<P> Late on in the war, as a result of statistical analysis, Freeman Dyson, put forward a case for the removal of the majority of the Lancaster's defensive armament . He argued that this would reduce the overall loss rate as it would have the benefit of increasing the Lancaster's cruise speed by up to 50 mph (80 km / h) (assuming the bomb load was not increased at the same time), and thus make the bomber harder to shoot down . He also considered that the modification would be justified regardless of the envisioned decreased loss rate as, by requiring fewer crew to serve as defensive air gunners, that would be a lower number of human losses incurred with each aircraft lost . </P> <P> Only the FN - 5A nose turret which was similar to the FN - 5 used on the preceding Avro Manchester, the Vickers Wellington and the Short Stirling remained unchanged during the life of the design, except in instances where it was removed entirely . </P> <P> The ventral (underside) FN - 64 turret quickly proved to be dead weight, being both difficult to sight because it relied on a periscope which limited the gunner's view to a 20 degree arc, and too slow to keep a target within its sights . Aside from early B Is and the prototype B IIs, the FN - 64 was almost never used . When the Luftwaffe began using Schräge Musik to make attacks from below in the winter of 1943 / 1944, modifications were made, including downward observation blisters mounted behind the bomb aimer's blister and official and unofficial mounts for . 50 in (12.7 mm) machine guns or even 20 mm cannon, firing through the ventral holes of the removed FN - 64 . The fitting of these guns was hampered as the same ventral position was used for mounting the H2S blister, which limited installations to those aircraft fitted with bulged bomb bays which interfered with the H2S . </P> <P> The mid-upper (dorsal or top) turret was an FN - 50 on early examples and the very similar FN - 150 with improved sights and controls on later examples . On all but the earliest examples this turret was surrounded by a coaming which provided a track for a cam operated interruptor device which prevented the gunner from shooting the tail of his own aircraft . The Mk . VII and late Mk . X Lancasters used the heavier, electrically - controlled Martin 250 CE 23A turret equipped with two . 50 inch machine guns which was mounted further forward to preserve the aircraft's longitudinal balance, and because it had an internal mechanism to prevent firing on the aircraft itself, it did not require a coaming . Other experimental turrets were tried out, including the FN - 79 and the Boulton - Paul Type H barbette system . </P>

Where did the name lancaster bomber come from