Page 245 - Wind Energy Handbook

STATIONARY BLADE LOADING 219

set according to the turbine class (see Section 5.1.2). Finally, the lifetime cycle counts
obtained for operation in the different wind speed bands are combined and added
to those calculated for start-ups, shut-downs and periods of non-operation.

5.6 Stationary Blade Loading

5.6.1 Lift and drag coefficients

Maximum blade loadings are in the out-of-plane direction and occur when the
wind direction is either approximately normal to the blade, giving maximum drag,
or at an angle of between 128 and 168 to the plane of the blade when the angle of
attack is such as to give maximum lift.
In the absence of data on drag coefficients for airflow normal to the blade,
designers formerly utilized the drag coefficient for an infinitely long flat plate of 2.0,
with an adjustment downwards based on the aspect ratio. Thus, on a typical blade
with a mean chord equal to one fifteenth of the radius, the length to width ratio
would be taken as 30, because free flow cannot take place around the inboard end
of the blade. Following CP3 (British Standard, 1972), the British code for wind
loading on buildings, this would give a drag coefficient of 1.68. More recently, field
measurements have shown that such an approach is unduly conservative, with
drag coefficients of 1.24 being reported for the LM 17.2 m blade (Rasmussen, 1984)
and 1.25 for the Howden HWP-300 blade (Jamieson and Hunter, 1985). The Danish
Standard DS 472 (1992) stipulates a minimum value of 1.3 for the drag coefficient.
The choice of lift coefficient value is more straightforward, because aerofoil data
for low-angle attacks is more generally available and is, in any case, required for
assessing rotor performance. The maximum lift coefficient rarely exceeds 1.6, but
values down to as low as 1.1 will obtain on the thicker, inboard portion of the blade.
The minimum value of lift coefficient of 1.5 specified in DS 472 for the calculation of
blade out-of-plane loads is therefore probably conservative.

5.6.2 Critical configuration for different machine types

It was shown in the preceding section that the maximum lift coefficient is likely to
exceed the maximum drag coefficient for a wind turbine blade, so consequently the
maximum loading on a stationary blade will occur when the air flow is in a plane
perpendicular to the blade axis and the angle of attack is such as to produce
maximum lift. For a stall-regulated machine, this will be the case when the blade is
vertical and the wind direction is 708–808 to the nacelle axis, whereas for a pitch-
regulated machine the blade only needs to be approximately vertical with a wind
direction at 108–208 to the nacelle axis.