Alongside the prototype-minded HD.5 of 1918, Hanriot of France was also developing the equally-experimental HD.6 twin-seat biplane fighter during the last few months of World War 1 (1914-1918). This form borrowed some of the design elements used by the HD.5 but went further in incorporating enough changes to make it wholly unique in the Hanriot lineup. The aircraft was still in its early phases of development when the Armistice of 1918 was signed to end the war - thus it went on to have little impact on the outcome and never entered serial production.
Though taking the HD.5's dual-bay wing design, tandem-seat open-air cockpits, and multi-machine gun approach, the HD.6 was made dimensionally larger with all-new design lines. The engine cowling at the nose was rounded as opposed to the HD.5's squared-off look. The upper wing member of the biplane configuration still resided low against the fuselage and, as in the HD.5, both the forward and trailing edges were cut-out for maximizing pilot and gunner vision. The tail unit utilized a similar single-finned rounded vertical plane and ground-running was possible by a conventional two-wheeled with tailskid "trail-dragger" arrangement.
Proposed armament was slightly dissimilar: 2 x 7.7mm Vickers Machine Guns were mounted in fixed, forward-firing positions over the nose as usual, and the rear cockpit entertained 2 x 7.7mm Lewis Machine Guns on a trainable mounting (T.O.3) as expected, but the floor of the rear gunner's position offered a cut-out in which a third 7.7mm machine gun could fire out onto targets below and behind the aircraft as a new defensive measure. The forward-facing machine gun pair was also synchronized to fire through the spinning propeller blades.
One of the chief design aspects of the HD.6 was its engine, essentially a pairing of Salmson 9Z 9-cylinder radials of 260 horsepower each to produce the singular Salmson 18Z twin-row, water-cooled air-cooled radial unit with a total output rating of 530 horsepower. This drove a typical wooden two-bladed propeller at the nose and was a completely experimental setup by the company to pull every ounce of power out of the fighter therefore producing a high-performance, well-armed aircraft by standards of the period.
However, such an engine arrangement was fraught with issues and delays proved common to the HD.6's development to the point that flight-testing of the design was not had until the Spring of 1919 - the war had ended back in November of 1918 and the demand for new fighters among the major players of the war was at an all-time low. Additionally, it was found that the twin engine pairing, despite the increase horsepower, added little to performance. As such, development of the HD.6 only ran into the summer of 1919 and it was given up for good shortly thereafter - the sole, flyable prototype being the only concrete work had on the program.
As built, the aircraft had an overall length of 29 feet, a wingspan of 44.6 feet, and a height of 9.5 feet. Empty weight reached 1,790lb against a Maximum Take-Off Weight (MTOW) of 2,755lb being reached. The HD.6's service ceiling was listed at 23,000 feet and range was out to 375 miles.
Design of this interesting biplane is attributed to Emile Dupont, as was the HD.5 detailed elsewhere on this site.