From 'Flying Guns – World War 2: Development of Aircraft Guns, Ammunition and Installations 1933-45':
"The B 29 finally appeared with five turrets: Front upper, front lower, aft upper, aft lower, and tail turret; as initially foreseen the tail also had a 20 mm cannon in addition to the twin .50”s. The tail guns were exclusively controlled by the tail gunners in his own compartment, but the other guns were operated from four sighting stations, one in the nose and three in a compartment aft of the wing. Each gunner could simultaneously operate two turrets, as the situation of the moment demanded. The “master gunner” was the upper gunner in the aft compartment, and he assigned turrets to gunners with his control panel.
The gunners had to track the attacking aircraft from their sighting stations, which had a reflector gunsight that generated signal outputs by a “Selsyn” system. An analog computer used the elevation, azimuth and range inputs from the gunner to calculate the lead and the parallax compensation, and aimed the gun turret with an Amplidyne drive unit. The computer took into account the effects the air density, the airspeed and the angle of the guns relative to this airspeed had on the bullet trajectory. A factor that could not be taken into account was the flexibility of the bomber's fuselage itself, and its tendency to expand or shrink locally as temperature varied. These caused a variable misalignment between the sighting station and the gun turret. The heating effect was large enough to make alignment of the guns on a butt outdoors, exposed to the sun, impracticable. The guns had to be harmonised indoors to meet specifications."
Tony Williams: Military gun and ammunition
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