Induced Drag - Historical Perspective

Many modem aircraft missions require high values of aerodynamic efficiency with aircraft having wings of relatively restricted span lengths. In many of these missions, the aircraft must operate at relatively large values of the lift coefficients, and the large induced drag associated with the small span consequently results in relatively low values for the operational aerodynamic efficiency. In endeavoring to increase the flight efficiency of such aircraft, it becomes necessary to investigate more complex and unconventional wing forms which might offer the possibility of securing appreciable reductions in the induced drag, subject to the restriction of limited span length. Induced drag is associated with the shedding of vorticity along the span of a finite lifting wing and, in particular, in the wing-tip region. For most subsonic aircraft configurations, induced drag contributes about 50 percent of the total drag of the aircraft throughout its flight profile. As a result, there has been a strong interest in NASA and the aircraft industry in developing methods of reducing the induced drag. Past studies have focused on in-plane and some limited out-of-plane concepts. The results have shown that the well-designed modern CTOL wing is nearly optimum and that the use of out-of-plane devices can result in decrease in the induced drag.

In this paper, some of the past research will be reviewed as well as some of the current research activities. The capabilities of the current computational methods will be briefly reviewed and some forecast of potential high pay-off research for the future will be suggested.