Frank W. Caldwell and variable-pitch propeller development, 1918-1938

The variable-pitch propeller was a critical propulsion technology to the aeronautical design revolution of the late 1920s and early 1930s. The new device bridged the gap between the two major advances of that revolution in the sky: aerodynamic drag reduction and increased engine power. The leader in this development was Frank Walker Caldwell (1889-1974). During his tenure as the government's chief propeller engineer (1917-1928) and his work in industry (1929-1938), Caldwell oversaw the invention, development, and innovation of the metal, multipiece propeller, the ground-adjustable pitch propeller, and the hydraulically actuated two-position controllable-pitch and constant-speed propellers. In the process, he made a major contribution to fundamental aeronautical engineering through his work on propeller testing facilities and techniques. Caldwell achieved this while recognizing the importance of the propeller within the synergy of the larger technical system of the airplane. This paper documents Caldwell's critical role in the development of the variable-pitch propeller in the United States from World War I to the outbreak of World War II.