Momentum Theory Analysis in Hovering Flight

The helicopter, or any other rotating-wing vehicle, must operate in a variety of flight regimes. These include hover, climb, descent, or forward flight. In addition, the helicopter may undergo maneuvers, which may comprise a combination of these basic flight regimes. In hover or axial flight, the flow is axisymmetric and the flow through the rotor

is either upward or downward. This is the easiest flow regime to analyze and, at least in principle, it should be the easiest to predict by means of mathematical models. It has been found, however, that even with modern mathematical models of the rotor flow, accurate prediction of hovering performance is by no means straightforward. Although it must be remembered that the actual physical flow about the rotor will comprise a complicated vortical wake structure, as previously shown in Fig. 2.2, the basic performance of the rotor can be analyzed by a simpler approach that has become known as the Rankine-Froude momentum theory. The momentum theory approach allows the derivation of a first-order prediction of the rotor thrust and power, and the principles also form a foundation for more elaborate treatments of the rotor aerodynamics problem.