Asymmetrical Aerofoils

. When a force is transferred by a moving solid object to another solid object, the second object will generally move in either the same direction or in a direction at a small angle (less than 90 degree) to the direction of motion of the first object, unless subjected to another force. However, the method by which forces are transferred from a fluid to a solid object is very different.

Wind turbines are operating in an unconstrained fluid, in this case air. To understand how they work, two terms from the field of aerodynamics will be introduced. These are “drag” and “lift”. (Fig1)

An object in an air stream experiences a force that is imparted from the air stream to that object . We can consider this force to be equivalent to two component

It is the shape of the ‘under surface’ or high pressure side of the sections that identifies the type. Asymmetrical aerofoils are optimized to produce most lift when the underside of the aerofoil is closest to the direction from which the air is flowing. Symmetrical aerofoils are able to induce lift equally well (although in opposite directions) when the air flow is approaching from either side of the chord line (the length, from the tip of its leading edge to the tip of its trailing edge of an airfoil section).

These structural requirements generally mean the airfoil needs to be thicker than the aerodynamic optimum, especially at locations towards the root (where the blade attaches to the hub) where the bending forces are greatest. Fortunately, that is also where the apparent wind is moving more slowly and the blade has the least leverage over the hub, so some aerodynamic inefficiency at that point is less serious than it would be closer to the tip. Having said this, the section can’t get too thick for its chord length or the air flow will ‘separate’ from the back of the blade - similar to what happens when it stalls – and the drag will increase dramatically.