Principles of Flight - Drag (1)

Drag is the force opposite to the forward moving Thrust (i.e. flight direction). There are two types of drag - Induced Drag and Parasite Drag.

Induced Drag is actually "induced" from Lift. Still remember how Lift is formed? One of the reasons is due to the pressure difference below (higher pressure) and above (lower pressure) the wing. Air flows from the bottom to the top through the wing tip. The wingtip vortices are resulted causing the lift component to tilt backward.

To reduce induced drag, one can lengthen the wing so that air cannot spill over the wingtip easily. For example, glider has long wing which minimizes wingtip vortices. 

On the other hand, induced drag can also be reduced by using winglets.