An introduction to the helicopter
This is a 1920s NACA Technical Memorandum, issued by the National Advisory Committee for Aeronautics in the United States, and compiled as an engineering introduction to early helicopter designs for aeronautical engineers and researchers. The record gives only a title and a 1925 date, so the pictures carry the content: they show rotor layouts, airframe drawings and photographs of experimental machines. A designer could take their precise technical line drawings, rotor diagrams and period engineering typography as a model for schematic, diagrammatic illustration.
