The theories and results described in this chapter have shown that using certain assumptions and approximations, the application of the conservation laws of aerodynamics in an integral form has permitted an understanding of the factors that influence the basic performance of the helicopter rotor. The so-called momentum theory has allowed a quantification of the thrust and power of a lifting rotor, and it has been shown how these quantities are related to the downwash (inflow) velocity through the rotor. The momentum method has permitted a preliminary evaluation of rotor performance in hover, climb, and descent. It has been shown that the disk loading is a kev Darameter governing rotor Derformance. and
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the need for a low disk loading is essential to give a helicopter good hovering efficiency. The ideas of power loading and figure of merit have been introduced as quantities that can help in designing the rotor as well as to compare the relative efficiency and performance of two different rotor designs.
To account for various nonideal effects that have their origin in the viscosity of the air, it has been shown how the basic momentum theory can be modified empirically to give a methodology that is in substantially better quantitative agreement with experimental measurements of rotor performance, while still retaining the simplicity of the overall method. The basic momentum theory has also been extended to forward flight, for which numerical solutions are generally required to solve for the inflow through the rotor disk. These numerical techniques have been examined, along with the identification of limitations in their use. Finally, the ideas embodied in momentum theory have been extended to coaxial and partly overlapping rotors such as tandems. It has been shown why there are rotor-on-rotor interference effects associated with these designs, which can reduce the net aerodynamic performance of the rotor system.
Despite the advantages of momentum theory in providing clarity of insight into the basic aspects of the helicopter rotor problem, it has many limitations. For example, it provides no information about the distribution of loads over the blade or as to how the rotor blades should be designed (i. e., the planform, twist, thickness distribution, airfoil sections.) to produce a given performance. Furthermore, the effects of blade motion (i. e., flapping) on the rotor behavior have not yet been considered. However, these are limitations that can be overcome by using more advanced methods based on a blade element analysis of the rotor blades, which is considered in the next and subsequent chapters.