Schräge Musik

In World War II, Schräge Musik was the name Germans gave to upward-firing that the Luftwaffe mounted in night fighter aircraft. A similar fitment was used by the Imperial Japanese Army Air Service and Imperial Japanese Navy Air Service and known by a different, undocumented name in their twin-engined night fighters. The Luftwaffe and the IJN air arm had their first victories with fighter-mounted upward-firing autocannons in May 1943.

Schräge Musik derives from the contemporary German colloquialism for shaky, off-tune music; literally, it translates to slanted or oblique music.

Night fighters used this device to approach and attack Allied bombers from below, outside the bomber crew's usual field of view or fire. Most of the Allied bomber types of that era which were used for nocturnal bombing missions (primarily the Avro Lancaster) lacked effective ventral armament, leaving them easy prey to attacks from below, an advantage the Luftwaffe capitalized on.

An attack by a Schräge Musik-equipped fighter was typically a surprise to the bomber crew, who only realized a fighter was close by when they came under fire. Particularly in the initial stage of operational use, until early 1944, Allied crews often attributed sudden fire from below to ground fire rather than a fighter.

During World War I, pusher configured fighter aircraft with flexibly mounted forward-firing machine guns enabled gunners to discover the principle of zero deflection shooting in which the fall of fire appears to follow a straight rather than curved path, greatly simplifying the business of hitting a moving target from a moving weapon platform.

When later, conventionally arranged, single-seat biplanes featured machine guns mounted on the top wing to fire over the disk of the propeller (in the absence of deflector plates or synchronization gear), arrangements had to be made to reload a weapon otherwise limited in flight to a single magazine. The Foster mounting used with air-cooled Lewis guns not only allowed the guns to be drawn and tipped back to reload but also be "locked at any angle within a 90° arc from horizontal to vertical". Whether by accident or design, this feature allowed the guns to be locked at 45° elevation, an 'ideal' angle at which the ballistic properties of World War I rifle-calibre guns and typical biplane aircraft speeds combined to give the appearance of shooting straight and true. This is illusory since, seen by an objective observer, the bullet's true path is a parabola (but so modified by the relative movement of the aircraft that shots appear to follow a straight line). Since shots taken at that angle generally hit their mark, requiring no calculation, compensation, deflection or 'aiming off', it was common for the guns of the Nieuport 11 and Nieuport 17 fighters, especially in British service, and Royal Aircraft Factory S.E.5 to be so arranged, allowing attacks to be made on enemy bombers or reconnaissance aircraft from the "blind spot" below the tail.

...
Wikipedia