All parts of an aircraft generate parasite drag, but the wings, as the producers of lift, generate an additional form of drag which is inextricably bound to their lift-producing function, and which is called induced drag. This type of drag bears the name induced drag because, in producing lift, the regions of differing pressure, above and below the wings, induce vortices, which vary in strength, dimension and drag effect with varying angle of attack. Because it is inseparable from lift, induced drag is also known as lift-dependent drag.
In dealing with parasite drag, we have considered airflow in two dimensions only: that is, airflow in the sense of the aircraft’s longitudinal axis, passing from front to rear, and airflow in the vertical sense, when considering upwash, upstream of the wing, and downwash, as the air leaves the aft sections of the wing, downstream.
But, in real life, a wing is not infinitely long. It must have wingtips. In the region of the wingtips, airflow is of a different nature from the two-dimensional flow that we have been considering up to this point. The finite length of the wing, and the consequent presence of wingtips, means that, at the wingtips, air flows from the under-surface of the wings (from the region of higher pressure) to the upper-surface of the wings, in a spanwise direction, modifying the airflow across the whole length of the wing, causing the type of vortices , and generating induced drag. It is this three-dimensional flow, then, which lies at the root of induced drag.
Trailing Edge and Wingtip Vortices.
Where the spanwise deflections in the airflow combine with the main longitudinal airflow, at the wing’s trailing edge, they meet at an angle to each other to form vortices at the trailing edge. When viewed from behind, these vortices will be rotating clockwise from the port (left) wing and anti-clockwise from the starboard (right) wing. The vortex at each wingtip is particularly large and strong.
It is the combination of trailing-edge and wingtip vortices which are the cause of induced drag.
Both wingtip and trailing edge vortices will become larger and stronger as the pressure difference between the upper and lower wing surfaces increases. Consequently, if we consider normal operating angles of attack (that is: below the stalling angle of attack), the vortices will increase in size and strength with increasing angle of attack.
This fact is the first factor in the explanation of why, unlike parasite drag, induced drag, in straight flight, increases as airspeed decreases (increasing angles of attack), and decreases as airspeed increases (decreasing angles of attack).
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