Twitter
Facebook
While the winner of the January 1918 fighter competition for the German air force was the immortal Fokker D VII, another fighter was a close second and more of a technological breakthrough. This fighter was the Siemens Schuckert Werke (SSW) DIII (yes, the same Siemens company) and its breakthrough was the contra-
rotating rotary engine.
The rotary engine seems distinctly odd today, but it was the engine of choice for many World War I fighters. The rotary’s crankcase and cylinders, with the propeller attached, rotated in its entirety, instead of just turning the propeller shaft. This provided many advantages: smooth running, excellent cooling and, best of all, high power for low weight.
The disadvantages were that there was no throttle, per se – power was controlled by cutting fuel to the engine with a “bleep switch” – and the major problem, torque. The torque of the spinning cylinders made rotary engine fighters like the Sophwith Camel and Fokker Triplane able to turn incredibly fast in one direction, but the torque put a clear limit on how big a rotary could be, since the torque of a heavy rotary would simply overpower the flying controls.
The obvious advantages of the rotary led Harald Wolff, the chief designer at the Siemens company, to try and offset the torque with a bi-rotary engine. This engine, the Siemens Halske rotary, differed from traditional rotaries in that the crankshaft rotated in one direction at 900 rpm, while the crankcase and cylinders rotated in the opposing direction, also at 900 rpm, for a combined RPM of 1,800.
The engine was rated at 160 horsepower and maintained this power at high altitude due to a high compression ratio. The torque was almost eliminated; this allowed a slower propeller speed for the same power and, as a bonus, it also used a normal carburetor, which could be controlled by a conventional throttle.
To maximize the power, the prototype D.III was fitted with a large two-bladed propeller and long undercarriage so the prop could clear the ground, but this made the D. 111 easy to flip over when landing on a grass strip and hard to handle on the ground, so the configuration was changed to a innovative four-blade propeller and shorter landing gear.
From the outside, the SSW D.III was a short, stubby biplane dubbed the “flying beer barrel,” but its unconventional engine gave it performance that belied its appearance, especially its phenomenal rate of climb (for the time) and high altitude performance of the high compression engine. It could reach more than 26,000 feet, about 1,200 feet higher than the Fokker VII’s maximum altitude, and at heights above 13,120 feet was considered faster and more maneuverable than D.VII.
The German air force placed an order for an initial batch of 50 D.IIIs, which arrived by the end of February 1918. They were sent to Jagdgeschwader II for evaluation, where it was well liked by German pilots for its high rate of climb, vital in a dogfight, and because it could attack enemy intruders at the then extraordinary altitudes of 5,000 to 6,000 meters.
However, the D.111 was soon plagued by problems with it revolutionary Siemens Halske engine. After less than 10 hours of normal running, the engines began to overheat, the pistons seized and the cylinder heads disintegrated, resulting in complete destruction of the engine. The D. IIIs were withdrawn from service and replaced by Fokker D.VIIs while the problem was examined.
Rudolf Berthold, the commander of Jagdgeschwader II, was not happy, saying that when it worked the combination of SSW D.III airframe and Sh.III engine was “faultless” and asking that “the Siemens fighter be made available again for front-line use as quickly as possible for, after elimination of the present faults, it is likely to be become one of our most useful fighter aircraft.”
The problem was that the Siemens Halske engine, like all rotary engines, used castor oil for a lubricant. But castor oil was in short supply because of the British naval blockade, and the Voltol-based synthetic substitute was inadequate for engine lubrication.
Once the lubricating oil problem was solved, the Siemens Halske passed a full 40-hour endurance test in June 1918 and the SSW fighters began to return to the front. During the time Siemens had modified the D. IIIs to the D. IV with a different tail fin, a cut away engine cowling for better engine cooling, and balanced ailerons
The bulk of the fast climbing SSW D.IVs to the home defense squadrons, but of the 280 ordered only 123 were completed by the end of the war. Still, in October 1918 it was officially described as “superior by far to all single-seaters in use.”
At the end of the war, the Allies were anxious to try this little fighter but found, as many German front line pilots had found, it was tricky to land. The first three Allied pilots to fly SSW D. IVs after the Armistice found themselves hanging upside down by their shoulder harnesses a few seconds after touchdown when the SSW flipped over! The SSW D.III/IVs were marked down by the Allies as an “airplane of potential threat” and almost all surviving machines were completely destroyed in 1919.
(Dr. Michel is currently deployed downrange.)
