Page 193 - Wind Energy Handbook

THE LIFT COEFFICIENT 167

behaviour but a much greater improvement can be obtained by giving thickness to
the aerofoil together with a well-rounded leading edge.

A3.8 The Lift Coefficient

The lift coefficient is defined as

Lift
C l ¼ (A3:8)
1 2
rU A
2
U is the flow speed and A is the plan area of the body. For a long body, such as an
aircraft wing or a wind turbine blade, the lift per unit span is used in the definition
and the plan area is replaced by the chord length.

Lift=unit span r(ˆ 3 U) rðUc sin ÆU
C l ¼ ¼ ¼ ¼ 2ð sin Æ (A3:9)
1 2 1 2 1 2
rU c rU c rU c
2 2 2
In practice,

C l ¼ a 0 sin Æ (A3:10)

2.2

1.8

1.4
Without boundary layer
1
With boundary layer
-C p
0.6

0.2

-0.2

-0.6

-1
0 0.2 0.4 0.6 0.8 1
x/c
Figure A3.18 C l Æ Curves for a Symmetrical Aerofoil