Correspondence with Harold W. Zipp (Boeing Airplane Company), Hays rotary aircraft design drawings, and records of Hays/Boeing negotiations and agreement, 1942-1943, Box: 5, Folder: 13. Russell Randolph Hays papers, RH MS 564. University of Kansas. Kenneth Spencer Research Library. https://archives.lib.ku.edu/repositories/3/archival_objects/278165 Accessed October 06, 2024.