Lift and angle of attack

Relative wind and angle of attack

The relative wind is the airflow the wing feels, which is opposite to the airplane's flight path. The angle of attack (AoA) is the angle between the wing's chord line (an imaginary line from leading edge to trailing edge) and the relative wind.

Angle of attack is not the same as pitch attitude. An airplane can be nose-high with a low AoA (in a steep climb at speed) or nose-level with a high AoA (sinking slowly at low speed).

How a wing makes lift

A wing accelerates the air flowing over its curved upper surface, which lowers the pressure there (Bernoulli's principle). The wing also deflects air downward, and pushing air down pushes the wing up (Newton's third law). Both explanations describe the same thing happening.

The amount of lift depends on the angle of attack, the airspeed (lift increases with the square of airspeed), the air density, and the wing's area and shape. In flight, you control lift mainly with angle of attack (the elevator) and airspeed (power and pitch).

The critical angle of attack

As you increase the angle of attack, lift increases, but only up to a point. At the critical angle of attack, the airflow can't follow the upper surface anymore. It separates, lift drops sharply, and the wing stalls.

This is the most important idea in this unit: a wing stalls when it exceeds the critical angle of attack, regardless of airspeed, attitude or power. The published stall speeds only apply to specific conditions (1G, a given weight and configuration). In a steep turn or a sharp pull-up, the wing can stall at a much higher airspeed.

To recover from a stall, you must reduce the angle of attack (lower the nose by relaxing back pressure or pushing forward). Power helps, but reducing AoA is what unstalls the wing.