The Unsteady Aerodynamics of Turbine Yaw and Over-Speed Protection - Small Wind Turbines: Analysis, Design, and Application - page 147

drive can be used to turn the turbine away from the wind to limit the aerodynamic
loads on the rotor during extreme events such as storms, cyclones, typhoons, or
hurricanes.
On the other hand, most small wind turbines are ''free yaw'' systems, in which
the rotor is located upwind of the generator and tower, and a tail fin keeps the
turbine pointing into the wind. This simpler yaw system reduces the cost of small
turbines, and is easier for the owner to maintain. An alternative to a tail fin is to
have the blades downwind of the tower which can cause ''tower shadow'' effects
and reduce the fatigue life of the blades. Despite their mechanical simplicity, tail
fin aerodynamics can be complex since the yaw rate is now unregulated and at the
mercy of the wind. Free yaw behaviour is largely a function of the tail fin design
especially during starting when the blades provide little yaw stability.
An examination of all the small wind turbines produced over the years would
show a wonderful array of tail fin designs, but most are simply variations of a flat
plate acting as a wing. Swept wing designs are common and their simplest form,
the delta wing, has been the subject of considerable study [ 3 - 5 ]. Aside from being
aesthetically pleasing, easily manufactured, and strong, delta wings have favour-
able and well-known aerodynamic characteristics. For this reason, the theory of
tail fins is developed in terms of delta wing aerodynamics. The most notable
feature of delta wings is their high stall angle, which in the context of a tail fin
suggests high restoring moments on the turbine up to yaw angles of around 408.
Figure 8.2 defines the basic geometry of a delta wing tail fin: The chord is c, the
span is b, and the aspect ratio, AR,is2b/c. Theoretically, the centre of pressure on
a delta wing is 2c/3 from the apex, so the moment distance r, should be measured
from this point to the yaw axis. In practice, 2c/3 is usually small compared to the
tail boom length and can often be neglected. Figure 8.3 shows the lift and drag
characteristics for the delta wing closest in geometry to the tail of the 500 W
turbine in Fig. 6.1 ; note that a delta wing is three dimensional so the lift and drag
are in Newtons. This data was compiled from the experimental studies cited in
Table 8.1 .
There are at least two reasons why high angle data is needed even though a tail
fin should operate near a = 0. First, an actual operation sequence of the 500 W
turbine from Chap. 6 will be presented later to show that high yaw angles occur in
practice, usually at low wind speeds before and during starting. Second, it is
possible to have a failure of the yaw bearing mechanism which may result in
excessive wind loads on a stationary turbine at high incidence. This possibility is
discussed further in Chap. 9 .

Fig. 8.2 Basic geometry of a
delta wing tail fin. r is the
distance from the yaw axis to
the centre of pressure

yaw axis

r

b

x

c