Automatic Gun Sight

Theory and Principles of Operation

War Department officials have recently approved release of a limited amount of information concerning the Automatic Computing Sight designed and produced by the Sperry Gyroscope Co.

This sight is an integral part of the Sperry-designed upper local and lower ball turrets with which the Boeing B-17 Flying Fortresses are equipped. The automatic sight makes the fire of twin .50-calibre machine guns installed in these turrets effective at ranges up to 1,000 yards, whereas normally the effective range might be only half as much or less.

In comparison with the effective range of any enemy pursuit planes, this gives AAF gunners a decided advantage — an advantage that enables them to destroy Axis attackers before they can get close enough to deal telling blows at our bombers. In disclosing this information to the fifth annual convention of the Aviation Writers Association held in New York recently, Thomas A Morgan, President, Sperry Corp, pointed out that the sight was another tribute to the vision of the US military officials.

Army experts obsolete enemies' weapons

"Although the sight has been a closely guarded secret," said Mr Morgan, "it is not new. Previous to our entry into the war, United States armament experts realized that their job was to obsolete the weapons of our enemies. Determined to give our fighting men the finest weapons, they argued, among other things, for more adequate protection for our bombers.

"In the early stages of the war," he continued, "bombers mounting .30-calibre machine guns tried to shoot it out at close ranges with more maneuverable, faster, enemy attackers.

"This resulted in heavy losses of expensive bombers and trained crews. Our military experts argued for heavier .50-calibre guns and got them. And, late in 1939, they asked Sperry engineers to design a precision sight which was delivered for tests in the record time of six months — in May of 1940."

Mr. Morgan pointed out that early tests proved the computing sight superior to previous types and the engineers predicted even better results if the Sperry Automatic Sight was used in conjunction with power turrets. This resulted in the modern Flying Fortress which is equipped with Sperry-designed upper local and belly turrets powered by hydraulic motors, designed and manufactured by Vickers, Inc, Detroit, a Sperry Corp subsidiary.

The sight-turret combination was demonstrated to the Aviation Writers by Hugh Willis, General Sales Manager of the Sperry Gyroscope Co.

One of the big advantages of the sight is that it takes the element of guess out of aerial gunnery. The effectiveness of other sights lacking the automatic feature depends largely on the inherent skill of the gunner, and requires long periods of training.

Computing sight makes calculations

Experience teaches the operator of a hand-held gun how to estimate the various calculations that lead to direct hits. The computing sight makes the calculations for the gunner and does the job quicker and more accurately.

The computing sight automatically figures the "fall" of the bullet because of gravity, the ballistic deflection caused by wind, and the amount of "lead" required to hit a target.

Duck hunters realize that to hit a moving object, the gunner cannot sight directly on the target. If he does, the bird will be out of the line of fire by the time the shot reaches its intended objective. Shooting from a fast moving platform at a faster moving target makes the problem more involved. Even though the muzzle velocity of .50-calibre shells is 2,700 feet per second, the correct computation of lead, fall, and deflection is the essential ingredient of successful aerial gunnery.

So far as the gunner is concerned, when using the automatic device, he seems to aim directly at the target. The sight, however, points the guns automatically and continuously at the spot where the target will be by the time the bullets arrive. The gunner makes no mental calculations. Trained to recognize the various types of enemy pursuit planes, he has only to adjust the sight control to correspond to the dimensions of the attacker. If, for instance, the attacker has a wing spread of forty feet, the operator twists the knobs to "40" on the dial. It is much like tuning a home radio.

Peering through the reflecting glass atop the sight, he must get the two illuminated vertical lines on the target — a line at the outermost tip of each wing. This he does by controlling movement of the turret and by operating a foot treadle which spreads the verticals to keep them on the wing tips as the pursuit plane, moving at terrific speed, roars closer, thus automatically and continuously setting the proper range in the computing sight.

Lead, fall, and deflection are automatically and continuously computed on the basis of rate of turret movement transmitted to the sight. With this information it instantaneously calculates the relative speed between target and bomber and multiplies the finding by the time of flight of the projectile.

This article was originally published in the August, 1943, issue of Air Tech magazine, vol 3, no 3, pp 32-36.
The PDF of this article includes detail photos of the gunsight installation in a top turret and of a ball turret. It also includes some 19 diagrams illustrating different aspects of the parameters used by and the computations made by the Sperry Automatic Computing Sight.