Interrupter gear

from Wikipedia, the free encyclopedia
How the interrupter gear works

The interrupter gear is a coupling between the propeller shaft and the machine gun (MG) of a fighter aircraft , which ensures that the burst of fire from the machine gun synchronized in this way is interrupted if a projectile from the MG would hit a propeller blade passing in front of the muzzle .


In order to enable the pilot to remove jams, to align the direction of the gunshot parallel to the pilot's line of sight and to concentrate the weight of the weapon and ammunition close to the center of gravity, it was important for the first fighter aircraft to position the armament directly in front of the cockpit . Since single-engine aircraft with pull-propellers were the most effective design for fighter aircraft, the machine gun had to be able to shoot through the propeller circle without damaging the running propeller.

Technical solutions in front of the interrupter gear

MG 08/15 with interrupter gear, exhibited in the Deutsches Museum in Munich

Since there was no technical solution for this, fighter pilots resorted to the following, sometimes very daring or unusual methods in the First World War :

  • Airplanes in the pusher configuration with pusher propellers were used, which had a free field of fire forward for a movable machine gun or a gun. z. B. the British Vickers Gunbus , the French Voisin L (a Voisin of this type shot down a German aircraft in an aerial combat for the first time on October 5, 1914) or the German Otto C types and the test aircraft from Schwade or August Euler . The engine and propeller were attached to the stern of the fuselage nacelle and the struts for the horizontal and vertical stabilizers were guided past the propeller on the side. This complex solution was aerodynamically unfavorable and considerably reduced the flight performance of the "lattice hulls".
  • In the case of two-seaters, the observer, who was often seated in front, was equipped with a movable machine-gun, which of course had to fire past the propeller circle. The observer aimed dangerously and with a very limited field of fire with the machine-gun between the struts and bracing or, if he was located in the rear cockpit, he could only work with the machine-gun when turning the aircraft. The attempt on the German side to combat enemy aircraft by using so-called "large-scale combat aircraft" proved to be tactically ineffective: Heavy three-seater large aircraft such as the Gotha GI were equipped with movable machine guns or on-board cannons. These could defend themselves, but to catch, attack and pursue enemy aircraft, such large combat aircraft were much too slow and cumbersome.
  • The French company SPAD pursued a particularly unusual and daring solution : the shooter sat in a gondola suspended in front of the propeller with a free field of fire forward, which not only impaired the propeller's traction and prevented any communication between the crew, but also with the rotating one for the observer Propeller in the rear became a deadly danger in crash landings.
  • It proved most efficient to mount machine guns either higher or laterally, or at an angle to aim past the propeller circle. Reloading was difficult in the first case, and aiming with the angled MG in the second. Nevertheless, this remained at least as additional armament until 1918 standard for numerous Allied fighters, u. a. with the SE5 and the Sopwith Camel .
  • At the beginning of 1915, however, the highest combat strength was demonstrated by aircraft with machine guns firing unsynchronized through the propeller circle: the propeller blades were protected by sheet steel bullet deflectors. This made a frontal attack and pursuit against enemy aircraft possible.

With these procedures, the Allies had won air supremacy at the front in early 1915; more and more unarmed and defenseless German and Austro-Hungarian aircraft fell victim to them, so that tactical aerial reconnaissance in the depths of the enemy airspace became almost impossible. Nonetheless, all of the procedures mentioned turned out to be makeshift and inefficient. In order to enable the propeller circle to be shot through in a frontal attack on an enemy aircraft, an interrupter gear was required that synchronized the engine and machine gun.

MG synchronization

The most dangerous fighter aircraft with rigid, forward-firing machine guns suitable for frontal attacks were French Morane-Saulnier single-seaters, in which the propeller was protected against hits with sheet steel bullet deflectors. Since the beginning of 1915 u. a. the so equipped Escadrille MS 23 squadron on the western front hunted German aircraft unhindered. On April 19, 1915, however, the well-known French pre-war aerobatic pilot Roland Garros , who had already scored five kills, got into German defensive fire with a Morane-Saulnier L equipped in this way via Courtrai . Garros had to make an emergency landing near Ingelmunster on the German side, and his plane was immediately taken to Berlin. Anton Fokker and other designers were invited to examine the Morane and were given the task of copying or recreating the machine. Helmuth Förster, captain and adjutant of the field flight chief, handed over a Parabellum machine gun and ammunition to Fokker . Fokker's attempts to attach baffles to a German aircraft propeller proved unsuitable in the bullet test because of the penetration power of the German steel-jacketed projectiles; his engineers Heinrich Lübbe , Curt Heber and Leimberger took up an invention patented in 1913 by the engineer Franz Schneider von Luftverkehrsgesellschaft (LVG) . Within two days, Fokker and his engineers succeeded in designing a mechanism that connected the trigger of the Parabellum MG to the rotating motor axis via a camshaft. Fokker took one of his currently available A.III single-seaters with a 59 kW UI Oberursel rotary engine, equipped it with the synchronized MG, attached the aircraft to his sports car, drove from Schwerin to Döberitz and carried out his "invention" General Staff personally.

Shortly thereafter, Fokker delivered its new hunting monoplane to the front, the use of which not only won the German air supremacy until the beginning of 1916 (" Fokker-Plage "), but also revolutionized fighter aviation.

Further development

The mechanical interrupter gear had the disadvantage of noticeably reducing the cadence of the synchronized on-board weapons due to the delay between the interruption and continuation of the burst of fire . This disadvantage was only reduced to a minimum in the late 1930s with the introduction of electrically ignited cartridges that received an ignition pulse from an ignition magnet driven by the engine. This technology was used by German fighters during World War II and was used to synchronize machine guns and on-board cannons .

From the 1930s it was also possible in aircraft construction due to more stable metal constructions to accommodate on-board guns in the wings, which eliminated the need for synchronization. Furthermore, designs were also used in which the barrels of large-caliber weapons were placed through the propeller hub (e.g. Messerschmitt Bf 109 from variant F, Jakowlew Jak-3 , Bell P-39 ).

With the introduction of jet aircraft , the propellers also eliminated the need for synchronization. More modern military aircraft types with propeller drive either have two engines in the wings ( Rockwell OV-10 , FMA IA 58 ) or are equipped with converging on-board weapons or suspension points for weapon containers outside the propeller circle ( Douglas A-1 , Soko J-20 Kraguj , Embraer EMB 312 ).

References and comments

  1. The kuk Luftfahrtruppen MGs had in August 1914, with appropriately positioned in the observer cockpit light experiments, but the flight characteristics of the suffered so much that the experiments were terminated at low-performing aircraft under the added weight of the MG. After more powerful engines were available in 1915, the British BE2a or the German Aviatik CI were equipped accordingly.
  2. cf. the French SPAD A types, which, however, were very unpopular with the crews and were mainly used by the less picky Russian air force
  3. z. B. the British Sopwith Tabloid or Bristol Scout , the French Nieuport 11 or the Russian Mosca MB
  4. z. B. the French Morane-Saulnier L and N or in individual cases the British Sopwith Tabloid and SE2
  5. Fokker's aircraft were similar to the Morane: General Erich von Falkenhayn , head of the Supreme Army Command , was able to personally observe Fokker's flight demonstrations in 1914
  6. German Imperial Patent 276,396
  7. Aviation , VI. Year, No. 20/1914, pp. 804 ff.
  8. As chief engineer of the LVG, Schneider had already built the two-seater LVG EI monoplane a few months earlier with an MG synchronized by an interrupter gearbox, but this was lost on the way to the front testing. The fact that Fokker developed the gearbox within 48 hours is obviously a legend, especially since, according to his own admission, he had no idea about weapon technology. In addition, Fokker is said to have said later that he found a disengaged synchronization gear in the French Morane. (see Frank T. Courtney: Flight Path , London 1973). This version cannot be ruled out, because in addition to the Russian engineers Poplawko and Smyslow-Dybowskij, Raymond Saulnier had also built such a transmission for the Hotchkiss MG in France, which Louis Peyret had already presented to the military authorities in 1914, but his efforts had failed because of this. that the French ammunition fired too unevenly.


  • Enzo Angelucci, Paolo Matricardi: Airplanes from the beginnings to the First World War . Wiesbaden 1976, ISBN 3-8068-0391-9
  • JM Bruce: The Fokker Monoplanes (Profile No. 38), Profile Publications Ltd., 1965
  • Peter M. Grosz: Fokker EI / II (Windsock Datafile No. 91). Albatros Publications, Berkhamsted, Herts, UK 2002. ISBN 1-902207-46-7 .
  • Peter M. Grosz: Fokker E.III (Windsock Datafile No. 15). Albatros Publications, Berkhamsted, Herts, UK 1989. ISBN 0-948414-19-7 .
  • Karlheinz Kens, Hanns Müller: The aircraft of the First World War 1914-1918 , Munich 1973, ISBN 3-453-00404-3
  • Günter Kroschel, Helmut Stützer: The German military aircraft 1910-1918 , Wilhelmshaven 1977, ISBN 3-920602-18-8
  • Kenneth Munson: Combat Airplanes 1914-1919 , 2nd edition, Orell Füssli Verlag, Zurich 1976, ISBN 3-280-00824-7