This chapter has described some basic aerodynamic characteristics of airfoil sections and has provided a basis for assessing the potential impact of airfoil design and selection on helicopter rotor performance. Methods of geometrically defining airfoils have been described following the NACA approach of combining camberlines and thickness envelopes. This is justified because many of the airfoils used on current helicopter rotors have their origin in the NACA sections. Also, the ideas of combining loading distributions associated with thickness and camber allow the primary effects of geometric shape on the chordwise loading and overall aerodynamic characteristics to be assessed. However, the primary discussion in this chapter has been centered around published measurements of static airfoil characteristics. These results, while from more than one primary source, allow a good basis from which to compare airfoil behavior, and for the most part are considered relatively unbiased by the wind tunnel test facility or by testing techniques. For these latter reasons, it has not been considered fruitful to dwell on the relative merits of airfoils designed by the various competing helicopter manufacturers.
Airfoils designed for helicopter applications have traditionally been obtained through a long evolutionary process, in which various levels of theory and experimental measurements have been combined in the pursuit of airfoil shapes with higher values of maximum lift, better lift-to-drag ratios, lower pitching moments, and higher drag divergence Mach numbers. It has been shown that, in general, these requirements are conflicting, making the design of general purpose rotor airfoils extremely challenging. Instead, various families of airfoils have been developed and optimized to meet the specific needs of different parts of the rotor blade. For example, airfoils with high camber and moderate thickness, which give high values of maximum lift, are used between 60 and 85% of blade radius. Much thinner airfoils, perhaps even those with supercritical-like shapes, give relatively high drag divergence Mach numbers and have been designed for the tip region of the blade (>85%/?). The use of different airfoils along the blade is made easier today because of computer-aided design and composite manufacturing capability, which involves only small additional costs over a blade made with a single airfoil section.
The principles of integrating surface pressure and shear stress distributions to obtain lift, drag and pitching moment coefficients on airfoils have been reviewed. Representative airfoil characteristics have been discussed, along with the limits of conventional linearized theories in predicting this behavior. Because of the importance of low pitching moments in the design of helicopter airfoils, the principles of defining the aerodynamic center and center of pressure have been reviewed. The influence of Reynolds number and Mach number on airfoil characteristics have been highlighted. Although these parameters have both dependent and interdependent effects on airfoil behavior, it has been possible to isolate the basic effects and assess their significance on maximum lift and other important airfoil characteristics. Because the design of airfoils with high values of maximum lift can result in smaller and lighter rotors with lower solidity, the geometric shape of the airfoil and other factors affecting maximum lift have been discussed in detail.
While an improved understanding of airfoil characteristics will usually lead to an improved analysis capability of existing rotor designs, and may lead to new rotors optimized for greater performance in both hover and high-speed forward flight, the performance of the rotor cannot be completely parameterized on the basis of static (steady) considerations alone. Therefore, in the next two chapters, the important role of unsteady aerodynamics on the problem of airfoil behavior are discussed.