Foster mounting

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A Lewis gun on a Foster mounting fitted to an Avro 504K Night Fighter.

In early 1916 Sergeant Foster of No. 11 squadron RFC devised a sliding rail mounting for the upper wing Lewis Gun on a Nieuport 11. It enabled the gun to be pulled down so that its breech was conveniently in front of the pilot - making it much easier to change ammunition drums or to clear stoppages in flight. The mounting was readily adaptable to several other types of biplane fighter aircraft - including in particular the Nieuport 17 and 24 (in British service) and the S.E.5a, and became a standard fitting for these aircraft and others. ^[1]

Although it was possibly not the original intention, it also became possible to fire the Lewis gun forwards and upwards at an angle, and thus to attack an enemy aircraft from behind and below - a favourite tactic of the ace Albert Ball, and an interesting precursor of the Schräge Musik tactics used by German night fighters in the Second World War.

When fired directly forwards, the trigger of the Lewis Gun was controlled by a Bowden cable - when fired in an upward direction the pistol grip was held to steady the weapon, which was fired by using the trigger manually. Twin Foster mountings were occasionally used, although the recoil and vibration of two Lewis guns firing simultaneously was considered a dangerous strain for the lightly braced centre section of a typical biplane of the time.

Due to the gun's line-of-fire being outside of the propeller arc, Foster-mounted guns had the advantage of not requiring the use of any synchronising gear. On the other hand the mounting could only be fitted to types with a suitable configuration, and was a little less stable and more subject to vibration than a mounting in the fuselage, resulting in a more widely spread pattern of fire.

The illustration shows a Foster mounting adapted to the Avro 504K night fighter. This aircraft had a much larger gap between the top of the fuselage and the upper wing than most types fitted with this type of mounting - in an S.E.5a for instance the gun was much more easily reached, and the mounting shorter and neater.