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An introduction to the helicopter (1925)
NASA Technical Reports Server (NTRS) · leaf 63
N.A.C.A. Technical Memorandum No.340 Figs.10,11,13/15 * 16 Fig,16 Karman Helicopter without turrent. , 7035 A.S.NASA Technical Reports Server (NTRS)
Internet Archive · NASA Technical Reports · Public domain (US government work). Free to use for anything.
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