THE conception of the rotating wing for the sustentation of aircraft has been known for the past few centuries (ref. 1), but not until recently has practical success been achieved. Modern developments are due principally to Juan do la Cierva (1895–1936), whose rotaplane, or “autogiro,” was the first successful aircraft with rotating wings. Although the patent literature on the subject is extensive, relatively little attention has been devoted to the underlying physical principles, a discussion of which will be given in the following pages.