Vietnam Choppers (Revised Edition): Helicopters in Battle 1950-1975 (General Aviation)

"After many years of experimentation the concept of rotary-wing flight, originally documented by Leonardo da Vinci, was finally realized during the 1930s; but it was not until the closing months of World War 2 that the helicopter was actually utilized for military purposes, with the deployment of the Sikorsky R-4 to the 1st Air Commando in the Far East for casualty evacuation and the retrieval of airmen shot down behind enemy lines."