Editing Lift-induced drag (section)

==Vortices==
When producing lift, air below the wing is at a higher pressure than the air pressure above the wing. On a wing of finite span, this pressure difference causes air to flow from the lower surface, around the wingtip, towards the upper surface.<ref name="Understanding Aerodynamics">{{cite book |last=McLean |first=Doug |date=2012 |title=Understanding Aerodynamics: Arguing from the Real Physics |isbn=978-1119967514}}</ref>{{rp|8.1.1}} This spanwise flow of air combines with chordwise flowing air, which twists the airflow and produces vortices along the wing [[trailing edge]].<ref name="McLean"/>{{rp|4.6|quote=The induction myth is more complicated and involves a serious misunderstanding of cause and effect. The trailing vortex sheet and the rolled-up vortex cores are often talked about as if they were the direct cause of the velocities everywhere else in the flowfield, and of induced drag, but this is misleading. It is true that in order for the large-scale flow pattern of Figure 3.1 to exist, there must be a vortex sheet shed from the trailing edge, but the vortex sheet is not a direct physical cause of the large-scale flow; it is more of a manifestation.}}<ref name="McLean"/>{{rp|4.7|quote=The induction myth leads us to think of induced drag as being "caused" by the vortex wake, and thus to think that by doing something very local to change the flow in the core of the “tip vortex” we can have a large effect on the induced drag.}}<ref name="Understanding Aerodynamics"/>{{rp|8.1.4, 8.3, 8.4.1}}

The vortices reduce the wing's ability to generate lift, so that it requires a higher angle of attack for the same lift, which tilts the total aerodynamic force rearwards and increases the drag component of that force. The angular deflection is small and has little effect on the lift. However, there is an increase in the drag equal to the product of the lift force and the angle through which it is deflected. Since the deflection is itself a function of the lift, the additional drag is proportional to the square of the lift.<ref name="Clancy"/>{{rp|Section 5.17}}

The vortices created are unstable,{{what|date=March 2022}} and they quickly combine to produce [[wingtip vortices]] which trail behind the wingtip.<ref name="Clancy"/>{{rp|Section 5.14}}