INDUCED DRAG OF WINGS

3- 4-1 Drag of Wings of Finite Span

The total drag of a wing of finite span, Sec. 1-3-2, is composed of profile drag and induced drag:

D = Dp + Di (3-124)

The profile drag Dp is created by friction effects. It is almost independent of the wing aspect ratio. A procedure for the theoretical determination of the profile drag has been developed in Sec. 2-5-2. Experimentally, the profile drag can be determined through wake measurements (momentum-loss measurements). The induced drag Dz- exists only at a wing of finite span. It is created by the flow processes at the. wing tips and can be determined from the laws of inviscid flow. In Sec. 3-2-1 it has been demonstrated that the induced drag is proportional to the square of the lift.

The drag coefficient cD —D/Aq„ of a wing with elliptic lift distribution is, from Eq. (3-326):

cD=cDP+cDi (3-125 a)

= cDp + ^ (3-1256)

where A, from Eq. (3-4n), is the wing aspect ratio.

There are two methods available for the determination of the induced drag. They differ in their physical concepts. In the first method, the induced drag is found from the pressure forces that act on the wing itself. In the second method, the induced drag is obtained from energy considerations. The latter approach allows the determination of the induced drag of only the whole wing. Conversely, the first method produces, within the framework of simple lifting-line theory, the local distribution of the induced drag. Truckenbrodt [86] summarizes the state of the art of the drag of wings. Basic considerations to the drag problem stem from Jones [38]. Also, the comprehensive compilation of experimental data of the wing aerodynamics by Hoerner [30] must be mentioned.