Wing-tip vortices are created by span-wise airflow over the upper and lower surfaces of a wing/aerofoil that meet at the wing tips as turbulence and therefore induces drag, especially on a swept wing.
Span-wise airflow is created because a wing producing lift has a lower static pressure on the upper surface than on the lower surface. At the wing tip, however, there can be no pressure difference, and the pressure is equalized by air flowing around the wing tip from the higher pressure on the lower surface to the lower pressure on the upper surface. There is therefore a span-wise pressure gradient, i.e., pressure changing along the wing span.
What are the effects of span-wise airflow over a wing?
1. Creates wing-tip vortices.
2. Reduced aileron (wing control surface) efficiency.
3. Reversed span-wise airflow increases disturbed airflow on the wing’s upper surface at the tip, contributing to a wing-tip stall.
the effects of wing-tip vortices
1. Create aircraft drag (induced drag because the vortices induce a downward velocity in the airflow over the wing, causing a change. in the direction of the lift force so that it has an induced-drag component; therefore, it creates a loss of energy).
2. Vortices create turbulence, which may affect the safety of other aircraft within approximately 1000 ft below or behind the aircraft.
3. Downwash affects the direction of the relative airflow over the tailplane, which affects the longitudinal stability of the aircraft.
preventing span-wise airflow on a wing, especially a swept wing
Fences and vortex generators. These items direct the airflow over the wing’s upper surface perpendicular to the leading edge.
purpose of vortex generators/fences
To reduce span-wise airflow and thereby reduce its effects. One of the effects of span-wise airflow over the wing is reduced effectiveness of the ailerons due to the diagonal airflow over the control surfaces. Vortex generators are located on the upper surface of a wing to create a slightly disturbed airflow perpendicular to the leading edge of the wing, which helps to maximize the effectiveness of the control surfaces, especially the ailerons. Fences also help to maximize the effectiveness of the control surfaces in a similar yet cruder manner. However, they are used normally to reduce the reverse span-wise airflow on the upper wing surface from reaching the wing tips, thus reducing the airflow, which contributes to a wing-tip stall. Vortex generators are used in other areas of the aircraft where a disturbed airflow is required (disturbed air tends to be denser and of a slower velocity), such as inlets to some types of auxiliary power units (APUs).
Winglets are aerodynamically efficient surfaces located at the wing tips. They are designed to reduce induced drag. They dispense the span-wise airflow from the upper and lower surfaces at different points, thus preventing the intermixing of these airflows that otherwise would create induced-drag vortices.