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Consider an <math>n</math> bladed wind turbine. Each [[blade]] is separated angularly from a neighbouring blade by <math>360/n</math> degrees. That is, for a 3-bladed wind turbine, the blades are 120 degrees apart.
The [[torque]] acting on the blade is defined as the z-component of <math>\textbf{r}\times\mathbf{F}</math>, where '''r''' is the radius from the axis of rotation (in this case the hub), and '''F''' is the force acting on the blade. If the torque is defined as the z-component of this cross product, then the torque is simply ''rF''<sub>perp</sub> where ''F''<sub>perp</sub> is the force perpendicular to the radius vector, or tangential to the instantaneous velocity of the blade (See figure below)
From the figure above, it can be seen that the torque, ''T'', due to gravitational forces acting on a single blade is given by the following expression:
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The [[covariance]] function of a sum of [[Sinusoidal model|sinusoids]] is itself a sum of sinusoidal functions. Thus, the power spectral density function is a set of Dirac delta functions. The locations of these are at multiples of ''n''. Thus, on a power spectrum, deterministic processes such as gravitational loading manifest themselves as spikes. This can be seen from analysing generator torque.
====Blades====
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