Investigations on the amount of downwash behind rectangular and elliptical wings (1936)
NASA Technical Reports Server (NTRS) · leaf 24
Figs. 15,16,17 N.A.C.A. Technical Memorandum Ho* 787 … Figure 17.- Elliptical wing; downwash factors for the longitudinal axes through the origin of the coordinates for different cC (mean values) •
NASA Technical Reports Server (NTRS)
Internet Archive · NASA Technical Reports · Public domain (US government work). Free to use for anything.