Using the IEC Simple Load Model for Small Wind Turbines - Small Wind Turbines: Analysis, Design, and Application - page 178

M yB ¼ 1

4 C d qU e50 A proj ; B R

ð 9 : 23 Þ

where the drag coefficient C d is taken as 1.5. If the blades are rotating, M yB is due
to the lift created on the blades due to variations in the wind direction 24 :

M yB ¼ 1

6 C l ; max qU e50 A proj ; B R

ð 9 : 24 Þ

If there is no value available for C l,max then a value of 2 is used. Next is the thrust
caused by the wind loading on the blades. For a parked rotor, the analogue of
( 9.14 )is 25

F x shaft ¼ 1

2 NC d qU e50 A proj ; B

ð 9 : 25 Þ

For a spinning rotor 26

F x shaft ¼ 0 : 17NA proj ; B k e50 q U e50

ð 9 : 26a Þ

where 27

k e50 ¼ X max pR = 30U e50 ð Þ ð 9 : 26b Þ

This load case also covers the maximum bending moment on the tower base
due to the thrust loading on the turbine calculated above. It must also include the
wind load on components such as the nacelle and tower, obtained, for all turbine
components, from the same basic equation 28 :

F ¼ 1

2 C f q U e50 A proj

ð 9 : 27 Þ

where A proj is the perpendicular projected area of the component against the wind
and C f is the force coefficient from Table 9.5 . This is the first of only two direct
references to the tower. The second, in Sect. 11.1 of IEC 61400-2, requires that
''support structures shall also meet local codes and regulations…'' This may cause
difficulties because most ''local codes'' use lower partial load safety factors than
IEC 61400-2.

24

(IEC 40).

25

(IEC 41).

26

(IEC 42).

27

(IEC 43).

28

(IEC 44).