The Compressibility Bubble
This is a NACA technical note issued by the National Advisory Committee for Aeronautics, a US government research body, with figures prepared for aerodynamicists and engineers studying airflow over wings. The pages show schlieren flow photographs of the NACA 4412 airfoil at various angles of attack and Mach numbers, with the rotated figure captions printed alongside each image. A designer could take from them the dark-and-light banding of shock and flow boundaries, a stark monochrome vocabulary for visualising invisible air movement around a streamlined form.
