Minimum Parasite Drag Estimation Methodology

The practiced method to compute CDpmin is first to dissect (i. e., isolate) the air­craft into discrete identifiable components, such as the fuselage, wing, V-tail, H-tail, nacelle, and other smaller geometries (e. g., winglets and ventral fins). The wetted
area and the Re of each component establishes skin friction associated with each component. The 2D flat-plate basic mean skin-friction coefficient, CFbasic, corre­sponding to the Re of the component, is determined from Figure 9.19b for the flight Mach number. Sections 3.5.1 and 9.7.1 explain the worked-out examples carried out in this book for fully turbulent flow, as shown in Figure 9.19.

The various ACf arising from the 3D effects (e. g., supervelocity) and wrapping effects of the components are added to the basic flat-plate CFbasic. Supervelocity effects result from the 3D nature (i. e., curvature) of aircraft-body geometry where, in the critical areas, the local velocity exceeds the free-stream velocity (hence, the term supervelocity). The axi-symmetric curvature of a body (e. g., fuselage) is per­ceived as a wrapping effect when the increased adverse pressure gradient increases the drag. The interference in the flow field is caused by the presence of two bod­ies in proximity (e. g., the fuselage and wing). The flow field of one body interferes with the flow field of the other body, causing more drag. Interference drag must be accounted for when considering the drag of adjacent bodies or components – it must not be duplicated while estimating the drag of the other body.

The design of an aircraft should be streamlined so that there is little separation over the entire body, thereby minimizing parasite drag obtained by taking the total CF (by adding various ACF, to CFbasic). Hereafter, the total CF will be known as the CF. Parasite drag is converted to its flat-plate equivalent expressed in f square feet. Although it can be easily converted into the SI system, in this book, the FPS system is used for comparision with the significant existing data that uses the FPS system. The flat-plate equivalent f is defined as:

fcomponent — (Aw X CF)component

where Aw is the wetted area (unit in ft2).

The minimum parasite drag CDpmin of an aircraft is obtained by totaling the con­tributing fs of all aircraft components with other sundries. Therefore, the minimum parasite drag of the aircraft is obtained by:

(CDpmin ) — (E fcomponent + sundries^ / SW — (Aw X CF)component/S^j (9.8)

The stepwise approach to compute CDpmin is described in the following three sub­sections.