NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

0201-20-2300

NBS REPORT

May 5, 1954 3279

Report

on an

Investigation of the Feasibility

Forced-Air Cooling

of an

NBS-Modified

AVQ-2A Aircraft Searchlight

j

J. A* LeReche

and

J. W. Lane, Sr.

Naval Aircraft Lighting Group

Photometry and Colorimetry Section

National Bureau of Standards

Washington 25, D, C.

Test No. 2.1N-19/53

Sponsored by

Lighting Section

EL 521

Bureau of Aeronautics

Dep

1

' the Navy

(J. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

Th. publication, rs Approved for public release by the

„„ t , „ prolllb , ted

unless permission'll Director of the National Institute Ot andards, Washington

25 .D.C, Suchper Standards and Technology (NIST) port has been speciflof

tally prepared If t on October 9, 2015 sport for Its own use,

Investigation of the Feasibility

of an

NBS-Modified Forced Air-Cooled AVQ—2A Aircraft Searchlight

I. INTRODUCTION

The AVQ-2A Searchlight, modified with an NBS modification

kit (see NBS Report No. 1354> Instruction book for modification

of the AVQ-2A Aircraft Searchlight) , is equipped with a copper

obturator-probe weighing about four pounds. This mass acts as

a heat reservoir for energy emitted from the carbon arc during

the 30-second "On” period of the five-minute duty cycle. The

purpose of this test was to determine the feasibility of using

forced air to cool the obturator-probe in an effort to permit

continuous operation of the AVQ-2A searchlight. Water-cooling

of the obturator-probe has been tried and found to be successful. However because of the added weight of the coolant as well

as of the necessary auxiliary equipment, and because of the difficulty of maintaining a liquid-tight system subjected to the

vibratiois and wide temperature gradients encountered, an effort

was made to use air alone as the cooling medium.

II. MATERIALS TESTED

Fig. 1 is a copy of the shop drawing originally prepared

for fabrication of an air-cooled obturator-probe. As finally

worked out the design was slightly modified to call for fewer

holes of larger diameter than shown. This was made necessary

by the difficulty encountered in drilling the long, smalldiameter holes called for. A further refinement incorporated

later was an end plate with a large nozzle to permit the attachment of a large air hose.

III. TEST HIOCEDURE

The obturator-probe to be tested was mounted on an AVQ-2A

searchlight lamp mechanism, and was operated at the rated current of 120 amperes at 70 volts. The temperature rise of the

obturator-probe was measured with a calibrated iron-constantan

thermocouple located in contact with the back of the upper half

of the obturator-probe. The thermocouple was connected to a

recording potentiometer and the temperature rise of the obturatorprobe thereby recorded during each run.
Air for the purpose of cooling the obturator-probe was

obtained from three alternate sources? a) the laboratory air

supply of about 8 lbs/square inch through a 3/l6 inch pipe,

b) a 1 l/2 ESP high-pressure compressor, and c) a 1 l/3 HP

portable blower. An air current shield was provided to minimize the effects of air currents in the neighborhood of the

arc. This shield consisted of a brass baffle plate attached

to the obturator-probe at the side opposite to that from which

the air entered.

IV. TEST RESULTS

The amount of air provided by the laboratory supply or

the 1 l/2 HP compressor was not sufficient to cool the

obturator-probe by an appreciable amount. The 1 l/3 or portable blower provided a supply of air at a sufficiently high

speed to accomplish a certain amount of cooling. The aircooled obturator-probe reached a temperature of about 700°C

after a period of about four minutes as shown in Fig. 2.

After that time the temperature remained fairly constant as

long as the searchlight was operating normally.

V. DISCUSSION

Although the cooling of the obturator-probe was partially

successful, the test indicated that more extensive experimentation and development of air cooling of the AVQ-2A searchlight

in its pre sentr design would not be practical. It is felt that

a satisfactory air cooling system should limit the maximum

temperature of the obturator-probe to about 400°C. This would

require a blower of considerable size in addition to adequate

air ducts to the obturator-probe. The cooling system would

have to be sufficiently well constructed and rigid to prevent

stray air currents from disturbing or extinguishing the arc.

In addition to the obturator-probe, other parts of the arc

mechanism would have to be cooled in order to insure continuous operation without injury to the arc lamp.

In other experimental work directed toward improving arc

performance, it has been found that increasing the angle of the

negative carbon from 16° to 45° improves the stability of operation. In addition, it reduces the amount of heating of searchlight parts and therefore lowers the demand on a cooling system.

Another experimental change tried out in the AVQ-2A searchlight was a redesigned positive head. The obturator-probe was

replaced in Its obturation function by a hollow air-cooled

cylindrical copper contact sleeve, about an inch in overall

diameter, which encased the positive carbon and delivered
current to it. Since the area of the material exposed to the

arc was much smaller, less heat was absorbed. A separate aircooled copper probe was provided to assume the electrical

sensing function formerly accomplished by the obturator-probe.

This new positive head and probe arrangement when operated

in conjunction with the 45° negative carbon trim operated satisfactorily with only the laboratory air supply ( 8 lbs/sq. in.

through a 3/l6 in. pipe).

VI. CONCLUSIONS

1) Air cooling of an AVQ-2A searchlight for continuous

duty operation is not practicable without extensive modifications of both the positive and the negative heads of the arc

mechanism.

2) The combination of a liquid coolant heat transfer

medium and air would be a more practicable modification of the

AVQ-2A searchlight for continuous duty operation.

3) Redesign of the sire mechanism to improve operation

and minimize overheating of the components should precede any

effort to incorporate a cooling system.
~

.
10-32 (4 Holes')

C OUMT ERS I WK

ORIGINAL DATE OF DRAWING DfC. /| 1^52

REVISIONS

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1 1

2

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K1BS Test No H N - 1 1 /s 3

PJ^CE 1 NOMENCLATURE REO'D NATIONAL BUREAU OF STANDARDS

WASHINGTON 23. D. C. Air-Cooled Oetoraor

FOR

AVQ-2A SeRRCHU&HT

MODEL TYPE SCALE r . r DIMENSIONS IN INCHES

{(Jnlet s o' Heritiw inaahed)

TOLERANCES

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