As air flows around an aerofoil the pressure differential setup over the upper and lower surfaces produces a force. This force acts perpendicular to relative airflow and is known as lift. In steady level flight, lift exactly balances the aircraft’s weight. For a given airspeed, a lower weight requires less lift.
To understand fully how the aerodynamic force of lift acts on an aircraft, it is necessary to study the effect of airflow. In principle it does not matter whether an aircraft is moving through the air, or whether air is flowing a stationary aircraft, since the result is the same. Airflow can either be streamline or turbulent in nature.
Streamline flow existing when molecules of air follow a steady path, with the molecules flowing in an orderly pattern along streamlines around an object as shown in figure 1 below
If a sudden change in the direction of the airflow occurs, the streamline flow breaks down and becomes turbulent flow
Turbulent or unsteady/eddying flow is defined as the state of airflow when succeeding molecules can no longer follow a streamlined flow pattern and instead travel along a path different than the preceding molecules.
To help understand how airflow can be used and utilized to generate lift, we must first discuss Bernoulli’s theorem
Bernoulli’s Theorem uses the principle of conservation of energy. It states that when a fluid flows at a steady rate through a pipe, its total energy remains constant, since energy can neither be created nor destroyed. At any point in a pipe, the total energy is a combination of:
When considering airflow at a given height, changes in potential energy are negligible and can be ignored.
The theorem states that, if a steady stream of air flows through the restricted section of a venture(restricted) pipe, its velocity increases and vice versa. Any rise in velocity results in an increase in dynamic pressure and a reduction in static pressure and vice versa as shown in figure 2 below.
The Value of Lift
The value or quantity of lift is based among the following factors
The angle of attack is the angle between the free stream relative airflow and the chord line of an aerofoil section and the chord line is the straight line between the most leading point and the most trailing point of the wing as shown in Figure 3 below.
Changes in the angle of attack causes the velocity and pressure of the flow to vary as the air passes over the upper and lower surfaces which in return affects the pressure differential that exists and hence the amount of lift developed
Weight is the force of gravity on all objects within the gravitational field of planet earth, pulling us towards the ground at all times. Weight along with other forces counteract lift by pulling the aircraft towards the ground which means that the lift needed to maintain a certain altitude must equal the weight of the aircraft.
Weight equals the mass of an aircraft multiplied by the gravitational force of 10 Newton
As an aircraft proceeds in flight, the engines constantly consume fuel which has the effect of reducing the aircraft’s weight therefore reducing the lift required to maintain a specific altitude and the increasing the ability of climbing to higher altitudes (Optimum altitude) where more lift can be generated. More on optimum altitudes in a later article
To summarize the process of generating and controlling lift, an increase in air flow around an aerofoil will cause an increase in differential pressure therefore an increase in lift and vise versa.
An increase of the angle of attack increases the differential pressure therefore increasing lift until a certain angle where the streamline flow transforms into turbulent flow and the aerofoil fails to produce any lift.
Lift must equal weight to maintain a specific altitude and lift must exceed weight for an aircraft to climb to a higher altitude.