Relative Wind and Angle of Attack

Relative Wind: The relative wind is the velocity and direction of the airflow with reference to the airplane. The relative wind flows opposite the direction of the air in motion. If the airplane is straight and level,the relative wind is horizontal,on descend it is from slightly upward and on climb it is from slightly downward. 

Angle of Attack: α (AOA): Angle of attack is the angle formed between the chordline and direction of the relative wind striking the airfoil. The Greek letter α (alpha) is used to denote this angle. The positive lift can be obtained if AOA increased. Each AOA produces a particular lift coefficient,which increases up to a maximum value. The lift begins to decrease at a critical angle of attack,which varies with the airplanes type and its configurations. 

The airflow about a symmetrical airfoil at 0 degree AOA attack will have a streamline pattern over the upper and lower surfaces of the airfoil. The airflow about a chambered airfoil has the ability to produce non symmetrical pressure distribution even at 0 degree AOA. Because of the positive chamber,the distance from the leading edge stagnation point to trailing edge stagnation point is greater over the upper surface than over the lower surface. 

Angle of Incidence: The angle of incidence is the angle formed by the intersection of the wing chord line and the longitudinal axis of the aircraft. It is fixed angle for a particular airplane and mostly designed with positive angle of incidence. 

Pitch Angle(Attitude): Angle of Attack (AOA) is sometimes confused with pitch angle(attitude) or flight path angle. Pitch angle is the angle between the airplane nose and the horizon.