Model Tests on the Economy and Effectiveness of Helicopter Propellers
A 1925 NACA technical report from the NASA Technical Reports Server, issued by the National Advisory Committee for Aeronautics for aeronautical engineers and researchers, testing scale-model helicopter propellers for efficiency. The plates are plain photographs and line drawings of blades, hubs and tail vanes, with a power-against-angle graph. A designer could draw on their clean technical diagram style, with simple pale grey fields and precise dimensioned linework.
