The ground effect on lifting propellers
A 1937 technical report from NASA's Technical Reports Server, a scanned engineering paper on how the ground affects lifting propellers, written for aeronautical engineers. Its one picture is a hand-drawn graph of a normalised lift ratio plotted against a dimensionless height ratio, with a handwritten formula at top right; a designer might borrow its plain axis-and-curve diagram style as a simple scientific chart motif.
