COAST. I ARTILLERY JOURNAL, - Air Defense Artillery

COAST ARTILLERY BOARD NOTES 495
not be relied on to illuminate a target for a sufficient length of time to
enable fire thereon by normal daytime methods.
2. Present searchlight-aeroplaneoperationsbeing carried out by the 61st
Artillery (A.A.) in conjunction with night bombardment airplanes from
Langley Field are so successful, from an antiaircraft viewpoint, that more
considerationof night firing at illuminated aeroplanes is warranted.
3. a. It is evident that even should a single plane be illuminated the
normal position finding system probably will be inoperative because of the
irregular course which will be adopted by the aviator in an effort to escape
from the beam. One of a formation so illuminated would, of course, be
more or less restricted to a regular course, because of the necessity for
maintaining the formation.
b. Even should the course become irregular it is still possible and essential
that fire be opened with estimated deflections. For such fire to be of
any value the altitude, which is the basis of the determination of fuze range,
must be accurately determined. With an accurate altitude and a Range
Indicator, gun type, the most effectivenight fire possible at a target flying
an indeterminate course may be improvised.
c. Should the target escape from the beam the determination of its altitude
while illuminated will be of great value in the continuation of the
fire against the unseen target whether by "fire by sound" or barrage methods.
2. Letter, Office,Chief of Coast Artillery, dated June 8, 1926, directed that
"the Coast Artillery Board submit a similar discussion on this subject giving its
opinions at the present time, based on the developmentsthat have been made in
the operation of searchlights and listening apparatus during the past year."
II-DISCUSSION.
3. Since actual use in warfare antiaircraft searchlights, hoth in material and
n::ethods,have gained in efficiency. The beams have greater intensity and greater
range; distant control and improved hand control facilitate operation of "following,"
and improved carbons, automatic feed, and the protection given by barrel
type make the arc more dependable. Original searching by guesswork has progressed
by stages through searching by aid of single horns, paraboloids, etc., to the
use of exponential horns utilizing the binaural sense of the listener. Prediction
based on the apparent source of sound and corrections for sound lag now enable
searchlight batteries to point the light beams at or in close proximity to the plane.
4. Exercises in the past year have demonstrated that, regardless of the
maneuvering of the plane endeavoring to cross a searchlight area to reach an
objective, only a small percentage reach the objective unilluminated. Airplane
tactics, such as zigzag courses, gliding with motors throttled, sound interference
from another plane leaving the searchlight aera, have failed to prevent the lights,
assisted by sound locators,picking up and illuminating the large majority of targets.
5. Targets once illuminated have been easily held in the beam when three
lights were used. Frequently two lights sufficed. The primary targets for searchlights
and guns are loaded bombers. Such planes are limited in their maneuverability
by their weight and their mission. Once illuminated they have little
probability of escaping the searchlight beams.
6. An illuminated bomber maintaining rectilinear flight for any period over
sixty seconds while within range of a 3-inch gun battery has the probabilities
against escaping unhit. Using the conservativefigure of 5% probability of hitting
(recent practices have all exceeded this) twenty rounds, or five rounds per gnn,
are necessary for one hit. Five rounds per gun require sixteen seconds firing time
which with average time of flight of 15 seconds and 8 seconds dead time gives 39