Computing and compensating sights for aerial gunnery

by Major General Follett Bradley, USAAF, Ret

At the time this article was written, American Army Air Forces operating from England and Italy against war factories in Germany had shot down 641 Nazi fighters in one week, with a loss to us of 232 heavy bombers and 37 fighters. They had also largely destroyed important aircraft assembly plants, ball bearing factories and other facilities essential to the continued operations of the German Air Force, including Berlin itself.

The Luftwaffe is vigorously sought by our Air Forces wherever it may be, and is being obliterated. Enemy airplanes are found and destroyed in many places; in process of manufacture by bombing of assembly plants and factories which make component parts; finished airplanes in their airdromes, and sources of supply and maintenance by bombing and strafing; and in flight by gunfire.

This article deals specifically with the problems of flexible aerial gunnery from our bombers against enemy fighters. Any means or combination of means which will improve the effectiveness of our bomber gunfire will advance the day when the Luftwaffe no longer exists. More effective gunnery will save ammunition and hence permit penetrations to smash targets now too deep inside Germany. The bombing of all targets would be more accurate as more effective gunnery would reduce the disturbance to the bombardiers of enemy fighter attacks during the bombing run. With a given force of bombers more targets could be destroyed and less fighter cover would be necessary.

Flexible aerial gunnery against airplanes in flight has been one of the Air Forces biggest headaches ever since that day in 1912, when the inventor of the Lewis machine gun fired a machine gun from an airplane for the first time in history. The fighter plane is designed around its guns. To shoot those guns accurately is the primary reason for its existence. This is not true of any other type of airplane. The bomber is designed to carry a heavy load of bombs to great distances, at high speed and at high altitude. Guns are not the reason for its existence but are carried for its own protection to permit it to drive through enemy fighter opposition to its target. The result has been that all bombers have their guns emplaced as a secondary consideration, if not as an afterthought.

The famous Flying Fortress was so named because at the time of its birth in 1935 it carried more machine guns in more gun positions than had ever been done before. These first B-17s had the astonishing total of five .30-cal guns mounted in five gun positions! They had no turrets and no guns of any kind in the nose or in the tail. It is now no secret that the latest B-17 and its cousin, the B-24, carry about thirteen .50-cal machine guns in about seven positions. There is no point from which an enemy fighter can attack without being brought under fire.

The obstacles to hitting an airplane in flight with a bullet are many and complex. These obstacles make the anti-aircraft gunners' problem difficult, but the aerial gunners' problem is even more difficult. In addition to all the obstacles faced by the anti-aircraft gunner, the aerial gunner must make allowance for many additional factors caused by his firing from a moving platform.

Solution of the flexible aerial gunnery problem is subject to errors caused by the following variables:

  1. Muzzle velocity which is a function of weight, moisture content and temperature of the powder.
  2. Weight, shape and type of bullet.
  3. Condition of the bore.
  4. Type, design and condition of the mount.
  5. Vibration and rigidity of the airplane.
  6. Drift and external wind.
  7. Air resistance.
  8. Own speed and direction of movement.
  9. Target speed and path, and direction of movement.
  10. Position of target in azimuth and elevation.
  11. Gravity drop.
  12. Range.
In addition, there are large errors inherent in any system of estimation and smaller errors in any sight.

To read the above is like reading a medical book with its descriptions of the many ills to which man is subject. One wonders how he manages ever to be free of disease, but by training and attention to the rules of health we avoid serious sickness most of the time. Likewise, through training in use of the equipment, and knowledge of the obstacles to be overcome, gunnery problems can be solved and the target struck in spite of errors.

It is more than thirty years since first a machine gun was fired from an airplane. It is hundreds of years since the invention of gun powder started men to study exterior and interior ballistics. Realizing the problem of aerial gunnery, Sperry had developed and made satisfactory tests of the K sight prior to the outbreak of World War II. The first of this family of sights, K-3 and K-4 for Sperry upper and lower turrets, respectively, and the K-9 for turrets not designed by Sperry, give a mechanical and nearly instantaneous practical solution to the gunnery problem against an enemy aircraft which is flying a straight path. Improved models, lighter and more accurate will soon be available. We use the term computing for this family of sights to distinguish from the compensating sights whose function is discussed later.

To keep any computing sight within the weight and space limits imposed by use in aircraft, and to make it simple enough for one man to use in the split seconds available in air combat, certain assumptions and compromises must be made. Of the groups of variables listed, the first six are assumed to be known constants or are disregarded and the seventh is taken as a partial constant by the K-3, K-4 and K-9 sights. Experience indicates that these assumptions are reasonable, as the induced errors tend to cancel out and keep the resultant error within small limits. These sights also assume the path of the target to be a straight line and when the gunner ranges the target smoothly and continuously, the sight automatically computes the resultant angular deflection caused by the last five variables listed, combines it with the assumed constants, and aims the guns.

Quite recently gunnery training programs have undergone a revolutionary change. It had been assumed formerly that to attack a bomber, a fighter always flew on a straight line collision course. As the fighter's speed is greater than that of the bomber, the bomber gunner had to fire ahead of the fighter to hit him. Scientists have known for many years that a fighter should fly on the pursuit curve to be most effective in attacking a bomber. This path is not a straight line but a curve of the tractrix family. A report on a sight proposed to meet his type of attack was written by Mr Carl Holschuh of the Sperry Research Laboratory about 1938. This type of sight was not favored by the Air Corps then because it would be effective against only the one type of attack. At that time and without the background of combat experience which we now have, the Air Corps believed the automatic computing sight was sufficiently accurate for any type of fighter attack. This deficiency has been rectified, and eventually every gunner will be equipped with a computing or compensating sight.

This future plan does not help the GI gunner who is equipped with non-computing and non-compensating sights. He has to use his ring and bead the best he can. Gunnery instructors did not know or had overlooked the fact that an enemy fighter on the pursuit curve must be led toward the rear. The poor combat results obtained forced a restudy of the whole system of gunnery instruction and resulted in the modern zone or position system of firing which recognizes that the effect on the bullet of the forward speed of the bomber is much greater than the forward speed of the fighter even though the actual speed of the fighter is greater than that of the bomber. Gunners are now taught that when they do not have a computing or compensating sight in working order, they use their fixed ring and bead and actually "lead" the fighter toward the rear of the bomber.

Training makes the student thoroughly familiar with the equipment he will have in combat and its most effective use. It must always keep pace with new discoveries and tactics and with new equipment. Training schools and training units are furnished the first items of any newly developed gun sights in order that gunnery instructors and student gunners may become competent in use of the sights, and mechanics in their maintenance, by the time they are installed in airplanes en route to combat.

This article was originally published in the September, 1944, issue of Air Tech magazine, vol 5, no 3, pp 23-24.
The original article includes a drawing and a photo.
Drawing credited to Sperry Gyroscope, Inc; photo credited to Boeing Aircraft Co.

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