During 1915, designer Gustave Delage of the Société Anonyme des Établissements Nieuport had fitted a Nieuport 10 fuselage with tri-plane wings of unusual fore-and-aft geometry for experimental purposes, the arrangement being patented on 10 January 1916. It was test flown from a field next to the factory at Issy-les-Moulineaux, just a few miles from the famous Eiffel Tower. This was the first of the extraordinary Nieuport tri-planes
There were several Nieuport triplane attempts. The first was based on a Nie. X 2-seater, the others on the Nie 17 airframe. This is a model of the 2nd triplane, which stayed with the French and differed from the later one that went to the RFC in having a Lewis gun and no cutout in the upper wing.
Progressive development of the first of the extraordinary Nieuport tri-planes led in 1916 to an even more unorthodox triplane arrangement in which the rniddle wing, attached to the forward ends of the upper fuselage longerons, was foremost and the upper wing rearmost.
Utilising a Nie 17 fuselage, powered by an Le Rhône 9J engine fitted with a large cône de pénétration ' (ie spinner) on the propeller and armed with a single synchronized Lewis gun, this triplane, designated Nie-11C (or 11.000) was officially tested by the S.T.Aé (Section Technique de l'Aéronautique) late in 1916, but the unusual configuration proved to offer poor handling and was not ordered for the Aviation Militaire.
Evolved in parallel with the HD.5 and of generally similar configuration, but larger and more powerful, the HD.6 two-seat fighter was powered by a 530hp Salmson 18Z two-row radial water-cooled engine. This was essentially two Salmson 9Z engines on a common crankcase and flight testing was delayed by difficulties with this experimental power plant, eventually commencing in the spring of 1919. Armament consisted of two synchronised 7.7mm Vickers guns for the pilot and three 7.7mm Lewis guns for the gunner, two on a rotating mount and one firing through a trap in the fuselage floor. The pilot, seated beneath a cut-out in the upper wing, was offered a singularly poor field of vision. Performance did not show a significant improvement over that of the more compact and simpler HD.3, and development was discontinued by the late summer of 1919.
HD.6.C 2: Enlarged two-seat fighter protoype, powered by a 530-hp (395-kW) Salmson 18Z radial piston engine. Only one was ever built, not demonstrating significantly better performance than the HD.3.
The Hanriot HD.7 was a French single-seat fighter prototype based largely on the Hanriot HD.3, using the wings and tail surfaces from the earlier model.
The aircraft was designed to replace the SPAD S.XIII fighter. However, after its first flight in 1918, it was found to be of good performance, but inferior to its main competitor, the Nieuport-Delage NiD 29, which then entered service production for the Aéronautique Militaire in 1918. No further production occurred.
A single-seat reconnaissance fighter derived from the two-seat HD.3, the HD.9 was placed in the broad category of Avions de Corps d'Armee (thus being the HD.9 Apl) and its armament consisted of a single synchronised 7.7mm Vickers machine gun. The airframe was basically that of the HD.3 and the installation of the 260hp Salmson 9Z radial engine was identical, but fuel capacity was considerably increased. The first example of the HD.9 was completed in November 1918 as the initial aircraft built against an order for 10 machines. However, its career was cut short by the Armistice and there is no evidence that all nine remaining aircraft were completed.