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WikiPedia: Sikorsky R-4 http://www.nationalmuseum.af.mil/factsheets/factsh... http://www.IndianaMilitary.org/ http://www.plastikowe.pl/galerie/lotnictwo/sikorsk... http://www.rafmuseum.org.uk/sikorsky-r-4b-hoverfly... https://web.archive.org/web/20071202060801/http://... https://commons.wikimedia.org/wiki/Sikorsky_R-4?us...