Gotha WD.7

The Gotha WD.7 (Wasser Doppeldecker - "Water Biplane") was a twin-engine maritime patrol floatplane developed during World War I by Gothaer Waggonfabrik (Gotha) for the Imperial German Navy's (Kaiserliche Marine) Naval Air Service (Marine-Fliegerabteilung). The prototype was captured by the French on its first combat mission in April 1916 after it was forced to make an emergency landing after an engine failed. Despite this seven additional WD.7s were ordered and were used for training torpedo bomber pilots and for trials. Only a single aircraft is known to have survived the war.

Background and description
A decade after the Wright Brothers made the first heavier-than-air flight in 1903, obvious missions for aircraft were reconnaissance and ground attack and the consequent need to negate the enemy's attempts perform them against your own troops. Although synchronizer gear to allow machine guns to fire between the propeller blades as they spun was under development in multiple countries, a successful system had yet to be fielded. This meant that the machine gun placed on a rotating mount with a field of fire unimpeded the propellers was the only way that one aircraft could shoot down another. This relegated the pilot to merely flying within range of an enemy aircraft while his gunners would attempt to destroy their opponent with their machine guns and autocannon. Designers and military aviators likened this to warships at sea which maneuvered to bring their weapons to bear. Thus were born the aerial cruisers or battleplanes (Kampfflugzeuge) built by Imperial Germany, Great Britain and France.

At the beginning of 1914, the Imperial German Army's Imperial German Air Service (Die Fliegertruppen des deutschen Kaiserreiches) began discussions with its Inspectorate of Flying Troops (Inspektion der Fliegertruppen (Idflieg)), the Transport Technical Investigation Commission (Verkehrstechnische Prüfungs Kommission (VPK)) and aviation industry executives about the wartime role of aircraft. In March they reached a consensus outlining three broad roles for aircraft: The Central Division (Zentral-Abteilung) of the General Staff approved the VPK's recommendations on 28 April, directing that aircraft be developed for the Type II and III categories as quickly as possible as the Type I requirement was already met by the existing B-type aircraft. The Air Service scheduled a competition to select the best Type II aircraft in November and another in early 1915 for the Type III aircraft as those larger and more complex aircraft would require more time to design and build, not least because two engines would be required as Germany lacked engines powerful enough to lift that weight on their own. The start of World War I in August disrupted these plans, although many companies had already made considerable progress with their Type III designs. Rather than hold a competition, Idflieg decided to order small numbers of prototypes from the various manufacturers.
 * Type I; a fast two-seater reconnaissance or bomber aircraft.
 * Type II; a short-range, two-seat, very maneuverable aircraft intended to fly at low altitudes and armed for self-defense.
 * Type III; a long-range aircraft able to carry 450 kg of useful load for six hours within range of enemy fire.

The Naval Air Service decided that it wanted floatplane equivalents of the Kampfflugzeuge and ordered one prototype from Gotha on 10 May 1915. Designed by Karl Rösner, the WD.7 was a tractor-configuration two-bay biplane with 120 hp Mercedes D.II straight-six engines mounted on the leading edge of the lower wing. The radiators were located above each engine. The aircraft retained the design of the WD.3's nose gunner's position, but it had an entirely new fuselage with the pilot's cockpit behind the gunner's position. It also used the same style of twin-tail structure. The prototype kept the central vertical stabilizer as well, but this was eliminated in the production aircraft. Its floats were attached to the lower wing via struts directly below the engines. A pair of lateral struts reinforced the floats, but also precluded the aircraft from carrying bombs or torpedoes underneath the fuselage. The gunner was armed with a 7.92 mm Parabellum MG 14 machine gun on a ring mount.

History
The prototype was delivered to Naval Air Station Flanders I (Seeflugstation Flandern I) at Zeebrugge in Occupied Belgium on 8 February 1916, but inclement weather prevented operational missions for several months. The aircraft was tasked with the morning reconnaissance mission off the Belgian and French coastlines on 2 April, but one engine broke down over Calais, France, and the pilot was able to make an emergency landing north of the port. He attempted to taxi back to Ostend, Belgium, but the WD.7 caught fire and was captured by a French destroyer south of Dunkirk, France.

Seven additional WD.7s were ordered on 24 February; the first six of them used 100 hp Mercedes D.I engines, but the last aircraft was fitted with 120 hp Argus As.II engines. Delivered in June–August, most were used to train torpedo bomber pilots at the seaplane bases in Warnemünde, Apenrade, Norderney and Flensburg. Several aircraft were retained by the Seaplane Experimental Command (Seeflugzeug-Versuchs-Kommando) for testing, including temporarily fitting a WD.7 with Austro-Hungarian 130 hp Hiero engines. One aircraft is known to have been used for trials of the Becker 20 mm autocannon beginning in late 1916. A 37 mm (1.5 in) autocannon built by DWM was also tested aboard these aircraft. When the Allies inspected the German seaplane bases in December 1918, they recorded a single surviving WD.7 at Hage. Its ultimate fate is unknown, but it was likely scrapped.

The Naval Air Service wanted to conduct comparative trials between single- and twin-engined versions of the same airframe with the same total power. It ordered the WD.8 reconnaissance floatplane prototype in July 1915 that substituted a single 240 hp Maybach Mb.IVa engine in the nose. It was not successful and the prototype was later sold to the Ottoman Empire.

Variants
WD.8: one prototype of a single-engine version, powered by a Maybach Mb.IV.