Page 198 - Wind Energy Handbook

172 AERODYNAMICS OF HORIZONTAL-AXIS WIND TURBINES

angle of attack is no longer zero; zero lift occurs at a small negative angle of attack
of approximately 48.
The centre of pressure, which is at the 1 4 chord position on symmetrical aerofoils
1
lies aft of the chord position on cambered aerofoils and moves towards the trailing
4
edge with increasing angle of attack. However, if a fixed chordwise position is
chosen then the resultant force through that point is accompanied by a pitching
moment (nose-up positive, by convention). If a pitching moment coefficient is
defined as

Pitching=unit span
C m ¼ (3:12)
1 2
rU c
2
then there will be a position, called the aerodynamic centre, for which dC m =dC l ¼ 0.
1
Theoretically, the aerodynamic centre lies at the chord position and is close to this
4
point for most practical aerofoils. The value of C m depends upon the degree of
camber but for the NACA4412 the value is 0:1, note that pitching moments are
always negative in practice (nose down) despite the sign convention. Above the
stall there is no aerodynamic centre, as defined, and so the pre-stall position
continues to be used to determine the pitching moment coefficient, which then
becomes dependent upon Æ.