September 1939
From The Space Library
Igor I. Sikorsky made initial flights with the first successful single-main-rotor helicopter, precursor of the R-4 two-place design procured in 1942 by the AAF.
World's largest balloon, the Star of Poland, was unable to make stratospheric flight because of the German invasion. The United States had provided helium gas in August for this Polish effort and several American experts, including A. W. Stevens, provided technical assistance.