Junkers F 13 F 13a

In 1920 the F 13 was given its first substantial change: a wing of greater span and area, meant to improve handling and leave more room for payload. Aircraft modified that way were designated F 13a, and they were the first to leave the Dessau line as a commercial article rather than a demonstrator.

The new wing came with a new engine. In place of the prototype's Mercedes D IIIa the F 13a carried the BMW IIIa, also a water-cooled inline six, offering more power on take-off and at altitude. It powered the first aircraft actually sold and is the engine associated with the demonstration flights of the early years.

This configuration had to make its way in an awkward market. The Inter-Allied Aeronautical Commission of Control banned aircraft construction in Germany through 1921 and 1922, so Junkers looked for orders abroad: Austria, Poland and the United States in 1919, then SCADTA in Colombia and the United States Post Office Department. It was on this airframe and engine that a Junkers-Larsen machine set a world endurance record in 1921.

The F 13a fixed the formula that held for thirteen years: the same corrugated metal airframe, the same four-seat cabin, and an engine chosen to suit the customer and the market. From here on the differences between sub-types were coded with two letters — the first for the fuselage, the second for the engine.