Of the four primary forces that enable an airplane to fly, lift is fundamental to flight. All vertical movement of the aircraft is a consequence of a relative imbalance between the weight of the aircraft and the force of lift produced by the airplane wings.
The wings of an aircraft are airfoils designed to create this force of vertical motion. Any surface that alters the airflow to the advantage of a produced force in a particular direction is termed as an airfoil. Airplane wings, designed as airfoils, achieve this by interacting with the remote airflow to produce the desired lift.
Understanding Air at the Molecular Level
Air is composed of molecules moving randomly at high speeds. With respect to airplane wings, these molecules exert a force on the airfoil, whenever they come in contact with it. This force or pressure is called static pressure, which entails the total force exerted by the molecules of an air mass.
A parcel of air does not remain stationary. Its movement involves the movement of the very molecules within it. When the airplane wings are subjected to an air mass that is stationary, the molecules of air strike the wing at angles that are perpendicular or nearly perpendicular.
Perpendicular interaction of molecules against the wing exerts a greater force or a greater static pressure when compared with molecules striking the wing at relatively slanted angles. Air molecules striking at slanted angles contribute to a relatively lower static pressure.
Bernoulli’s principle, which states that “high flow velocity gives a low static pressure” is in line with this concept. The higher the velocity of airflow over airplane wings, the lower would be the static pressure exerted on the wing.
Airplane Wings Creating Lift
The force of lift completely depends on the interaction of air molecules with airplane wings. To harvest this force, the wings of an aircraft must be designed accordingly. Consider a single plate exposed horizontally to a streamlined airflow at a considerable speed.
- The plate is exposed to the airflow.
- Air molecules slide past the plate.
- A very few molecules interact with the plate, and that interaction too is at a slanted angle providing low static pressure.
Airplane wings are never built in-line with the fuselage of the aircraft. These airfoils are always installed at an angle (generally around 4 degrees). This angle of the wing is termed as the angle of inclination, depicting the particular angle at which the wing is inclined. When this inclination is applied to the plate in supposition:
- Air molecules strike against a greater portion of the plate.
- Much of the plate is exposed to perpendicular strikes of the air molecules.
- The air mass exerts a reactional force that manifests itself in terms of lift and drag.
The plate, in this instance, represents airplane wings that are typical airfoils designed to harvest this reactional force. An overview of how airplane wings create lift is as under:
- Whether the speed of the wings itself in the air mass, or the movement of the air mass relative to the wings. the wings have a relative speed to the air mass.
- The air strikes against the inclined wing and speeds up over the wing. This movement of air (upwards and over the wing), is called upwash.
- Upwash restricts the random movement of air, causing the molecules to flow in a relatively streamlined manner and, thus speeding it up.
- As explained earlier, high speed motion of air causes its static pressure over the airplane wing to decrease.
- This creates an area of low pressure over the airplane wings and a relatively higher pressure under them.
- This pressure differential exerts an aerodynamic force on the wing which basically has two components; lift and drag.
- The lift component acts perpendicular to the direction of the airflow.
- Drag is parallel to the airflow.
Various airfoil shapes and designs have been researched, each having a different effect. The most efficient for the production of lift by airplane wings is called a “well-cambered airfoil,” having a curvature near the leading edge on the upper surface of the wing.
The following video demonstrates the interaction between the forces responsible for the flight of an airplane.
Federal Aviation Administration, Flight standards Service. Airplane Flying Handbook. (2004).
Federal Aviation Administration, Flight standards Service. Pilot’s Handbook of Aeronautical Knowledge. (2008).
Trevor, T. Aeroplane General Knowledge and Aerodynamics. Aviation Theory Centre. (2004).