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and it is well known that AR partially determines the drag. This obviously cannot
be a lift-induced effect as there is very little, if any, lift when a = 90. A data
correlation that reproduces the flat plate results in Table 7.2 of [
20
] was apparently
proposed by Hoerner:
C
D
;
max
¼
1
:
11
þ
0
:
018 A
ðÞ
for AR\50
¼
2
:
0
ð
4
:
14
Þ
for AR
50
Based on this Viterna and Corrigan proposed—see [
21
],
C
D
¼
C
D
;
max
sin
2
a
þ
B
2
cos a
ð
4
:
15a
Þ
where
cos a
s
B
2
¼
C
D
;
s
C
D
;
max
sin
2
a
s
ð
4
:
15b
Þ
and
2
C
D
;
max
sin 2a
þ
A
2
cos
2
a
sin a
C
L
¼
1
ð
4
:
16a
Þ
where
sin a
s
cos
2
a
s
A
2
¼
C
L
;
s
C
D
;
max
sin a
s
cos a
s
ð
4
:
16b
Þ
and the subscript ''s'' indicates a matching point at low a, often within the stall
region. Despite the wide use of these equations, the author believes that they are
more complex than is warranted by the data, as demonstrated in Figs.
4.5
and
4.6
,
which suggest the effects of Re, aerofoil geometry, and possibly the variations in
turbulence levels, wind tunnels, and measuring techniques. In addition,
Chap. 3
noted that blade element theory assumes the independence of the elements. If this
is correct then the forces on each element cannot depend on the aspect ratio of the
blade.
Chapter 6
analyses turbine starting, which involves high angles of inci-
dence,
using
Eqs.
4.6
,
4.7
,
and
4.14
-
4.16
and
compares
the
predictions
to
measurements.
While moderate aspect ratios are not of much interest for small wind turbines,
small values are.
Chapter 7
shows that the prototype shapes of most tail fins are
slender delta wings or low AR rectangles. That chapter considers their lift and drag
in considerable detail.
4.6.1 Further Reading
A good description of the early NACA work on aerofoil design is found in Abbott
and von Doenhoff [
22
]. All the NACA and NASA reports referred to in this topic
can be downloaded from the NASA Technical Report Server website
http://naca.larc.nasa.gov/search.jsp
. General information on the fundamentals of
aerofoils with aeronautical application can be found in topics such as Bertin and
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