The Siemens-Schuckert Werke S.S.W. D III was a biplane fighter powered by the unusual Siemens-Halske Sh III engine, and that served as a home defence interceptor in Germany late in the First World War.
The D III was a development of the earlier S.S.W. D II. This aircraft had been designed specifically for the Sh III engine, but although the airframes were ready early in 1916 the engines didn't arrive until June. When the engines were available they were quickly installed, and flight tests began. These showed that the aircraft had a very impressive rate of climb, although its level speed wasn't quite so good.
The basic D II was followed by a series of modified prototypes, some of which were originally ordered as D IIs. The D IIc kurz (short) had a 27ft 10 3/4in wing span and a wing area of 209 sq ft. The D II c lang (long) had a wing span of 29ft 6 3/8in, but the upper wing had a smaller chord, giving it a wing area of 195 sq ft. The D IIc short had more impact on the design of the S.S.W. D III, while the D IIc long, with two nearly identical wings, developed in the S.S.W. D IV.
The D IIc kurz began testing on 22 October 1917, and was soon redesignated as the D IIIc kurz. In December 1918 an order was placed for a pre-production series of twenty D IIIs, based on the kurz. This was followed in February 1918 by an order for thirty production aircraft.
The D.III was of conventional construction for the period. The fuselage had four main longerons, linked by circular transverse bulkheads. These bulkheads were linked by diagonal formers, which were covered with three-ply plywood to give the fuselage its circular cross section. The result was a very rigid structure. Although the S.H.III produced less spin than a normal rotary engine, there was still some, and the vertical tail was asymmetric in an attempt to counter this. The pre-production and production aircraft used a four bladed airscrew with a smaller diameter than the airscrews used on the prototypes. This allowed Siemens to reduce the height of the undercarriage.
The engine cowling was originally a full circle, but the bottom half was cut away to improve cooling. The propeller had a large spinner, which was later given four louvers between the roots of the four blades of the propeller, again in an attempt to improve cooling.
The wings used two box spars, with 1.5mm plywood ribs. The wings were linked by V struts built of spruce. The upper wing was larger than the lower wing.
The first pre-production aircraft were delivered in January 1918, and in April-May 1918 forty one of the first fifty went to JG II for operational trials, where most were used by Jasta 12 and Jasta 15.
In general the pilots approved of their new aircraft, reporting that it had sensitive controls, good flying characteristics and a very rapid rate of climb. On the negative side the engine easily overheated and then seized up. Rudolph Berthold, then commander of JG II, reported that the aircraft had its flaws, but that 'it is an urgent requirement that this fighter be made available for Front Line use as soon as possible', at least as long as the overheating could be solved.
After these early tests the S.S.W. D IIIs were returned to the factory, where they were given new engines and had the lower cowling cut away to improve cooling. A third batch of thirty aircraft also had their cowlings modified. The D III returned to operational service in July 1918, where they were used by the Kampfeinsitzer Staffel home defence squadrons Kestas 2, 4b, 5, 6 and 8. The D III's high rate of climb made it an excellent interceptor, although its top speed of 112.5mph was less impressive.
In December 1917, when Idflieg issued the first order for the D III they also ordered Siemens to begin work on the D IV, which was based on the D II/III lang, with its longer but shorter chord wings. A two-bay D V was also ordered, but got no further than the prototype stage.
Engine: Siemens Halske Sh III
Span: 27ft 7 7/8in
Length: 18ft 8 1/2in
Height: 9ft 2 1/4in
Empty weight: 1,175lb
Max speed: 112.5mph
Climb Rate: 3,280ft in 1.75min
3.75min to 2,000m/ 6,560ft
6min to 3,000m/ 9,840ft
9min to 4,000m/ 13,120ft
13min to 5,000m/ 16,400ft
20min to 6,000m/ 19,680ft
Service ceiling: 26,240ft
Endurance: 2 hours
Armament: Two synchronised Spandau machine guns