Gun Sight Mark 18


GUN SIGHT MARK 18

The Gun Sight Mark 18 is an electrically operated, computing, reflector-type sight designed for use in power turrets. The gunner makes the proper settings on the control units, after which it is only necessary for him to "range and track" the enemy plane, for the sight automatically figures the proper point of aim. It makes no difference whether or not the enemy plane is in a pursuit curve, or whether the gunner is strafing a stationary target. This sight handles all situations equally well. It is not necessary for a gunner to be familiar with the internal construction of the Mark 18

Restricted

S-53



Gun Sight or the theory of how it computes deflections. Gunners should never dismantle or open up the sighting head or any of the other units. Only experienced maintenance men are allowed to replace units, alter adjustments or check for malfunctions, except as specified in "Boresighting" and "Gunner's Check."

Fixed reticle and gyro reticle

GENERAL DESCRIPTION

The Gun Sight Mark 18 is really two sights in one. It has two optical systems and two reticles. When you look at these reticles through the reflector plate they appear to be projected out into space. The reticle on the left which you view with your left eye resembles the inner ring of the Mk 9 reflector sight. This is the fixed reticle. The reticle on the right side of the reflector plate which you see with your right eye consists of a center "pipper" which is surrounded by six diamond-shaped pips arranged in the form of a circle. This is the gyro reticle or movable reticle.

GYRO OR MOVABLE RETICLE

The circle formed by the six diamond-shaped pips of the gyro or movable reticle can be made to spread out and close in. When you operate your turret the gyro reticle (the center pipper and six diamonds) moves around on the reflector plate. If you use the Mk 18 sight properly the gyro reticle automatically computes the correct deflection and gets hits for you. All you have to do is keep the enemy framed or "spanned" in an imaginary circle drawn so that it cuts the inside points of the six diamond pips of the gyro reticle. The center pipper of the gyro reticle, also forms the center of the imaginary circle.

THE FIXED RETICLE
The fixed reticle is essentially the same as the inner ring of the Mk 9 reflector sight. The only difference is that the center of the fixed reticle is a cross instead of a round pipper as in the Mk 9. The fixed cross indicates the direction that your guns are pointing. The fixed reticle is used for aligning the two reticle systems, for boresighting, for maintenance checks (also gunner's checks) and as a standby sight in case of malfunction or failure of the gyro reticle. Sighting with the fixed reticle is the same as with the Mk 9 sight, except it has only a one-rod ring whereas the Mk 9 has a one-rod and a two-rod ring.

Restricted

S-54



THE CONTROL UNITSMark 18 gun sight showing contrrols
The complete Mk 18 sight installation consists of a number of units. Only those units that are of concern to you as a gunner are discussed.
SIGHTING HEAD
The sighting head is in reality the sight. It contains the reflector plate through which you view the fixed and gyro reticles.
The lever on the right side of the sighting head is the sun filter control handle. Depressing this handle lowers the sun filter. On bright days the sun filter may be raised by lifting up the control handle.
On the left side of the sighting head is the fixed reticle mask lever, which when depressed will blank out the ring of the fixed reticle and leave only the center cross showing. Below the fixed reticle mask lever is the range cable sheave, which is mechanically connected to the movable reticle and controls the movement of the six diamond-shaped pips. It is numbered in feet from 600 to 2400.
Directly in front of you on the sighting head is the target span scale numbered from 30 to 120. These numbers represent the size of the target in feet. The target span lever can be moved from side to side to correspond with the known target dimension.
Just under the target span scale and lever is a double-latched door which opens downward. Depress both latches to open the door, and mounted inside are two lamp bulbs which provide light for the two reticles.
On the front of the sighting head (side farthest from you) is a small plastic container filled with blue material. This is the silica gel cell which

Restricted

S-55



dries out the air entering the sighting head and prevents the sight from fogging up. The large cup or gyro pot to the side of the silica gel cell houses the gyro and electromagnets of the computing mechanism.
The small cup beneath the silica gel cell is the gyro motor housing. It houses the electric motor which turns the gyro.
On the sighting head next to the gyro motor housing are two small screws. One screw cannot be turned unless the silica gel cell is removed. These two screws adjust the fixed mirror within the sighting head so that the fixed reticle can be aligned with the gyro reticle. The adjusting screw underneath the silica gel cell gives horizontal adjustment while the other gives vertical adjustment.
An azimuth boresighting ring on the bottom of the sighting head is equipped with notches to permit movement of the sighting head. To move the sighting head right or left the number of notches necessary to harmonize the sight to the guns, loosen the mounting bracket bolt projecting from the bottom of the sighting head. To absorb shock, an "anti-vibration mount" is provided on the underside of the sighting head near the mounting bracket.
Mark 18 gun sight showing contrrols
SELECTOR-DIMMER
This unit consists of a four-point switching device and a rheostat. It is usually mounted near your left hand or on some other place you can reach easily. The "selector-dimmer unit" enables you to turn on either the fixed reticle or the gyro reticle, or both at the same time. On the face of the unit a pointer can be turned to any one of the following four positions: FIXED—only fixed reticle visible in reflector plate. This switch position is used for boresighting, maintenance checks, gunner's checks and

Restricted

S-56



when the fixed reticle is used as a standby sight. FIXED & GYRO—fixed and gyro reticles visible in reflector plate. This switch position is used for day firing, for alignment of the two reticle systems and for gunner's checks. GYRO DAY—only gyro reticle visible. This switch position is optional for daytime firing. GYRO NIGHT—only gyro reticle visible. This switch position is used for night firing because the combat ranges are necessarily short. On the outer circumference of the selector-dimmer unit is a knurled ring (dimmer) which operates the rheostat controlling the brightness of the reticles. Turn the dimmer clockwise for greater brightness or counterclockwise for dimming the reticles.
ALTITUDE AND AIRSPEED UNITAltitude and Airspeed Unit
This unit is mounted so that its numbers are visible and control pointers easily reached. The upper half of this unit is the airspeed section and is numbered in knots from 0 to 250.
The indicated airspeed (IAS) of your plane is "set in" by moving the pointer to the correct figure.
The lower half of the unit is the altitude section and is numbered in thousands of feet from 0 to 40. The altitude of your plane is "set in" by moving the pointer to the correct figure.
OPERATING THE MARK 18 SIGHT
Order of tunring on switchesFor Day Firing
BEFORE YOU SEE THE ENEMY
1 Turn on turret Main Power Switch (see Turret Section for location).
2 Turn on Sight Power Switch.
3 Turn Selector-Dimmer to "FIXED & GYRO" position; then turn knurled ring (dimmer) of this unit until the reticles have the desired amount of brightness.
4 Adjust the altitude and airspeed settings of your plane on Altitude and Airspeed Unit. During an attack, however, if your pilot is taking evasive action, do not use precious time resetting the altitude and airspeed.
5 Raise the sun filter if it is needed.

Restricted

S-57



WHEN YOU SEE THE ENEMYWingspan setting

1 Wingspan Setting

As soon as you recognize the enemy plane, "set in" his wingspan on the target span scale. For example, if you recognize the enemy plane to be the Jap fighter "Hamp" you should know that his wingspan is 36 feet. Move the target span lever to the proper position on the target span scale. If you were attacking or being attacked by a Jap "Betty," set the target span scale at 70 feet. If you were being attacked by both, you must change the wingspan setting when you shift from one to the other. It is very important that you know enemy planes and their wingspans. Keep your knowledge of recognition up to date.

Mark 18 gun sight assemblyFoot pedal operation

Restricted

S-58



Ranging views of six diamonds

2 Ranging

After you have "set in" the correct wing-span of the enemy plane you must "range and track" him properly if the sight is to compute the deflections. You "range" the enemy by the use of the two foot pedals attached to cables in the turret. If you push down on the right pedal the "aiming" or "ranging" circle (an imaginary circle drawn through the inside points of the six diamond pips of the gyro reticle) gets larger. In other words, the six diamonds move farther away from the center pipper, thus making the aiming circle larger. This is minimum range. If you push all the way down on the left pedal the diamonds move closer toward the center pipper and the aiming circle is smaller. This is maximum range. It is very important that you always keep the enemy "spanned" or framed within the aiming circle if you want to get hits.
NOTE: Be sure you frame the enemy plane within a circle, not within a six-sided figure formed by joining the pips with straight lines. Let's assume that you are in a bomber making 200 knots at an altitude of 20,000 feet. You have already "set in" the airspeed and altitude of your plane. As you are cruising along you see a Jap "Hamp" heading toward you at an approximate range of 4000 feet. You immediately adjust the target span scale to 36. Then place the center pipper of the gyro reticle right on his nose and press the left pedal all the way down to make the ranging circle smallest. At first, Hamp's wingtips may not extend all the way across the ranging circle, as he may not yet be within range. As he gets closer he will appear to get larger, and soon his wingtips


Restricted

S-59



will touch each side of the ranging circle. He is now "spanned" and in range. Open fire. He appears to get larger as he gets closer. It will be necessary for you to make the aiming circle get larger and larger in order to keep it just touching his wingtips. You do this by pressing down on the right pedal gradually and smoothly. You must track him carefully and keep him accurately spanned or you will not get hits. Then just keep the center pipper right on the spot you want to hit and watch Hamp fall to pieces. If an enemy gets within 600 feet of you the ranging circle will not get any larger. This does not mean that the sight is not accurate within 600 feet. It is very accurate. All you have to do now is hold the right pedal all the way down and keep the pipper where you want to hit. Fuselage and mass ranging

Sometimes it may not be possible for you to span on a full-square view of the enemy's wingspan. If you have a full-square view of the enemy plane's fuselage instead of the wings, you still set the wingspan of the enemy on the target span scale. However, keep the pipper on the cockpit and one edge of the aiming circle on his tail and you will have him "spanned" properly. This is because enemy fighters are approximately one-half as long from cockpit to tail as from wingtip to wingtip.
When the position of the enemy does not present a full-square view of either its wings or fuselage, set in his wingspan as usual; then do your spanning on whatever target dimension appears longest to you. In some cases he may appear wider than long or longer than wide, and in others the enemy plane may simply appear as more or less a mass in the sky. When spanning such a target keep the ranging circle a little larger than necessary to touch the longest dimension of the target that is apparent to you— about 1/10 the diameter of the ranging circle on each side of him. As soon as the angle of the enemy changes so that you can see his full wingspan or full fuselage, use the procedure for wingspan and fuselage ranging.
The ranging mechanism may not always be controlled by foot pedals. Other types may be developed and used. However, the principles will be the same regardless of the type of control used.

3 Tracking

As the attacking enemy moves in, you must operate your turret to keep him framed within the ranging circle. This is referred to as "tracking." Frequently it will be necessary for you to move your turret rapidly at first to pick up your target. This is called

Restricted

S-60



"slewing." Always keep the right pedal fully depressed while slewing the turret, regardless of the range of the enemy. Otherwise the gyro reticle will "tumble" (drift entirely off the reflector plate) and the sight will be out of commission momentarily. Therefore, when slewing pick up the target with the fixed reticle and swing ahead of it along its line of apparent motion. The fixed reticle shows where your guns are pointing. To help the gyro reticle to "catch up," reverse the direction of slewing along the target path; then depress the left pedal to frame the target with the gyro reticle. Track smoothly for a moment and then open fire. When you are on the target your tracking must be smooth and deliberate. The sight will not compute properly if your tracking is jerky. After you have spanned the target always track a short distance before opening fire to enable the sight computing mechanism to figure deflection.

For Night Firing

1 Turn on turret Main Power Switch.
2 Turn on Sight Power Switch.
3 Turn Selector-Dimmer to "GYRO NIGHT" position; then turn knurled ring (dimmer) of this unit until the gyro reticle has the desired brightness. It should not be very bright.
4 As soon as you see the enemy, put the center pipper on him and keep firing as long as you can see him.
NOTE: There is no need for the sun filter. No setting is required on altitude and airspeed unit; pointer may be in any position. No setting is required on target span scale; target span lever may be in any position. Ranging is not necessary and it makes no difference what you do with the foot pedals. The sight is set for a fixed range of 450 feet, as the range during night firing will rarely be greater than that.

Firing with Fixed Reticle

If for any reason the gyro reticle system does not function properly or fails to function, use the "fixed reticle" as a standby sight. To use the fixed reticle turn selector-dimmer to the "Fixed" position; turn the dimmer to adjust the brightness of the reticle; and then use the fixed reticle the same way you would a Mk 9 sight. The fixed reticle has a 35 mil radius—one rod.

BORESIGHTING

Boresighting the Gun Sight Mark 18 establishes a parallel relationship between the center of the fixed cross of the fixed reticle and the bore axes of the guns. Before boresighting this sight, the fixed and gyro reticles must be aligned. They are aligned when the center pipper of the gyro reticle and the center of the fixed cross of the fixed reticle are super- imposed at an infinite distance with no windage, gravity or relative speed allowances in the system.

ALIGNMENT CHECK

1 Turn the selector-dimmer unit to the "FIXED" position.
2 Turn the turret so that the fixed cross of the fixed reticle bears on some distant object

Restricted

S-61



3 Turn the selector-dimmer to "GYRO NIGHT." The center pipper of the gyro reticle should bear within one mil upon the object on which the fixed cross was sighted. (The diameter of the pipper is approximately two mils.) If the sight fails the above test, the fixed mirror is out of adjustment.

ALIGNMENT ADJUSTMENT

1 Secure the optical gage in position on the sighting head. The gage brings the patterns of both reticles into one line of vision when, without the aid of a distant reference point, both reticle patterns are observed at the same time.
2 Turn selector-dimmer to "FIXED & GYRO."
3 Remove the silica gel cell.
4 Manipulate the range pedals until the range unit sheave is at the 1250 mark. Secure the sheave in this position. The range unit sheave is connected to the range pedals by the range pedal cables. The range unit sheave is numbered from 600 to 2400 feet to correspond with the graduations on the range sheave of the sighting head. The reading of 1250 should be the same on both sheaves. If they are not the same, make adjustments to synchronize them.
5 Disconnect the GREEN wire in the range section of the junction box.
6 Settings of altitude and airspeed are immaterial, and the turret may be turned to any convenient position in azimuth and elevation.
7 Look through the optical gage. Bring the center of the fixed cross to bear upon the center pipper of the gyro reticle by adjustment of the two screws located near the gyro motor housing.
NOTE: The fixed cross is made to move horizontally by adjusting the topside screw and is moved vertically with the lower screw. If no optical gage is available, pick out some distant point (at least 1000 ft. away) and bring the center pipper to bear on this point. Adjust the screws until the center of the fixed cross bears on the same point.
8 When the two reticles are aligned, release the range unit sheave and slowly manipulate the range pedals to move from maximum to minimum range. Observe the pipper and cross; the pipper must not move more than two mils. If the pipper moves more than two mils, replace the sighting head and return to an overhaul shop. The diameter of the pipper is approximately two mils.
9 Remove the optical gage. Replace the GREEN wire in range section of junction box. Replace the silica gel cell.

BORESIGHTING PROCEDURE

1 Level the airplane athwartships and longitudinally.
2 Turn the turret so that the guns are at a right angle to the center line of your plane.
3 Using a spirit level, adjust the nuts on the rear mounting post so that the guns are level. Make sure the level is aligned with the center line of the gun barrel. Secure the nuts on the rear mounting post.
4 Swing the turret to any convenient position, and adjust the guns so that both bore axes bear upon a distant object (at least 1000 ft. away). Do not disturb the elevation setting; the guns need only be adjusted horizontally.


Restricted

S-62



5 Adjust the sighting head so that the center of the fixed cross bears upon the same distant object.

NOTE: It is not necessary to allow for gravity drop of the projectile. The sight automatically compensates for this.
To adjust the fixed sight axis in azimuth, loosen the center bolt in the bottom of the mounting bracket and rotate the sighting head to the desired position. The mounting bracket incorporates a differential toothed arrangement for large and small movements. An adjustable azimuth boresighting ring on the underside of the anti-vibration mount has 36 large teeth on its underside face which mate with the teeth on the top face of the mounting bracket. The ring also has 109 small teeth on its topside which mate with teeth on the anti-vibration mount.
A movement of the sighting head and azimuth boresighting ring of one large tooth represents 10°. A movement of the sighting head alone of one small tooth represents 3.30°. Therefore, if the sighting head and azimuth boresighting ring are moved one large tooth, and then the sighting head alone is moved in the opposite direction three small teeth, a total movement of 0.1° of the sighting head is made. Since 0.1° is approximately 1.7 mils, the differential toothed arrangement permits adjustment in azimuth to within .85 mils.
No elevation boresight fittings are provided with the sight. These fittings are integral parts of the turret and therefore will be peculiar to different installations. The manner of adjusting the fitting will have to be determined by personnel making the boresight.

GUNNER'S CHECK

The most expert and painstaking maintenance of this gun sight, as of any gun sight, will not insure accurate results if the turret is operating badly. Make sure your turret is checked and serviced frequently.

PRE-FLIGHT INSPECTION

1 Test the sun filter's operation and the fixed reticle mask lever. Check that all glass surfaces are clean; use lens paper for cleaning. Make sure the range cable sheave does not rotate when the sighting head is moved back and forth on its sliders or when the guns are elevated or depressed.
2 Check the silica gel cell; its color must be BLUE. When the gel is dry, its color is blue.
The gel begins to lose its blue color and turns pinkish white as it absorbs moisture. If the gel is not blue, it must be replaced. The moist gel may be dried out by removing it from the cell and heating mildly. Heat the gel in a metal container over a low flame until it regains its original blue color.
3 Inspect the light bulbs; if glass surfaces have blackened, replace them. Until service experience dictates otherwise, it is recommended that bulbs be replaced after each five hours of sight operation. Be careful to keep the light bulb reflecting cavities clean.
4 Examine the sheaves and range cables between the sighting head and foot pedals, making sure there is no interference with

Restricted

S-63



smooth operation. Check that the fully depressed position of the left and right pedals against the stops corresponds to 2400 and 600 feet respectively on the range unit sheave. Check that the override springs do not "give" before the pedals reach the stops. Check that 1250 feet on the range unit sheave corresponds to 1250 feet on the range cable sheave on the sighting head.
5 Examine the selector-dimmer unit pointer to see that it turns to each of its four positions. See that the dimmer ring rotates from dim to bright properly; check its action against the stops. Examine the altitude and airspeed unit pointers to see that they turn easily and cover the scales.
6 Examine all electric cables to the junction box and to the various units to see that they are secure.

PIPPER CHECK

Use the optical gage if there is no distant object to employ as a target. The figures that follow are not to exact scale, so the positions are relative. For clarity, the six diamond-shaped pips are not shown. The check will be easier to understand if you remember that the fixed cross is boresighted to the guns; therefore, the allowance between the fixed cross and gyro pipper illustrates the angle between the sight axis and the gun bore. Unless your plane is connected to an external power supply, operate the turret manually—do not use plane batteries.

ALIGNMENT

If the cross and pipper are not aligned, make the necessary adjustments. (See Boresighting Instructions.)

MAKE PIPPER CHECK AS FOLLOWS

SELECTOR-DIMMER UNIT

FIXED

1 Turn the selector to "FIXED."

2 Note the fixed reticle pattern. The ring may be blacked out by action of the mask lever. The gyro motor should not run. See Figure 2.

3 Run the dimmer ring from dim to bright and note the increase in reticle brilliance.

Fig 1 & 2


Restricted

S-64



FIXED & GYRO

1 Turn the selector to "FIXED & GYRO."

2 Both fixed and gyro reticles should be visible and the gyro motor should run. The gyro reticle pipper should be directly below the fixed cross; this is the gravity allowance. See Figure 3.


Fig 3

3 Turn the turret slowly in azimuth to the right and note that the gyro reticle pipper lags the fixed cross. See Figure 4. Continue to turn the turret at the same speed and shift rapidly to short range. Note that the lag effect is less at short range. See Figure 5. Repeat the above while rotating the turret in the opposite direction and at the same speed. The deflections should be the same as the above, but in the opposite direction.


Fig 4 & 5

GYRO DAY

1 Note the azimuth deflection, which indicates that the guns are pointing ahead of the sight axis. The fixed reticle disappears and the gyro reticle remains stationary when the selector is turned to "GYRO DAY." The faded reticle in Figure 7 is for reference only.

2 Note the difference between Figure 3 and Figure 6. Windage deflections are not generated when the airspeed is zero.


Fig 6 & 7

Restricted

S-65



FIXED NIGHT

1 When the selector is turned from "GYRO DAY" to "GYRO NIGHT" the gyro reticle pattern should shift to the left as in Figure 9. Faded reticle is for reference only. Movement can be observed by selecting a distant reference point in the field of view corresponding to the position of the fixed cross. No windage or gravity deflections are generated.


Fig 8 & 9
RANGE UNIT

FIXED NIGHT

1 Note that as the range is changed from maximum to minimum, the gyro pipper shifts to the left and almost touches the fixed cross. This illustrates the smaller allowance needed between the sight axis and gun bore at short ranges. Also note that at short range the allowance for gravity drop is less.


Fig 10
ALTITUDE AND AIRSPEED UNIT

ALTITUDE CHECK

1 Note that as the altitude control pointer is turned from zero to maximum altitude the gyro reticle pipper moves to the left toward the fixed cross. Its position is halfway between the fixed cross and the zero altitude position. This illustrates that at high altitudes, the air being less dense, the bullet travels faster, so a smaller allowance is needed. See Figure 11.


Fig 11

Restricted

S-66



AIRSPEED CHECK

1 As the airspeed control pointer is turned from maximum to minimum, the gyro reticle pipper moves to the left, and at zero airspeed it lies directly below the fixed reticle cross. This illustrates the fact that at zero airspeed no windage allowances are needed.


Fig 12

NOTE: In some turrets the guns cannot be positioned at cardinal points of azimuth and/or elevation. Example: The depression of the guns in an upper deck turret is very small. In such cases, follow the check throughout the allowable positions; i.e. the pipper should be in the direction indicated on the figures.

AZIMUTH UNIT

1 With turret at 0° or 180° the gyro reticle pipper should be directly under the fixed cross as in Figure 13. When firing directly fore or aft, no azimuth windage allowances are needed.

Fig 13

2 With the turret at 90° azimuth, the gyro reticle pipper moves to the right of the fixed cross as shown in Figure 14. With the turret at 270° azimuth, the gyro reticle pipper moves to the left of the cross as shown in Figure 14. Both positions illustrate the guns pointing ahead of the sight axis.

Fig 14
ELEVATION UNIT

In the early production units, there is a dead zone beyond 82V2° elevation and depression; so, the pipper deflection will drop to zero. If this condition exists, only elevate or depress 82V2° when making the test. In later units there is no dead zone.
1 With the turret at zero azimuth and guns at zero elevation, the gyro pipper should be directly below the fixed cross.

Fig 15

Restricted

S-67



2 With the turret at zero azimuth, 90° elevation, the pipper shifts to a point directly above the cross. If the guns are swung to maximum depression (90°), the gyro pipper will shift to a point considerably below the cross. The positions are illustrated in Figure 16. Note that in both positions the guns point ahead of the sight axis.

Fig 16

3 With the turret at 180° azimuth and the guns at 0° elevation the pipper shifts to a point directly below the fixed cross.

Fig 17

4 With the turret at 180° azimuth and the guns at maximum elevation (90°), the pipper shifts to a point directly below the fixed cross. If the guns are swung to maximum depression (90°), the gyro pipper shifts to a point directly above the fixed cross. Positions are illustrated in Figure 18. Note that in both positions the guns are pointed ahead of the sight axis.

Fig 18

5 With the control positions as indicated under Figure 19, there is the allowable gravity deflection but no azimuth windage allowance.

Fig 19

6 As the guns are elevated or depressed, the gyro pipper moves toward the center of the fixed cross. At 90° elevation or depression, the pipper should be at the center of the cross. This action indicates that the gravity shunt is working properly.

Fig 20

Restricted

S-68


Previous Section - Table of Contents - Next Section

Transcribed and formatted by Thomas Wildenberg for the HyperWar Foundation