BMW IIIa

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BMW IIIa
The Smithsonian National Air and Space Museum's preserved BMW IIIa, shown with quick-change propeller hub
Type
Inline engine
Manufacturer BMW
First run 1917
Major applications Fokker D.VII

BMW IIIa was an

Bayerische Flugzeugwerke AG, who, at the time, were exclusively an aircraft engine manufacturer. Its success laid the foundation for future BMW engine designs. It is best known as the powerplant of the Fokker D.VIIF
, which outperformed any allied aircraft.

Design and development

On 20 May 1917,

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 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 military authorities were responsible for the fact that the first BMW product was designated with a III instead of an I. As early as 1915, the

IdFlieg German military aviation inspectorate introduced uniform model designations for aero engines, with the Roman numeral referring to the performance class. IdFlieg's Class 0 (zero) engine power category was for engines of up to 100 bhp (75 kW), such as the Gnome Lambda
-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 twin-barrel "high-altitude carburettor", enabling the engine to develop its full power high above the ground. Burning a special high

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.

BMW IIIa at the Luftwaffenmuseum

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

Jasta 11, the "Red Baron's" squadron. Ernst Udet, squadron leader of Jasta 11 in World War I, 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 it came too late.[1]

About 700 engines were built by BMW, however, a large demand for the new BMW IIIa aircraft engine in

Rüsselsheim
.

On September 13, 1919,

Ju F 13 powered by a BMW IIIa aircraft engine.[citation needed
]

Applications

Specifications (BMW IIIa)

Data from Smithsonian NASM BMW IIIa specifications

General characteristics

  • Type: 6-cylinder, inline, water-cooled, piston engine
  • Bore: 150 mm (5.9 in)
  • Stroke: 180 mm (7.1 in)
  • Displacement: 19.085 L (1,165 cu in)
  • Length: 170.2 cm (67.01 in)
  • Width: 50.8 cm (20 in)
  • Height: 105.4 cm (41 in)
  • Dry weight
    :
    293 kg (646 lb)

Components

  • Valvetrain: SOHC (single overhead camshaft)
  • Fuel system: Two-barrel altitude compensating (to 2 km) carburetor
  • Cooling system: Water-cooled

Performance

See also

References