On the elementary relation between pitch, slip, and propulsive efficiency
A 1920 NASA technical report on the elementary relation between propeller pitch, slip, and propulsive efficiency, issued for aeronautical engineers and researchers. Its two pictures are graphs with labelled curves plotted against scales of pitch or slip, with energy and efficiency axes, lettered families of curves (a through f), and a dense grid of gridlines. A designer might take from them the look of technical charts: fine grid lines, lettered curve families, and scale annotations set against a plain background.
