Page 325 - Wind Energy Handbook

NACELLE LOADING 299

It is instructive to compare the moments acting on the nacelle due to determi-
nistic loading for three-bladed and two-bladed machines. The fluctuations of out-
of-plane root bending moment due to wind shear and yaw misalignment are
approximately proportional to the cosine of blade azimuth for an unstalled blade.
Substituting M Y1 ¼ M 0 cos ł, M Y2 ¼ M 0 cos(ł 2ð=N) etc into Equations (5.122)
and (5.123) yields M YN ¼ 1:5M 0 and M ZN ¼ 0 for a three-bladed machine,
whereas the corresponding results for a rigid hub two-bladed machine are
M YN ¼ M 0 (1 þ cos 2ł) and M ZN ¼ M 0 sin 2ł cos ç. Thus the moments on the
nacelle are constant for a three-bladed machine, but continually fluctuating with
amplitude M 0 for a rigid hub two-bladed machine. Parallel results are obtained
for M Y1 ¼ M 0 sin ł which approximates to the out-of-plane root bending moment
due to shaft tilt – again for an unstalled blade. The full comparison is given in
Table 5.6 below.
It is clear that the moments acting on the nacelle due to deterministic loading are
much more benign for a three-bladed rotor than for a two-bladed rotor with rigid
hub.
In the case of three-bladed machines, the standard deviation of shaft bending
moment due to stochastic rotor loading is independent of the rotating axis chosen
(Section 5.10.3), so the standard deviation of the resulting moment on the nacelle
will take the same value about both the nacelle y- and z-axes.

5.11.2 Cladding loads

Except in the case of sideways wind loading, cladding loads are not usually of great
significance. They may be calculated according to the rules given in standard wind
loading codes. For sideways wind loading, a drag factor of 1.2 will generally be
found to be conservative.

Table 5.6 Comparison Between Nacelle Moments due to Deterministic Loads for Two- and
Three-bladed Machines
Nacelle moments resulting from Nacelle moments resulting from
out-of-plane blade root bending out-of-plane blade root bending
moment fluctuations due to wind moment fluctuations due to shaft
shear and yaw misalignment tilt approximated by: M Y1 ¼ M 0 sinł,
approximated by: M Y1 ¼ M 0 cos ł, M Y2 ¼ M 0 sin(ł 2ð=N) etc
M Y2 ¼ M 0 cos (ł 2ð=N) etc

Nacelle nodding Nacelle yaw Nacelle nodding Nacelle yaw moment,
moment, M YN moment, M ZN moment, M YN M ZN
Three-bladed 1:5M 0 Zero Zero 1:5M 0 cos ç
machine
Two-bladed, M 0 (1 þ cos 2ł) M 0 sin 2ł cos ç M 0 sin 2ł M 0 (1 cos 2ł)cos ç
rigid hub
machine