Induced Drag and Total Aircraft Drag
Equation 3.19 gives the basic definition of drag, which is viscous-dependent. The previous section showed that the tip effects of a 3D wing generate additional drag for an aircraft that appears as induced drag, mi. Therefore, the total aircraft drag in incompressible flow would be as follows:
aircraft drag = skin-friction drag + pressure drag + induced drag
= parasite drag + induced drag (3.30)
Most of the first two terms does not contribute to the lift and is considered parasitic in nature; hence, it is called the parasite drag. In coefficient form, it is referred to as CDp. It changes slightly with lift and therefore has a minimum value. In coefficient form, it is called the minimum parasite drag coefficient, CmPmm, or CD0. The induced drag is associated with the generation of lift and must be tolerated. Incorporating this new definition, Equation 3.30 can be written in coefficient form as follows:
Cm = CDp + CDi (3.31)
Chapter 9 addresses aircraft drag in more detail and the contribution to drag due to the compressibility effect also is presented.