 The first flight covered 200 m at an altitude of 10 m.

During the First World War he invented the plane-mounted Le Prieur rocket launcher for bringing down observation balloons.This weapon system, which allowed an airplane to fire a single volley of rockets in close succession (the design planned for simultaneous launch, but technical unreliability made it impossible in 1916) was remarkably effective against the German observation balloons, and was only phased out when tracer rounds and incendiary bullets for the on-board machine guns (with similar efficiency and larger ammunition capacity) became widespread among the Allied air forces near the very end of the war.Le Prieur also patented a number of designs for mechanical lead computing sights for both ship to ship and anti-aircraft guns.