Albatros D.XI prototype showing type designation displayed as part of serial number marking
The Idflieg designation system was used to designate German heavier-than-air military (as opposed to naval) aircraft from the early days of the Luftstreitkräfte to the end of World War I. The system necessarily evolved during this period, as new aircraft types were produced.
At times complicating the identification of wartime German aircraft is the fact that German manufacturers not only typically used their own designations; but also sometimes gave their experimental productions unofficial "Idflieg-style" class numbers, perhaps in the hope of production orders, or even built on them. The "Rs" (giant seaplane), and "CLs" (two seater seaplane) designations of the Zeppelin-Lindau company are examples, as are the unofficial "Dr" designations of the experimental Euler triplanes, which in fact were never given official designations.
The German Navy had their own system for classification of aircraft types, but did not use this, nor the Luftstreitkräfte system, to designate particular aircraft types, preferring to use manufacturers' designations. Airships were totally outside either system, being individually numbered in the same way as German destroyers and submarines, mostly in the "L" series.
As well as serving to identify types, the Idflieg type designations were also often included as part of German aircraft serial number markings.
A – originally, simply monoplanes. "A" type aircraft (for example the Rumpler Taube and Fokker M.5) were not limited by any official limiting specification, apart from their wing layout, although in practice most "A" class aircraft were unarmed two seat reconnaissance or training aircraft. Exceptions were the single-seat Fokker "A" types that became the famous "E" class fighters when they were armed with synchronised machine guns. After 1915 the "A" class became extinct, as examples reached obsolescence and were discarded. Later monoplanes were included in the "D", "C", or "CL" classes, with operationally equivalent biplanes.
B – originally, inclusive of all biplanes. Again, this designation was not connected to any official specification, apart from the wing layout. In practice, specifications applied to later types limited aircraft retaining the "B" designation after 1915 to low powered unarmed two-seaters, mostly used for training and other second line duties.
C – two-seat armed biplanes (designation introduced by April 1915). This was the first new designation to be introduced after the outbreak of war, and also the first to have a defining specification. In order to reduce the vulnerability of early German military aircraft to Allied types equipped with machine guns, "C" types were armed with a rearward firing machine gun operated by the observer and (later) a forward firing synchronized machine gun for the pilot. An engine of more than 150 hp was also specified (later "C" types typically had over 200 hp). A number of future German fighter aces obtained their first victories in the "C" type
CL – lightweight "C" class aircraft (designation introduced early 1917). Later "C" types became progressively larger – the "CL" specification was intended to provide smaller aircraft, nimble enough to be used as a two-seat fighter aircraft. In practice, the "CL" types were mainly used for close support. Engine power of a "CL" was limited to less than 200 hp – and total loaded weight to under 360 kg.. In other respects "CL" types were similar to "C"s – in fact serial and type numbers generally fell in the same sequence.
CN – "C" class aircraft modified to carry a heavier bomb load for use at night. Designation replaced by "N".
D – single-seat armed aircraft, specifically intended for use by the new jagdstaffeln or fighter squadrons (designation introduced in 1916). Until late in 1918, when designations for fighter aircraft were simplified, "D" implied an armed doppeldecker, or biplane. By the end of the war all single seat fighters were designated as "D" types, as distinctions based on wing layout were abandoned.
DJ – Armoured "D" class aircraft. The only type in this class was the prototype AEG DJ.I and the designation may not have been official.
Dr – single-seat armed triplane, or Dreidecker (designation introduced in late 1917 – abandoned in late 1918). The first two Fokker Dr.I aircraft were in fact designated "F.I". By the end of the war all new single-seat fighters were designated as "D" types, regardless of wing layout.
E – armed monoplane (designation introduced in 1915 – abandoned in late 1918), from Eindecker. This was initially simply the monoplane version of the "C" class armed biplane, having the same relationship to the "C" class as the "A" had to the "B", and several early "E" types were two-seaters. In practice, due largely to the success of the single seat Fokker "E" types, which were single seat fighters, the "E" class came to mean a single seat fighter monoplane (i.e. the monoplane equivalent of the "D" class). In late 1918 the last "E" type, the Fokker E.V was redesignated the "D.VIII", and other late war monoplane types (such as the Junkers CL.I) were also designated in their "functional" class, in line with the abandonment of designations based on wing layout.
F – single-seat armed triplane (designation used briefly in 1917). Applied only to the Fokker F.I, which was redesignated the "Dr.I" by the time it went into production.
G – armed biplane bomber aircraft with two or three engines (groß–"large") (designation used from 1916). These aircraft were originally designated K (see below).
GL – faster twin-engined aircraft suitable for use as a day-bomber or long range reconnaissance (designation introduced in 1918). Bore a similar relationship to the "G" as the "CL" bore to the "C". Weight and wing span were reduced, and crew was limited to two – the forward gunner’s cockpit being eliminated.
J – armoured dual-roleliaison aircraft and ground attack aircraft (designation introduced in 1917). Most examples resembled "C" types in general layout – differing only in being fitted with armour to reduce vulnerability to ground fire. The exception to this was the Junkers J.I, designed specifically as an armoured aircraft.
K – armed biplane bomber aircraft with two or three engines (Kampfflugzeug–"battle aircraft"). Designation introduced in 1915 and replaced by "G" by early 1916.
N – two-seat single engined night (Nacht) bomber. Basically a "C" type aircraft with longer wing span to enable a heavier war-load. Designation introduced in 1918, superseding "CN" specification.
R – a large bomber aircraft with at least three, in some cases up to six engines (Riesenflugzeug–"giant aircraft"). An important distinguishing feature from the "G" class (apart from size) was that all engines should be accessible in flight to permit running repairs.
The system ceased to apply with the end of German military aviation following the Armistice, and aircraft of the newly reborn Luftwaffe would be designated according to the RLM aircraft designation system, although in the nineteen twenties and thirties the Fokker company designated its new military types with "Idflieg" style numbers, so as to continue war-time sequences. In particular Fokker two seat military reconnaissance aircraft continued the wartime "C" series, while Fokker single seater fighters were given numbers in the "D" series exploiting the reputation of the wartime D.VII.