Page 65 - Fluid Power Engineering

Aerodynamics of W ind T urbine Blades 43

R
Lift Lift R

α Drag
ν ν Drag

FIGURE 4-3 Symmetrical airfoil with positive angle of attack and a
nonsymmetrical airfoil with zero angle of attack. Air flow around both types of
airfoil result in lift and drag. R is the resultant aerodynamic force.

increases to the free-stream speed and pressure decreases to free-
stream static pressure.
Since the airfoil is symmetric, the speed and pressure are identical
on the upper and lower surface of the airfoil. There is no imbalance
of force along the y-axis. Since the fluid is inviscid, there is no friction
force and the static pressure along the x-axis is equal and opposite.
When the airfoil is tilted at an angle to the fluid flow, as shown
in Fig. 4-3, then there is an imbalance in the pressure along the y-axis
resulting in a lift force. In an ideal fluid, the pressure remains balanced
along the x-axis and, therefore, there is no net force along the x-axis.
α is called the angle of attack.
Whentheairfoilisnotsymmetricalandtheuppersurfaceiscurved
more than the lower surface, then a lift force occurs because the pres-
sure decrease and speed increase in the upper surface is larger than
the pressure decrease and speed increase in the lower surface. As a
convention, lift force is perpendicular to the direction of wind and
drag force is parallel to the direction of wind. Positive α or nonsym-
metrical airfoils cause airplanes to fly and wind turbines to produce
energy. Each type of blade has an optimal value of α that produces
maximum lift and minimal drag (see Fig. 4-4). This is discussed in
greater detail later in the chapter.

Maximum lift
Lift force Stall Drag force
Lift R

α Drag
ν Minimum drag
Angle of attack, α Angle of attack, α

FIGURE 4-4 Relationship between lift and drag force and angle of attack for
an airfoil.