Geometric Parameters, Reynolds Number, and Basic CF Determination
The Re has the deciding role in determining the skin-friction coefficient, CF, of a component. First, the Re-per-unit-length speed and altitude is computed. Then, the characteristic lengths of each component [i. e., Re — (pTOLcompV(XJ)/дто] are determined. The characteristic length L of each component is as follows:
axial length from the tip of the nose cone to the end of the tail cone (Lfus) the wing MAC
the MACs of the V-tail and the H-tail
axial length from the nacelle-highlight plane to the nozzle-exit plane (Lnac)
Figure 9.19 shows the basic 2D flat-plate skin-friction coefficient, CFbasic, of a fully turbulent flow for local and average values. For a partial laminar flow, the CFbasic 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.