What is the vortex and the induced drag ?

This always exists in the parallel direction of the relative wind. It is the main cause of the whirlwinds that appear on the trailing edge: the air passing on the top skin of a wing has a tendency to pass over the lower surface. This is because the pressure on the top skin is lower than the pressure outside of the wing-tips.

On the other hand, the relative wind flows under the outside of the wing because the pressure on the lower surface is greater than the one outside of the wing. The air tries to turn around the wing-tips from the lower surface to the top skin. The way to explain why a higher aspect ratio is better than a shorter one, would be to say that the greater the aspect ratio, the smaller the air quantity escaping outside from the wing-tips is. The air turning around the wing-tip is not useful to produce lift; this is often called "a marginal loss".
As these two airflows, the one from the top skin and the one from the lower surface, meet on the trailing edge at a certain angle, they create whirlwinds turning clockwise (seen from the back) behind the left wing and counter clockwise behind the right wing. All the vortices from the same side have a tendency to gather in order to form a great vortex that escapes from each wing-tip. This is called tip vortex or easer vortex.
Many pilots have seen this vortex or, more precisely, the central part of them that condensation makes perceptible. The air humidity condensates because of the pressure loss in the heart of the vortex. You must not confuse these visible vortices at take-off with those created by the exhaust gas of engines at altitude.
If we now watch the direction of rotation of the vortex we perceive that there is a draught towards the top at the outside of the wing span and a draught downwards behind the trailing edge. You must not confuse this downwards draught with the normal deflexion that occurs. In this last case the deflexion downwards always goes with an upwards deflexion in front of the wing so that the final direction of draught is not modified. But in the case of tip vortex the deflexion occurs upwards outside of the wing and not in front of it, so that the draught leaving the wing is finally directed downwards. By consequence, the lift which is perpendicular to airflow is slowly inclined towards the back and contributes to drag. Result of the vortex, this part of drag is called induced drag.

Other possible applications:

· Airplane wing
· Racers wing
· Glider wing
· U.L.M. wing
· U.A.V wing
· Stabilizing boat wing
· Rudder blade
· Lipp rotor wing
· Submarine airfoil
· Hydrofoil wing
· Hydraglider wing
· Tidal energy wing
· Airfoil racercars, F1
· Windmill blade
· Windmill submarine blade
· Helicopter blade
· Horn Sail, etc...

Source: minix.frAdded: 22 February 2011