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induced drag[in′düst ′drag]
in aerodynamics, the part of the aerodynamic resistance of a wing that is caused by the vortices whose axes originate on the wing and are directed downward against the flow. These so-called free vortices of the wing originate from the flow of air near the wingtips (Figure 1) from the region beneath the wing to the region above it. Above the wing, the airflow near the tip results in a flow directed from the tips toward the plane of symmetry; below the wing, from the plane of symmetry toward the tip. As a result, each particle in the wake, behind the wing, rotates about an axis that passes through the particle and is parallel to the velocity vector v of the oncoming flow; here the direction of rotation is opposite for the left and right halves of the wing (Figure 2). Thus, a continuous system of vortices that emanate from every point of the wing surface arises.
In the region between the end planes of the wing, free vortices induce velocities directed downward, and when the flow induced by the free vortices is superimposed on the oncoming flow, it directs it downward through the angle Δα (the flow rake angle). Since the lift of the wing should be perpendicular to the oncoming flow, it is deflected downward through the same angle a (Figure 3). Resolving this force into components along and perpendicular to v, we obtain the induced drag dQindand the lift dY. If the wing has infinitely large span, there is no induced drag.
REFERENCEPrandtl, L. Gidroaeromekhanika, 2nd ed. Moscow, 1951. (Translated from German.)
N. IA. FABRIKANT