Induced Drag
Induced drag is a direct result from the production of lift and therefore it is also called lift dependant drag. As lift increases so does the induced drag. How does induced drag occur? As we know lift is produced by differences in pressure above and below the wing. As lift is produced, a lower pressure above the wing and a higher pressure below the wing have the tendency to flow to eachother. Figure 1.1 and 1.2 visualizes this effect.

Pressure distribution of a wing
Figure 1.1 Wing pressure distribution Figure 1.2 Pressure patterns above and below

Due to the pressure difference between the upper and lower wing, air spills around the wingtip causing the flow over the wing to deflect towards the fuselage while the flow beneath it is deflected towards the wingtip. When these flows meat at the trailing edge of the wing a series of turbulent flows called vortices are formed, which accumulate at the tip creating a large wingtip vortex. These vortices increase in strength as lift increases.

Induced drag is the inverse to the square root of speed. This means that if the speed is decreased by half, the induced drag fourfolds, this in contrast to parasite drag. At speeds nearing the stall speed, induced drag is therefore the the major cause of drag at low speeds but reduces as speed increases. Under certain conditions it is possible to observe aerodynamics and marvel at the physics of it all. On take-off all aircraft produce vortices at rotation in order to produce sufficient lift to get the aircraft airborne. If the air is moist, the pressure drop may be enough to cause condensation making vortices visible.

Factors which affect induced drag are:
- wing shape;
- aspect ratio;
- lift and weight;
- speed;

Figure 1.3 - Induced drag curve
Wingtip Modification
Installation of additional wingtip devices or wingtanks can reduce the leakage of the airflow around the wing ends and thus reduce the induced drag. Also the installation of wing fences can reduce spanwise flow and thus the induced drag. Figure 1.3 and 1.4 show an example of a wingtip modification.

Figure 1.4 - Boeing 747 wingtip modification
Figure 1.5 - Boeing 737 wingtip modification

Induced Downwash
As the air leaves the trailing edge of the wing the net effect of the vortex is the deflection of the airflow downward. This means that the average relative wind is inclined downward and rearward and lift is inclined aft. The rearward component of lift is therefore called induced drag (figure 1.5).
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