In comparison to airframes and engines, there has been little or no discussion of propellers in the vast sea of literature on the history of flight. As a result, the main body of evidence for this project consisted of archival and published primary sources. Secondary materials addressing the technical development of propellers and flight and broader issues in the history of technology had their place as well. The following essay discusses the sources central to this work.

Primary Sources

The primary sources used in this book reflect the interplay between airframe, engine, and propeller specialists and their clients and collaborators, the airline industry and national governments. Delving into the private and public records of all the historical actors revealed a broader context of innovation while understanding the role of the propeller. That research methodology also made up for a paucity of resources in critical areas while still facilitating a book whose sum was greater than its individual parts. In other words, while a propeller manufacturer's records were not available regarding a particular design, that information would appear in the files of a government research organization or an aircraft manufacturer that would be the ultimate user of that product.

Archives

Boeing Company Archives, Seattle, Washington, Long Beach, California, and St. Louis, Missouri

The Boeing Company maintains the world's premier corporate aerospace archive for itself and its legacy companies, which include Douglas, McDonnell, and North American. The collections include the personal papers of executives and engineers, subject files for each aircraft model, and general marketing materials. Boeing was a member company of the United Aircraft and Transport Corporation during the crucial years of 1929–1934. The papers of engineer, Claire Egtvedt, include the verbatim transcripts of the corporation's Technical Advisory Committee meetings, which discuss in detail the technical direction of its member companies. In the case of the variable-pitch propeller, aircraft and engine designers debated the merits of the new technology with the specialists dedicated to their development. The files on the Boeing Model 200, Model 221A, and Model 247, and Douglas DC-2 generated the documentary evidence for how the variable-pitch propeller became an integral part of those modern airplanes.