BMW IIIa - Design and Development

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 SOHC in-line six-cylinder, just as the earlier Mercedes D.III was, 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 100 bhp (75 kW), such as the Gnôme Lamba-clone 80 hp (60 kW) Oberursel U.0 rotary engine, Class I was reserved for engines from 100 to 120 bhp (89 kW), with Class II for engines of between 120 and 150 hp (110 kW). The BMW engine was 185 bhp (138 kW) 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 200 horsepower (150 kW) up to an altitude of 2000 meters -a decisive advantage over competitors' engines.

German and British horsepower ratings apparently differed. Postwar British tests put the rating of the BMW IIIa at 230 hp. This corresponds to British ratings of the Mercedes DIIIa engine being rated by the British as 180 hp (German rating of 170 hp) and the DIIIau at 200 hp (German-180 hp). This discrepancy may explain the significant difference in performance of the BMW IIIa equipped Fokker D.VIIF both against Mercedes powered D.VII's and their Allied opponents. The standard German Pferdstärke metric horsepower unit was expressed in the early 20th century as being a unit of almost exactly 735.5 watts, while the British unit for mechanical horsepower was based on the older 33,000 ft-lb/min figure, which translates to 745.7 watts instead.

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:

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 (eight people on board, 6750 meters) in a Ju F-13 powered by a BMW IIIa aircraft engine.