Fitting of an upward-firing autocannon or machine gun to an interceptor aircraft
 () was a common name for the fitting of an upward-firing autocannon or machine gun, to an interceptor aircraft, such as a night fighter. The term was introduced by the German during World War II. was previously a German colloquialism, meaning music that featured an unusual tuning and/or time signature (e.g., jazz). The standard usage of the adjective is often translated as "slanting" or "oblique", but its slang usage is often translated as "weird" or "strange".
The first such systems were developed (though not widely employed) in World War I as anti-Zeppelin defenses by the French and British, in an era when fighters struggled to match the altitude capacity of the German airships and were forced to devise means to attack from below. The later resurrection of the concept by the Germans was inspired by observed weaknesses in the standard British night bomber aircraft of the WW2 era (the Avro Lancaster and Handley Page Halifax), which lacked ventral ball turrets in order to save weight, making them vulnerable to covert approaches and attacks from below under the cover of darkness. In keeping with the plans of the Allied Combined Bomber Offensive, the American B-17 Flying Fortress and B-24 Liberators operating in Europe (factory-equipped with ball turrets) typically bombed by day, thus experiencing far fewer encounters (and relative losses) from . Later similar Japanese experiments with upward-firing cannons on their night fighters in 1944 (intended to target the American B-29 Superfortress fleet firebombing Japan by night) were largely fruitless, owing to the B-29's notably superior speed and altitude.
In the initial stages of its operational use by German air crews, from mid-1943 to early 1944, many attacks using achieved complete surprise while destroying many British bombers. The crews that survived such attacks, during this period, often believed that damage and casualties had been caused by ground-based anti-aircraft artillery (AA or AAA), rather than fighters, and much confusion resulted until the cause was successfully pinpointed.
 ;templatestyles src="Template:TOC limit/styles.css" / ;
Background.
World War I.
During World War I, pusher-configured fighter aircraft with flexibly-mounted forward-firing machine guns (especially the Royal Aircraft Factory F.E.2s), enabled gunners to discover the principle of zero-deflection shooting. When firing upward at roughly a 45° elevation, when the attacking aircraft and its target are travelling at about the same speed and the range is fairly short, the trajectory will appear straight. The bullets' true path is a parabola, but the movement forward of both aircraft, and the air passing the aircraft counter the tendency of the round to arc down after leaving the muzzle so it appears to follow a straight line, simplifying accurate sighting which then requires no deflection or leading of the target.
The pilots of Royal Aircraft Factory B.E.2 night fighters, after trying various schemes for attacking the Zeppelin raiders of 1915–1916, hit on the idea of firing a mixture of explosive and incendiary bullets into the body of the airship "from below". For this purpose a air-cooled Lewis gun was mounted in front of the pilot, firing upward. Exploitation of this led to the destruction of six German airships between September and December 1916. Later British night fighters were similarly armed with upward-firing guns.
Several tractor-configured single-seat biplanes of the time featured machine guns mounted on the centre section of the top wing to fire over the radius of the propeller to bypass the need for synchronization gears). Both the French mountings and the British Foster mounting allowed a machine gun to be tipped back to reload and whether by accident or design, this allowed the gun to be held at an intermediate angle (ideally about 45°) and fired upward, steadying the gun and firing it with the "normal" trigger rather than the remote Bowden cable used for forward firing. These could then be used to attack enemy aircraft from the blind spot below the tail. Most notable of the airplanes used were the Nieuport 11, 16 and 17 and 23 fighters from 1915 onwards, and the tactic was continued in British service, with the Royal Aircraft Factory S.E.5s and Sopwith Dolphin. That Dolphin entered service near the end of World War I, and was delivered with a pair of Lewis guns on a cross-tube connecting the upper wing spars. British ace Albert Ball in particular was a great exponent of this technique, The Germans copied the arrangement in 1917, when Gerhard Fieseler of "Jasta" 38 attached two machine guns to an Albatros D.V, pointing upwards and forwards.
Interwar years.
The Boulton Paul Bittern was a twin-engined night fighter (designed to Specification 27/24) with an armament of barbette mounted guns, that could be angled upwards for attack against bombers, without having to enter a climb. The first of two Bittern prototypes flew in 1927, though performance was poor and the development stopped.
The Westland C.O.W. Gun Fighter (1930) and Vickers Type 161 (1931) were designed in response to Air Ministry specification F.29/27. This called for an interceptor fighter operating as a stable gun platform for the Coventry Ordnance Works 37 mm autocannon produced by the Coventry Ordnance Works (COW).
The COW gun had been developed in 1918 for use in aircraft and had been tested on the Airco DH.4. The cannon fired shells and was to be mounted at 45 deg or more above the horizontal. The tactic was to fly below the target bomber or airship and fire upwards into it. Gun firing trials with both types went well, with no detriment to airframe or performance, although the Westland prototype displayed "alarming" handling characteristics. Neither the Type 161 nor its competitor, the Westland C.O.W. Gun Fighter were ordered, and no more was heard of this use of the aerial COW gun.
Similar logic lay behind the later Vickers Type 414 twin-engined fighter. This aircraft, which can be seen a natural successor to the Vickers COW gun fighter, combined a streamlined monoplane two-seater fighter airframe with a remotely controlled nose-mounted 40mm cannon that could be elevated for no-allowance shooting.
While turret fighters like the Boulton Paul Defiant and the naval Blackburn Roc addressed the same threat – enemy bombers attacking the UK – the approach was very different: upward-firing guns and no-allowance shooting are separate and distinct, and the equipment that can do the one can, generally speaking, be arranged so as to do the other (unless the fixed armament is automatically triggered, as in the photo-cell firing arrangements detailed below). On paper at least, the advantages of flexible aim and weight of fire from a two-seater were clear: the pilot is not overburdened, several fighters could be brought to bear on a target together, and there are two pairs of eyes per aircraft. However, the weight of a powered turret and air gunner imposed performance penalties.
The RAF put the Defiant into service in 1939, intending to use it against bombers, despite the bombers' numerous gun positions. However, the unexpected German territorial gains in France meant that bombers were escorted by fighters. Despite being utterly outclassed as a day fighter, when moved to the night-fighter role it had some success, typically attacking from below and slightly ahead of the bomber, well outside its field of defensive fire.
Meanwhile, in the United States, the twin-engine Bell YFM-1 Airacuda was designed as a "bomber-destroyer", touted as "a mobile anti-aircraft platform". Its armament included mainly forward-firing M4 37mm cannon, with an accompanying gunner mounted in a forward compartment of each of the two engine nacelles. Theoretically, the cannon could be slewed, aimed and fired at an oblique angle but flight tests and operational evaluation, disproved the theory: the type proved troublesome and except for initial flight testing in 1937, where full armament was carried, the nacelle cannon armament and the accompanying gunner–loaders were eliminated in the final development aircraft.
World War II.
German developments.
"Oberleutnant" Rudolf Schoenert of 4./NJG 2 decided to experiment with upward-firing guns in 1941 and began trying out upward-firing installations, amid scepticism from his superiors and fellow pilots. The first installation was made late in 1942, in a Dornier Do 17Z-10 that was also equipped with the early UHF-band version of the FuG 202 "Lichtenstein B/C" radar. In July 1942, Schoenert discussed the results of his experiment with General Josef Kammhuber, who authorized the conversion of three Dornier Do 217J-1s, to add a vertical armament of four or six MG 151s. Further experiments were carried out by the Luftwaffe flight testing centre at Tarnewitz on the Baltic Sea coast through 1942. An angle between 60° and 75° was found to give best results, allowing a target turning at 8°/sec to be kept in the gunsight.
Schönert was made CO of II./NJG 5, and an armourer serving with the "Gruppe", "Oberfeldwebel" Mahle, developed a working arrangement with the unit's Messerschmitt Bf 110s, mounting a pair of MG FF/M cannon in the rear compartment of the upper fuselage, firing through twin holes in the canopy's glazing. Schönert used such a modified Bf 110 to shoot down a bomber in May 1943. From June 1943, an official conversion kit was produced for the Junkers Ju 88 and Dornier Do 217N fighters. Between August 1943 and the end of the year, Schönert achieved 18 kills with the new gun installation. Before the introduction of "Schräge Musik" in 1943, the "Nachtjagdgeschwadern" (NJG, night fighter wings) were equipped with heavy fighters fitted with radar in the nose and a combination of front-firing and defensive weapons.
In the standard interception, the fighter approached the target from the rear to get into a firing position, presenting the night fighter crew with a much smaller target, a problem compounded by the Royal Air Force bombers (such as the Whitley and Wellington medium bombers) first being fitted with twin-gun hydraulic tail turrets, later upgraded to four guns to fend off just such attacks.
While the small calibre made these tail turret guns less effective than hoped, rear-gunners also maintained a watch for fighters and the pilot would make evasive manoeuvres such as corkscrews. Night-fighter pilots developed a new tactic to avoid the turret guns: instead of approaching directly from the rear they would approach about below the bomber, pull up sharply and start firing when the nose of the bomber appeared in the gunsight. As the fighter slowed and the bomber passed over them, its wings were sprayed with cannon or machine gun rounds. While effective, this manoeuvre was difficult to perform, there was a risk of collision and, if the bomb load exploded, it could destroy the night fighter. Systems similar to the original "Schräge Musik", such as the "Sondergerät 500" or "Jägerfaust", were tested on day fighters and other airframes, with the largest-calibre upward-firing aerial ordnance in German service, based on the quintuple-launcher of the 21 cm Nebelwerfer infantry barrage rocket, the experimental heavy-bomber based "Grosszerstörer" (heavy destroyer) also under test.
The "Jägerfaust" system, firing projectiles vertically into the lower sides of bombers, was triggered by an optical device as the pilot passed beneath the target. This was tested on the Fw 190, and was destined for installation in the Messerschmitt Me 163B and the Me 262B. The definitive night fighter version of the Messerschmitt Me 262, the Me 262B-2, was also designed to carry such an installation but it did not work and was not used operationally. Trials with the Me 163 were promising, with six operational aircraft modified. On 10 April 1945, a Halifax bomber was shot down by Fritz Kelb flying a "Jägerfaust"-equipped Me 163B, most probably from I. "Gruppe"/JG 400 operating from Brandis, Germany.
As experimental aircraft were developed as night fighters, such as the Horten Ho 229, a "Schräge Musik" system was incorporated from the start. The experimental Horten Ho 229 flying wing series was proposed for consideration, with a form of unusual upward-firing armament for testing on the V4 night fighter prototype, photoelectric fired vertically mounted rockets or recoilless guns, instead of cannon armament inspired by the "Jagdfaust".
Method of sighting guns.
In the Ju 88 G-6 night fighter, which was fast and manoeuvrable, the Revi 16N gunsight was modified to allow the pilot to aim at the target by placing a mirror above his head, parallel to a similar mirror placed behind the gunsight (where the eye would normally be), which was further to the rear, functioning together in the manner of a periscope. The Ju 88 G-6 was guided into position from sighting and final approach