Tolong jelaskan mengenai Varian dari Sikorsky R-4

McGowen, Stanley S. Helicopters: An Illustrated History of Their Impact (Weapons and Warfare Series). Santa Barbara, CA: ABC-CLIO, 2005. ISBN 978-1-85109-468-4.

Myall, Eric and Ray Sturtivant (ed.). The Hoverfly File. Tunbridge Wells, Kent, UK: Air-Britain (Historians) Ltd., 1998. ISBN 0-85130-262-9.

United States Air Force Museum Guidebook. Wright-Patterson AFB, Ohio: Air Force Museum Foundation, 1975.

Williams, Dr. James W. A History Of Army Aviation: From Its Beginnings To The War On Terror. Bloomington, IN: Iuniverse, 2005. ISBN 978-0-595-67396-4.