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144 'ACCURACY OF FIRE
Effect of danger-space and the 50 % zone can nowbanger- be established, and it will be easily under-space onChance of stood how increase of range decreases theHitting. probahilityof hitting. The angle of descent
increases with the range and danger-spacebecomes smaller and smaller, but the 50 %zone increases with the range, sincethere is more dispersion J of the pro-jectiles. The result of this is to reduce the
danger-space .value of the factor 0/ and Its50 /0 zone
corresponding percentage.
Example.-To find the probable per-centage of hits on a target 25 feet high at
3000 , 6000, and 9000 yards. The 50 % zonesat these ranges being 30, 50~and 7 0 yards,whilst angles of descent are 2, 4, and 8degrees.
The danger-spaces for a 25-foot targetwith these angles of descent are 175, 87,
and 43 yards .•'. Probability factors for these ranges
are given by 81, and J l which equal 6,30' 50 70
1.75, and "6.
These correspond to percentages of 100,76, and 32.I t is usual for the target to be possessed
of both height and depth as in Fig. VII, and
~
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ACCURACY OF FIRE 145
the available target on which we can calcu- Virtual
late our' chances of hitting is then referred Target.
to as the virtual target.
In Fig. VII, AmXM is a target of depthAM and height Am, and the range is suchthat'the angle o~ descent of the projectilesis bBA.
The danger-space of the vertical portionAm is clearly AM, but a shot passing overAm will not necessarily miss the horizontaltarget mX,-and in-faet can pass at a heightmb above the vertical target before missingthe horizontal at X.
The virtual target is therefore AB, whereAM is the dangerous space for the verticalportion and MB for the horizontal.
The danger-space ol. __ _target is somc- Permis-times referred to when dealing with deflection, sible
but in this case it is not an accurate expres- error inDeBee-sion. What is really meant is the maximum tion.
error in deflection allowable without missingthe target. --
Example.-,Target 100 ft. wide. Range 5000 yds.
,,'J Deflection dial graduated in knots for arange of 3000 yards .
.. Calculate first the angular deflectioncorresponding to I knot on the dial.
L
"
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Permis-sibleError inDeflec-tion.
146 ACCURACY OF FIRE
ExtTan. angle deflection =~.
6080 . . .E speed I knot -6- ft. per sec.3 00
t = time .of flight = 3 seconds (fromrange tables).
R range 3000 yds. 9000 ft .
•.. Tan. angle deflection6080 I I
3 600 X 3 X 9 000 1800.
The target is 100 feet wide and the rdnge is15.000 feet .
... Angle subtended from centre to edgeof target is
50 I
15.000 3°0.
I .
Since I knot is rerrescnted by 1800,
2.- must be the equivalent of 6 knots.300
From edge to edge of the target willrequire. therefore, 12 knots alteration of the
sight dial, and the permissible error is, ofcourse, 6 knots.
A knowledge of the danger-space is thusof great value, since if it is necessary to
= = =
= = =
=-= =
=
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ACCURACY OF FIRE 147
correct for elevation or direction we can be
certain that if a shot has missed for eitheror both reasons, a correction of an amountequal to these danger spaces will not be toomuch; and if we have only missed by asmall amount, we shall be certain of hittingwith the next round.
At long ranges it may often be impossibleto tell for certain if one is hitting; and allthat can be hoped for is to place the shotabout the target and be governed by thenumber of misses. This may not soundvery hopeful until we seek the aid of the
probability zone and its factors, as thefollowing example will show.
Target Ab at range R, angle of descent M.Length of 50 % zone known.
AB here represents the danger-space of the
n. target.Mean point of impact should therefore be
midway between AD, as at M.
For simplicity let us assume that the50 % zone is of length AD. Then the maxi-
mum number of hits we can hope to obtainis 50 %. Twenty-five % of our shots willfall short and 25 % over.
. If, now, while firing we obtain everyfourth shot short, it is fair to assume that
1.2
Percent-age ofShortShots.
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Accuracyof Fire inPractice.
148 ACCURACY OF FIRE
two are hitting and one is going over. An,error in range equal to AM, i.e. the mean
point of impact moving to A, would at oncebe shewn, since we should now be obtainingtwo shots short and one hit out of ourbroadside of four.
So much for the theorefical accuracy ofour shooting and the' assistance which snch
knowledge gives us in practice.
The practical side of the question canbe summed up as follows :-
(a) A knowledge of present conditiolls anda proper appreciation of recent results,
together with the capacity of forecastingthe future of affairs so that :-
(b) The necessary information may be' de-duced and transmitted rapidly to thegun positions, where .
(c) Advantage can be taken of such infor-mation as' will enable us' to' obtain fullvalue from our armament.
(b) is concerned with the organisation ofthe control staff, the instruments empl~ycd,and methods of transmission,' whilst (c)
depend3 upon the accuracy of sights andguns, the ability of . the gun-layers. andthe skill of the guns' crews.
Both (b) and (c) are o~tside 'the scope of
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