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=== Power spectral density functions ===
==== Drivetrain components ====
The drive train of a wind turbine comprises the hub, the low speed shaft, the gearbox, the high speed shaft, and the generator. The torque at the hub is strongly influenced by the rotor dynamics. The instantaneous hub torque is found by summing all the torques from all the blades of the wind turbine at any instant in time.
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.
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The covariance function of a sum of 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 ====
For analysis of torque on a single blade, the spikes occur at <math>k'\Omega_0</math> where ''k''' is 1,2,3,..<ref>Aerodynamics of Wind Turbines, Martin O. L. Hansen</ref> This can be seen from taking the autocovariance of equation 1, and then taking the Fourier transform of this result.
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