At the same time, the air on the upper surface has a tendency to flow in toward the fuselage and off the trailing edge. This air current forms a similar vortex at the inboard portion of the trailing edge of the airfoil, but because the fuselage limits the inward flow, the vortex is insignificant. Consequently, the deviation in flow direction is greatest at the outer tips where the unrestricted lateral flow is the strongest.
Just as lift increases with an increase in AOA, induced drag also increases. This occurs because as the AOA is increased, there is a greater pressure difference between the top and bottom of the airfoil, and a greater lateral flow of air; consequently, this causes more violent vortices to be set up, resulting in more turbulence and more induced drag.
In Figure 4-10, it is easy to see the formation of wingtip vortices. The intensity or strength of the vortices is directly to the weight of the aircraft and inversely to the wingspan and speed of the aircraft. The slower the aircraft, the greater the AOA and the wingtip vortices. Thus, an aircraft will create vortices with maximum strength occurring during climb, and landing phases of flight. These to a particularly dangerous hazard to flight, turbulence.