Foster mounting
The Foster mounting was a device used on some Royal Flying Corps fighters in World War I. It let a machine gun, usually a Lewis gun, fire over the spinning propeller instead of through it. Different aircraft had different versions, but all used a quadrant-shaped I-beam rail on which the gun could slide back and down in one motion. This made it easier to change ammunition drums and to clear jams. It also allowed the gun to be fired upward or at an angle, so a fighter could attack an opponent from below.
Before reliable synchronization gear to time the gun with the propeller, engineers tried many methods to fire past the propeller. Even after synchronization became available, there were reasons to avoid it: gears could fail, and firing certain ammunition through the propeller arc could be dangerous. Some guns, like the Lewis, were hard to synchronize. So firing over the propeller remained an option, especially on biplanes where the upper wing provided a convenient base. However, this kind of mount was not as sturdy as a forward-fuselage gun mount, and vibration could affect accuracy. A key challenge was giving the pilot or gunner access to the breech to reload drums or clear jams.
Early top-wing Lewis guns on light scouts were fixed in place, or pilots had to stand up to reach the drum. A hinged setup that let the breech swing back into the cockpit so the drum could be changed from the seat was preferable. Several versions of this idea helped French and British fighters in early 1916.
In early 1916, Sergeant Foster of 11 Squadron RFC improved the French hinged upper-wing mount on the Nieuport 11 and 16 by replacing the awkward double hinge with a quadrant-shaped I-beam rail. This rail became the defining feature of all later Foster mounts. With the breech sliding back and down, the pilot could reach it for drum changes or jams. The gun could also be fired obliquely forwards and upwards, allowing a fighter to attack from behind and below, a tactic used by several aces such as Albert Ball.
The Foster mounting stayed in use on later Nieuport models (17 and 24), though the quadrant sometimes needed strengthening. Adapting the setup to the S.E.5a wasn’t straightforward: the longer, stronger rail had to be fitted, and the top of the propeller arc sometimes had to be raised because of the engine’s thrust line. Upward firing remained possible, but it required skill and careful handling in combat. On the S.E.5, the Foster gun was sometimes used in addition to a synchronized Vickers gun, rather than as a replacement. A two-gun arrangement was considered, but the wing would have needed reinforcement to cope with the extra recoil and vibration.
The Foster idea also found other uses. In 1918, trainees on S.E.5 aircraft could use a Foster Lewis setup for the Hythe camera gun. The 2F.1 version of the Camel used an over-wing Foster arrangement as an anti-airship weapon, and some night fighters used modified mounts to prevent muzzle flash from dazzling the pilot. There were field mods like the Sopwith Comic, which used Foster-style mounts to replace a synchronized gun. The Avro 504K night fighter used a modified Foster arrangement too.
Some aircraft, such as the B.E.12b night fighter, used makeshift mounts that weren’t very reliable, and attempts to add a Foster-mounted Lewis to the Bristol Fighter could cause interference with navigation instruments.
This page was last edited on 2 February 2026, at 14:52 (CET).