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power coefficient
that can be achieved with a potential rotor configuration. This was done
previously for a drag-driven translator (Eq. (5-3)), a lifting translator (Eq. (5-5)), the Ran-
kine-Froude actuator disk (Eq. (5-15b)), and the Glauert optimum HAWT rotor (Table 5-2).
With the added effects of finite blade number,
B
, and realistic airfoil lift and drag properties,
C
L
and
C
D
, estimates of maximum power coefficient are best done with empirical equations
such as the following [Wilson
et al.
1976]:
l
B
0.67
1.48 + (
B
0.67
- 0.04) l + 0.0025 l
2
1.92 l
2
B
1 + 2 l
B
D
/
L
(5-40)
C
P
,max
= 0.593
-
where
D
/
L
= ratio of
C
D
to
C
L
at the design angle of attack; drag-to-lift ratio
Figure 5-17 illustrates the application of Equation 5-40. The Glauert ideal HAWT perfor-
mance (
B
® ¥ and
D
/
L
= 0; data from Table 5-2) forms an upper bound.
Figure 5-17. Typical effects of the number of blades and the design drag-to-lift ratio on
the maximum power coefficient of a HAWT.
Equation (5-40); [Wilson
et al.
1976]
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