Wind Turbine Blades

Wind Turbines extract energy from the force of the wind on an aerofoil, in this case a turbine blade. The relative motion between the air flow and the turbine blade, is the same as for the aircraft wing, but in this case the wind is in motion towards the turbine blades and the blades are passive so that the external thrust provided by the moving air flow is in the opposite direction to the thrust provided by the aircraft wing. The turbine blades thus experience lift and drag forces, similar to the aircraft wing, which set the blades in motion transferring the wind energy into the kinetic energy of the blades

The turbine blades are connected to a single rotor shaft and the force of the wind along the length of the blades creates a torque which turns the rotor.

As with aircraft wings, the magnitudes of the lift and drag on the turbine blade are dependent on the angle of attack between the apparent wind direction and the chord line of the blade.

The dynamics of wind turbines is however slightly more complex than the dynamics of a simple wing because the direction of the gravitational force on the turbine blade changes with the rotation of the turbine rotor.

In a "theoretical" turbine with a single blade operating with a constant wind force, the magnitude and direction of the lift and drag with respect to the aerofoil profile will be constant throughout the full 360° rotation of the turbine rotor but the direction of the lift with respect to the ground will depend on the position of the rotor. The magnitude of the gravitational force on the blade will also be constant for any position of the rotor but the horizontal position of the centre of gravity of the blade with respect to the centre of the rotor will vary as the rotor turns. The net effect of these forces on the rotor torque depends on the position of the rotor.

·         When the blade is horizontal and moving upwards it is moving against the force of gravity which is pulling the blade downwards so that the net lifting force on the blade and the resulting torque on the rotor is reduced.

·         After 180° rotation of the rotor, the blade is once more horizontal but upside down and moving downwards so that the "lifting force" due to the wind is in the opposite direction and reinforces the downwards gravitational force so that the torque on the rotor is increased.

·         When the blade is vertical, either at the top or the bottom of its cycle, the gravitational force is perpendicular to the lifting force and passes through the centre of the rotor shaft and hence has no effect on the torque which is purely due to lift.

Practical turbines however have multiple blades which balance each other, so that the gravitational effects cancel out and the torque on the rotor is constant.