WHEN considering a flat plate of finite width at a low angle of attack, it is observed that, downstream from the plate, where the air flow from the suction and pressure sides join, a transverse velocity appears. On the pressure side there is an outward flow toward the ends of the plate, while on the suction surface the component is inward. Since all streamlines have the same origin, in the case of steady flow the pressure where the flow over upper and lower surfaces joins must be the same and no velocity ‘jump’ is possible. It follows then that any velocity change must be purely transverse and that this will extend to the ends of the plate. The same phenomenon occurs with wings of finite span and the vortices are formed continually in flight. These vortices absorb energy, for which power has to be supplied continuously—in other words the induced drag caused solely by the vortex formation appears.
Goossens, E. (1947), "Downwash Behind Wings of Finite Span: Simplified Formulae Developed by a Belgian Engineer Who Completed his Work Early in 1946", Aircraft Engineering and Aerospace Technology, Vol. 19 No. 8, pp. 244-248. https://doi.org/10.1108/eb031537
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