2 - Rotor aerodynamics in axial flight

Abstract

One of the most important aerodynamic problems of the helicopter is the determination of the loading of rotor blades. For this, it is essential to know the local components of airflow at any station along the blade, which requires knowledge of the air velocity induced by the lift of the blades. While developing the analysis, reference is made to well-known and fundamental theorems, laws, and equations in aerodynamics. Performance of the rotor can be gained from a simple flow pattern, which can be treated by momentum methods. The method is known as the classical actuator disc theory. The “ideal” blade loading does not occur in practice and the vortex wake does not conform to the simple helical surface. The vortex ring state occurs when the rate of descent is of the same order as the induced velocity in hovering flight. Recovery is by reducing the collective pitch and attaining a forward flight velocity component, moving the rotor into unrecirculated air. An element of a rotor blade is regarded as an elementary aerofoil. The lift characteristics of the blade can be used to obtain a relationship between thrust and induced velocity, enabling both to be evaluated. The calculations follow closely the standard methods of aerofoil theory but the rotor analysis is simplified.