Page 102 - Wind Energy Handbook

76 AERODYNAMICS OF HORIZONTAL-AXIS WIND TURBINES

induction factor is lower in the presence of drag than without because the blade
drags the fluid around in the direction of rotation, opposing the general rotational
reaction to the shaft torque.
From the torque/angular momentum Equation (3.50) the blade geometry para-
meter becomes
2 2
N c 4º ì a9(1 a)
ºC l ¼ (3:70)
2ð R W C d
(1 a) ºì(1 þ a9)
U 1 C l
Figure 3.24 compares the blade geometry parameter distributions for zero drag
and a lift/drag ratio of 40 and, as is evident, drag has very little effect on blade

0.8
Blade geometry parameter 0.48
0.64

0.32

0.16
0
0 1 2 3 4 5 6 7 8 9 10
Local speed ratio
Zero drag
L/D = 40
Figure 3.24 Span-wise Variation of the Blade Geometry Parameter with and without Drag

40

32
Inflow angle 24

16

8

0
0 1 2 3 4 5 6 7 8 9 10
Local speed ratio
Zero drag
L/D = 40
Figure 3.25 Variation of Inflow Angle with Local Speed Ratio with and without Drag