Bernoulli's Principle explains the force which is (wrongly) called 'lift'. The force is perpendicular to the aerofoil, whether the aerofoil is top-towards-groun

Bernoulli's Principle explains the force which is (wrongly) called 'lift'. The force is perpendicular to the aerofoil, whether the aerofoil is top-towards-ground (plane flying upside down) or top-towards-sky (plane flying right way up). The angle of attack is always measured relative to the airflow, whether that relative airflow is upwards or downwards or sideways. Note that at zero degrees of angle of attack, the wing still provides 'lift' due to the Bernoulli Principle. At negative degrees angle of attack, the wing moves 'downwards' relative to the airflow, when the angle of attack is positive the wing moves 'upwards' relative to the airflow. NOTE: the terms 'upwards' and 'downwards' here are directions relative to the aerofoil/wing and have nothing at all with the orientation of the wing to the ground. (If a plane is flying upside-down, 'downwards relative to the wing' would lead to an increase in the plane's altitude.) When the angle of attack exceeds roughly 15 degrees relative to the airflow the fluid ('air', in other words) becomes turbulent which destroys the 'lift' and the wing is said to be 'stalled'. Being stalled is a dangerous situation and leads to aircraft literally falling out of the sky.