The Unsteady Aerodynamics of Turbine Yaw and Over-Speed Protection - Small Wind Turbines: Analysis, Design, and Application - page 158

Typical yaw data when the turbine in Fig. 6.1 is producing power is shown
in Fig. 8.10 —Wright [ 21 ] shows much more data for various tail fin shapes and
a range of other parameters. Generally, the simulations (even including an
extra linear term for the coning) are less accurate when the turbine is producing
power,

which is hardly surprising in view of the list immediately above

(Fig. 8.10 ).
In discussing the data in Fig. 8.7 , it was suggested that the damping ratio of the
tail fin whose details are in Table 8.1 , was too low. Examples 8.4 and 8.6 show
that it is difficult to alter the damping ratio by changing the planform. The one
possibility that has not yet been considered is to alter the tail boom length.

Example 8.5 Can changing the tail boom length influence the damping?
Attempted Answer Equation 8.11b gives f * b(r ? c/3) 2 /(rI) 1/2 if A and K
remain constant. Keeping the tail fin unchanged but increasing r can only be of
limited help as for r/c 1, I * r 3 (see Exercise 8.6), and f becomes independent
of r.

8.6 High Yaw Rates

Equation 8.1 shows that gyroscopic loads in the rotor shaft and blade root are
proportional to the product of rotor speed and yaw rate. This suggests that one way
to summarise a large amount of yaw data is to plot yaw rate against rotor speed and
wind speed as done in Fig. 8.11 for the 500 W turbine. Chapter 9 discusses the
IEC SLM which specifies a maximum yaw rate dependent on rotor area. For the
500 W turbine, this is ±2.99 rad/s =±171/s which is assumed to occur at
the design X. The SLM maximum gyroscopic moment depends on the product of
these two angular speeds. Figure 8.11 shows positive and negative extreme yaw
rates in excess of this value.
Up to wind speed of around 10 m/s, the maximum yaw rate increases almost
linearly with wind speed, possibly as a consequence of the linear relationship

12 600

10 400

6

4

200

0 0

90

60

30

0

-30

-60

-90

0 20 40 60 80 100 120

time (s)

Fig. 8.10 Comparison of computed (thick red line) and actual (thick blue line with more
variations than the computed) yaw behaviour when turbine is producing power. The thin black
line is the wind direction. On the upper part of the figure the rotor speed is above the wind speed
(which starts at nearly 8 m/s) for most of the record. Figure from Wright [ 21 ]