observers for the field artillery. On 29 August 1917 the 3rd Aero Squadron left Fort Sam Houston for Fort Sill with 12 Curtiss R4 airplanes under the command of Capt. Weir. It was re-designated as Squadron A, Post Field, Okla. on July 22nd, 1918. It was demobilized, due to the end of World War I, on Jan 2nd 1919. Today, the 3rd Flying Training Squadron, which traces its lineage to the 3rd Aero Squadron, trains pilots at Vance Air Force Base, Enid, Okla. The 4th Aero Squadron was also sent to Post Airfield that summer. The 4th operated as an observation school for the field artillery until it was demobilized on Jan 2nd, 1919. Today, the 394th Combat Training Squadron at Whiteman AFB, Mo., traces its lineage to the 4th Aero Squadron. A variety of units were created, inactivated, assigned and reassigned as the Army's aviation assets grew. In 1922, Fort Sill was considered the busiest airport in the U.S. Aviation at Fort Sill added lighter-than-air ships to its inventory when Company A, 1st Balloon Squadron, arrived on 5 September 1917 from the Balloon School in Omaha, Nebraska. The company split to form the 25th and 26th Balloon Companies on 16 February and April 2nd, 1918. In order to meet the demand for trained aerial observers for field artillery, a Balloon Corps Training School was set up at Post Field in 1918. During World War I, the school trained 751 officers and created 89 companies, of which 33 were deployed to Europe. The school used balloons and fixed wing aircraft for aerial observation. Both sausage-shaped "captured" balloons and spherical-shaped "free" balloons were used in the 1920s and 30s. The balloonists were trained on free flight on the "free" balloons, but they had to stay within 50 miles (80.5 km) of post and 8,000 feet (2,438 m). The tethered or "captured" balloons were for observation only – connected to winch trucks on the ground by cable and transported at speeds as high as 60 miles an hour (96.6 km/h). They were inflated with hydrogen and operated at a maximum height of 4,300 feet (1,310.6 m). They observed and relayed fire-corrective information to special operation trucks. At this time, balloon companies were a corps-level asset. The Army of World War I included an aero squadron in every corps. Other auxiliary units for a corps were an anti-aircraft machine-gun and anti- aircraft artillery