INDUCED DRAG

The two major components of the total drag of an airplane are the induced drag and the parasite drag. The parasite drag is the drag not directly associated with the production of lift. This drag, expressed as a coefficient, is

nearly constant and approximately equal to the drag for an airplane lift coefficient of zero. As the lift coefficient takes on a value different than zero, the drag coefficient will increase. This increment in Cd is defined as the induced drag coefficient, Q. Thus, for an airplane,

Cd = Сц, + Сц (4.14)

Here Сц, is the parasite drag coefficient and is not a function of CL. On the other hand, the induced drag coefficient, CD|, varies approximately as the square of CL. This dependence will be derived later.

Strictly speaking, this definition of Сц is not correct. Although it has become practice to charge to CDi any drag increase associated with CL, some of this increase results from the dependency of the parasite drag on the angle of atfack. What, then, is a more precise definition of Сц? Very simply, the induced drag at a given CL can be defined as the drag that the wing would experience in an inviscid flow at the same Cl. D’Alembert’s paradox assures us that a closed body can experience no drag in an inviscid flow. However, as we saw in the last chapter, a wing of finite aspect ratio generates a trailing vortex system that extends infinitely far downstream. Thus, the system in effect is not closed, because of the trailing vortex system that continuously transports energy across any control surface enclosing the wing, no matter how far downstream of the wing this surface is chosen.