Bristol Mk. IV Upper Gun Turret

 This turret, designated the Bristol Type B.I Mk.IV, was one of a series of Bristol-designed turrets. The B.I turret was powered from a hydraulic pump mounted on the port engine. When the turret was not in use the cupola could be partially retracted to reduce drag.

The turret was controlled in elevation and rotation by means of a tee-shaped control column. A squeeze lever on the left handle operated the main hydraulic valve supplying pressure to the system. On the right handle was the gun firing trigger that operated solenoids attached to the guns. The turret was rotated by a bicycle-type steering movement – turned to the left it rotated to the left, and vice versa.

The guns were elevated and depressed by a twisting movement of the handles, the gunner’s seat being linked with the gun elevation system. A hydraulic ram operated the seat, which was linked by an arm to the gun cradle. When the gunner twisted the handles towards him the ram extended, lowering the seat and elevating the gun cradle. This system ensured that the gunner’s eyes were always in line with the gunsight.

The gunner entered the turret through a hatch forward of the turret. He stowed his parachute on a rack, then stooped down under a bulkhead and up into the turret. He then stepped into his seat and fastened a lap strap.
Airframe protection was by two separate systems, a lever that cut off a valve before the gun barrels hit the airframe, and a switch on the main gun pillar that cut off the firing system when the guns approached the danger zone.
 The Bristol Bolingbroke nose section on display in the Museum is part of the complete aircraft owned by the Museum. The Bristol Mk.IV turret would be fitted to this aircraft.

Technical Details B.I Mk.IV:

Motive power: hydraulic, pressure 700 psi
Armament: Two 0.303 in (7.7 mm) Browning Mk.II
Ammunition: 400 rounds per gun
Field of fire: Traverse 120°, elev. 60°, depression 32°
Rotational speed: 50 degrees/sec
Weight of turret: 390 lb (177 kg)
Gunsight: Mk.IIIA reflector sight