The Norden Bomb Sight manual

Linear

Mil

North

Hook

Fig. 43

surface when the earth

has moved through the

arc

represented by the distance from B to

1. To the

observer it would appear that the spin axis had

changed its position.

Actually the rotation of

the earth has caused

the vertical to change rela-

tive to any point of reference, such as the verti-

cal spin axis of the gyro.

The observer does not

sense this, and so it appears to him

as though the

bottom of

the

gyro's spin axis actually moved to-

ward the east.

A linear mil is an

angle which from the ver-

tical subtends 1/1000th of

its altitude along its

base line. Determine the finear distance subtend-

ed and then any angle,

no matter how far from the

vertical it may be tormed, which subtends this

same linear distance in the same problem

is also

a mil. Actually,

farther the angle proceeds

away from the vertical, the smaller it must become

to subtend the same linear distance.

To reiterate: In computing the amount of re-

lative displacement of the gyro spin axis with the

reference vertical (to the earth's surface) there

are two units of measurement

that can

be consid-

They are degrees and mils.

A degree is l/360th of the circumference of

a circle.

for our purpose,

is an

angle

that will

subtend

A mil 1000th of its altitude along

its base line.

vertical (that is a mil, is the angle formed from

(cat and thich

is determined from

the

subtends 1/1000th of its

altitude), then

as earh

mil angle is formed

far-

ther from

the vertical it becomes progressively

smaller in order to subtend the fixed mil

dis-

be shown mathematically that

the angular increase between the gyro spin axis

and the vertical continues to increase at the same

the spin axis 10 min-

to precess the first 4°

from

the vertical, it

will take it 10

minutes to precess go

the vertical.

farther away the spin

axis deviates

vertical, the greater linear

mii distance

it precesses

for any given period

of time.

North hook is or can be caused by (1) flex-

ible lead

drag,

(2)

bearing drag,

and (3) pendul-

ousness of the

vertical gyro. Notice Fig. 43 for

position of the bearings.