The Central Fire Control System The B-29 was equipped with a unique method of operating its machine guns, called the Central Fire Control system. The fact that the plane was pressurized precluded the gunners from having physical access to the guns. So, the General Electric Corporation was contracted to provide a means of remotely operating the guns. GE had experience stabilizing shipboard naval guns in the 1930s, which gave them a head start. Each of the five turrets is controlled using a sighting station, some of which can control more than one turret. This allows the aiming and the firing of two turrets simultaneously. Each turret and each sighting station has two elevation selsyns, for coarse and fine resolution, and two azimuth selsyns, also for coarse and fine resolution. A selsyn is a rotary transformer used to measure angular position. As the gunner moves the sighting station to track the target, an error signal is developed because the sighting station selsyns and the turret selsyns are not in line. This error signal is first amplified using vacuum tubes in the servo amplifier and then further amplified in an amplidyne, which is a rotary generator-amplifier. The direct current output voltage of the amplidyne is sent to the azimuth and elevation motors in the turret, moving it until it is aligned with the sighting station, causing the error signal to go to zero. Because the sighting stations and the turrets are separated from each other, an aiming correction must be applied for the distance between the sighting station and its turret. This is called a parallax correction, and it is supplied by an electro-mechanical computer that modifies the sighting station selsyn signals before sending them on to the turret selsyns. In addition the parallax correction, the computer also adjusts for target speed, target range, aircraft speed, and air density. Aircraft speed and air density are supplied by the Navigator, who sets the aircraft speed, aircraft altitude, and outside temperature on a unit called the Altitude and Airspeed Handset. Target speed and target range are supplied by the sighting station. The sighting station has a display of a circle of dots with a center dot. The gunner keeps the center dot on the target aircraft and the circle of dots on the target's wingtips. As the gunner tracks the target, the sighting station's gyroscopes allow the computer to estimate the target's speed. The circle of dots on the target's wingspan give the computer a range estimate. With all this information, the computer can adjust the turret's position so that it leads the target and compensates for bullet ballistics. The gunner fires the guns electrically by depressing a button on the sighting station. Each turret has its own computer, adjusted for parallax with its primary sighting station, five in all.

The paths of these various signals converge in one of four control boxes located in the nose, waist, and tail. The control boxes allow the switching of sighting stations between their primary turret and their secondary turret. Sighting stations are located in

the nose, at each of the two blisters in the waist, at the top of the plane, also in the waist, and at the tail. Turrets are located above and below the cockpit, above and below the aft waist, and in the tail. The forward gunner, who is also the bombardier, has primary control of the upper and the lower forward turrets. The upper gunner has primary control of the upper aft turret. The blister gunners have primary control of the lower aft turret, and must choose which of the two sighting stations controls the turret. If the forward gunner releases control of his guns, the blister gunners get control of the lower forward turret, and the upper gunner gets control of the upper forward turret. If the tail gunner releases control of his turret, then either of the blister gunners can also fire the tail turret. The tail gunner fires only the tail turret. The upper turret gunner is called the CFC gunner and is in control of the defense of the airplane, calling out which gunner should fire which turrets. Gunners coordinate among themselves using the intercom system. All of the turrets are operable on T-54, the Museum's B-29, and may be operated from various locations inside the fuselage, and from two locations outside the plane, giving a better view of their movement. Also, a complete, additional upper turret has been mounted on a stand outside the aircraft, and can be operated from the outside sighting stations. A recording of the actual firing of a .50 caliber machine gun is amplified and sent to speaker located in each of the turrets when that turret is fired. The operational checkout of our CFC system started with all of the restored components for one turret-sighting station combination assembled on a test bench, including a turret mounted on a test stand. As each combination was made operational, it was mounted in the aircraft. Three of the control boxes were completely fabricated, as none were available. The tail turret was a very major project, with complete physical construction required. Extensive work was done to the amplidynes and motors, and all of the active electronic elements were refurbished.