While the French Air Service of World War 1 (1914-1918) ultimately rejected the Hanriot HD.1 in favor of the SPAD S.VII biplane fighter (both detailed elsewhere on this site), the type went on to see operational service under the national flags of others such as the United States, Belgium, and the Kingdom of Italy with total production reaching 1,200 units before the end. Utilizing the same framework of this very capable biplane, France-based Hanriot developed a purpose-built "floatplane fighter" variant under the designation of "HD.2" before the end of 1917 primarily for use by the French Navy (the "Aeronavale").
The aircraft carried a Clerget 9B rotary engine of 130 horsepower and more-or-less maintained the structural form and function of its predecessor. The single-bay, biplane wing arrangement was of unequal span with forward-cranked parallel struts. The mainplane members were situated well-ahead of midships, concentrating the center-of-gravity towards the front of the aircraft. The engine was fitted to a relatively short nose section with the pilot's open-air cockpit directly aft. The fuselage has slab-sides and was generally devoid of obstructions throughout. The tail unit used the same rounded planes as the HD.1 model. The chief physical difference between the two aircraft, of course, was the HD.2's twin floatplane undercarriage which allowed for the requisite water landings and take-offs. A whole new strut network was affixed to the underside of the aircraft for this purpose.
Despite being a floatplane, the aircraft was intended to retain the fighter-like capabilities of the HD.1 so it was well-armed through 2 x 0.303 caliber Vickers Machine Guns in fixed, forward-firing mounts atop the fuselage and synchronized to fire through the spinning propeller blades.
For its evaluation phase, the HD.2 prototypes were tested with different floatplane lengths, with wheeled landing gear arrangements, and - in some cases - different engine installations. Trials were undertaken at Dunkirk with easy access to the water and under controlled circumstances. This work was had throughout the early-to-mid part of 1918 and continued into September. The Armistice to end the war was eventually signed in November of 1918 but the legacy of the HD.2 went on for a time longer.
Basic production forms of the French Navy were designated simply as "HD.2" but there emerged several other experimental forms. The "HD.12" was a one-off model fitting the Le Rhone 9R rotary piston engine of 170 horsepower as well as a wheeled undercarriage for land-based operations. The "HD.27" was another one-off, this time powered by a Hispano-Suiza 8Ac engine of 180 horsepower. The "H.29" saw two built to a standard intended for ship-borne use, these aircraft being powered by a Hispano-Suiza 8Ab series engine.
As completed, the HD.2 had a crew of one. Structurally, it had a running length of 23 feet, a wingspan of 27.10 feet, and a height of 10.1 feet. Empty weight reached 1,100lb with a Maximum Take-Off Weight (MTOW) of 1,540lb possible. Power was from a Clerget 9B series engine of 130 horsepower used to drive a two-bladed wooden propeller at the nose. Maximum reachable speed was 113 miles per hour with a range out to 185 miles and a service ceiling of 15,750 feet.
The United States Navy (USN) contracted for the purchase of ten HD.2 floatplane fighters which, after they had been delivered an operated for a time, were eventually modified locally to a land-based fighting form by the Naval Aircraft Factory (NAF), operating as land-based trainers out of Langley Field under the designation of "HD.2C". At least one was used to experiment with ship-based / ship-launched aircraft which became a design standard of larger warships seen in the Second World War (1939-1945).