Heinz J. Nowarra, "German Helicopters, 1928-1945", Schiffer Military History Book Series | ISBN 0887402895 | 1999 Year | PDF | ~12 Mb | 48 Pages
| “ | Among the makers of these unusual aircraft were Focke-Wulf, Flettner and Wiener-NaustŠdter. B/W photographs, line drawings. | ” |