Lift and Drag
Lift and Drag
Airflow over any surface creates two types
of aerodynamic forces— drag forces, in
the direction of the airflow, and lift forces,
perpendicular to the airflow. Either or both
of these can be used to generate the forces
needed to rotate the blades of a wind
turbine. Click here for a simple way to
demonstrate lift and drag using a piece of paper.
Drag-based wind turbine
In drag-based wind turbines, the force of the wind pushes against a surface, like an open sail. In fact, the earliest wind turbines, dating back to ancient Persia, used this approach. The Savonius rotor is a simple drag-based windmill that you can make at home (Figure 1). It works because the drag of the open, or concave, face of the cylinder is greater than the drag on the closed or convex section.
Lift-based Wind Turbines
More energy can be extracted from wind using lift rather than drag, but this requires specially shaped airfoil surfaces, like those used on airplane wings (Figure 2). The airfoil shape is designed to create a differential pressure between the upper and lower surfaces, leading to a net force in the direction perpendicular to the wind direction. Rotors of this type must be carefully oriented (the orientation is referred to as the rotor pitch), to maintain their ability to harness the power of the wind as wind speed changes.