і lie uiauc сіоніст uicuiy dsz і; is a puwcnui шиї iur me aeruuynaiiiic analysis ui helicopter rotors. It forms the basis for nearly all modern computational methods used for rotor performance, airloads, and aeroelastic analyses. The basic ideas consist of representing the airloads on 2-D sections of the blades, and integrating their effect to find the performance of the rotor as a whole. This allows tremendous flexibility in the rotor analysis and also allows the effects of airfoil shape to be examined, along with the effects of the Reynolds number and the Mach number, and even some elementary effects associated with nonlinear aerodynamics and stall.
On the basis of certain assumptions, it has been shown how the combined blade element momentum theory (BEMT) can provide analytic results about how to design the rotor in terms of optimum blade planform and blade twist distribution to enable maximum hovering efficiency. Whereas the simple momentum theory shows that the rotor should be designed for low disk loading, the blade element approach allows the trade-offs associated with the interrelated effects of disk loading, blade tip speed, blade loading, blade twist, and blade planform to be examined. While the BEMT theory is by no means complete, it paves the way
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to help design the rotor from the onset that gives these methods great practical utility, and they can also be used as check cases for other and more advanced types of methods.
In forward flight, the BET allows for the calculation of the nonaxisymmetric airloads over the rotor disk. Besides the need to account for the effects of blade flapping motion (which are considered next in Chapter 4), this chapter has also introduced the ideas that more accurately representing the effects of the rotor wake is one key to the successful use of the BET. The use of other than uniform inflow models provides a better representation of the effects of the rotor wake and a better overall physical picture of the rotor aerodynamics problem. While inflow models are not completely rigorous, especially at low forward flight speeds, they give
reasonable descriptions of the induced velocity field over the rotor disk and may be well suited for the analysis of many rotor problems. Their computational simplicity allows for easy integration into blade element based rotor models and may allow for the inclusion of unsteady aerodynamic effects as well (see Chapter 8). However, it must be remembered that in many situations where the individual tip vortices come close to the disk (especially in low speed forward flight, during maneuvers, or in descents) the induced velocity distribution is considerably more complicated than can be prescribed by these simple inflow models. The use of prescribed – or free-vortex methods provides the fidelity necessary under these conditions, albeit at much greater computational expense (see Chapter 10).
However, before the rotor problem in forward flight can be fully enunciated, it is necessary to consider properly the effects of the rotor blade motion on the aerodynamic forces. Because the aerodynamics and blade motion are intrinsically coupled, they must be solved simultaneously as a system. This then allows the calculation of rotor trim, that is, the control inputs required to orient the rotor in a direction to meet vehicle lift, propulsion, and control requirements. These issues are considered in the following chapters.