Using the IEC Simple Load Model for Small Wind Turbines - Small Wind Turbines: Analysis, Design, and Application - page 175

When the blades are vertical, the blade inertia, J B , about its axis contributes almost
nothing to the yaw moment of inertia. When the blades are horizontal, blade inertia
contributes significantly to yaw inertia. The significant variation in inertia alters
the maximum loads on the blade and shaft in the manner reflected by the first term
in ( 9.11 ). For N C 3, the inertia varies much less with azimuthal position of the
blades and can usually be ignored as suggested by the simple analysis in Exercise
8.1. This statement implies that ( 9.12 ) approximates both the average and maxi-
mum moment on the shaft of a turbine with three or more blades, whereas ( 9.11 )
gives the maximum moment for a two-bladed turbine. It is also worth noting
that the gyroscopic moment can give rise to a fatigue load, particularly if the tower
has a resonant frequency matching the blade passing frequency or the natural
frequency of yaw. There is no specific load case in the SLM covering tail fin
fatigue.
The derivation of ( 9.11 ) and ( 9.12 ) (and all the SLM equations) is given in
Annex F of IEC 61400-2. The gyroscopic terms are mentioned very briefly in
Burton et al. [ 2 ], and treated in much more detail by Eggleston and Stoddard [ 4 ].
To complicate matters even further: note that the gyroscopic terms do not include
any contributions from the yaw acceleration or the angular acceleration of the
blades. The former makes a contribution to M yB that is out of phase with the
Coriolis term and hence will not alter its magnitude. The latter contributes only to
the lead-lag motion of the blade, that is motion in the direction of the rotation
where most blades are very stiff. Current knowledge of the extra terms is too poor
to justify any attempt at quantification.
The gyroscopic moments acting on the main shaft will be transmitted to the
tower. These loads are discussed in Chap. 10 on tower design.

9.2.3 Load Case C: Yaw Error

This load occurs when the turbine operates with a mean yaw error, as was the case
of the furling turbine described in the previous chapter. For simplicity, this load
case considers a single yaw error of 30. Yaw error causes a bending moment on
the blades according to 15 :

" #

2

M yB ¼ 1

4
3k design

1
k design 8 qA proj ; B C l ; max R 3 X design

1 þ þ ð 9 : 13 Þ

where A proj,B is the projected area of the blades which can be taken as the planform
area, and C l,max is the maximum lift coefficient. If no value is available assume
C l,max = 2.0.

15

(IEC 31).