The QF 3.7-inch AA was Britain's primary heavy anti-aircraft gun during World War II. It was roughly the equivalent of the German Flak 8.8 cm and American 90 mm, but with a slightly larger calibre of 3.7 inches, approximately 94 mm. Production began in 1937 and it was used throughout World War II in all theatres except the Eastern Front. It remained in use after the war until AA guns were replaced by guided missiles beginning in 1957.
The gun was produced in two versions, one mobile and another fixed. The fixed mounting allowed more powerful ammunition, Mk. VI, which gave vastly increased performance. Six variants of the two designs were introduced. The gun was also used as the basis for the Ordnance QF 32-pounder anti-tank gun variant used on the Tortoise heavy assault tank.
History
Background
During World War I, anti-aircraft guns and anti-aircraft gunnery developed rapidly. The British Army eventually adopted the QF 3-inch 20 cwt as the most commonly used type. Shortly before the end of the war, a new QF 3.6 inch gun was accepted for service but the end of the war meant it did not enter production. After the war, all anti-aircraft guns except the three-inch gun were scrapped.
However, the war had shown the possibilities and potential for air attack and lessons had been learned. The British had used AA guns in most theatres in daylight, as well as against night attacks at home. They had also formed an AA Experimental Section during the war and accumulated much data that was subjected to extensive analysis. After an immediate post-war hiatus, the army re-established peacetime anti-aircraft units in 1922. In 1925, the RAF established a new command, Air Defence of Great Britain, and the Royal Artillery's anti-aircraft units were placed under its command.
In 1924–5, the war office published the two-volume Textbook of Anti-Aircraft Gunnery. It included five key recommendations for heavy anti-aircraft (HAA) guns: Shells of improved ballistic shape with HE fillings and mechanical time fuses Higher rates of fire assisted by automation Height finding by long-base optical rangefinders Centralised control of fire on each gun position, directed by tachymetric instruments, which incorporated the facility to apply corrections of the moment for meteorological and wear factors More accurate sound-location for the direction of searchlights and to provide plots for barrage fire
Two assumptions underpinned the British approach to HAA fire. First, aimed fire was the primary method and this was enabled by predicting gun data from visually tracking the target with continuous height and range input. Second, that the target would maintain a steady course, speed and height. Heavy anti-aircraft units were to engage targets up to . Mechanical, as opposed to igniferous, time fuses were required because the speed of powder burning varied with height so fuse length was not a simple function of time of flight. Automated fire ensured a constant rate of fire that made it easier to predict where each shell should be individually aimed.
During the 1920s, Vickers developed the Vickers range clock (Predictor No 1), an electro-mechanical computer that took height and range data from an optical rangefinder, applied corrections for non-standard conditions and was used by its operators to visually track a target, its output predicted firing data and fuse setting via the "mag-slip" electrical induction system to dials on each gun in a battery, the gun layers moved the gun to match pointers on the dials. The three-inch AA guns were modified accordingly.
QF 3.7
In 1928, the general characteristics for a new HAA gun were agreed, a bore of firing shells with a ceiling of . Financial stringency led to no action being taken until the 1930s, when the specification was enhanced to a shell, muzzle velocity, a ceiling, a towed road speed of , maximum weight of eight tons and an into action time of 15 minutes.
In 1934, Vickers-Armstrongs produced a mock-up and proceeded to develop prototypes of the weapon, which was selected over a competitor by the state Design Department and passed acceptance tests in 1936. The weight specification was exceeded, the muzzle velocity not achieved and the mechanical time fuse, No. 206, was still some years from production. The igniferous No. 199 had to be used and its lesser running time limited the effective ceiling. The gun passed the trials by April of the following year, and the gun production started later in the year.