Angle of Attack

The angle of attack of a turbine blade is the angle between the direction of the apparent or relative wind and the chord line of the blade. For an aircraft wing, it is the angle between the direction of motion of the wing and the chord line of the wing.

At very low angles of attack, the airflow over the aerofoil is essentially smooth and laminar with perhaps a small amount of turbulence occuring at the trailing edge of the aerofoil. The point at which laminar flow ceases and turbulence begins is known as the separation point.

Increasing the angle of attack increases the area of the aerofoil facing directly into the wind. This increases the lift but it also moves the separation point of laminar flow of the air above the aerofoil part way up towards the leading edge and the result of the increased turbulent flow above the aerofoil is an increase in the drag.

Maximum lift typically occurs when the angle of attack is around 15 degrees but this could be higher for specially designed aerofoils.

Above 15 degrees, the separation point moves right up to the leading edge of the aerofoil and laminar flow above the aerofoil is destroyed. The increased turbulence causes the rapid deterioration of the lift force while at the same time it dramatically increases the drag, resulting in a stall.

 The graph opposite shows the lift and drag at different angles of attack experienced by a Clark Y aerofoil, a type widely used in general purpose aircraft designs. When moving through the air at constant speed, as the angle of attack is increased, both the lift and the drag increase until the aerofoil reaches a critical angle when the lift suddenly falls away and the aerofoil begins to stall, in this case, as the angle of attack approaches 20 degrees.