NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

0201-20-2300 March 26, 1954 3176

REPORT

ON

AN INVESTIGATION OF THE OPERATING CHARACTERISTICS

CF

AN NBS OETURATCR-PROBE FOCUS CONTROL

IN

AN AN/AVQ-2A AIRCRAFT SEARCHLIGHT

by

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 21N-I79/53

Sponsored by

Lighting Section

EL-521

Bureau of Aeronautics

Department of the Navy

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, repr

unless permission tsol

25, D.C. Such permi

cal iy prepared if tha

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 2015

part, is prohibited

ldards, Washington

>rt has been speclfiort for its own use,

An Investigation of the Operating Characteristics

of an NBS Obturator-Probe Focus Control in an AN/AVQ-2A Aircraft Searchlight

1. INTRODUCTION

In 1951 } the Naval Aircraft Lighting Group at the National

Bureau of Standards developed a modification kit for the AN/AVQ-2A

aircraft searchlight. This modification was intended to improve

the reliability of operation of the searchlight. The principal

component of the kit was the obturator-probe, a disc of copper

which replaced the obturator of the searchlight and at the same time acted as a probe control to maintain the positive carbon in

focal position automatically. The probe method of focus control

replaced the thermostat-optical system that is standard on the

AN/AVQ-2A searchlight. Ten modification kits were fabricated at

the National Bureau of Standards and furnished to the Navy p or field evaluation. Two evaluations were made, one at the Key West

Naval Air Station and one at the San Diego Naval Air Station. The

reports of both evaluations were favorable and it was recommended

that the modification be incorporated into the searchlights.

Recently, it was found at NBS and at the Strong Electric Corp.

that under certain operating conditions, a deposit forms on the

controlling edge of the obturator-probe, leading occasionally to

reduction in the accuracy of focus control, and, in the most

extreme cases, to loss of control altogether. Accordingly, the

investigation herein reported was undertaken to ascertain the con- ditions under which these deposits form, and, if possible, to

redesign the obturator-probe so as to minimize or eliminate the

difficulty, and generally improve its operation.

2. MATERIALS TESTED

The AVQ-2A arc lamp mechanism was tested with several

obturator-probes modified by various changes from their original

shapes.

Tests were made with obturator-probes modified as follows:

1) Two one-ouarter inch holes were drilled fro’- the bottom

of the obturator-prote to the recess in the center^ Two more holes

of the same diameter were drilled from the top of the obturatorprobe to the recess. This was done to determine whether ventilating the recess would affect the formation of the deposit.

2) The controlling edge of the obturator-probe was raised

1/16 of an inch above its original design. This wa„ done to

remove the edge from the intense heat of the arc and the products

of combustion.

- 2 -

3) The controlling edge was rounded to present a smooth- curve to the positive tail flame. The screw mounting holes were

later elongated vertically in order to permit vertical adjustments of the obturator-probe over a range of about 7/32 of an

inch.

3. TEST PROCEDURE

The modified obturator-probes were mounted on a standard

NBS-modified AVQ-2A arc lamp mechanism and their performance com- pared with an obturator-probe of the original design. The values

of resistive ballast in series with the arc current were varied

in order to determine the relation between the value of ballast

and the tendency for the deposits to form on the obturator-probe.

It was found that the deposits formed most readily with smaller

amounts of ballast (stiffer power supply). Accordingly, the

bulk of the work was done with the minimum ballast practicable

in the laboratory, amounting to a voltage drop at rated current

of about 12 volts. This effectively limited the peak current on striking to about 350 amperes.

Two methods were used to evaluate the results and compare

the effectiveness of the modified obturator-probes. By one method,

the extension of the positive carbon from the obturator was measured

after each run. This method was used in evaluating the first- two

modifications of the obturator-probe.

With the second method, a 50 mm photographic lens was em- ployed to project the image of the arc on a screen placed at a suitable distance to obtain a magnification of about ten to one. A scale was drawn on the screen to represent l/32-inch movements

of the positive carbon. The image of the arc was observed during

the entire run during which the limits of control and the general

behavior of the arc were noted. This method of observation was used in the measurements made to determine the proper mounting

height of the third modification of the obturator-probe. The

amount of deposit on the controlling edge of the obturator-probe

was observed and noted during and after each run. The duration

of the strikes was varied in order to simulate the range of actual

operating conditions and to ascertain the conditions under which

the deposits are most likely to occur. Observations on about

400 runs were compiled for the results of the test. In each case the magnetic field surrounding the arc was adjusted by bending or aligning the positive bus beneath the arc to obtain a good tail

flame and a smoothly operating arc. The arc was operated at rated

current and voltage.

4. TEST RESULTS

The rate of formation of a deposit on the controlling edge

of the obturator-probe did not appear to be affected by the

addition of vent holes nor was it affected by raising the con- trolling edge.

- 3 -

It was observed that deposits on the controlling edge of the

obturator-probe could be placed in tx^o categories, soft and hard

deposits. The soft deposits resembled a very small feather protruding from the controlling edge of the obturator-probe, and

parallel to the positive tail flame. Its formation was rapid and

took place when the arc misbehaved because of an improperly placed

magnetic field or eccentricity in the negative and positive carbon

axes.

This deposit is apparently conductive, since it affected the

controlled position of the positive carbon as if it were an ex- tension of the obturator-probe. However, it is easily dislodged

by heat, convection currents, and probably by vibrations such as would occur in an aircraft. This type of deposit is not of much

concern since its formation can be controlled to a certain extent

and, in every case, it disappeared within about one half minute of

operation, ^he hard deposits of the second category resembled a small bulbous growth of carbon on the controlling edge of the

obturator-probe. This deposit, like the soft deposit, was found

to be electrically conductive and therefore affected the position

of control of the positive carbon. Its formation was rare when

the arc was operating normallyj however, it was almost certain to occur when the positive carbon burned back into the recess of the

obturator-probe because of malfunction of the mechanism or because

of an abnormally large number of strikes (of the order of six or more) of very short duration (about five seconds). Violent strikes

such as occur when the arc power supply does not have the proper

droop characteristics (see NBS Report No. 1872, "Dr^op Characteristics of Power Supplies for the AVQ-2A Searchlight”), or with carbons still hot from a previous strike, also increas ; the possibility

of deposits of the hard variety. These deposits formed comparatively

slowly over a period of about one or two minutes of actual operation, intermittent or continuous. They disappeared in about the same length of time apparently due to the intense heat. The situation became grave when the positive carbon burned back into the

recess of the obturator-probe and the deposits formed on the con- trolling edge extended downward sufficiently to block the advance

of the positive carbon. Continued operation under this condition

may result in the formation of additional deposits and possible

arcing between the obturator-probe and the negative carbon.

Increasing the vertical distance between the controlling edge

of the obturator-probe and the positive carbon did not appear to

have an effect on the formation of deposits on the obturator’s

controlling edge. When deposits did occur, it took a longer time

for them to extend down to the positive carbon and cause serious

- 4 -

malformation of the arc. However the location of the tip of the

positive carbon was not controlled as accurately with the controlling edge further from the carbon.

Rounding the controlling edge of the obturator-probe did not

prevent the formation of deposits. However the deposit was more

widely distributed and took longer to build up in sufficient quantity

to affect the control position of the positive carbon.

* . The effect on iocus control of varying the distance between the

controlling edge of the obturator-probe and the positive carbon is

shown in Fig. 1. The average variation of the limits of control for

groups of ten runs for various vertical separations is shown. The

standard deviation for individual runs is also indicated on the

graph.

5. CONCLUSIONS AND RECOMMENDATIONS

The modification of the obturator-probe controlling edge is

recommended as shown in Fig. 2 (Drawing 1354-1A), ” Obturator-probe

for AVQ-2A Searchlight”. The rounded controlling edge will not

prevent the formation of a deposit, but will tend to minimize its

deleterious effects and will result in improved operation. The one- eighth inch spacing between the controlling edge of the obturatorprobe and the positive carbon was determined to be optimum design

for good control, reliability of operation, and reasonable safeguard

against the formation of deposits in sufficient amounts to cause actual contact with the positive carbon.

Deposit build-up on the controlling edge of the obturatorprobe can be minimized by careful adjustment for proper arc operation. This adjustment should include, if necessary, bending or alignment of the positive contact bus located under the arc in order

to obtain a smoothly operating arc with a tail flame that is not

split and produces little or no soot. Attention should also be

given to the arc current and voltage as well as to the power supply

characteristics with respect to ballast of proper droop regulation.

(See NBS Report No. 1872).
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NO. NOMENCLATURE NO. REQ’D

NATIONAL BUREAU OF STANDARDS

WASHINGTON 23. D. C. 5>ipc Vie /v OBTU KPiT OP. — Probe

K\at eRjal : Cavity- Fkee,

Elec.t'r.o l v r ic. Tough T itcm

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AVQ-2A 5 E «RCH U I a H T

MODEL TYPE **“ 1

1 DIMENSIONS IN INCHES

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CHECKER

TOLERANCES

(UnU’tB of hrrwi»" »pmc»fu»il) DECIMALS ±.0U3 FRACTIONS r.015

PROJECT ENGH PROJECT ENGR.

SUBMITTED BY

CHieE. SEC ANGLES

DO NOT SCALE THIS PRINT

EXAMINED BY

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DIV. SEC. THIS PRINT ISSUED

APPNOVEO BY

CHIEE. OIV 2.1

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