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BMW IIIa was an inline six-cylinder, water-cooled aircraft engine, the first-ever product from BMW GmbH. Its success laid the foundation for future BMW success.

Design and development

On 20 May 1917, Rapp Motorenwerke (which later that year became BMW GmbH) registered the documentation for the construction design for the new engine, dubbed BMW III. Designed by Max Friz and based on the Rapp III engine, it was an in-line six cylinder, which guaranteed optimum balance, therefore few, small vibrations. It was designed with a high (for the era) compression ratio of 6.4:1.

The military authorities were also responsible for the fact that the first BMW product, due to its power output, was designated with a three instead of a one. At the beginning of 1917 the IdFlieg introduced uniform model designations for aeroengines. The Roman numeral referred to the performance class. IdFlieg's Class 0 engine power category was for engines of up to Template:Convert, such as the Gnôme Lamba-clone Template:Convert Oberursel U.0 rotary engine, Class I was reserved for engines from 100 to Template:Convert, with Class II for engines of between 120 and Template:Convert. The BMW engine was Template:Convert and was assigned to category III.

The engine was successful, but the real breakthrough came in 1917, when Friz integrated a basically simple throttle butterfly into the "high-altitude carburettor", enabling the engine to develop its full power high above the ground. Burning a special high octane fuel of gasoline blended with Benzole, the carburettor adjusted the richness of the fuel-air mixture according to the aircraft's altitude. It enabled the engine, now dubbed BMW IIIa, to develop a constant Template:Convert up to an altitude of 2000 meters - a decisive advantage over competitors' engines.

The first design drawings were available in May, and on 17 September the engine was on the test rig. After a successful maiden flight for the IIIa in December 1917, volume production started up at the beginning of 1918.

The ability to gain power at higher altitudes was why this engine had unique superiority in air combat. It was primarily used in the Fokker D VII and in the Junkers Ju A 20 and Ju F 13. When equipped with the BMW IIIa engine, the Fokker D VII could outclimb any Allied opponent it encountered in combat. Highly maneuverable at all speeds and altitudes, it proved to be more than a match for any of the British or French fighter planes of 1918. The water-cooled in-line 6 cylinder engine's reputation grew very quickly after is abilities were proven in air combat by Jasta 11, the "Red Baron's" squadron. Ernst Udet, squadron leader of Jasta 11 in WWI, acknowledged the outstanding performance of the BMW IIIa engine:

There can be no doubt that the BMW engine was the absolute highlight in power unit development towards the end of the war. The only bad thing was that is came too late.[1]

About 700 engines were built by BMW, however, a large demand for the new BMW IIIa aircraft engine in Munich (coupled with a lack of production capacity) caused part of the production to be transferred to the Opel factory in Rüsselsheim.

On September 13, 1919, Franz Zeno Diemer Set up a world altitude record for a passenger aircraft (8 people on board, 6750 meters) in a Ju F-13 powered by a BMW IIIa aircraft engine.


Specifications (BMW IIIa)


See also


  1. Barker, R. (2002); The Royal Flying Corps in World War I; Robinson. ISBN 1-84119-470-0

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This article is licensed under the GNU Free Documentation License.
It uses material from the Wikipedia article "BMW IIIa".