## Aerodynamic Forces: Definitions of Lift and Drag

Wind turbines exploit the aerodynamic forces which arise when the wind blows on the rotor blades, and the blades move relative to the wind.
Any body which is immersed in a fluid stream will be subject to forces and moments (turning forces). The forces and moments depend on the shape of the body and its orientation and its movement or rotation in relation to the stream.

• The force parallel to the steam is called drag (air resistance).
A moment around this axis is called a rolling moment.
Drag is given a positive sign, when it pulls in the same direction as the stream. Drag is a loss that must be overcome with another force, if an object is to move against a stream.
• A force perpendicular to the stream is called lift.
A moment around this axis is called a yaw moment.
As the the implies, the sign is usually considered positive, when lift pulls upward. (Lift on a sail on a sailboat is usually measured sideways). Lift often performs some useful function, such as carrying the weight of a aircraft, or driving a boat forward.
• The force perpendicular to the other two is called the side force.
A moment around this axis is called a pitch moment.
If symmetric objects (e.g. cars) move directly against the wind, there is no side force impact on their movement. The side force concept is mostly used in connection with turning aircraft, but is not used much in connection with wind turbines.

Understanding Aerodynamic Forces
It is important to understand that the forces described above are simply practical definitions. They are not neccessarily different physical phenomena. When air molecules bounce against or slide along the surfaces of an object they tend to move and/or turn that object.
In order to have a common frame of reference when we discuss this, physicists use the terms which are indicated above.

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Updated 29 September 2002
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