Definition of Method of Measurement of Supporting and Control Surface Areas, Special Report
A 1931 technical report from the NASA Technical Reports Server, a government aeronautics document that defines how wing and control surface areas are measured for aircraft. Its pictures are plain line diagrams of aircraft control surfaces, with hatching marking the measured area and labels for the hinge line and effective chord. A designer could take from them the clean, hatched-section diagram style typical of early engineering reference drawings.
