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Glenn’s Computer Museum

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K-3 Gunsight
Figure 1
K-3 Gunsight
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Figure 9
K-3 Gunsight
Figure 2
K-3 Gunsight
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K-3 Gunsight
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Sperry K-3 & K-4 Computing Gun Sights

The K-3 is a computing gunsight used in used in Sperry upper gun turrets used on the B-17 bomber. The K-4 is the same device with a different mount used in the Sperry lower gun turrets on the B-17. Figure 1 shows our two K-3's plus a computing element from a third. The other pictures show various views of these devices. Figure 9 shows a page from the Gunner's Information File illustrating the lower turret with the K-4 sight right in the middle.

The gunner inputs range information by estimating the size of the plane and adjusting its image in the attached optical sight so that the image fits withing reticles. The gunner then tracks the target with the optical sight by moving the K-3 (mounted on a movable head) keeping the plane image centered in the reticles. The sight movements cause the computing unit, which, based on the range information and built-in ballistics data, to calculates the deflection, or lead, for aiming the guns and moves the turret accordingly.