July-August - Air Defense Artillery School

40
Where ex: 0 is 800 mils. Lm is % (206), or 154 mils.
(Actually 161.)
The foreg~ing treatment of angular travel leads should
be of interest to all officers dealing with the subject. Howeyer,
from the standpoint of practical application, the principles
here developed may be summarized as follows:
For machine guns developing muzzle \'elocities of 2600-
2800 ft./sec. and for all target courses having midpoint
ranges (Omln) up to 1200 yards .
THE COAST ARTILLERY ]OllRJ\'AL
(l) The angular trm'el lead (Lm) in mils at the midpoint
is 14 mils for each 20 mi.jhr. target speed;
that is, 70, 140. and 210 mils respectively for
speeds of 100, 200, and 300 miles per hour. It
should be of interest to note that these values are
likewise correct for "lateral" lead as required on
Central Tracer Control equipment, since target
travel at the midpoint is seen as horizontal. Although
this similarity to lateral lead occurs only at
the midpoint, it may provide a valuable check in
using sight equipment where lateral and vertical
leads must be set separately. (For guns having
shorter times of Hight than the Cal. .50 a similar
constant may be determined, varying directly according
to the time of Hight for 1000 yards; for instance,
the constant for the 40mm gun is 13 mils for each
20 mi.,lhr., and the midpoint lead for 200 mi.jhr, is
130 mils.)
(2) The angular travel lead (L.n) for an approaching
target is % of the midpoint lead when the angle of
approach is 45 degrees (800 mils), and ~ of the
midpoint lead when the angle of approach is thirty
degrees (533 mils). These rules are sufficiently accurate
for practical use.
0) Leads for targets on the receding leg of a course are
not generally considered important. However, a
receding angle of approach of 45 degrees requires
about % of the midpoint lead, and a receding angle
of approach of thirty degrees requires about % of
the midpoint lead.
( -+) The variation of lead between the main target positions
considered above is a smooth curve, as indicated
by Fig. No.4. It may also be shown on the
forward area sight, as in Figure No. 7 c. In using
forward area sights the mil values are never considered,
the lead being regarded in terms of speed
rings (full speed values at midpoint) or fractions
thereof.
(5) If linear lead must be considered (due to inadequate
sights, or in engaging ground targets) the above rule
for midpoint angular lead still furnishes the clue.
The lead in yards for 1000 yards range is equal to
the numerical lead in mils, and linear leads at other
midpoint ranges are in direct proportion to the
range. For instance, a 20 mi.jhr. target at 1000
yards requires a midpoint lead of 14 yards. If at
600 yards, the linear lead is .6 x 14, or 8.4 yards.
Approaching at 45 degrees, the linear lead is 1.5
times the midpoint lead. Receding at 45 degrees
the linear lead is 2.0 times the midpoint lead. Ordinarily
these linear leads are finally expressed in
1~._ .~~~::7
. ,- -~- m~;fJS: 0
~
•• ~-.
~. T.
PI.;If~ ~C,,' klL.
lI!I> ...y 7)--,.. .. ~/.s ~ 6oz. ~, .. ~,
'-RONT R€.AR ~ E8.S2!:a.
j, Twn ,&0,,";/10,. e~0/ ro'-JNI::7,.o! 4~ S;.i'h~.s.1
(2) 0 @ 0 @F/,M
(i""
C. cross/nQ CourJe W//h FA .5:
~
Figure 7
target lengths rather than in yards, the length
target being assumed. Obviously it is necessaryal
that the range be known or estimated.
It should be remembered that in the use of I.T.C. tra\,
leads either linear or angular, are important largely fr
the standpoint of establishing an opening lead. Subst
quent adjustments must be made on a basis of tracer obser
vation; if the tracers cannot be seen through the sights
will be necessary to look around the sights.
FORWARD AREA SIGHTS
The forward area sight is at this time the only practi
method known by which a gunner can instantly appl
angular leads of known value to antiaircraft targets. Fi
No. 7 a indicates the principle by which angular leads a
established by the sight. The target must be carried in t
front sight as always appearing to Hy towards the center
the sight; thus the lead is always established in the line 0
Hight, or, to express it differently, the sight is always point
at a spot directly ahead of the target. The better types
front sights have radial elements (Fig. No.7 b) with whic
(or between which) the target may be aligned in order t
accomplish the above purpose. The amount of angul
lead is determined by the radial distance of the target ima
from the center of the sight, as measured by the circu
"speed rings." The rear sight must invariably be alig
011 the target, and not with the center of the front sight.(~
Fig. No.7 c left). In cases where the rear sight is deSign
with rubber eyecup for application of the eye direct!
against the rear sight the problem is simplified and. ~
target appears in the front sight as in Fig. No. 7 c, ng ht
Fig. No. 7 c, illustrates the principles of taking I
on any target not diving directly at the gun. (For this lat
type of target no initial lead is necessary.) Since the use
forward area sights involves actually laying the sightS