Page 264 - Wind Energy Handbook

238 DESIGN LOADS FOR HORIZONTAL-AXIS WIND TURBINES

loads in very low wind is approximately equal to the maximum rearward out-of-
plane moment due to the thrust loading in combination with centrifugal loads
during operation in rated wind.

Gyroscopic loads

When an operating machine yaws, the blades experience gyroscopic loads perpen-
dicular to the plane of rotation. Consider the point A on a rotor rotating clockwise
at a speed of Ù rad=s, as illustrated in Figure 5.16. The instantaneous horizontal
velocity component of point A due to rotor rotation is Ùz, where z is the height of
the point above the hub. If the machine is yawing clockwise in plan at a speed of
¸ rad=s, then it can be shown that point A accelerates at 2Ù¸z towards the wind,
assuming the rotor is rigid. Integrating the resulting inertial force over the blade
length gives the following expression for blade root out-of-plane bending moment

A Ω z 2 Ω z Λ
ψ
Z
Ω

Ω z
2 Ω z Λ

Ω = Speed of rotor rotation
Λ = Speed of yawing
Λ

Figure 5.16 Gyroscopic Acceleration of a Point on a Yawing Rotor