## 5 HAWT Blade Design
### Aerodynamics
The classification shows the geometric profile of a NACA aerofoil where the 1 st digit refers to maximum chamber to chord ratio, 2 nd digit is the camber position in tenths of the chord and the 3 rd & 4 th digits are the maximum thickness to chord ratio in percent [24]. The emergence of wind turbine specific aerofoils such as the Delft University [23], LS, SERI-NREL and FFA [6] and RISO [26] now provide alternatives specifically tailored to the needs of the wind turbine industry. The angle of attack is the angle of the oncoming flow relative to the chord line, and all figures for \(C_{L}\) and \(C_{D}\) are quoted relative to this angle. The use of a single aerofoil for the entire blade length would result in inefficient design [19]. Each section of the blade has a differing relative air velocity and structural requirement and therefore should have its aerofoil section tailored accordingly. At the root, the blade sections have large minimum thickness which is essential for the intensive loads carried resulting in thick profiles. Approaching the tip blades blend into thinner sections with reduced load, higher linear velocity and increasingly critical aerodynamic performance. The differing aerofoil requirements relative to the blade region are apparent when considering airflow velocities and structural loads (Table 6). An aerodynamic phenomenon known as stall should be considered carefully in turbine blade design. Stall typically occurs at large angles of attack depending on the aerofoil design. The boundary layer separates at the tip rather than further down the aerofoil causing a wake to flow over the upper surface drastically reducing lift and increasing drag forces [6]. This condition is considered dangerous in aviation and is generally avoided. However, for wind turbines, it can be utilised to limit the \begin{table}
\begin{tabular}{l c c c} \hline \hline \multirow{2}{*}{**Parameter**} & \multicolumn{3}{c}{**Blade Position (Figure 2)**} \\ \cline{2-4}  & **Root** & **Mid Span** & **Tip** \\ \hline Thickness to chord ratio (\%) (\(\frac{d}{c}\) Figure 2) & \(>\)27 & 27–21 & 21–15 \\ Structural load bearing requirement & High & Med & Low \\ Geometrical compatibility & Med & Med & Med \\ Maximum lift insensitive to leading edge roughness & & High \\ Design lift close to maximum lift off-design & & Low & Med \\ Maximum CL and post stall behaviour & & Low & High \\ Low Aerofoil Noise & & High \\ \hline \hline \end{tabular}
\end{table} 
Table 6: The aerofoil requirements for blade regions [26]. maximum power output to prevent generator overload and excessive forces in the blades during extreme wind speeds and could also occur unintentionally during gusts. It is therefore preferable that the onset of the stall condition is not instantaneous for wind turbine aerofoils as this would create excessive dynamic forces and vibrations [1]. The sensitivity of blades to soiling, off design conditions including stall and thick cross sections for structural purposes are the main driving forces for the development of wind turbine specific aerofoil profiles [1,26]. The use of modern materials with superior mechanical properties may allow for thinner structural sections with increased lift to drag ratios at root sections. Thinner sections also offer a chance to increase efficiency through reducing drag. Higher lift coefficients of thinner aerofoil sections will in turn lead to reduced chord lengths reducing material usage [Equation (3)].