The lift created by an aircraft wing is a result of one of Newton’s laws of motion: for every action, there is an equal and opposite reaction.
Air approaches the wing in the opposite direction to flight. As air flows over the wing it is forced to change both direction and speed, with the end result being a downwards deflection – known as downwash.
The equal and opposite reaction to the downwash is an upwards force on the wing – known as the total reaction. The total reaction acts upwards and slightly backwards, which can be resolved into two vectors – lift and drag.
Lift is at a right angle to the relative airflow and drag is parallel to it. We will focus on the lift for now and discuss the drag in a later topic.
You will often hear people debating about whether it is the pressure differential or the downwash that creates lift. In truth, both the pressure difference and the downwash exist and the combination of both leads to the total reaction force.
The pressure differential that results from air flowing past the wing leads to the higher pressure air ‘expanding’ downwards and the lower pressure air being forced downwards by the surrounding higher pressure air above it. The result of this is the downwash at the back of the wing and the equal and opposite reaction to the downwash: the total reaction.
The total reaction is the combination of all the forces created by the airflow along the whole aerofoil. For simplicity, we combine the forces across the wing such that the total reaction acts at one point along the aerofoil, known as the centre of pressure.
The centre of pressure is not a fixed point. It moves along the aerofoil as angle of attack is changed.