Parasite Drag Definition
The components of drag due to viscosity do not contribute to lift. For this reason, it is considered “parasitic” in nature. For bookkeeping purposes, parasite drag is usually considered separately from other drag sources. The main components of parasite drag are as follows:
• drag due to skin friction
• drag due to the pressure difference between the front and the rear of an object
• drag due to the lift-dependent viscous effect and therefore seen as parasitic (to some extent resulting from the nonelliptical nature of lift distribution over the wing); this is a small but significant percentage of total aircraft drag (at LRC, it
All of these components vary (to a small extent) with changes in aircraft incidence (i. e., as CL changes). The minimum parasite drag, CDpmin, occurs when shock waves and boundary-layer separation are at a minimum, by design, around the LRC condition. Any change from the minimum condition (CDpmin) is expressed as ACDp. In summary:
parasite drag (CDp) = (drag due to skin friction [viscosity]+drag due to pressure difference [viscosity]) = minimum parasite drag (CDpmin)
+ incremental parasite drag (ACDp) (9.1)
Oswald’s efficiency factor (see Section 3.12) is accounted for in the lift-dependent parasite drag. The nature of ACDp is specific to a particular aircraft. Numerically, it is small and difficult to estimate.
Parasite drag of a body depends on its form (i. e., shape) and is also known as form drag. The form drag of a wing profile is known as profile drag. In the past, parasite drag in the FPS system was sometimes expressed as the drag force in pound force (lbf) at 100 ft/s speed, represented by Dioo. This practice was useful in its day as a good way to compare drag at a specified speed, but it is not used today. These two terms are not used in this book.
The current industrial practice using semi-empirical methods to estimate CDpmin is a time-consuming process. (If computerized, then faster estimation is possible, but the author recommends relying more on the manual method at this stage.) Parasite drag constitutes one-half to two-thirds of subsonic aircraft drag. Using the standard semi-empirical methods, the parasite drag units of an aircraft and its components are generally expressed as the drag of the “equivalent flat-plate area” (or “flat-plate drag”), placed normal to airflow as shown in Figure 9.1 (see Equation 9.7). These units are in square feet to correlate with literature in the public domain. This is not the same as air flowing parallel to the flat plate and encountering only the skin friction.
The inviscid idealization of flow is incapable of producing parasite drag because of the lack of skin friction and the presence of full pressure recovery.
Figure 9.1. Flat plate equivalent of drag