Rotating Aerofoils and Flaps

Abstract

Some aerodynamic properties of auto-rotating or forcibly rotated cylinders and aerofoils were investigated even prior to the beginning of aviation and, somewhat surprisingly, it was Maxwell who wrote the earliest (1853) known paper on the subject. That entirely non-mathematical paper tried to explain the curious behaviour of an oblong card which, left to fall freely in the air, started immediately to rotate about its longitudinal axis while deviating from the vertical in the horizontal direction perpendicular to the axis of rotation. Steady “terminal” conditions are soon reached in which speed, rate of rotation, and path angle, are all constant. This simple experiment revealed at once two important properties of an “aerofoil of large aspect ratio“: (1) ability to autorotate, and (2) ability, when rotating in an air stream, to create an asymmetrical pressure distribution resulting in a force normal to both axis of rotation and mainstream velocity. About the same time, the famous “Magnus effect” on rotating circular cylinders was discovered, but it was not before the Kutta-Joukowski theory of aerodynamic lift appeared some 50 years later that a more general understanding of the “lift through circulation” principle became possible. Magnus effect found important applications in ballistics, and one serious (although not economically successful) attempt was made to apply it in maritime navigation (Flettner's rotorship). As to the use of aerofoils rotating about a horizontal transverse axis for generating lift in aircraft, very little indeed has been done apart from some early publications. The concept of a wing with rotating flap as a lift-augmenting or control device arose from suggestions by Riabouchinsky and Chappedelaine, and there were some early French and German patents based on this concept. However, the matter was soon abandoned. It was taken up again during the last war in Germany, where some interesting experimental and theoretical research was done on a small scale. The subject has apparently never attracted any attention in Britain until 1957 when a short review of the existing information was given by Crabtree. This was followed by the lecturer's own effort which concentrated on the relevant mathematical theory.