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2.6.  ANGLE OF ATTACK

The angle of attack is the angle at which an airfoil passes through the air. This angle is measured between the chord of the airfoil and the relative wind, as shown in figure 2.6. The chord is an imaginary line from the leading edge to the trailing edge of an airfoil. Increasing the angle of attack deflects the airstream and causes an upward pressure on the underside of the airfoil. This in turn increases the speed of the airflow over the topside of the airfoil. As air-flow-speed increases, pressure on the foil's top side is further reduced. The upward pressure on the foil's underside and pressure reduction on the top side combine to furnish lift.

Figure 2.6. Angle of Attack.

The angle of attack and angle of incidence are measured angles.

Angle of incidence (fixed-wing aircraft) is the angle between the airfoil chord line and the longitudinal axis or other selected reference plane of the aircraft.

Angle of incidence (rotary-wing aircraft) is the angle between the chord line of a main or tail rotor blade and the plane of rotation (tip path plane). It is usually referred to as the blade pitch angle. For fixed airfoils, such as vertical fins or elevators, the angle of incidence is the angle between the chord line of the airfoil and a selected reference plane of the helicopter.

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