The Unsteady Aerodynamics of Turbine Yaw and Over-Speed Protection - Small Wind Turbines: Analysis, Design, and Application - page 157

is little information on the Re-dependence of lift and drag for low AR flat plates.
At low a, the centre of pressure on a rectangular wing is approximately 0.2c.Itis
reasonable to assume from these results that tail fin shape has only a second order
effect on tail fin performance. Nevertheless, it would be highly desirable to have
lift, drag, and centre of pressure data for more possible tail fin shapes, and to
explore their yaw behaviour in experiments like that shown in Fig. 8.5 .
Example 8.4 In terms of increasing the damping ratio of the tail fin, is it better to
use an arrow or a diamond shape rather than a delta wing?
Attempted Answer The data fit in Fig. 8.9 gives information on the variation in K 1
in Eq. 8.8 with shape, but give nothing on K 2 . Negative indentation increases the
lift slope K 1 , which, apparently paradoxically, decreases the damping by
Eq. 8.11b , unless the associated reduction in I counteracts this. A similar can-
cellation occurs the other way; a positive indentation reduces K 1 but increases I.At
this point, the best advice is to set up a wind tunnel test similar to that shown in
Fig. 8.5 and measure the response of the new shape. Fortunately these tests are
straightforward.
8.5 Rotor Effects on Yaw Performance
The contribution to yaw moments from the aerodynamic forces on the rotor is
difficult to determine, and a satisfactory understanding does not yet exist. The
thrust and the centre of thrust during yaw determine the moment on the rotor.
Standard blade element calculations suggest that in the absence of azimuthal
variations in inflow, no yaw moment is generated by an unconed rotor, that is a
rotor whose blades are not bent in the wind direction, either by the aerodynamic
loads or by design. Coning the blades tends to stabilise the rotor in yaw much like
dihedral gives roll stability to aircraft wings [ 19 ]. Unpublished BET calculations at
the University of Newcastle assuming azimuthally-uniform inflow gave a steady
restoring moment on a coned 5 kW rotor that was linear in yaw angle below 608.
This is an interesting result, because it implies the linear second order yaw
behaviour also applies to a turbine when producing power. Coning can occur either
by the downwind deflection of flexible blades under load, or by pre-setting. By
combining the BET results with USB, the main change due to coning is to increase
the damping which is in agreement with the measurements of Bechly et al. [ 4 ].
Other influences on rotor yaw moment include the following [ 20 ]:
• Dynamic stall—hysteresis in the aerodynamic performance of the blades (par-
ticularly in stall) as they rotate
• Skewed wake induction effects
• Vertical wind component or rotor tilt
• Vertical and horizontal wind shear
• Turbulence—a combination of the above wind conditions varying randomly in
time