# Pitch

Pitch (or blade pitch or pitch angle or blade pitch angle) is the rotational angle (turning) of a blade relative to the rotor plane (i.e. the pitch does not vary along a blade, as opposed to twist and structural twist that typically varies along the blade). As a rule-of-thumb, the blade pitch angle is approx. the angle of the chord of the blade's outer most station relative to the rotor plane. This assumes that the convention of setting the twist angle of the outer most station to zero or approx. zero.

The pitch affects the aerodynamics because it affects the angle between the chord line of any station and the rotor plane. Note that to find this angle the twist of the station must be taken into account in addition to the blade pitch. For a deformed case (i.e. not the initial, unstrained, unstressed case), the structural pitch must also be added to blade pitch and station twist.

Traditionally, up to approximatively 2005, wind turbines used fixed pitch, i.e. the blades were bolted to the hub with a fixed pitch. Thus, the pitch did not change during operation.
To our knowledge, all modern industrial size wind turbines are Variable Speed Pitch Regulated (VSPR), i.e. the pitch angle is changed to control (reduce) the power transferred from the wind to the generator at above rated wind speeds

To find the (undeformed) angle of the chord of a station along a blade, the twist of that station must be added to the blade pitch. The twist is given in the blade definition (see Blade shape file), see Twist.

In a physical wind turbine, pitching a blade has two effects;
1.  Aerodynamic: The angle-of-attack of the stations along the blade are changed and thus the loading (lift and drag) on each and every station changes. The chnage is different from station to station.
2. Structural dynamics: The structure of the blade is rotated and changing the structural (and dynamical) properties and behavior of the WT.

Pitch angle vs. twist angle vs. structural twist angle
1. Pitch angle is a blade parameter and can change during a simulation if the WT is pitch regulated. For a pitch regulated WT, the pitch angle of a blade is changed by a motor, called the pitch actuator.
2. Twist angle is a station parameter and changes along the blade. It is constant during a simulation. Twist is an essential parameter when a blade is defined (see Blade shape file).
3. Structural twist angle is a FEM frame element parameter that changes along the blade and changes during a simulation due to torsional deflection/deformation of the blade. It is normally referenced to the blade root (see Blade structure file).

Implementation of aerodynamic pitch and structural pitch in Ashes:
In a physical WT the aerodynamic pitch and the structural pitch are by definition identical and it must have pitch actuator motors. However, this is not necessarily the case in Ashes, the pitch system can be modelled with or without pitch actuators.
• If the pitch system is modelled with pitch actuators, then the Ashes model behaves as the physical WT and the aerodynamical pitch and the structural pitch are identical.
• If there are no actuators then there is no structural pitch. However, there will still be a (small) structural twist.
Note: if the Ashes model has stiff blades (see Analysis), there will be no structural and no structural twist, since the blades are not modelled in FEM. There will only be aerodynamic pitch