 In 1923, he was transferred to Maxwell Field in Montgomery, Alabama for aerial mapping missions; while there, he flew the first air mail route between Montgomery and New Orleans, Louisiana.After four years at Maxwell, Knapp was assigned to Kelly Field in San Antonio, Texas, where he was a flying instructor at the flight school.Having been promoted to commandant of the Air Corps Primary Flying School at nearby Brooks Field in 1929, he was appointed commanding officer of the 44th Observation Squadron in the Panama Canal Zone in 1931.