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The loads on both [[horizontal-axis wind turbine]]s (HAWTs) and [[vertical-axis wind turbine]]s (VAWTs) are [[cyclic]]; the [[thrust]] and [[torque]] acting on the blades depend on where the blade is. In a horizontal axis wind turbine, both the apparent [[wind speed]] seen by the blade and the [[angle of attack]] depends on the blade's position. This phenomenon is described as [[Rotational sampling in wind turbines|rotational sampling]]. This article will provide insight into the cyclic nature of the loads that arise because of rotational sampling for a horizontal axis wind turbine.
Rotational sampling can be divided into two parts: [[Deterministic system|deterministic]] and [[Stochastic process|stochastic]]. Deterministic processes present themselves as spikes on a [[Power spectral density|power spectrum]], where as stochastic processes spread over a wider [[frequency range]].
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The [[drivetrain]] of a wind turbine comprises the hub, the low speed shaft, the [[Transmission (mechanical device)|gearbox]], the high speed shaft, and the generator. The torque at the hub is strongly influenced by the [[Rotordynamics|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
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)
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