Helicopter problems
A 1937 technical report on helicopter problems, issued through the NASA Technical Reports Server as a record of aeronautical research for engineers. The pictures are plain line diagrams: one shows a spherical coordinate construction with axes, an ellipse and angle markings, the other shows streamlines of axially symmetrical ideal flow around a rotor, with a caption. A designer might take from them the clean, technical vocabulary of flow lines and orbital geometry for abstract patterns.
