Estoppey D-series

The Estoppey D-series was a line of inter-war era bombsights developed by Georges Estoppey of the US Army Air Corps' McCook Field starting with the D-1 of 1922. A key feature was the use of a pendulum to keep the bombsight correctly oriented towards the ground even as the aircraft maneuvered, and dashpots to keep it from swinging around in turbulence. In testing in 1923, it proved more accurate than the US Navy's Mark III or the Army's Mk. I, and twice as accurate as the older Michelin bombsight designs. A series of minor updates led to the first widely used production model, 1925's D-4, of which several hundred were produced. The design was among the most advanced of its era but was still inaccurate to the point of being useless over 8,000 feet (2,400 m) altitude.

Estoppey continued working on the design, introducing models for both the Army and Navy. The D-5 of 1929 was a very different design, using the new tachometric concept, but it proved to have even lower accuracy than the D-4 and was cancelled. The D-7 of 1931 was similar to the D-4 but added illumination for night use, an automatic pilot direction indicator that signalled the pilot which direction to turn, and automatic bomb release to improve timing. While effective, the Army was expecting production of the Sperry bombsight which offered dramatically better accuracy. When the Sperry proved difficult to produce, in 1938 the Army ordered a last production run of a much more basic update of the D-4 as the D-8, which was the most produced version with just over 10,000 produced.

Estoppey became infamous for a lengthy and acrimonious series of legal battles with the Army and Navy over royalties for his concepts. As most of the designs were produced while he worked for the Air Corps, he was not eligible for royalties. On leaving their employ in 1926 he immediately began demanding fees from both the Army and the Navy, eventually receiving several thousand dollars.