Page 375 - Wind Energy Handbook

TEETERING 349

times the yaw moment due to out-of-plane load. As e is typically about one tenth
of the tip radius, it is seen that the yaw moments due to in-plane loads are at
least an order of magnitude smaller than those due to out-of-plane moments, so
that the introduction of the teeter hinge results in a very significant reduction.

(e) Tower. The fatigue loadings due to the M Y moment and M Z torque will clearly
be significantly reduced at the top of the tower if the rotor is teetered, but the
effect will be negligible towards the base where thrust loads dominate the
moments.

6.6.2 Limitation of large excursions

Some limitation on teeter excursions has to be provided, if only to prevent collision
between the blade and the tower. If the teeter hinge is located close to the axis of the
blades, with the low-speed shaft passing through an aperture in the wall of the hub
shell (see Figure 6.6), then the maximum teeter excursion is governed by the size of
the aperture.
The teeter response to deterministic and stochastic loads is considered in Section
5.8.8. Although it is evident that a permitted teeter angle range of the order of 58
will accommodate the vast majority of teeter excursions during normal operation, it
is usually impracticable to accommodate the largest that can occur. Hence, in order
to minimize the occurrence of metal-to-metal impacts on the teeter end stops,
buffers incorporating spring and/or damping elements normally have to be fitted.
These also perform an important role in limiting the much larger teeter excursions
that would otherwise arise during start-up and shut-down, when the centrifugal
restoring moment is reduced.

6.6.3 Pitch–teeter coupling

As described in Section 5.8.8, the magnitude of teeter excursions can be reduced by
coupling blade pitch to teeter angle, in order to generate an aerodynamic restoring
moment proportional to the teeter angle. This can be done simply by setting the
teeter hinge at an angle, known as the Delta 3 angle, to the perpendicular to the
rotor axis. Alternatively, on pitch-controlled machines, pitch–teeter coupling can be
introduced by actuating the blade pitch by the fore-aft motion of a rod passing
through a hollow low-speed shaft (see Figure 6.6).

6.6.4 Teeter stability on stall-regulated machines

At first sight, it might be thought that the teeter motion of a stalled rotor would be
unstable because of negative damping resulting from the negative slope of the C l –Æ
curve post-stall. However, two-dimensional aerodynamic theory is a poor predictor
of post-stall behaviour, and it has proved possible to design teetered rotors that are
stable in practice, such as the Gamma 60 (Falchetta et al., 1996) and Nordic 1000