Aircraft technical Basics: TM 1-409, Aircraft Armament and Pyrotechnics, 1941: IX. - Aircraft Machine-Gun Sights

Section IX - AIRCRAFT MACHINE-GUN SIGHTS

49. General.-Aircraft machine-gun sights are designed to enable the gunner to fire with accuracy under all possible combat conditions. However, in order to use the sights effectively, the aerial gunner must have a thorough knowledge of the forces which affect the trajectory or flight path of the bullet; the adjustment necessary to compensate or correct for these forces; and the proper use of the various types of sights.

50. Forces affecting projectiles fired from guns. - a. There are four important forces affecting a projectile fired from a gun mounted on a stationary base.

(1) The first of these forces is the propellent charge, which gives the bullet its initial velocity and sends it out of the gun.

(2) No matter how great a force is utilized in giving the bullet its initial velocity or at what tremendous speed the bullet leaves the gun, the bullet starts to drop away from the line of bore as soon as it emerges from the muzzle. This drop is due to the second force gravity.

(3) The third force affecting the bullet is air resistance. If a bullet were discharged horizontally in a vacuum, it would never lose its initial velocity until pulled to earth by gravity. The retardation or loss of speed of the bullet due to the density of the air through which it passes becomes more pronounced as the bullet continues along its course. This force in combination with gravity causes an increase in the amount of drop of the bullet as the range increases.

(4) The. fourth force. and the least important, is drift; that is deflection caused by the rotation or spin of the bullet. As a bullet passes through the bore of a rifled gun, it has impressed upon it the moment of rotation about its longer axis. The caliber .30 and .50 machine guns are rifled with a right-hand twist which rotates the bullet from left to right as observed from the rear. This rotation causes the bullet to drift to the right of a straight course. This drift may be disregarded at short ranges.

b. So far, only those primary forces which may be said to affect all projectiles have been considered. In addition to these, the aerial gunner has many more with which he must be familiar. Whenever a gun is fired from a moving mount, the movement of the mount becomes a force which affects the flight path of the bullet. Inasmuch as an airplane is capable of movement in all directions at greatly varying speeds, problems confronting the flexible aerial gunner are considerably magnified, since the flexible guns may be fired at all angles to the flight line of the airplane.

c. For purposes of illustration and clarity, it is advisable to consider separately the forces imparted to a bullet by the movement of the airplane. However, since there are always four or more affecting the bullet simultaneously, it will be found very difficult to isolate any one force and attempt to devise means of correcting for that one force without taking all the others into consideration. It must be remembered that the forces caused by the movement of the airplane are in addition to those primary forces already observed.

d. When a gun is fired forward, parallel with the flight line of the airplane on which the gun is mounted, the initial velocity of the bullet is increased by the speed of the airplane. For instance, fire-control tables show that. the caliber .30, M1 bullet, fired horizontally from a Browning machine gun, will drop 83 inches at a range of 500 yards. The bullet leaves the gun at approximately 2,600 feet a second. If that gun is fired forward when mounted on an airplane flying 200 miles an hour, then it will be found that the bullet drops only 63 inches at a range of 500 yards. The bullet leaves the gun at the same velocity as when fired from a stationary base; its speed in relation to the ground has been increased by 200 m. p. h. or approximately 300 feet a second. The difference in drop is due principally to the fact that the bullet fired forward from the airplane covered the range of 500 yards in less time than the bullet fired from the stationary gun. The bullet fired from the airplane was not exposed to the force of gravity for as long a period of time. As a result, it was not pulled toward the earth as great a vertical distance.

e. If the flexible aircraft machine gun is fired horizontally to the rear, from an airplane flying at a speed of 200 m. p. h., then the initial velocity of the bullet, in relation to the ground, will be decreased by that amount of speed of the airplane. The bullet will require more time to travel 500 yards than when fired from a stationary gun, and it will drop approximately 103 inches in that period.

f. Insofar as the drop of the bullet is concerned, only the two extreme conditions, namely, firing directly forward and firing directly to the rear, have been considered. It may be easily seen that when a gun is fired from any of the intermediate positions in the 180°, the effect on the bullet caused by the movement of the airplane will vary. The amount of bullet drop will increase as the gun is swung from front to rear. At 90° from the flight line of the airplane, the drop in inches will be similar to that occurring when firing horizontally from a stationary mount. The initial velocity of the bullet is not affected by the speed of the airplane unless the gun is pointed toward the front or toward the rear of a line extending 90° from the flight line of the airplane.

g. When a bullet is fired from a moving base, such as an airplane in flight, at different angles to the line of flight, not only the velocity of the projectile is increased or decreased according to the firing angle, as explained above, but there is also under these conditions an effect on the trajectory of the bullet. Thus, when the gun is fired in the direction of airplane flight, the forward movement of the airplane gives additional impetus to the bullet and this tends to keep the trajectory close to a straight line. However, when the gun is fired at 90° to the flight line, the forward movement of the airplane causes the bullet, immediately after discharge, to move in the direction of the airplane until it gradually loses this momentum, and the air resistance becomes strong enough to cause lag making the trajectory assume a curve.

51. Sights in general.-A satisfactory sight for machine guns must automatically, by mechanical or optical means, correct for all the forces which affect a bullet in flight when fired from a flying aircraft. In addition it should correct for the movement, if any, of the target. The perfect sight would insure hits on any target, regardless of the gun or target speed or direction, as long as the gunner held the sights on the target. Until such a sight is developed the gunner must combine his experience and knowledge of gunnery with the sights at his disposal.

52. Purpose of harmonization.-Harmonization is the alinement of the guns in relation to the sight so as to cause the line of sight and the trajectories of the projectiles to intersect at a desired range. The range at which the sight and gun are harmonized may be decreased or increased by elevating or lowering the gun in relation to the line of sight. The drop of the bullet at various ranges must be known in order to harmonize the sight and guns.

53. Adjustment of sights.-Aircraft machine-gun sights, as used by the pilot on pursuit type airplanes, are rigidly mounted on the airplane and cannot be moved by the pilot while the airplane is in flight. Therefore, in harmonizing the sights for a definite target which is to be a given distance from the airplane at the time of fire, the flight attitude of the airplane must be considered. There are charts furnished with each airplane and attached to it which will indicate the flight attitude of the airplane at given speeds. If the speed is known in advance the attitude may be determined, and by placing the airplane in this attitude on the ground and causing the sight to be moved to a level position, the line of sight will be placed parallel to the line of flight. Once this is done the pilot will be able to fly a straight course into the target and there will be a resultant high percentage of hits. If the line of sight is not parallel to the line of flight it will be impossible for the pilot to fly a level course into the target, and there will be a vector applied to the projectile for which no adjustments have been made. The result will be that there will be very few hits and the airplane will be flying an untrue course to the target. Therefore, it is absolutely essential that the line of sight be placed parallel to the line of flight before the guns are moved or brought into any relationship with the line of sight.

54. Adjustment of guns.-By reference to the preceding paragraphs, we find that the projectiles start falling away from the line of bore as soon as they leave the muzzle of the gun, and that the line of sight must. be parallel to the line of flight of the airplane. Therefore, it is obvious that it will be impossible for the guns to be placed parallel to the line of flight or the line of sight. By reference to the ballistic charts furnished, it is possible to determine the exact amount of bullet drop at any given range. By using this measurement, combined with actual measurements made on the airplane, it will be possible to determine the exact position of the line of bore of the gun in relation to the line of sight of the airplane. By using the figures obtained, a right angle triangle can be formed, and using trigonometric functions the angle of elevation necessary to cause the line of bore to be the proper distance above the target at the target range. Using the angle obtained with a gunners quadrant, the correct angle of elevation may be transferred to the gun, causing the projectiles, when fired from the gun under the conditions established, to strike the center of the target.

55. Sights on pursuit type airplanes.-Pursuit type airplanes are normally used as interceptors and their usual target will be another aircraft in the air with them, or some fixed point on the ground. The enemy aircraft will be at the same altitude, or in any case, in a direct line with the flight line of the airplane when the projectiles are fired. For this reason, it is considered that the point of origin as well as the point of impact are in the same plane. Since the line of sight is parallel to the line of flight and the enemy aircraft is in this line, the only drop of the projectile is that caused by gravity, for which compensation has been made before the airplane left the ground. The guns are set to converge on one given point at some definite range in front of the airplane. This will cause the fire of the guns to be enfiladed or crossed at this point. If the range estimation is correct, the fire of both guns will be effective on the target. If the estimate is incorrect, there will be an excellent chance of having one of the guns effective.

56. Sights on attack type airplanes.-Attack gunnery differs from pursuit gunnery in that the target is not in the same lateral plane as the flight line of the airplane, and the fire is spread rather than being enfiladed. This causes the line of sight to differ from the line of flight of the airplane. The airplane is flown at an altitude of 75 feet above the target and fire is conducted at a range of 1,000 yards. Therefore, the line of sight must be first set. parallel to the line of flight and then lowered to compensate for this 75-foot altitude at 1,000 yards range. The line of bore of the gun is set so that the area covered ahead of the airplane at 1,000 yards range will be 50 feet. Thus, instead of the guns being toed in, they will be toed out to produce this result.

57. Flexible sights.-Flexible gun sights are mounted on the guns and are moved as the gun is moved. They are operated by some flight member of the crew other than the pilot. Basically, the sights are adjusted in the same manner as are fixed sights. Since most of the fire is conducted at an angle to the line of flight, there are two parts to the sight. The rear sight is usually a ring sight, and the front sight is either a fixed post sight or a wind vane type sight, depending on the preference of the gunner. By the proper use of the rear ring sight, the gunner is able to compensate for the speed of the enemy aircraft. If the wind vane sight is used, the gunner can compensate for the speed of the airplane on which the gun is mounted. This assists the gunner in properly leading the target, and compensates for the relative movement of enemy aircraft.

58. Duties.-The duties of the armament officer regarding sights will consist principally of supervision of the work as it is done on the equipment, and maintaining an accurate record of the placing and harmonization of the sights and their operation. These reports will be gathered from the officers flying the aircraft involved and will be a consensus of opinions of the officers involved.