough dust in a few minutes to raise

their pressure drop to ten in. W.CJ. or more and require bypassing of the filter to maintain the combustion air supply rc

the engine . Eve?., a prefilter on the Air-Maze type would not

extend the cleaning interval to mere than about an hour under

such conditions. Consequently, for dusty areas, it appears

logical to equip the aircraft with a device having a greater
.
28-

dust-holding capacity even at the expense of a few pounds additional weight „ The most promising air cleaners for dusty areas

appear to be the cyclonic self-cleaning type, and the oil bath

filter type described in this report. While these devices have

a higher pressure drop than the initial values for the Air-Maze,

Pram, and Purolator units, it is comparable in magnitude or a

little lower than that attained by these three filters after a

few minutes operation in dusty atmospheres.

The Farr cyclonic self-cleaning air cleaner can function

indefinitely with an efficiency of about 92 percent and a pressure drop of approximately 8.5 in. W.G. The Donaldson'oil bath

filter with some modifications can operate with an efficiency

above 95 percent for mixed coarse and fine dust for periods

approaching 100 hours flight time under severe dust conditions

without exceeding a pressure drop of six in. W.G. The use of

one of these types of air cleaner would probably preclude the

necessity for by-passing the air cleaner even under severe dust

conditions. Both of these proprietary air cleaners could be

reduced in weight by the choice of other metals and the use of

lighter gage material.

The exhaust gas venturi required to provide continuous

scavenging or bleed-off from the Farr device would be simple in

principle and would not increase the back pressure on the engine

objectionably. Neither would it add excessively to the weight

of the air cleaning system.

By establishing two classes of air cleaners, the choice of

which would depend on the dustiness of the flying conditions in

the area of use, the advantages of simplicity and light weight

would be gained for aircraft using improved landing fields and

the advantages of adequate dust-holding capacity and continuous

protection to the engine's would be secured for aircraft in dusty

areas

The development of a dry cloth filter to be used on both

the H-13 and L-19 has led to a group of knitted and napped

fabrics which show good filtering efficiency and a dust-holding

capacity that corresponds to about five times that of the air

cleaners presently used. Another advantage of the dry cloth

filter as compared to the Air-Maze would be the ease of cleaning,

and it would be superior to the paper filters as it does not

need frequent replacement.
.* '

•
"29

The development of an oil bath air cleaner for the H-13 is

progressing and it is planned to provide this air cleaner with

a dust-holding capacity sufficient to last 100 hours flight time

under the most severe dust conditions . It is estimated that an

experimental model of an oil bath air cleaner for the model H-13

helicopter can be completed for flight testing during 1957

The investigations on dry cloth filters made thus far were

restricted to single layer fabrics. Preliminary studies, howt*

ever, seem to indicate that higher filtering efficiencies can be

obtained even with increased air velocity by using double layers

of cloth. Future tests are, therefore, scheduled to determine

combinations of fabrics which will produce the best performance

in an air cleaner.
,*
U. S. DEPARTMENT OF COMMERCE

Siiu lui.r Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. A si in, Director THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards at its headquarters in Washington,

D. C., and its major field laboratories in Boulder, Colorado, is suggested in the following listing of

the divisions and sections engaged in technical work. In general, each section carries out spe- cialized research, development, and engineering in the field indicated by its title. A brief

description of the activities, and of the resultant reports and publications, appears on the

inside front cover of this report.

WASHINGTON, D. C.

Electricity and Electronics. Resistance and Reactance. Electron Tubes. Electrical Instruments. Magnetic Measurements. Dielectrics. Engineering Electronics. Electronic Instrumentation.

Electrochemistry.

Optics and Metrology. Photometry and Colorimetry. Optical Instruments. Photographic

Technology. Length. Engineering Metrology.

Heat and Power. Temperature Physics. Thermodynamics. Cryogenic Physics. Rheology

and Lubrication. Engine Fuels.

Atomic and Radiation Physics. Spectroscopy. Radiometry. Mass Spectrometry. Solid State

Physics. Electron Physics. Atomic Physics. Nuclear Physics. Radioactivity. X-rays. Betatron. Nucleonic Instrumentation. Radiological Equipment. AEG Radiation Instruments. Chemistry. Organic Coatings. Surface Chemistry. Organic Chemistry. Analytical Chemistry.

Inorganic Chemistry. Electrodeposition. Gas Chemistry. Physical Chemistry. Thermochemistry. Spectrochemistry. Pure Substances.

Mechanics. Sound. Mechanical Instruments. Fluid Mechanics. Engineering Mechanics.

Mass and Scale. Capacity, Density, and Fluid Meters. Combustion Controls.

Organic and Fibrous Materials. Rubber. Textiles. Paper. Leather. Testing and Specifications. Polymer Structure. Organic Plastics. Dental Research.

Metallu rgy. Thermal Metallurgy. Chemical Metallurgy. Mechanical Metallurgy. Corrosion.

Metal Physics.

Mineral Products. Engineering Ceramics. Class. Refractories. Enameled Metals. Concreting

Materials. Constitution and Mieroslruclure.

Building Technology. Structural Engineering. Fire Protection. Healing and Air Conditioning. Floor. Roof, and Wall Coverings. Codes and Specifications.

Applied Mathematics, Numerical Analysis. Computation Statistical Engineering. Mathematical Physics.

Data Processing Systems. SEAC Engineering Group. Components and Techniques. Digital

Circuitry. Digital Systems. Analogue Systems. Application Engineering.

• Office of Basic Instrumentation ® Office ol Weights and Measures

BOULDER, COLORADO

Cryogenic Engineering. Cryogenic Equipment. Cryogenic Processes. Properties ol Materials.

Gas Liquefaction.

Radio Propagation Physics. Upper Atmosphere Research. Ionospheric Research. Regular

Propagation Services. Sun-Earth Relationships.

Radio Propagation Engineering. Data Reduction Instrumentation. Modulation Systems.

Navigation Systems. Radio Noise. Tropospheric Measurements. 1 roposplicrii Analysis. Radio

Systems Application Engineering.

Radio Standards. Radio Frequencies. Microwave Frequencies. High Frequency Electrical

Standards. Radio Broadcast Service. High Frequency Impedance Standards. Calibration

Center. Microwave Physics. Microwave Circuit Standards.