When an airfoil is flown at a positive Angle of Attack (AoA), a pressure differential exists between the upper and lower surfaces of the airfoil.

The pressure above the wing is less than the pressure below the wing. Air always moves from high pressure toward low pressure, and the path of least resistance is toward the airfoil’s tips, there is a spanwise movement of air from the bottom of the airfoil outward from the fuselage around the tips.

As the air curls upward around the tip, it combines with the downwash to form a fast-spinning trailing vortex. These vortices increase drag because of energy spent in producing the turbulence.

As AoA increases, induced drag also increases. This occurs because there is a greater pressure difference between the top and bottom of the airfoil, and a greater lateral flow of air. This causes more violent vortices to be set up, resulting in more turbulence and more induced drag.

The intensity or strength of the vortices is directly proportional to the weight of the aircraft and inversely proportional to the wingspan and speed of the aircraft. The heavier and slower the aircraft, the greater the AoA and the stronger the wingtip vortices.

The pressure compensation between wind upper and lower surface results in induced drag by wing tip vortices.