Figure 9.19 shows the basic 2D flat-plate skin-friction coefficient, CF basic, of a fully turbulent flow for local and average values. For a partial laminar flow, the CF basic correction is made using factor f1, given in Figure 9.20. It has been shown that the compressibility effect increases the boundary layer, thus reducing the local CF. However, in LRC until the Mcrit is reached, there is little sensitivity of the CF change with Mach number variations; therefore, the incompressible CF line (i.e., the Mach 0 line in Figure 9.19b) is used. At HSC at the Mcrit and above, the appropriate Mach line is used to account for the compressibility effect.

The methodology presented herein considers fully turbulent flow from the LE of all components. Here, no credit is taken for drag reduction due to possible laminar flow over a portion of the body and lifting surface. This is because it may not always be possible to keep the aircraft surfaces clear of contamination that would trigger turbulent flow. The certifying agencies recommend this conservative approach.

The basic CF changes with changes in the Re, which depends on speed and altitude of the aircraft. The chapter introduction in Section 9.2 explains that a subsonic aircraft CDpmin computed at LRC would cater to the full flight envelope during Phase 1 of a project.

9.7.2 Computation of Wetted Areas

Computation of the wetted area, Aw , of the aircraft component is shown herein. Skin friction is generated on that part of the surface over which air flows, the socalled wetted area.

Lifting Surfaces

These are approximate to the flat surfaces, with the wetted area slightly more than twice the reference area due to some thickness. Care is needed in removing the areas at intersections, such as the wing area buried in the fuselage. A factor k is used to obtain the wetted area of lifting surfaces, as follows:

Aw = k × (reference area, S − the area buried in the body),

where k = 2.02 for t/c = 0.08%

=2.04 for t/c = 0.12%

=2.06 for t/c = 0.16%

The factor k may be interpolated linearly for other t/c ratios.

Fuselage

The fuselage is divided conveniently into sections – typically, for civil transport aircraft, into a constant cross-section mid-fuselage with varying cross-section frontand aft-fuselage closures. The constant cross-section mid-fuselage barrel has a wetted area of Aw f mid = perimeter × length.

The forwardand aft-closure cones could be sectioned more finely, treating each thin section as a constant section “slice.” A military aircraft is unlikely to have a constant cross-section barrel, and its wetted area must be computed in this way. The wetted areas must be excluded where the wing and empennage join the fuselage or for any other considerations.