Aerofoils: Lift, Drag, and Circulation - Small Wind Turbines: Analysis, Design, and Application - page 57

Chapter 4
Aerofoils: Lift, Drag, and Circulation
4.1 Introduction
The analysis in Chap. 3 assumed that blade elements behave as aerofoils. Thus
wind turbine thrust and power depend upon the lift and drag coefficients, C l and C d
respectively, of the aerofoil sections that comprise each blade. For a great many
aerofoils, these coefficients are known from wind tunnel investigations, and, at
least in principle, can be used immediately for power and thrust calculations.
Although computational analysis is routine for aerofoils, the author's opinion is
that computed C l and C d results are not to be trusted, particularly at the low
Reynolds numbers that characterise small wind turbines. Some experimental data
are also not to be trusted because of poor quality equipment or test procedures,
e.g. [ 1 ]. It is important to attempt to establish the veracity of any aerofoil data used
for analysis or design.
This chapter considers the basic features of aerofoils and emphasises aspects of
their performance that are significant for small wind turbine application. These are
behaviour at high angles of attack and low Reynolds number, Re, as well as the
relation between lift and circulation. A very good introduction to low Re aero-
dynamics at low angles is Chap. 2 of [ 2 ].
It is important to remember that an aerofoil is two-dimensional and that, as
indicated in Chap. 3 , there are always assumptions involved in applying two-
dimensional data to rotating, complex three-dimensional geometries such as wind
turbine blades. Some of these assumptions will be considered in the next chapter.
4.2 Geometry and Definition of Aerofoils
Figure 4.1 shows four members of the NACA ''four digit'' series. NACA—the
U.S. National Advisory Committee on Aeronautics—was the forerunner to NASA,
and was very active in aerofoil development after the First World War. The four